26/29 : contact patch traction vs. rolling resistance- Mtbr.com
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  1. #1
    CDB
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    26/29 : contact patch traction vs. rolling resistance

    I am aware that if you have two tires, one 26" and one 29", each the same tread pattern, air pressure, and rider weight, there are some differences that affect the ride.

    I know the 29er tire has a longer contact patch, providing more tread contact at any given moment, and that the larger outer radius of the 29" wheel rolls over small bumps w/ less resistance. I hear multiple times from folks on this forum that 29er would have lower rolling resistance, as well as greater traction. My question is, how does that come? Is it due to the radius, and/or the contact patch.

    As you gain traction, does that create rolling resistance? A direct correlation?

    How does "overall rider weight per square inch of tire surface contact" factor into the equation? Would a 26er tire, due to less surface contact, effectively have greater traction? Wouldn't each of those knobs have more weight driving down into the dirt per square inch vs. spreading that load out over more knobs w/ a 29er tire? Again, remember I'm assuming the same tread pattern, rim width, casing size, and pressure.

    The reason for my curiosity is from doing some very steep climbs over the weekend on the 29er, in the neighborhood of 32-38% steady climbs over consolidated, firm dirt w/ the occasional little bit of loose rock on top. I got to thinking that maybe my smaller 26er tires would bite in a bit better. Maybe it was in my head.

    Anyone else have informed opinions on this?

  2. #2
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    Quote Originally Posted by CDB
    The reason for my curiosity is from doing some very steep climbs over the weekend on the 29er, in the neighborhood of 32-38% steady climbs over consolidated, firm dirt w/ the occasional little bit of loose rock on top. I got to thinking that maybe my smaller 26er tires would bite in a bit better. Maybe it was in my head.

    Anyone else have informed opinions on this?
    Cloxxki did an explanation here + read the comments
    http://twentynineinches.com/2006/06/...-and-traction/
    and some thoughts with center of gravity
    http://twentynineinches.com/2006/06/...ces-1-braking/

  3. #3
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    so i am wondering when in the maturity of 29ers these kind of questions will stop.
    Tires for real rides: www.terrenetires.com

  4. #4
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    informed... No

    Opinion... Yes

    you will climb said hill better with the bike YOU BELIEVE will climb it better. Mind over matter man. Especially true of climbing, it is a mental game more than a physical one....

    And concerning hard facts and physics.

    unless you are spinning out on your 29er then you don't need to worry about any of that...
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  5. #5
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    Quote Originally Posted by centerridgerider
    informed... No

    Opinion... Yes

    you will climb said hill better with the bike YOU BELIEVE will climb it better. Mind over matter man. Especially true of climbing, it is a mental game more than a physical one....

    And concerning hard facts and physics.

    unless you are spinning out on your 29er then you don't need to worry about any of that...
    I disagree. There are limits to bike design. A given rider will be able to negotiate steeper hills with perfect technique on one bike then on the other. I've seen and experienced this. On most types of steep hills, 29" is an obvious advantage. This is not subject to opinions. The tall and clumsy 29" rider in a group cleans the tough hills the shorties on 26" don't.
    Not a hard rule, but certainly an average.

    To understand a difference better, take it to an extreme. Imagine attacking a steep hill on a perfect MTB with 20" wheels. Wheelsize behaves proportionately.
    Klok - XC - Skate - Ski

  6. #6
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    Quote Originally Posted by CDB
    I know the 29er tire has a longer contact patch, providing more tread contact at any given moment, and that the larger outer radius of the 29" wheel rolls over small bumps w/ less resistance. I hear multiple times from folks on this forum that 29er would have lower rolling resistance, as well as greater traction. My question is, how does that come? Is it due to the radius, and/or the contact patch.

    As you gain traction, does that create rolling resistance? A direct correlation?

    How does "overall rider weight per square inch of tire surface contact" factor into the equation? Would a 26er tire, due to less surface contact, effectively have greater traction? Wouldn't each of those knobs have more weight driving down into the dirt per square inch vs. spreading that load out over more knobs w/ a 29er tire? Again, remember I'm assuming the same tread pattern, rim width, casing size, and pressure.
    I would be interested in making some "fingerprints" of similar 26" and 29" tires to see how different the shape of the contact patch actually is. We always refer to the longer contact patch of the 29" tire, but I'll bet the difference isn't that notable.

    That said, at equal pressures, both will have equally sized contact patches, despite their different shapes. To deform a 29" tire the "long way" ought to take less physical deformation of the rubber than to deform a 26" tire the "squat way," and I figure that would result in a rolling resistance advantage to the bigger tire.

    But I wonder if that advantage is canceled out due to me running my 29" tires at slightly lower pressures that I previously ran my 26" tires?!? FWIW, I've never done side-by-side comparisons using similar makes of tires in different diameters, and my 26" bikes were substantially different geometries than the 29" bikes, so any head to head observations are moot -- except that I believe I get better traction on the 29"er.
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  7. #7
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    I think Shiggy once posted some contact patch research he carried out.

    A larger diameter tire should be deformed more at the "tip" of the tire, where the smaller one is deformed more along the sides of the casing.

    I also run lower pressures with 29", and for sure this is going to offset rolling resistance in aprt, full, or more than full.
    I recently rode a to me very familiar trail on a heavier friend's 29" bike with Racing Ralphs, 2.4F, 2.25R. Slightly higher pressures than I would have preferred, although the grip was alrady more than I know what to do with. I and really know how to blast those trails
    Anyway, the rolling resistance was shockingly low compared to my already fast Wweirwolf 2.55/Karma 1.9 combo. Perhaps with the RR's, a slightly higher psi is just worth it more. My WW/Karma combo required the lower psi to reach optimal grip, I put them lower halfway into the first lap for that.
    With 29", I remain amazed by how well I can coast over super-winding singletrack with naturally created berms. Swoop, swoop, you seem to keep rolling a few corners further. The RR's were worth one left-right before I felt like applying pedal pressure again even over the combo my own bike ran.

  8. #8
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    Rolling resistance refers to the tires ability to absorb terrain changes in order to maintain momentum, so pressure and wheel size are the key factors that determine RR. So if 26/29 tire pressure is the same, the 29er will still have lower rolling resistance because it is bigger.

    At equal pressure, the 26/29 contact patch is the same. You have to lower the pressure in the 29er to achieve a bigger contact patch (which you should since the 29er volume is bigger). This contact patch is related to friction which is related to traction. So a 29er tire that achieves similar deformation as a 26er (via lower pressure) will have more contact and hence more traction.
    My rides:
    Lynskey Ti Pro29 SS
    RM Suzi Q 90 RSL
    KHS Team 29
    S-Works Roubaix
    KHS CX 550 cyclocross

  9. #9
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    Threads like this make me shoot animals for no good reason.
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  10. #10
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    I dunno about al the scientific data, experiments etc., all I know is that I climb way better since I moved to a 29er. I can easily clean climbs that gave me horrors before, stall out and still maintain traction and continue on. Make it technical and holy crap no comparison IMHO.

