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  1. #76
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    recently ran 17psi in 2.1 Renegade's on a wider rim (Easton Haven)...it was a wierd ride - if i was super smooth everything was good. But, if I gave some serious input then the tire squished too much and I could feel some roots hit.

    So the deal is, with UST run low psi until the above starts happening. 1.5bar, and 21psi are magic numbers for bigger tires.

    I am 180lbs, so I am heading 24psi in the front, 25-26psi rear. These lower PSI's give more traction and thus you can run a lower hieght knob, and a faster wider tire.

    To the people that WONT GO TUBELESS, then they are at a dead end, I would say they can only go down to 30psi with a bigger casing and 35psi with a 2.0 sized tire...and thats only at my 180lbs wieght....any heavier then up the PSI.

    later...LC

    PS: UST rims, and any tire with sealent works - Guetto tubeless add 5-10psi just for reliablity!

  2. #77
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    Quote Originally Posted by twenty6black
    recently ran 17psi in 2.1 Renegade's on a wider rim (Easton Haven)...it was a wierd ride - if i was super smooth everything was good. But, if I gave some serious input then the tire squished too much and I could feel some roots hit.

    So the deal is, with UST run low psi until the above starts happening. 1.5bar, and 21psi are magic numbers for bigger tires.

    I am 180lbs, so I am heading 24psi in the front, 25-26psi rear. These lower PSI's give more traction and thus you can run a lower hieght knob, and a faster wider tire.

    To the people that WONT GO TUBELESS, then they are at a dead end, I would say they can only go down to 30psi with a bigger casing and 35psi with a 2.0 sized tire...and thats only at my 180lbs wieght....any heavier then up the PSI.

    later...LC

    PS: UST rims, and any tire with sealent works - Guetto tubeless add 5-10psi just for reliablity!
    No, it depends on the rider, tire, rim and terrain. I am currently running Panaracer GC CX 2.1s with tubes on 24mm rims under 25psi on hardpack to rocky terrain.180 pounds, hardtail bike. I can go lower with wider rims.

    I do/can not go lower with similar volume tubeless tires.

    p.s. Not all DIY conversions work. Standard tires without tubes require more pressure to prevent squirm. Use at your own risk.
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  3. #78
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    Quote Originally Posted by twenty6black

    To the people that WONT GO TUBELESS, then they are at a dead end, I would say they can only go down to 30psi with a bigger casing and 35psi with a 2.0 sized tire...and thats only at my 180lbs wieght....any heavier then up the PSI.

    later...LC
    Hey LC, I have to disagree with going tubeless is the only way to go.

    I run tubes and I don't have a problem running low pressure. As I stated earlier in the post I am 140ish with a 30ish lb XC bike. I have been running 2.1 Nevs @ 23 ish pounds the end of last racing season and now this season. I am not getting any more flats(very rare) compared to the tubless guys (flatting or burping) in my group. I can see that I am not spinning any more than the tubleless guys. I clear tough sections (without flatting) and I am one of the newest guys in the group.

    IMHO key things are TERRAIN---RIDER WEIGHT---RIDING STYLE combined .
    If you are not willing to experiment with pressure and changing the occational flat then pump em up to the sidwall specs and go, with a trade off on traction, comfort, and possibly speed.
    Maybe this is a good argument to go tubeless(if you just want to pump em up and go)( I believe sidewall pressure specs are lower for tubeless tires ???YES/NO???)

    Cheers,
    Paul

  4. #79
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    Quote Originally Posted by twenty6black
    recently ran 17psi in 2.1 Renegade's on a wider rim (Easton Haven)...it was a wierd ride - if i was super smooth everything was good. But, if I gave some serious input then the tire squished too much and I could feel some roots hit.

    So the deal is, with UST run low psi until the above starts happening. 1.5bar, and 21psi are magic numbers for bigger tires.

