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  1. #1
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    Tire pressure and rolling resistance

    Can someone pls advse if there is a difference in rolling resistance on off road terrain using same brand+type of tire, running on lower and higher pressure (say 20psi vs 30psi) ?

    if so, which will have a lower and higher rolling resistance ?

    thanks

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    Kozmos, it depends of a few factors, like tire construction (rubber/casing, sidewall rigidity, etc.), but basically, on hard pack you should go with the higher pressure to keep the rolling resistance low, and on soft terrain, you should favor a lower pressure. Schwalbe did some tests on that subject, and it's been discussed in this forum quite a bit. There's some info for ya ... Rolling Resistance | Schwalbe North America

    In the end, I think it's up to you to find what works best for you !!

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    Quote Originally Posted by whtdel View Post
    Kozmos, it depends of a few factors, like tire construction (rubber/casing, sidewall rigidity, etc.), but basically, on hard pack you should go with the higher pressure to keep the rolling resistance low, and on soft terrain, you should favor a lower pressure. Schwalbe did some tests on that subject, and it's been discussed in this forum quite a bit. There's some info for ya ... Rolling Resistance | Schwalbe North America

    In the end, I think it's up to you to find what works best for you !!
    Good info. In general, more pressure = less resistance. Obviously, it can get more complicated depending on how bumpy your trail is. Tires with lower air pressure roll over bumps more easily, but there's a point at which you start losing efficiency. I'd suggest doing some experimenting of your own, that way you know what works best on your trails with your setup.
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    Quote Originally Posted by lightjunction View Post
    Good info. In general, more pressure = less resistance. Obviously, it can get more complicated depending on how bumpy your trail is. Tires with lower air pressure roll over bumps more easily, but there's a point at which you start losing efficiency. I'd suggest doing some experimenting of your own, that way you know what works best on your trails with your setup.
    this test is the one you need to read for offroad applications.
    the conclusion is: less pressure = less resistance
    and that trend was clear all the way down to the lowest pressure tested (21 psi). in other words, there was no point at which you start losing efficiency.
    the trend is reversed on hard surfaces like pavement and... rolling resistance test drums, which is why the study I linked was surprising and hard to accept for many people.
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    Quote Originally Posted by kozmos View Post
    Can someone pls advse if there is a difference in rolling resistance on off road terrain using same brand+type of tire, running on lower and higher pressure (say 20psi vs 30psi) ?

    if so, which will have a lower and higher rolling resistance ?

    thanks
    I will add that the roughness of the surface matters, too. Generally, lower pressures roll better on rougher terrain. hardpack or soft.
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    Quote Originally Posted by meltingfeather View Post
    in other words, there was no point at which you start losing efficiency.
    When the pressure gets low enough that the tire casing begins to deform too much then the rolling resistance skyrockets...

    This would generally start to occur when the tire begins to "squirm" as you go around a corner.

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    Quote Originally Posted by jeffscott View Post
    When the pressure gets low enough that the tire casing begins to deform too much then the rolling resistance skyrockets...

    This would generally start to occur when the tire begins to "squirm" as you go around a corner.
    any evidence demonstrating what you're saying would be great.
    TIA


    could you explain "low enough," "too much," "skyrocket," and "the point at which the tire begins to 'squirm,'" in terms of quantities someone besides yourself could understand?
    Quote Originally Posted by pvd
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    Quote Originally Posted by meltingfeather View Post
    any evidence demonstrating what you're saying would be great.
    TIA


    could you explain "low enough," "too much," "skyrocket," and "the point at which the tire begins to 'squirm,'" in terms of quantities someone besides yourself could understand?
    Yes Dear

    Jump on your bike put 5 psi in your tires (say a 2.1 in tire) ride around then add another 5 psi....

    You will get it fast enough.

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    Quote Originally Posted by jeffscott View Post
    Yes Dear

    Jump on your bike put 5 psi in your tires (say a 2.1 in tire) ride around then add another 5 psi....