  11. #11
    CDB
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    Quote Originally Posted by LyNx
    I dunno about al the scientific data, experiments etc., all I know is that I climb way better since I moved to a 29er. I can easily clean climbs that gave me horrors before, stall out and still maintain traction and continue on. Make it technical and holy crap no comparison IMHO.
    I agree about the limitations of getting bogged down w/ minutia and technical data vs. the "subconscious perception stuff" and "trusting what you're feeling". But, since we all cannot get inside your head to actually "feel" what you are feeling, and we know nothing of your variables that affect WHY one particular tire works better for YOU, it is worth a very small amount to others. Things like history prior to 29, rider style, body type/weight, terrain type, tread pattern, air pressure, etc. are helpful bits of information that make a statement actually helpful to others. We're sharing experiences, but w/o the details, they offer very little. If you feel this conversation is not interesting to YOU, or discussion of the minute details is bland on your palate, feel free to simply NOT PARTICIPATE!

    It's kind of like describing a preference for one particular chewing gum flavor vs. another. Why do you like that flavor? "I don't know, I just kinda like it". Cool, but that doesn't help me (or others) UNDERSTAND why. Not meant to be directed specifically to YOU, but more of a thought to consider in GENERAL when participating in these forums. Certainly everyone's opinion is worthy of consideration, just remember that what you write will be read by someone else's eyes, and the question is, "how is this information actually going to help someone else?" The specifics are what make it more helpful, in my respectful opinion.

    Thank you all for your insights. I'd like to hear more about this subject. I know that when I was researching Schwalbe Little Alberts a few weeks ago, I came across an article, maybe German based, that showed an ink imprint of that same tread pattern in 26 and 29. I cannot place my finger on it now. Not sure that is helpful to this discussion, but I thought it was an interesting illustration. I'm not sure if they had identical air pressures, or if that was apparent on the "tread stamp".

    I am still curious to read some informed opinions regarding the relationships between the following variables:
    • a rider's mass (gravity pushing towards the earth)

    • unit of surface contact (tread contact patch)

    • traction (resistance to the tire spinning out in the dirt)

    • rolling resistance (less contact = more or less friction with a given tread pattern?)

    • how a rider's unit of mass relative to square inches of tire surface contact affects traction and rolling resistance.

  12. #12
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    Quote Originally Posted by CDB
    I came across an article, maybe German based, that showed an ink imprint of that same tread pattern in 26 and 29.

    • rolling resistance (less contact = more or less friction with a given tread pattern?)
    If anyone can find a link to those ink blots, or Shiggy's test that Cloxxki referred to earlier, please share.

    The German <i>Bike</i> I recall was good with rolling resistance tests, so perhaps this has been studied and reported on already. Anybody want to page Nino in the WW forum? I remember him being the go-to guy for tire resistance tests in the past.
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  13. #13
    CDB
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    Quote Originally Posted by serious
    Rolling resistance refers to the tires ability to absorb terrain changes in order to maintain momentum, so pressure and wheel size are the key factors that determine RR. So if 26/29 tire pressure is the same, the 29er will still have lower rolling resistance because it is bigger.
    So, is this the case still w/ a smooth surface, like pavement? Since there isn't really the "angle of attack" advantage present, which is the case w/ a 29er? Especially when considering your statement below, that the contact patch would be identical w/ the same pressure.

    Quote Originally Posted by serious
    At equal pressure, the 26/29 contact patch is the same. You have to lower the pressure in the 29er to achieve a bigger contact patch (which you should since the 29er volume is bigger). This contact patch is related to friction which is related to traction. So a 29er tire that achieves similar deformation as a 26er (via lower pressure) will have more contact and hence more traction.
    Are you saying that, due to the greater overall volume of a 29er tire (say, w/ a set casing size), it will deform less w/ a given load relative to the same casing size 26er tire?

    I know personally, when comparing a 26" Fast Trak to a 29" version, both 2.0, I felt that I needed lower pressures in the larger wheel to be able to compress the tire towards the rim in the same amount. That is my own personal gauge. I couldn't tell if it had to do w/ different thread counts of the casing though. I had the impression that my 29er version had a stiffer sidewall, so didn't really feel I was able to compare them fairly. Ordinarily I'd run 27-28ish psi in the 26" version and found that 22-24psi in the 29" version felt right. (I weigh 170).

    I am getting the impression that in order to achieve the same "tire compression" dimension (depth of compressive movement towards the rim), the larger diameter 29er wheel gains the same lateral tread contact, but a greater amount of lengthwise tread contact. Wouldn't that mean more surface area (traction), and thus more rolling resistance friction?

    Disclaimer, I am certainly not a rocket scientist.

  14. #14
    CDB
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    Quote Originally Posted by Speedub.Nate
    If anyone can find a link to those ink blots, or Shiggy's test that Cloxxki referred to earlier, please share.

    The German <i>Bike</i> I recall was good with rolling resistance tests, so perhaps this has been studied and reported on already. Anybody want to page Nino in the WW forum? I remember him being the go-to guy for tire resistance tests in the past.
    Here it is:
    https://forums.mtbr.com/showthread.p...30629#poststop
    https://www.twentyniner.ch/download/...bikes_0907.pdf

    Quote Originally Posted by marty_hd
    Babelfish is ass... here is my translation.

    Grammer police, etc. back off, I did it on the fly and didn't proof read.
    Marty


    From “BIKE” 9/07, original in German by Markus Greber, Pics by W. Watzke

    -snip-

    Traction and Rolling Resistance
    The practical test revealed very good traction for the rear tire. In order to explain these results in a theoretical manner we, in conjunction with Schwalbe, made a tire print of both tires. The tires had the same tire pressure and the same amount of force was applied to both when pressing them on the plate. Normally the contact area is independent of the tire size and should have been the same for both tires. Regardless, we obtained a significantly greater area with the 29er. The reason may be that the pattern and deformation of the nobs which had a positive affect on the larger “Little Albert”. Anyhow, the tire print explains the unnaturally high traction of the 29er.
    The last comparative evaluation concerned rolling resistance. In an offshoot of our tire test (issue 08/07) we used the resistance teststand from the company Schwalbe to test both tires. The 29er has better results here. The numbers: The 29er “Little Albert”, with 28.4 Watts, is in the league of the “Nobby Nic”, whereas the same tier in 26inches rolls a little worse than the free-ride :Big Betty”. The 29er rolls 14 percent easier than the 26er, a respectable amount.



    -snip-

    edit:
    Disclosure:
    Bike allowed www.twentyniner.ch to post the German language version of their 29er article "Is Bigger Better?" from their 09/07 issue.
    I am a mod in the www.twentyniner.ch forum.
    Any errors in the translation are mine but I tried to use the same words and keep the meaning of the sentences as close to the original as possible.
    The images of the charts are from Bike, issue 09/07.
    I ride 29ers.
    And here is my own personal photo of that same tread:
    Last edited by CDB; 05-07-2008 at 03:35 PM.

  15. #15
    fc
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    Quote Originally Posted by CDB
    I am aware that if you have two tires, one 26" and one 29", each the same tread pattern, air pressure, and rider weight, there are some differences that affect the ride.

    I know the 29er tire has a longer contact patch, providing more tread contact at any given moment, and that the larger outer radius of the 29" wheel rolls over small bumps w/ less resistance. I hear multiple times from folks on this forum that 29er would have lower rolling resistance, as well as greater traction. My question is, how does that come? Is it due to the radius, and/or the contact patch.