    I am 180lbs, so I am heading 24psi in the front, 25-26psi rear. These lower PSI's give more traction and thus you can run a lower hieght knob, and a faster wider tire.

    To the people that WONT GO TUBELESS, then they are at a dead end, I would say they can only go down to 30psi with a bigger casing and 35psi with a 2.0 sized tire...and thats only at my 180lbs wieght....any heavier then up the PSI.

    later...LC

    PS: UST rims, and any tire with sealent works - Guetto tubeless add 5-10psi just for reliablity!
    Depends on your terrain and style of riding.I'm 170 lbs, I ride forest trails[ no rocks or thorns] and have been riding tubes at 25 psi for 15+ years. One of my current fav tires for some conditions is a Nobby Nic 2.4 snakeskin. It gives best traction when run on the front 28mm rim with a tube at 20 psi.Ghetto tubeless I would probably run it 23 to 25 PSI[ no support from a tube].

  5. #80
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    Some useful info on tire pressures and rolling resistance here

    http://www.schwalbe.co.uk/shopdata/f...chInfo2-GB.pdf

  6. #81
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    Is my pump gauge ok?

    I didn't bother about tyre pressure until I read some threads and bought a full rigid 29. The absence of suspension fork drove me to low pressures.
    I don't have a gauge to measure the pressure at trails. At my first ride I reduced the pressure to the point I felt the bike rolling smooth; I was sure I could go lower without worrying about pinch-flats or damage the rims. When I returned home I checked the pressure at Nevegals 2.2 and my floor-pump gauge show 15psi front and 18psi rear (I know pump gauges are not so accurate). My weight is 158lbs (72kg).
    With my old 26 ht I was running my tyres at hight pressures and its ride was so harsh! Now I know how important is the "right" pressure in tyres. Maybe the right pressure is different for everyone but don't be afraid to try lower pressures, you will be surprised .

  7. #82
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    tubeless stuff

    Quote Originally Posted by gvs_nz
    Depends on your terrain and style of riding.I'm 170 lbs, I ride forest trails[ no rocks or thorns] and have been riding tubes at 25 psi for 15+ years. One of my current fav tires for some conditions is a Nobby Nic 2.4 snakeskin. It gives best traction when run on the front 28mm rim with a tube at 20 psi.Ghetto tubeless I would probably run it 23 to 25 PSI[ no support from a tube].

    yep...the above are good guidelines....everyone jumps into (onto) the tubeless topic - love it!

    Here is something that just happened to me....mud race had the Easton Haven's 21mm rim width, with 2.2 MK SS run them tubeless, so I can go low on the pressures - ANYWAY....didnt lower the pressure for the race - ended up runing 30+psi in the front and 35+psi in the rear !!! I kenw there was a problem right at the start - i could feel every rock and root.

    sooooo...I took it easy on the rock and roots - thing is, the mud sections I was getting good grip - knobs digging through to hard soil - and the double track BAAM - major speed!

    So the MK 2.2 clears the mud and digs for traction, and when run at higher pressures (higher for me) they roll amazingly fast.

    I have also run them at lower pressures and the traction goes UP - speed on hard pack goes down a wee bit.

    This race turned out good - more from good luck than good management.

    ...LC(cheers)

    PS: lesson is, higher pressure on a kobby tire are cool for mud events!

  8. #83
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    I run 50 psi front and rear on a Stumpjumper. I'm 215 with gear.

  9. #84
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    I'd like to chime in. Everyone who hasn't gotten that high volume tires and low pressure is the way to go by now, really should wake up and leave 1994.

    I weigh 215# without gear on, and I run 22psi in the rear and 18 in the front on trails with my FS with Big Betty 2.4's, and 25/23 psi on fire roads/pavement with my hardtail and Rocket Ron 2.4's. I roll faster than the 2,0"
    semi slick guys with their 50 psi pressures (off the tarmac at least), and have way more comfort.