    You will get it fast enough.
    cute.
    i can't ride with 5 psi in my tires on a rough surface because i'd pinch flat my tires or ding my rims. can you? do the math, if you can, on the contact patch you would need to support your weight.
    the rims probably wouldn't even leave the ground, so even if your unsupported theoretical point of "skyrocketing" resistance is there at 5 psi, it is completely irrelevant.
    in the test, rolling resistance decreased until they stopped testing at ~21 psi. anything below that point is irrelevant to anyone (myself included) who can't get there without pinch flatting.
    i said 'evidence,' not more of your typical, know-it-all, blathering fail.
    thanks anyway, though.
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    Quote Originally Posted by shiggy View Post
    I will add that the roughness of the surface matters, too. Generally, lower pressures roll better on rougher terrain. hardpack or soft.
    this is a good point.
    i'll add that surface softness plays an additional role:
    because soil hysteresis is much higher than that of a pneumatic tire (normally very little to none of the energy put into deforming the soil can be recovered), a lower-pressure tire that floats on the surface due to increased contact patch will also typically exhibit lower resistance on a soft surface, even if it is smooth.
    Quote Originally Posted by pvd
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  11. #11
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    Quote Originally Posted by meltingfeather View Post
    this test is the one you need to read for offroad applications.
    the conclusion is: less pressure = less resistance
    and that trend was clear all the way down to the lowest pressure tested (21 psi). in other words, there was no point at which you start losing efficiency.
    the trend is reversed on hard surfaces like pavement and... rolling resistance test drums, which is why the study I linked was surprising and hard to accept for many people.
    The problems with the test mentioned aren't the conclusions. It appears good enough for a master's degree and good enough for publicity. Science it is not.

    Also, the trend isn't "reversed" on hard surfaces and test drums. It is possible to explain both results in a coherent way. Higher pressures reduce rolling resistance but also pneumatic benefit. When there is insufficient pneumatic effect rolling suffers. It's easy to see that there will be an optimum overall pressure that will drop as the surface gets rougher.

    Ignoring jeffscott's hyperbolic language, there is a "knee in the curve" below which rolling resistance begins to increase more rapidly. Don't know how that relates to the MTB trail rolling problem though. I think it's safe to say that starting at 0 psi, as you add pressure rolling resistance decreases up to a point, after which it begins to increase. Where that point is depends on a number of factors, but it holds true for road and track tires as well. I though whtdel's advice in Post #2 was perfect.

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    Front and rear work differently, higher pressure on the front + rocks = more stalling out, where as on the rear the rear lifts over the bumps easier so I favour fairly hard on the rear which is where most of the resistance is especially when climbing, but softer on the front to prevent stalls / over the bars.

    Smoother the surface higher you want the pressure within reason, rougher then softer but not to a level the tire squirms or sidewalls collapse ofcourse.

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    Quote Originally Posted by craigsj View Post
    The problems with the test mentioned aren't the conclusions. It appears good enough for a master's degree and good enough for publicity. Science it is not.
    I agree with that. The trouble is bridging the gap between what is applicable on the trail and what is satisfactorily controlled for repeatable results in a lab.
    I have seen pictures of some "rough surface" test drums on the Finnish lab Wheel Energy's website, but I have not seen any results using them.
    One thing I've thought about in regards to this is that it would seem that some of the energy loss captured in the Schwalbe test is going to be the work associated with moving mass upward due to the tire not conforming as well to surface features in a way that the potential energy can not be recaptured, or at least not consistently or completely. I don't see how a conventional drum would capture that, and therefore can it (the Schwalbe results) even be called "rolling resistance" as it is typically understood?
    Quote Originally Posted by craigsj View Post
    Also, the trend isn't "reversed" on hard surfaces and test drums. It is possible to explain both results in a coherent way. Higher pressures reduce rolling resistance but also pneumatic benefit. When there is insufficient pneumatic effect rolling suffers. It's easy to see that there will be an optimum overall pressure that will drop as the surface gets rougher.
    I agree with this as well. What I meant was that generally resistance decreases with increasing pressure on smooth, hard surfaces; the opposite of what we see in the Schwalbe test. There are still many people who run skinny tires at high pressure off-road, thinking that it reduces rolling resistance.
    Quote Originally Posted by craigsj View Post
    Ignoring jeffscott's hyperbolic language, there is a "knee in the curve" below which rolling resistance begins to increase more rapidly. Don't know how that relates to the MTB trail rolling problem though.
    I'm not sure either. My point was to highlight that if the Al Gore hockey stick is below the ~21 psi where the Schwalbe test stopped, it is going to be irrelevant to most people because you will get to other limits (e.g., tire squirm or rim dings or pinch flats) first.
    Quote Originally Posted by craigsj View Post
    I think it's safe to say that starting at 0 psi, as you add pressure rolling resistance decreases up to a point, after which it begins to increase. Where that point is depends on a number of factors, but it holds true for road and track tires as well.
    I think that's probably safe to say too. Again, though, how relevant is the "knee" if it happens below what 99% of riders are going to run pressure-wise?
    Quote Originally Posted by craigsj View Post
    I though whtdel's advice in Post #2 was perfect.
    I agree.
    Last edited by meltingfeather; 02-02-2012 at 05:17 PM.
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  14. #14
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    Quote Originally Posted by meltingfeather View Post
    this test is the one you need to read for offroad applications.
    the conclusion is: less pressure = less resistance
    and that trend was clear all the way down to the lowest pressure tested (21 psi). in other words, there was no point at which you start losing efficiency.
    the trend is reversed on hard surfaces like pavement and... rolling resistance test drums, which is why the study I linked was surprising and hard to accept for many people.
    An experienced MTB rider once told me this very thing and it really makes a difference, although it seems counterintuitive. The tire tends to grab a surface more instead of bouncing off of it. The bounce causes a greater loss of momentum versus the tire conforming to the terrain. I'm a believer and I run about 24psi in the front, 28in the rear in Arizona rocky terrain. Tubed, 29er.