    As you gain traction, does that create rolling resistance? A direct correlation?

    How does "overall rider weight per square inch of tire surface contact" factor into the equation? Would a 26er tire, due to less surface contact, effectively have greater traction? Wouldn't each of those knobs have more weight driving down into the dirt per square inch vs. spreading that load out over more knobs w/ a 29er tire? Again, remember I'm assuming the same tread pattern, rim width, casing size, and pressure.

    The reason for my curiosity is from doing some very steep climbs over the weekend on the 29er, in the neighborhood of 32-38% steady climbs over consolidated, firm dirt w/ the occasional little bit of loose rock on top. I got to thinking that maybe my smaller 26er tires would bite in a bit better. Maybe it was in my head.

    Anyone else have informed opinions on this?
    On a perfectly smooth surface (like glass or a velodrome) I don't think a 29er wheel has less rolling resistance.

    But on a rough gravel surface, or one with bumps, holes and obstacles, the 29er will have less rolling resistance. The reason is it has better and angle of attack (5% is what I hear). Obstacles will stop your forward momentum and increase rolling resistance. A bigger wheel will counter these forces better.



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  16. #16
    CDB
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    Quote Originally Posted by francois
    On a perfectly smooth surface (like glass or a velodrome) I don't think a 29er wheel has less rolling resistance.

    But on a rough gravel surface, or one with bumps, holes and obstacles, the 29er will have less rolling resistance. The reason is it has better and angle of attack (5% is what I hear). Obstacles will stop your forward momentum and increase rolling resistance. A bigger wheel will counter these forces better.



    fc
    Yes, thanks for the "angle of attack" update. Aware of it and I mentioned this in a later posting in this thread, but failed to mention it in my original posting. I wanted to set that attack angle variable aside for now, and think only in the perspective of a smoother trail, maybe a bit loose, but shallow depth to that loose soil. For the sake of conversation, asphalt paved surface works fine. Does that make sense?

    Would the affect of "increased contact patch" create MORE surface friction rolling resistance ONLY while riding on a smooth surface, but as soon as the terrain surface becomes moderately uneven, suddenly the angle of attack factor kicks in and overcome the friction?

    While climbing a smooth but wet asphalt climb w/ knobbies, would you have more or less rolling resistance at 20psi vs. 30psi? Would you have more traction at 20 vs. 30psi? Does the gain in traction overcome the increase in rolling resistance? Would those answers change if the surface changed from asphalt to hard pack dirt w/a small amount of loose fines on top, kind of like what you'd find at Sea Otter, for example?

    I'm not trying to consider rougher terrain at the moment. Clearly a larger wheel and suspension can contribute other things there. Trying to isolate the variables of surface area and relationship to traction/rolling resistance.
    Last edited by CDB; 05-07-2008 at 03:51 PM.

  17. #17
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    Hope Let me elaborate then.................

    ................. even quote a 26" wheeled rider using DH type tyres. I ride all the varied terrain down here from smooth gravel/dirt roads to steep as$ gravel/dirt climbs. I use Nanos, Xmarks, Ignitors, doesn't matter the 29er just seems to shine. When I visited CO I was clearing stuff on these punny XC tyres that guys on much bigger and stickier tyres were having a hard time clearing - that's scientific enough for me Guys riding those big sticky tyres just couldn't believe what i could clear on the RIP mainly running a Nano or Xmark rear so as not to have "to much grip" as practice for racing enduros on fast tyres.

    Quote Originally Posted by CDB
    I agree about the limitations of getting bogged down w/ minutia and technical data vs. the "subconscious perception stuff" and "trusting what you're feeling". But, since we all cannot get inside your head to actually "feel" what you are feeling, and we know nothing of your variables that affect WHY one particular tire works better for YOU, it is worth a very small amount to others. Things like history prior to 29, rider style, body type/weight, terrain type, tread pattern, air pressure, etc. are helpful bits of information that make a statement actually helpful to others. We're sharing experiences, but w/o the details, they offer very little. If you feel this conversation is not interesting to YOU, or discussion of the minute details is bland on your palate, feel free to simply NOT PARTICIPATE! ................................

  18. #18
    CDB
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    Quote Originally Posted by LyNx
    ................. even quote a 26" wheeled rider using DH type tyres. I ride all the varied terrain down here from smooth gravel/dirt roads to steep as$ gravel/dirt climbs. I use Nanos, Xmarks, Ignitors, doesn't matter the 29er just seems to shine. When I visited CO I was clearing stuff on these punny XC tyres that guys on much bigger and stickier tyres were having a hard time clearing - that's scientific enough for me Guys riding those big sticky tyres just couldn't believe what i could clear on the RIP mainly running a Nano or Xmark rear so as not to have "to much grip" as practice for racing enduros on fast tyres.
    Good info. Maybe you're simply a better bike handler, or faster than them, regardless of the bike though. Are you saying that YOU, when comparing all those different tires at 26 and 29 format, are able to ride faster in all situations? Cool! I know that I can beat plenty of folks on big travel bikes, but certainly not all of them out in the world. Just happens to depend on who I'm riding around that particular day. A person on a fancy full suspension bike and large knobbies isn't necessarily faster than a person on a hardtail w/ semislicks, due to bike alone.

  19. #19
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    29ers have a bigger, much bigger, contact patch. Theoretically they are the same but in the real world it just ain't so.

    A translated excerpt from the German Bike magazine article.
    "The practical test revealed very good traction for the rear tire. In order to explain these results in a theoretical manner we, in conjunction with Schwalbe, made a tire print of both tires. The tires had the same tire pressure and the same amount of force was applied to both when pressing them on the plate. Normally the contact area is independent of the tire size and should have been the same for both tires. Regardless, we obtained a significantly greater area with the 29er. The reason may be that the pattern and deformation of the nobs which had a positive affect on the larger “Little Albert”. Anyhow, the tire print explains the unnaturally high traction of the 29er."

    A link to a translation of the entire article
    Scoll down to mart_hd's comment.
    http://forums.mtbr.com/showthread.ph...=contact+patch

  20. #20
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    Hope

    I was definitely fitter than most I rode with and had been training to climb, climb, climb for Leadville and Laramie. Skill level I would say was fairly comparable, with me pretty much having the lesser skill level definitely. They smoked me on the DH's, no questions there, especially on the wet granite, but on the climbs I guess a 30lbish FS 29er makes pretty easy work of most of it.

    Quote Originally Posted by CDB
    Good info. Maybe you're simply a better bike handler, or faster than them, regardless of the bike though. Are you saying that YOU, when comparing all those different tires at 26 and 29 format, are able to ride faster in all situations? Cool! I know that I can beat plenty of folks on big travel bikes, but certainly not all of them out in the world. Just happens to depend on who I'm riding around that particular day. A person on a fancy full suspension bike and large knobbies isn't necessarily faster than a person on a hardtail w/ semislicks, due to bike alone.

  21. #21
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    Some of us ride for the feel, the "escape", to turn off that mental noise.......
    And we do it on a 29er.