    With tubes, but on XTR Flow and Crest rims. I can't remember my last pinch flat. Must be years ago.

  10. #85
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    I find your weight is generally proportionate to your hand strength and therefore I use the it's soft enough to get a good squeeze on the front and only a moderate squeeze on the front

    This the lower the better mentality started from the Stan's users is well ridiculous, yes you can run it that low cause of no pinches but you've got to run it harder than with tubes cause 1 less layer of rubber, and ran really low, tires squirm, side walls give out randomly causing a sharp fall over sensation or even a fall over.

    That and modern tyres the casing is so soft, if you run them flat the knobs fold in on contact with the ground and you might aswell be running slicks, and slicks ran low on rocks / road would still out grip mtb tires, until you hit mud.

  11. #86
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    Dazed I'm your weight and with a tougher higher volume tire ( RQ 2.4 ) with a fat rim ( EX729 ) I get 24psi, 22psi I can feel the roots hitting the rim, 26 and it feels rigid ( the effect of HUGE tires ) no way can I get near 18 on the front without it feeling like it's flat.

    Obvously you have to allow hugely for guages being really in accurate at these kinds of pressure.

  12. #87
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    Quote Originally Posted by Turveyd
    Dazed I'm your weight and with a tougher higher volume tire ( RQ 2.4 ) with a fat rim ( EX729 ) I get 24psi, 22psi I can feel the roots hitting the rim, 26 and it feels rigid ( the effect of HUGE tires ) no way can I get near 18 on the front without it feeling like it's flat.

    Obvously you have to allow hugely for guages being really in accurate at these kinds of pressure.
    I have two standalone gauges (Schwalbe Airmax Pro and Topeak Shuttle Gauge), and I know what I feel. I may not ride very rough, but I don't flatten the tires.

    I doubt the RQ is any bigger or sturdyer than a BB. My tires aren't squirmy at all. The RoRo's would be at that pressure, but not the BB's.

    It's a very individual thing, of course.

  13. #88
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    The RQ2.4 is the biggest volume tyre I've seen and with 180tpi in the side walls ( Apex casing ) it's pretty solid aswell, the 29mm internal DH rim helps aswell.

    Shame there so expensive, but no visible wear on mine on the front so not really a worry if I can get a year out of it.

  14. #89
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    Those wide rims make a difference of course, even though the flows have Stan's "BST" bead hooks ( http://www.notubes.com/images/height-compare.gif )adding to the width and volume, 29mm is WIDE.

    Shiggy / mtbtires.com backs you up on the RQ being bigger, btw. Too bad, 'cause i don't think my Ventana frame can deal with ANYTHING bigger than the BB. ...And now I know something bigger exists!

  15. #90
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    I don't have issues with pinch flats and I have dropped my pressure based on this thread. Can't honestly say the RR is less with lower pressure. I recently showed up to a group ride (a really fast XC group) with my all mountain/rocky trail pressure and I very soon got a "you better put some air in your tires." from folks that just looked at my tires. Took like 50 psi rear to get them as firm as the rest of the group. Kept front to about 30. Wow, what a difference! I still couldn't keep up with them but it didn't feel like riding velcro tires on carpet anymore. I'll drop back down for my regular rocky bombing but for the XC stuff, I'm checking to see if some road wheels will fit.

    Most of the group were running 1.95s and few were larger than 2.1. Maybe "all around xc" is different from serious xc race?

  16. #91
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    Dazed, I run the RQ 2.4 in a 29er fork so loads of room Not many regular forks they'd work in, front only aswell I'd say, to draggy on the rear for me.

    I actually bought it to run with my Rigid bike, which uses the 29" wheel and a Ardent 2.4 ran soft ish, although not as big it still handles roots and rocks rigid better than the RQ2.4, but the RQ2.4 out grips it.