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    Quote Originally Posted by CCMTB View Post
    An experienced MTB rider once told me this very thing and it really makes a difference, although it seems counterintuitive. The tire tends to grab a surface more instead of bouncing off of it. The bounce causes a greater loss of momentum versus the tire conforming to the terrain. I'm a believer and I run about 24psi in the front, 28in the rear in Arizona rocky terrain. Tubed, 29er.


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    I agree with this... though it is hard to translate to or conclude from experience.
    The bounce versus conforming thing is what I was just getting at in my last post. I don't see how that could be captured on a conventional wheel drum, even on a rough surface.
    Conventional RR machines are really designed to look at hysteresis in the tire, which is somewhat easier to control for.
    Quote Originally Posted by pvd
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    Quote Originally Posted by meltingfeather View Post
    ... though it is hard to translate to or conclude from experience...
    This has to be the recurring theme of any of these discussions. The huge range of variables at play when riding offroad make any generalisations from experience difficult.

    What we do know is:

    - for anything other than a smooth surface, lower pressure will reduce rolling resistance (possible even below 21psi);

    - what happens to rolling resistance below 21psi is irrelevant as you can't run pressures below that without pinch flatting with tubes or rolling the tyre off the rim (tubeless).

    Where does that leave us? Everyone's riding terrain, style and suspension setup are different, so its up to the individual to do some experimentation on their own. Ideally "experiment" in a scientific way. Try different pressures over the same loop and look at your own empirical data (times, speed, heart rate etc.) as well as how the bike felt. This is the only way to arrive at a tyre setup that you know and like.

    Low pressure = low rolling resistance tells us that in our own personal tyre pressure experiments we should:

    - start at low pressures and work up as necessary;
    - increase pressure only as much as is required. eg. pinch flatting, burping, tyre squirm too much.

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    Extremely soft surfaces

    Totally agree with lowering the pressure on rough terrain ... hardpack or soft (as ccmtb and shiggy were expressing).

    But what gives me headaches has to do with very soft surfaces ... like sand or snow! Based on my experience, it seems like, on those surfaces, we really need to put our weight over the back wheel, to prevent the front wheel from digging in, thus stalling the bike. I mean ... we know the rolling resistance is directly proportional to the amount of ground that is being deformed under each tire, thus, evenly distributing our weight over both wheels should keep the total rolling resistance of the whole system at its minimum, but in this case (extremely soft surfaces), itís like the CG has to be located toward the rear axle to keep the momentum going. Does that make sense ?

    Also, I always wondered if this has anything to do with some of us complaining with an increase in rolling resistance as we lower our fork (for Fox-Talas users) ... thus moving the center of gravity to the front ?