    My LBS had great advice:
    "The physician heals, Nature makes well" - real fortune cookie

    CCCMB trail work for trail access - SLO, CA

  22. #22
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    Quote Originally Posted by GeoKrpan
    29ers have a bigger, much bigger, contact patch. Theoretically they are the same but in the real world it just ain't so.

    A translated excerpt from the German Bike magazine article.
    "The practical test revealed very good traction for the rear tire. In order to explain these results in a theoretical manner we, in conjunction with Schwalbe, made a tire print of both tires. The tires had the same tire pressure and the same amount of force was applied to both when pressing them on the plate. Normally the contact area is independent of the tire size and should have been the same for both tires. Regardless, we obtained a significantly greater area with the 29er. The reason may be that the pattern and deformation of the nobs which had a positive affect on the larger “Little Albert”. Anyhow, the tire print explains the unnaturally high traction of the 29er."

    A link to a translation of the entire article
    Scoll down to mart_hd's comment.
    http://forums.mtbr.com/showthread.ph...=contact+patch
    Wow, lets just toss out physics...

    ...but be careful, your cell phones and computers might stop working.

    PS: the link just spontaneously disappeared. I guess it must have violated some rule of physics too.

    <object width="425" height="355"><param name="movie" value="http://www.youtube.com/v/us7YB7eiOeQ&hl=en"></param><param name="wmode" value="transparent"></param><embed src="http://www.youtube.com/v/us7YB7eiOeQ&hl=en" type="application/x-shockwave-flash" wmode="transparent" width="425" height="355"></embed></object>

  23. #23
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    Quote Originally Posted by lidarman
    Wow, lets just toss out physics...

    ...but be careful, your cell phones and computers might stop working.
    Welcome to the strange & unusual world of big wheel physics....
    A man must have enemies and places he is not welcome. In the end we are not only defined by our friends but those against us.

  24. #24
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    I like my bike. It rides nice.

    How's that?
    Just a regular guy.

  25. #25
    CDB
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    Quote Originally Posted by slocaus
    Some of us ride for the feel, the "escape", to turn off that mental noise.......
    And we do it on a 29er.

    My LBS had great advice:
    The thing is, I do ride. In fact, my original question arose while riding my 29er actually. It's the thinking AFTER the ride that gets me. Especially when I do back to back rides comparing both 26/29" bikes. Most things have a reason for the way they work out. It's nothing mystical or supernatural. I have a curiosity and am brainstorming w/ others who think similarly about the same subject. People are capable of thinking intellectually and also being able to enjoy a nice ride and compete at the top level. In fact, that thinking is often what makes those few folks that little bit faster. Give it a try some time, instead of borrowing someone else's graphic and wasting your own time posting something that doesn't relate to the subject. Sure, you make a good point that over analyzing things by it self can be a dead end.

  26. #26
    CDB
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    Quote Originally Posted by ~martini~
    I like my bike. It rides nice.

    How's that?
    I'd say on a 1-10 scale, 10 being extremely clever, I'd rate it an 11. Good work!

    Just kidding. Thanks. That's short and sweet.

  27. #27
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    For me, the better steep climbing of 29" sems to come from the rear axle position relative to the BB, both horizontally and vertically. It's been well documented, but it I'll just say that I need a steeper hill to tumble back over with 29". Because I tumble back less strongly, I can apply more pedal pressur before I get that pesky unwanted front wheel lift.
    And even if I get step enough to get into trouble, I can get out of the seat and still get more traction than on the small wheeled bike.
    What it adds up to, is that I ride up stuff my most capable riding buddies can't, till they take my bike. I've had more than a few times that I cleaned a technical climb, and a friend could not on his 26" bike. First go on my bike without any adjustment time (just 10m leading up to the hill), and he cleans it all the way up. I have not seen such a thing work the other way around. Although sometimes a particular hill doesn't seem to work too well for me or my bike, and I struggle much like the riders around me.

  28. #28
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    Cool-blue Rhythm Roadies chime in please !

    Quote Originally Posted by CDB
    Yes, thanks for the "angle of attack" update. Aware of it and I mentioned this in a later posting in this thread, but failed to mention it in my original posting. I wanted to set that attack angle variable aside for now, and think only in the perspective of a smoother trail, maybe a bit loose, but shallow depth to that loose soil. For the sake of conversation, asphalt paved surface works fine. Does that make sense?

    Would the affect of "increased contact patch" create MORE surface friction rolling resistance ONLY while riding on a smooth surface, but as soon as the terrain surface becomes moderately uneven, suddenly the angle of attack factor kicks in and overcome the friction?

    While climbing a smooth but wet asphalt climb w/ knobbies, would you have more or less rolling resistance at 20psi vs. 30psi? Would you have more traction at 20 vs. 30psi? Does the gain in traction overcome the increase in rolling resistance? Would those answers change if the surface changed from asphalt to hard pack dirt w/a small amount of loose fines on top, kind of like what you'd find at Sea Otter, for example?

    I'm not trying to consider rougher terrain at the moment. Clearly a larger wheel and suspension can contribute other things there. Trying to isolate the variables of surface area and relationship to traction/rolling resistance.
    What you are trying to do is pure comparing the triathlon bicycles vs ordinary road bike isolating the rolling resistance only. Has someone seen a direc comparison between 650c vs 700c?
    Roadies would profit much as 650c would shred more weight and make their wheels stiffer, but they do not push for that , why?

    Even on perfect smooth flat surface a 29er with have more revolution momentum and will sustain the speed better with less additional mashing compared to 26er. When the gradient is getting much steeper (I already tossed that question in another thread), we do not know by how much steeper, there is only weight to be dragged- you fight with gravity.
    I surmize that such gradient must be really steep like 30% on perfect smooth tarmac to put a 29er at disadvantage, of course the key factor is- by how much the 29er is heavier.

  29. #29
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    Quote Originally Posted by CDB
    Would the affect of "increased contact patch" create MORE surface friction rolling resistance ONLY while riding on a smooth surface, but as soon as the terrain surface becomes moderately uneven, suddenly the angle of attack factor kicks in and overcome the friction?
    I'm a little unclear, but from what you wrote I think you're looking at <i>rolling resistance</i> and <i>traction friction</i> as the same thing.

    Whether the 29" (vs. 26") tire has an equal sized contact patch, or larger (as the <i>Bike</i> test portrays), the rolling resistance is derived from how much the carcass has to deform at the bottom of the roll -- like that car driving in front of you with the tire "pudge" due to low pressure. The energy that goes into deforming the sidewall and creating the bulge is what creates resistance.

    I really don't think rolling resistance lessens based on surface type: glass smooth or craggy lava rock, the footprint of the tire has to spread out to cover a particular area based on the weight of the load it is carrying.
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  30. #30
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    I think that climbing traction can be better on a 29'er wheel, as the longer contact patch spreads the distance that the force can be applied to the ground over a longer distance... So - if you are climbing on a packed surface, and there is a small rock, or piece of branch/stick/etc... the tire has more chance to deform and the contact patch "straddle" that small obstacle and have rubber ahead of, and behind the small item that could cause the tire to spin with a shorter contact patch. Being able to run a few psi lower pressure also helps with the tire being able to deform and absorb these imperfections and maintain grip on the ground that will not be moving. In looser conditions, the longer patch simply applies more of the tires surface area, and more knob edges, in a manner that allows more fore-aft pressure to be exerted from the tire to the ground. In the graphic posted in this thread, the larger diameter tire shows 22 pieces of knob present vs. about 16 in the smaller diameter tire (counting "measureable knob" print - not the "dots" of a point of a knob). That has to help in climbing and braking.