    Seriously pressure sensitive from roots hurt to you can't feel the roots with just the smallest amount of air let out, then to the jarring rim contacting sound no pinches yet though. and I bust my right thumb 2 weeks back so my own gauge is off

  17. #92
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    No gauge, huh... Well, I was surprised how "hard" 20 psi feels in a whooping big 2.4" tire. maybe you'll find that you're actually running lower pressures than you think if you check with a good gauge?

  18. #93
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    Mates got a gauge though, as I state your gauge very likely isn't accurate, it's obviously means the same for my mates.

  19. #94
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    And as I stated, I've got several gauges showing the same pressure.

  20. #95
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    Just discovered this thread. It is interesting that no-one has actually stated the physics first.

    1. For a given tyre pressure your contact patch size will stay the same regardless of the tyre size. Read that again. So a 2.5" tyre will have the same contact patch area as a 1.9" tyre @30psi, or @25psi or whatever.
    2. What will change with changing tyre size is the shape of the contact patch. The wider the tyre, the shorter and fatter the contact patch. But the size will stay the same.
    3. With the changing shape of the contact patch, the dynamics of the tyre will change in the 4 axis that a tyre works on (the "friction circle") - front and rear for traction/braking, and side-to-side for cornering.
    4. Generally speaking chose a narrow long contact patch for better traction & braking, and a wide short patch for cornering.

    That is the physics.

    What is also true, as someone has stated is that at lower pressures a tyre will deform to absorb a small obstacle where a tyre at higher pressure will resist deformation and ride up over the obstacle. Whether this is a "good thing" depends on the terrain, the speed and your objective.

    At one extreme, hard-packed surfaces, you work to present the smallest contact patch that you can. Using smaller tyres and higher pressures. There will be nothing faster. That is why roadies run 19c tyres @130psi for races.

    For other terrain, you need to balance speed, traction, cornering and braking to find the right combination to go fastest. This will be a combination of tyre size, pressure and tread pattern. There is no hard-and-fast rule here. You will find some tyres will be faster at higher pressure than other tyres run at lower pressure when sense says they shouldn't be. Why? Because the tyre manufacturer has developed a casing optimised for the conditions you are riding, and the tyres are fast.

    For this reason, I work on tread/compound first, size second, then pressures third.

    However, I have about 20 tyres in the garage and 3 sets of wheels setup. One with a general XC tyre-set, another with a more radical XC race set, and one with a wide software "traction" set for wet weather.

  21. #96
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    What is also true, as someone has stated is that at lower pressures a tyre will deform to absorb a small obstacle where a tyre at higher pressure will resist deformation and ride up over the obstacle. Whether this is a "good thing" depends on the terrain, the speed and your objective.

    At one extreme, hard-packed surfaces, you work to present the smallest contact patch that you can. Using smaller tyres and higher pressures. There will be nothing faster. That is why roadies run 19c tyres @130psi for races..
    I think it is said that you loose more energy in the vibration and bouncing of the bike from a harder tire (from pressure) than you do from the energy lost deforming a softer tire over a bump.
    It must also be noted that there is optimum tire pressures for road tires. Its not pump them up to next to the blowing point. For example at my weight my tire pressure for my Michilen Race is around 90 psi, while a heavier rider needs more.

    It's also funny how all your theories get thrown out the window buy smoking your best time running the bigger heavier tire with lower pressure(Nevegal 650g) on a day when its dry and fast and you figured being to lazy to switch to the lighter higher pressure tire (Karma 400g) was gong to hurt you
    Last edited by Rum Runner; 10-01-2010 at 06:10 AM.

  22. #97
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    Actually, based on my analysis of the physics involved, I found that I had a larger contact patch when running skinnier tires at high pressure. This was due to the fact that my feet were touching the ground far more often.
    Warning: may contain sarcasm and/or crap made up in an attempt to feel important.