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    For soft surfaces you want as much footprint as possible to avoid digging in. Lower pressures should help you stay on the surface. This will result in less rolling resistance than if you "dig in" and have to displace lots of soft material to the sides. Any displacement is extra mechanical work that you won't get back.

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    Quote Originally Posted by meltingfeather View Post
    this test is the one you need to read for offroad applications.
    the conclusion is: less pressure = less resistance
    and that trend was clear all the way down to the lowest pressure tested (21 psi). in other words, there was no point at which you start losing efficiency.
    the trend is reversed on hard surfaces like pavement and... rolling resistance test drums, which is why the study I linked was surprising and hard to accept for many people.
    21 psi isn't that low at race weight ( do they call out the weight of the test rider?). I'm really curious to see them finish the study and see what point rolling resistance goes the other way. I can't imagine it's not a bell curve type situation. To me that study was great to see but it made me ask more questions than it solved.

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    Quote Originally Posted by customfab View Post
    21 psi isn't that low at race weight ( do they call out the weight of the test rider?). I'm really curious to see them finish the study and see what point rolling resistance goes the other way. I can't imagine it's not a bell curve type situation. To me that study was great to see but it made me ask more questions than it solved.
    I'd bet 21 psi is lower than what 95% of people race.... maybe more.
    I'd be curious to see the results taken lower too, but I think there are very few people that actually ride that low.
    What pressure do you run in a lightweight 2.2?
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    Very interesting thread,
    I ve been testing many different tire pressure on my bike, riding the same loop over and over .
    The course is a only 4 ml long with a mix of roots sand and hard pack .
    I found that at my body weight 127 lb ( 58 kg) the best tire pressure for speed and control is
    17 psi front and 19/20 psi back on WTB Bronson 2.3 front and WTB nano race 2.1 tubeless.
    I couldn t beleve how faster and smoother my bike feels ! ( emprove my time record of over a minute ). For reference I ride a Giant xtc 29 small.
    thanks

  22. #22
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    21 psi really isn't that low. More and more people are racing large tires at really low pressure. Personally, if I am using a 2.2 Ikon with EXO protection I run 19-21 psi. Some of the guys on the 2.25 Racing Ralfs are racing sub 20.
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    Quote Originally Posted by LMN View Post
    21 psi really isn't that low. More and more people are racing large tires at really low pressure. Personally, if I am using a 2.2 Ikon with EXO protection I run 19-21 psi. Some of the guys on the 2.25 Racing Ralfs are racing sub 20.
    It'd be interesting to know. I tried posting a tire pressure poll, but when I click the "include a poll" box I don't see any place to input the poll options and it doesn't come up as a poll when I preview.
    If anyone wants to, this is I was going to say:
    Tire Pressure Poll

    What pressure do you run in your XC mtb tires?
    The reason I say "XC" is that I'm trying to rule out fat bikes, pavement slicks, etc.Options:
    < 20 psi
    20-25 psi
    25-30 psi
    30-35 psi
    >35 psi
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    Quote Originally Posted by meltingfeather View Post
    I'd bet 21 psi is lower than what 95% of people race.... maybe more.
    I'd be curious to see the results taken lower too, but I think there are very few people that actually ride that low.
    What pressure do you run in a lightweight 2.2?

    Well since tire pressure is directly related to rider weight it depends on what that factor is. If your a levi racing leadville you could certainly run pressure that low because your the size of a horse jockey. Me and you (irc) are around 200# and that kind of pressure would be silly. It would be nice if some smart person (like you) could come up with an equation to even tire pressure discussions. Something that would account for at least tire width and rider weight and ideally weight distribution as well. Obviously 21psi in a 2.2 or 2.4 is more pressure than it is in a 2.0 or a 1.9 because it's in PSI and the bigger tires simply have more I's. So that opens up another can of worms.

    To answer your question I typically run 2.1-2.25's in the rear and run them at about 35 psi. I really don't like the feeling of the tire squirming all over the place like it's flat. I still get some give out of the tire at that pressure and it holds it shape in a corner.