    The narrower contact patch may prove to be detrimental in a cornering situation or riding on camber with a heavy side-load, etc... The tire may tend to break loose sideways a bit more easily. I seem to notice that with the smaller section Michelin I am running now, but I think that most of that is due to knob pattern and the smaller section. I did not have any noticeable problem with a 2.3" Exiwolf on back. To really compare, the same tire in the same compound, section, etc... with only the difference in diameter should be compared. Even then it may be hard to come up with a real definition, as the bike frame geo and other variables will affect the ride and sensations. I just know that I like the way the 29'er climbs, and I do feel that the rear end in particular, seems to hook up a bit better, even if the pressures are not reduced in the larger wheels.
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  31. #31
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    Quote Originally Posted by ATBScott
    I think that climbing traction can be better on a 29'er wheel, as the longer contact patch spreads the distance that the force can be applied to the ground over a longer distance... So - if you are climbing on a packed surface, and there is a small rock, or piece of branch/stick/etc... the tire has more chance to deform and the contact patch "straddle" that small obstacle and have rubber ahead of, and behind the small item that could cause the tire to spin with a shorter contact patch. Being able to run a few psi lower pressure also helps with the tire being able to deform and absorb these imperfections and maintain grip on the ground that will not be moving. In looser conditions, the longer patch simply applies more of the tires surface area, and more knob edges, in a manner that allows more fore-aft pressure to be exerted from the tire to the ground.
    This sums it up.
    Attached Images Attached Images

  32. #32
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    Stiffer road wheels are actually not favorable. Everyone who watches european road classics can tell that they barely get through corners that 5 years ago were flown through.
    road wheels already being light, little weight is to be won by going a size smaller. Gain some RR, lose some weight, lose air drag, but gain it back from a longer head tube.
    I bet a criterium on any 650c set is going to be an unpleasant affair. Road bikes iMO are too manouvrable as they are.

  33. #33
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    The tires had the same tire pressure and the same amount of force was applied to both when pressing them on the plate. Normally the contact area is independent of the tire size and should have been the same for both tires. Regardless, we obtained a significantly greater area with the 29er.

    Did these geniuses consider the fact that "the plate" deformed to reflect the bigger contact patch of a bigger tire? In other words, a 29er with same pressure as a 26er will have a much larger contact patch on soft, muddy terrain. Duh!
    My rides:
    Lynskey Ti Pro29 SS
    RM Suzi Q 90 RSL
    KHS Team 29
    S-Works Roubaix
    KHS CX 550 cyclocross

  34. #34
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    Quote Originally Posted by serious
    ...a 29er with same pressure as a 26er will have a much larger contact patch on soft, muddy terrain. Duh!
    Are you serious?

    I kinda suspect that when they referred to "the plate," they were being somewhat literal, for the purpose of recording an impression of these tires. Steel plate? Plate of glass (or plastic)?

    I dunno, just sayin'...
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  35. #35
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    Quote Originally Posted by Cloxxki
    For me, the better steep climbing of 29" sems to come from the rear axle position relative to the BB, both horizontally and vertically. It's been well documented, but it I'll just say that I need a steeper hill to tumble back over with 29". Because I tumble back less strongly, I can apply more pedal pressur before I get that pesky unwanted front wheel lift.
    And even if I get step enough to get into trouble, I can get out of the seat and still get more traction than on the small wheeled bike.
    What it adds up to, is that I ride up stuff my most capable riding buddies can't, till they take my bike. I've had more than a few times that I cleaned a technical climb, and a friend could not on his 26" bike. First go on my bike without any adjustment time (just 10m leading up to the hill), and he cleans it all the way up. I have not seen such a thing work the other way around. Although sometimes a particular hill doesn't seem to work too well for me or my bike, and I struggle much like the riders around me.
    I've noticed that too. The front end of the 29er is less likely to lift up while riding up steep grades. It is also less twitchy and more resistant to getting batted left/right on large, loose rocks.

  36. #36
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    Quote Originally Posted by CDB
    I've noticed that too. The front end of the 29er is less likely to lift up while riding up steep grades. It is also less twitchy and more resistant to getting batted left/right on large, loose rocks.
    Me too. I don't know if it is the big wheels or the "long" chainstays that they require but I can stay in a relatively normal seated position on steep climbs that had my saddle playing "poky, poky " on my 26" bikes.

    G
    You can't depend on honest answers from dependant hands...

  37. #37
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    Quote Originally Posted by serious
    The tires had the same tire pressure and the same amount of force was applied to both when pressing them on the plate. Normally the contact area is independent of the tire size and should have been the same for both tires. Regardless, we obtained a significantly greater area with the 29er.

    Did these geniuses consider the fact that "the plate" deformed to reflect the bigger contact patch of a bigger tire? In other words, a 29er with same pressure as a 26er will have a much larger contact patch on soft, muddy terrain. Duh!
    "In order to explain these results in a theoretical manner we, in conjunction with Schwalbe, made a tire print of both tires."

  38. #38
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    Quote Originally Posted by lidarman
    Wow, lets just toss out physics...

    ...but be careful, your cell phones and computers might stop working.

    PS: the link just spontaneously disappeared. I guess it must have violated some rule of physics too.

    <object width="425" height="355"><param name="movie" value="http://www.youtube.com/v/us7YB7eiOeQ&hl=en"></param><param name="wmode" value="transparent"></param><embed src="http://www.youtube.com/v/us7YB7eiOeQ&hl=en" type="application/x-shockwave-flash" wmode="transparent" width="425" height="355"></embed></object>
    Logical fallacy?

  39. #39
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    Guys they said the tires had the same pressure !!! Even they acknowledged that contact area is independent of tire size. Common now.
    My rides:
    Lynskey Ti Pro29 SS
    RM Suzi Q 90 RSL
    KHS Team 29
    S-Works Roubaix
    KHS CX 550 cyclocross

  40. #40
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    Quote Originally Posted by Davidcopperfield
    This sums it up.


    That's a cool illustration. If you compare the two, you can see that there is definitely a little more tire touching the ground on either side of the "obstacle" in the 29er version. But, if you also look at the difference in dimension between the "hub" and outer radius of the tire circle vs. the dimension between the hub and the top of the obstacle, it looks to me like the tire is being pressed further towards the rim in the 29er case. In order for that illustration to show the benefits of increased traction of the 29er, it inadvertantly shows that the tire is possibly using much less air pressure, in order to compress the tread that much. Sure that gains more tread contact, but it maybe doesn't equate to what is trying to be proven.... that a 29er w/ the same psi as a 26er, will have more surface contact w/ the ground, or "traction". I may be wrong.

    Maybe another way to go about it is to imagine that little "pea" (which the tire is rolling over in the diagram) is a 1" diameter pipe laying on the ground. At what low pressure would the rim make contact w/ the pipe w/ the two wheelsize options, and at that pressure, which wheel size would be providing more traction at that point. Would the rim bang the pipe at the same pressure w/ the two different sized tires, as you lower it? Maybe that is exactly what the illustration shows.