  23. #98
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    Quote Originally Posted by Rum Runner
    It's also funny how all your theories get thrown out the window buy smoking your best time running the bigger heavier tire with lower pressure(Nevegal 650g) on a day when its dry and fast and you figured being to lazy to switch to the lighter higher pressure tire (Karma 400g) was gong to hurt you
    That is also a point that is worth re-affirming. It is very very hard to have a consistent measure to evaluate tyre/wheel performance against. A long time ago I setup a trial of aero rims as a few of my mates and I were deeply into triathlons. We have a evaluation circuit of about 5 klicks taking in a variety of types of surface, hill, flat and downhill (obviously). We were all very fit and strong, and yet there was about 5% variability in our circuit times on the same day, on the same equipment. It makes it very hard to scientifically measure with the "control" being so variable..

    I would also state the obvious. Feeling fast and being fast against the clock are different things...

  24. #99
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    Quote Originally Posted by trailville
    Actually, based on my analysis of the physics involved, I found that I had a larger contact patch when running skinnier tires at high pressure. This was due to the fact that my feet were touching the ground far more often.
    I think the key point for the physics is whether your training wheels are on or not???

  25. #100
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    Quote Originally Posted by skiwi
    Just discovered this thread. It is interesting that no-one has actually stated the physics first.

    1. For a given tyre pressure your contact patch size will stay the same regardless of the tyre size. Read that again. So a 2.5" tyre will have the same contact patch area as a 1.9" tyre @30psi, or @25psi or whatever.
    2. What will change with changing tyre size is the shape of the contact patch. The wider the tyre, the shorter and fatter the contact patch. But the size will stay the same.
    3. With the changing shape of the contact patch, the dynamics of the tyre will change in the 4 axis that a tyre works on (the "friction circle") - front and rear for traction/braking, and side-to-side for cornering.
    4. Generally speaking chose a narrow long contact patch for better traction & braking, and a wide short patch for cornering.

    That is the physics.

    What is also true, as someone has stated is that at lower pressures a tyre will deform to absorb a small obstacle where a tyre at higher pressure will resist deformation and ride up over the obstacle. Whether this is a "good thing" depends on the terrain, the speed and your objective.

    At one extreme, hard-packed surfaces, you work to present the smallest contact patch that you can. Using smaller tyres and higher pressures. There will be nothing faster. That is why roadies run 19c tyres @130psi for races.

    For other terrain, you need to balance speed, traction, cornering and braking to find the right combination to go fastest. This will be a combination of tyre size, pressure and tread pattern. There is no hard-and-fast rule here. You will find some tyres will be faster at higher pressure than other tyres run at lower pressure when sense says they shouldn't be. Why? Because the tyre manufacturer has developed a casing optimised for the conditions you are riding, and the tyres are fast.

    For this reason, I work on tread/compound first, size second, then pressures third.

    However, I have about 20 tyres in the garage and 3 sets of wheels setup. One with a general XC tyre-set, another with a more radical XC race set, and one with a wide software "traction" set for wet weather.
    Mfg have released snippets of their research over the years which are counter intuitive to the “Physics” as we thought we understood it. XC World Cup podium finishers using 2.4 tires have a little more info at their fingertips than we do.

    When comparing different width tires it's not the size of the contact patch which governs rolling resistance but the shape. It’s related to hysterisis not friction.

    When considering construction of tires and their rolling resistance the physical tread blocks have an affect but are secondary to the underlying tread region of the casing.

    Whether on or off road, there is trend towards wider tires having less rolling resistance. Aerodynamics aside, a 25mm road tire, even at 120 psi, has been shown to have 40% less rolling resistance than 19 mm road tire at the same pressure. A 1.7" tire at 60 psi has the same rolling resistance as a 2.4" tire at 30 psi. The rougher the ground the greater the effect. As much as 50 watts can be gained on rough terrain using the 2.4" tire at 23 psi over the the 1.7" tire at 60 psi.
    Last edited by gvs_nz; 10-09-2010 at 09:54 PM.

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