    Ok, so the rim wouldn't bang the pipe, but it would bottom out as it pinches the tire, and if there were a tube involved, would pinch flat the tube. In this day and age, w/ tubeless tech, that's not much a factor as protecting the rim from getting dented.

    Here's a few more basic questions to answer:

    • Will a 29x2.0, 25 psi tire compress in the same amount dimensionally towards the rim as an identically treaded 26x2.0 tire, while rolling over a 1" diameter round pipe?


    • Does the greater overall air volume in the 29er tire, even though at the same psi, mean that the tire would not compress as much at a given pressure?


    • If the deformation of the tire where it makes contact w/ the ground is the factor which increases or decreases tread surface contact, therefore affecting traction, would it not be wise to consider that you'd need to use lower pressures in a 29er tire to maintain as much contact patch as you'd have at that same pressure with a 26er?


    • If my thoughts above are in error, and a 29er tire actually has the same amount of surface contact given a set air pressure when compared to a 26er, but the difference is the width(26) vs. length (29), does the longer contact patch factor in more positively in regards to traction while climbing and slowing while braking?


    • Is the more significant factor for maintaining traction while riding over that theoretical 1" pipe actually the angle of attack? In other words, having a lower angle of attack enables the tire to roll over the obstacle w/o kicking up vertically so much, lifting more tread away from contact w/ the ground, reducing traction? (obviously, if a rider was getting that much tire to drape over the pipe, as per the illustration above, the rim would be dented. There would certainly be some tire "lift off" while rolling over that pipe at a normal riding pressure. Even if suspension were involved.)


    • Regarding rolling resistance, is there a direct corellation between "reduction of traction" vs. "reduction of rolling resistance"?


    • If, according to the Sheldon Brown and Wikipedia pages, rolling resistance is most significantly related to the amount of tire deformation at the sidewall, and a higher pressure will reduce that, therefore the less deflection of the tread casing relative to the amount of surface contact w/ the tread seems to be the better option. Right?

      And since a 29er tire, at a set pressure, will supposedly deform less laterally and more linearly than a 26er, then that's the reason for it's increased efficiency, when only looking at traction, pressure, and rolling resistance?


    I look forward to reading your thoughts. Hopefully you will have something more insightful than "shut up and ride" or "just ride it and don't worry about it". I guess you CAN say those things too, and I'll be sure to give you a little emoticon reaction, just to satisfy your cravings.

  41. #41
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    [QUOTE=CDB]
    • Will a 29x2.0, 25 psi tire compress in the same amount dimensionally towards the rim as an identically treaded 26x2.0 tire, while rolling over a 1" diameter round pipe?

    No, it will not compress in the same amount as there is more tyre to be pressed onto that 1" pipe. More tyre is in comtact with the pipe, more rubber to squeeze.

    Sure that gains more tread contact, but it maybe doesn't equate to what is trying to be proven.... that a 29er w/ the same psi as a 26er, will have more surface contact w/ the ground, or "traction". I may be wrong.
    Compare a 16" with 36er at the same psi. Bigger tyre will always have more traction and contyact patch. Tha benefit of larger tyre is to run a smaller pressure- why? see my previous reply. To boost the traction you are able, without bottoming out the rim, to run even lower pressure than on 16" bike.

    At what low pressure would the rim make contact w/ the pipe w/ the two wheelsize options, and at that pressure, which wheel size would be providing more traction at that point.
    always bigger tyre will provide more of everything. More rubber to squeeze before the pipe bangs the rim.

    Would the rim bang the pipe at the same pressure w/ the two different sized tires, as you lower it?
    No


    • Does the greater overall air volume in the 29er tire, even though at the same psi, mean that the tire would not compress as much at a given pressure?
    No, it is because of the tyre segment deformed around an object.

    Please someone else continue

  42. #42
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    [QUOTE=Davidcopperfield]
    Quote Originally Posted by CDB
    • Will a 29x2.0, 25 psi tire compress in the same amount dimensionally towards the rim as an identically treaded 26x2.0 tire, while rolling over a 1" diameter round pipe?

    No, it will not compress in the same amount as there is more tyre to be pressed onto that 1" pipe. More tyre is in comtact with the pipe, more rubber to squeeze.


    Compare a 16" with 36er at the same psi. Bigger tyre will always have more traction and contyact patch. Tha benefit of larger tyre is to run a smaller pressure- why? see my previous reply. To boost the traction you are able, without bottoming out the rim, to run even lower pressure than on 16" bike.


    always bigger tyre will provide more of everything. More rubber to squeeze before the pipe bangs the rim.
    I am not refuting your statement, but am struggling to comprehend it. Try this scenario:

    Imagine if you had three separate 12" lengths of wheel, same width rim, same width casing, same psi. The difference in the lengths would be that one would be straight (no radius), the second would be say 29", and the third would be very sharp radius, like 16".
    Each section of wheel would have the same amount of air volume inside and would be capped off square at the ends. (Yes, this requires some imagination ).

    The test experiment w/ these three study models would be to apply a consistent and measured downward force, laying the midpoint of the "tire" upon a perpendicularly laid 1" pipe with approximately the normal weight of a rider. Are you saying that with these three models, the pipe would make contact with the rim at different pressures, simply due to the radius of the tire/rim section?

    Or is it related to the ratio of amount of displaced air (by the pipe) relative to the overall remaining air volume inside a "real" tire of different diameters?

    In other words, the total amount of air inside a "16x2.0 @25psi" tire is much less than
    the amount in a "[email protected]" tire.

    Would it will be easier to accommodate that "displaced volume" of air inside the larger tire by compressing that air around the rest of the inside space of a larger tire? The smaller the available amount of interior space (smaller wheel) the harder it is to displace a set volume of air, or cram it into that same remaining space. IMO, when you are squeezing a tire, you are essentially doing that. You are increasing the psi across the remaining interior surface area of the tire by reducing the volume, even though the total amount of air inside remains the same. Squeeze a soft tire against the rim in the real world and the remainder of the tire feels firmer. That is because you've placed more air in less space.

    The above statement makes sense to me... but that would imply that a 29er at 25psi would be *edit* EASIER to pinch than a 26er at 25psi, no? And that completely conflicts w/ my own personal experience, which seems to be the opposite, being that I have to use lower pressure in order to get the same firmness in my fingers.

  43. #43
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    Quote Originally Posted by CDB
    The thing is, I do ride. In fact, my original question arose while riding my 29er actually. It's the thinking AFTER the ride that gets me. Especially when I do back to back rides comparing both 26/29" bikes. Most things have a reason for the way they work out. It's nothing mystical or supernatural. I have a curiosity and am brainstorming w/ others who think similarly about the same subject. People are capable of thinking intellectually and also being able to enjoy a nice ride and compete at the top level. In fact, that thinking is often what makes those few folks that little bit faster. Give it a try some time, instead of borrowing someone else's graphic and wasting your own time posting something that doesn't relate to the subject. Sure, you make a good point that over analyzing things by it self can be a dead end.
    You misinterpreted the smilies. It takes an over analytical technoweenie to call someone else an over analytical technoweenie.

    This is actually a great discussion, and I plan to follow it. I just wanted to add a counterpoint to it, since I was thinking so hard that my head was hurting.

    And the "Shut Up and Ride" is relevant, since it is one of the big advertisers on the front of MTBR, they help us have these discussions, in a round about way. They are my local LBS. They sold me 2 of my 3 29er bikes, and a gazillion parts. When I get too deep into these analysis there ("gotta go, it's getting deep, and my boots leak" ). they point to that logo on their sleeve and say, go.
    "The physician heals, Nature makes well" - real fortune cookie

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  44. #44
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    Quote Originally Posted by Cloxxki
    Stiffer road wheels are actually not favorable. Everyone who watches european road classics can tell that they barely get through corners that 5 years ago were flown through.
    road wheels already being light, little weight is to be won by going a size smaller. Gain some RR, lose some weight, lose air drag, but gain it back from a longer head tube.
    I bet a criterium on any 650c set is going to be an unpleasant affair. Road bikes iMO are too manouvrable as they are.
    Agreed. I have no intention of ever being behind someone on 650c wheels in a crit. I can't imagine the carnage of racing Downers Grove, magnified by a lower BB height...

    However, Cloxxki, there's a difference between a wheel being vertically stiff (absorbing bumps, or rather, not) and a wheel being torsionally stiff (not rubbing your brake pads).

    A carbon rim is going to be both, provided it is built up right. An aluminum box section rim, like an Ambrosio F20, is going to provide a much more compliant ride, yet be just as stiff side to side, if built on a good pair of Record or Dura Ace hubs with 32 spokes.

    I should also point out that some of the problems these days are to be found with the tire and rim interaction/selection. With some "modern" rims, you can't run anything bigger than a 25mm tub, or in some cases, even a 23. This basically eliminates a whole strata of 27-28mm tires (FMBs, Challenge, Dugast), many of which were originally created for the cobbled classics. So, it's not necessarily a tire problem, or a rim problem; it's a matter of being able to use them together, or, in this case, not being able to use them.

  45. #45
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    Quote Originally Posted by francois
    On a perfectly smooth surface (like glass or a velodrome) I don't think a 29er wheel has less rolling resistance.
    Not true according to these articles from long ago:

    http://www.precisiontandems.com/artbillwheelsize.htm

    http://www.gtgtandems.com/tech/700-26.html

    And talking about what happens at upper levels of roadracing I believe doesn't count. Take the TdF for instance, I believe the competitors are required to use 700c wheels. IIRC there was a time when 20" wheels were tried out and the aerodynamic advantages of being able to tuck in so tight to your teammates far outweighed any drawbacks, so the possibility was outlawed, despite what the links I posted say about 26ers.

  46. #46
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    It's my understanding and experience that a large wheel with similar tire will compress more on sharp objects, and less on blunt ones. A sharp object, in my mind, should increase psi in the tire less when hitting the rim than on a smaller wheel. The removed volume, especially with a suptle casing, is the same, but relatively less on a large wheel.
    At 30psi each, 29" does feel less comfortabe than it should be, over general fast trails or rough pavement. When I lower pressur to compensate, I just end up riding careful regarding sharper objects.
    The blunter rim does also seem to reduce number of snake bites per 10 rim hits, so it's mostly caring for my rims when I look where I roll.
    Klok - XC - Skate - Ski

  47. #47
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    Wow - my head is spinning...
    lots of good pts. made.

    2 things:

    if you drive off-road vehicles (like a 4X4), the traction can be predicted by "ground pressure". Over-inflated, hard tires slip more easily even though the contact area has more ground pressure per square inch (if you think a hard knobby tire will have better traction because it "digs in" more, you are mistaken because the dirt that the knob is biting into is weaker than the knob - the dirt gives way and you get no traction - the traction has to be distributed across more dirt). When you air down, the contact area grows (and your ground pressure decreases) and your traction (and handling) improves until you go too low and the tire starts rolling sideways on the rim. This is true for any tire size - off-road. Even if you are using a slick tire, it will have better traction with "just the right amount" of air pressure. Of course, dirt changes from something like concrete all the way to mud bog, so lots of variables.

    The pressure inside a larger tire will change less under deformation and intermittent loads than a smaller tire - which requires less work - which equals less rolling resistance at each instance of deformation (like running over a 1" pipe). Of course, deforming the entire tire because you are only running 2 psi increases the rolling resistance in general. Finding the optimal tire pressure goes back to ground pressure - which depends upon the weight of the vehicle. There are just so many variables that this topic could go in circles forever.


    For the 26/29 comparison, I think what people interpret as "rolling resistance" is that, to achieve the same ground pressure as the 26er, the 29er tire has to be inflated more, which is uncomfortable, so they air down. This has the effect of making a more comfortable (and probably more efficient) off-road experience while increasing traction, but actually increases rolling resistance on smooth surfaces like pavement or even hardpack. I think my 29er coasts slower (that is, not as far) than my old 26er on pavement, but it rolls better on uneven terrain.

    Good luck drawing a conclusion on this one - too many variables.

    -F

  48. #48
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    PS - I ran higher pressure on purpose off-road, and adjusted down during the ride until I liked it. I have an Ignitor up front and an Exi on the back. I ended up at 30 psi in both tires on 2 separate occasions on a fully rigid bike. I am 185#.

    -F

  49. #49
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    Are you saying that with these three models, the pipe would make contact with the rim at different pressures, simply due to the radius of the tire/rim section?
    As in the pictures the bigger tyre "overflows" or absorbs inwards the lying pipe. The object must exert more force on the carcass to compress it because there is more of it on and around the peak of that pipe.

    Or is it related to the ratio of amount of displaced air (by the pipe) relative to the overall remaining air volume inside a "real" tire of different diameters?

    In other words, the total amount of air inside a "16x2.0 @25psi" tire is much less than
    the amount in a "[email protected]" tire.

    Would it will be easier to accommodate that "displaced volume" of air inside the larger tire by compressing that air around the rest of the inside space of a larger tire? The smaller the available amount of interior space (smaller wheel) the harder it is to displace a set volume of air, or cram it into that same remaining space.
    Yes it is easier to press a bubble of air, yet the tyre is firmer and you must overcome more carcass to press the tyre to the rim on the larger wheel.

    More surface of tyre carcass offset the smaller air pressure resistance, thus the tyre yields to a lesser degree with the same force apllied f. ex. your palms.

    Imagine doing a drop from 1 meter shelf on numerous wheel-sized bicycles onto flat concrete. Starting from 10" up to 36"
    When rider weight is constant, the biggest tyre will compress the least becuse there is more carcass coming into contact with concrete. In 36" you could use much lower pressure and still do not hit the rim far beyond a point where a 10" wheel would be bottomed out and destroyed.

    when you are squeezing a tire, you are essentially doing that. You are increasing the psi across the remaining interior surface area of the tire by reducing the volume, even though the total amount of air inside remains the same. Squeeze a soft tire against the rim in the real world and the remainder of the tire feels firmer. That is because you've placed more air in less space.

    The above statement makes sense to me... but that would imply that a 29er at 25psi would be *edit* EASIER to pinch than a 26er at 25psi, no?
    If you perpendicularly put a sharp nail and then land onto it all tyres will be pierced, because the tyre carcass connot deform around this nail enough. But rocks with no neddle-like points with be absorbed more by the bigger tyres longitudinally. If you use fatter tyre they are absorbed more widely.
    Last edited by Davidcopperfield; 05-09-2008 at 07:31 AM.

  50. #50
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    We seem to be getting back to ground pressure.

    As the tire deforms (likely decreasing in volume) whether by a sharp blow or a dull impact, the contact patch increases, and the pressure inside the tire increases.
    The increase in tire pressure resists further deformation, but if the tire is of large volume, that resistance will not build as quickly as with a small volume tire. (I think a smaller tire would require more force (more work) to compress it over an irregularity.) BUT, since the ground pressure at the time of impact is being distributed over a larger area of the larger tire, the volume change is probably the same (or even less), just with less of a change in pressure = less work = less rolling resistance.
    I think the work involved in deforming the tire carcass is much less than the work to compress the air in the tire, unless you deal with something like a heavy 2.4" tire running at low pressure, where the deformation of the tread has it scrubbing against the ground as much as it is rolling.

    Wait... what was the question?

    -F

  51. #51
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    Quote Originally Posted by Davidcopperfield
    As in the pictures the bigger tyre "overflows" or absorbs inwards the lying pipe. The object must exert more force on the carcass to compress it because there is more of it on and around the peak of that pipe.
    I think I can decipher this, but I fail to understand the logic behind it, or what it adds to the questions I previously posted, or the theoretical models I laid out for sake of discussion. This isn't necessarily targeting your point being invalid, but possibly more so my failure to get what you're saying, and how it relates.

    How is there more rubber tire casing or tread making contact with the pipe, when comparing a 26 vs 29? Aren't they both the same size casing at 2.0, and same pressure? Are you saying that the outer surface of the tread will wrap more around either side of the 1" pipe? If we're talking the same kind of tire, the casing of a 29" shouldn't be more supple than that of a 26" tire, right? If we can agree that, due to the relative ease of displacing a small volume of air into a greater overall air volume in a larger tire, a 29er will compress more towards the rim when riding over the pipe -- then I can see that the tire might make more surface contact with the pipe.

    But how does that thinking apply to the '3 short 12" lengths of tread' experiment I posted previously? In other words, I wanted to see what role the radius played, using those short tire segments, vs. the total air volume of the complete wheels. Since each segment would have the same volume, you could narrow the discussion down to purely a radius issue. I'm trying to not diverge on a separate tangent here, desire being to get at the relationship between the radius of the wheel, vs. the effects of overall air volume with an entire tire.

    Does that make sense?

    Thanks for the thinking.

  52. #52
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    Getting back to the "TRAKTION" illustration, I noticed that they stated that both the 29" and 26" tires were inflated to the same pressure. In a more "real world" comparison, the higher volume 29" would be inflated to a lower pressure than the 26" to achieve the same optimum "feel". This would make the disparity between the contact patches even more dramatic.

    I also like the concept of "ground pressure" that Fleas has brought to the table, and I agree that there are too many variables to easily draw any firm scientific conclusions, only general theories. Though this theoretical discussion has been interesting, the anecdotal evidence is compelling. 29er's have better TRAKTION than 26ers. My experience, and other's, tells me so.
    Last edited by Titus Maximus; 05-09-2008 at 12:46 PM.
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  53. #53
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    Quote Originally Posted by Fleas
    Wow - my head is spinning...
    lots of good pts. made.

    2 things:

    -F
    Hopefully not spinning as much as mine! Thanks for the basic logic you've shared. I think so far, you have made the most sense (to me) and clarified it well. I think your points about ground pressure and how that increases/decreases as the wheel size (and resulting overall air volume) changes is clear. I also found it helpful to hear how you've clarified the relationship between the ground pressure and overall displacement while riding over a 1" pipe, as well as the contact patch.

    That's the main reason I started the thread, to hear from someone knowledgeable, to help me understand the "why". Whether that makes me faster or not, probably very insignificant, but it does satisfy my obsessive curiosity.

    Now, back to work!

  54. #54
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    New question here. Some drawings needed...

    Quote Originally Posted by CDB
    How is there more rubber tire casing or tread making contact with the pipe, when comparing a 26 vs 29? Aren't they both the same size casing at 2.0, and same pressure?
    More longitudinal contact patch on 29er.

    Are you saying that the outer surface of the tread will wrap more around either side of the 1" pipe?
    Bingo It dpeends on the pressure; at lower pressure of course, at high pressure both wheel should bounce but the bigger less owing to shallower angle of attack.
    Let someone verify it as I am not certain about this experiment.

    relative ease of displacing a small volume of air into a greater overall air volume in a larger tire, a 29er will compress more towards the rim when riding over the pipe -- then I can see that the tire might make more surface contact with the pipe.
    Yes bigger wheel tyre will wrap around certain object more rather than slide or skid on the top of it at least at lower pressure.
    i'm not sure how it will react when the pressure is the same.

    But how does that thinking apply to the '3 short 12" lengths of tread' experiment I posted previously? In other words, I wanted to see what role the radius played, using those short tire segments, vs. the total air volume of the complete wheels. Since each segment would have the same volume, you could narrow the discussion down to purely a radius issue. I'm trying to not diverge on a separate tangent here, desire being to get at the relationship between the radius of the wheel, vs. the effects of overall air volume with an entire tire.
    Please provide a draft drawing since my brain does not comprehend it. I cannot visualize this part of your speech.

  55. #55
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    Quote Originally Posted by Davidcopperfield
    Please provide a draft drawing since my brain does not comprehend it. I cannot visualize this part of your speech.
    I had already mocked it up on the side. These are shown as though you cut a cross section lengthwise and looked at the side view, like through the sidewall. I am aware that it isn't necessarily accurate, as far as how it represents the amount of tire pressed against the dirt w/ a constant vertical load. But the intent is to illustrate that the 12" length of each sample tire is the same, and would have the same interior air volume and pressure. The 16" and 29" are drawn as though the pipe is compressed the same distance "x" away from the rim. Also, I attempted to show the tire pressing at an amount, as though it were penetrating the soft dirt by about 1/2". That seemed like a reasonable approximation to make for illustrative purposes. I didn't now how to convey this w/ the flat "no radius" section. If the contact patch is the same, regardless of the radius, then how I have drawn it reveals the "length" of the contact patch, labelled as "A", "B", or "C". The 16" wheel has a shorter length contact patch, but would have a wider footprint. The accuracy of my drawing may be way off, relative to reality, but it at least kind of shows how i was imagining things. Maybe it will serve to reinforce what others have alluded to in this conversation, bring up some new ideas, or throw the whole thread on a worthless tangent. Whatever. The way I drew it, the tire casing wrapped around the pipe in the same way. Whether it really would do that, I can't say.

    Here you go:

  56. #56
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    Your picture shows that there is more contact patch on the 29er and it shows that the rim is not necessarily more pretected by tyre than on 26er. In order to visualizeit even more throw more sizes into the mix. Do the drawing with 20",24",26"27,5", 29" and 32 or even 36"
    Doing it you will see whether you are right or mistaken.
    I shall write something more when I'll be more versed.

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