  25. #25
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    Quote Originally Posted by meltingfeather View Post
    Tire Pressure Poll

    What pressure do you run in your XC mtb tires?
    The reason I say "XC" is that I'm trying to rule out fat bikes, pavement slicks, etc.Options:
    < 20 psi
    20-25 psi
    25-30 psi
    30-35 psi
    >35 psi
    But then this would circle back to the core premise for all tire discussions: what is the terrain & what is the riding style.

    Type of terrain (smooth or rough) / tire (casing & tread) / riding style (bashing or finesse) all have bearing on tire pressure.

    I have to run ~5psi higher than I like to preserve sidewall support for high G turns and landing jumps, which is a considered tradeoff with efficiency.

    The slowest rolling tire is the flat one.

    None of these studies address any of our particular needs. And like IndayFab said, raises more questions (although in a productive direction). But they do provide a data point that we can look to with some amount of trust to make a better informed decision. Prior to these tests, it was myth, marketing and hearsay, that created an environment which had Mountain Bike Action proclaiming the Nevagal as an efficient roller

    P

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    Quote Originally Posted by Mr.P View Post
    But then this would circle back to the core premise for all tire discussions: what is the terrain & what is the riding style.

    Type of terrain (smooth or rough) / tire (casing & tread) / riding style (bashing or finesse) all have bearing on tire pressure.

    I have to run ~5psi higher than I like to preserve sidewall support for high G turns and landing jumps, which is a considered tradeoff with efficiency.

    The slowest rolling tire is the flat one.
    It would provide some insight on the question of how many or what percentage of riders get below 20psi, whatever their reason.

    Quote Originally Posted by Mr.P View Post
    None of these studies address any of our particular needs. And like IndayFab said, raises more questions (although in a productive direction). But they do provide a data point that we can look to with some amount of trust to make a better informed decision. Prior to these tests, it was a myth, marketing and hearsay, and an environment which had Mountain Bike Action proclaiming the Nevagal as an efficient roller
    Agreed... and no study will. What the Schwalbe study did do for the first time that I'm aware of, is provide some basis for breaking down the "the more pressure = less resistance" paradigm that comes from road riding and conventional resistance testing.
    Quote Originally Posted by pvd
    Time to stop believing the hype and start doing some science.
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  27. #27
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    Quote Originally Posted by meltingfeather View Post
    What the Schwalbe study did do for the first time that I'm aware of, is provide some basis for breaking down the "the more pressure = less resistance" paradigm that comes from road riding and conventional resistance testing.
    What the study did was conflate rolling resistance and pneumatic effect which I don't consider a service. Understanding that the problem is one of balancing contradictory goals is much better than believing that MTB rolling resistance follows a difference set of rules. Now we have a bunch of trail riders who think that rolling resistance improves with lower pressure when in reality it is something else.

    There are not two different rolling resistance "paradigms", the same rules apply regardless of the surfaces you cycle on. Rough surface road riders, at least some of them, also understand that bigger tires and lower pressures work better. Road riders have largely given up on the really skinny tires though still seem be stuck on 23's. There's lots of effort on how to optimize around wider tires even for time trials. Cyclists of all sorts are increasingly aware that they've been misled for a long time on the value of undersized rubber.

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    Quote Originally Posted by craigsj View Post
    What the study did was conflate rolling resistance and pneumatic effect which I don't consider a service. Understanding that the problem is one of balancing contradictory goals is much better than believing that MTB rolling resistance follows a difference set of rules. Now we have a bunch of trail riders who think that rolling resistance improves with lower pressure when in reality it is something else.
    I see what you're saying. My counterpoint would be that prior to seeing these results, many trail riders ignored or didn't consider pneumatic effect and ran higher pressure to reduce "rolling resistance," without accounting for the fact that their energy input might be higher due to pneumatic effect. I consider that a service, even if the methods were crude.
    Quote Originally Posted by craigsj View Post
    There are not two different rolling resistance "paradigms", the same rules apply regardless of the surfaces you cycle on. Rough surface road riders, at least some of them, also understand that bigger tires and lower pressures work better. Road riders have largely given up on the really skinny tires though still seem be stuck on 23's. There's lots of effort on how to optimize around wider tires even for time trials. Cyclists of all sorts are increasingly aware that they've been misled for a long time on the value of undersized rubber.
    I see what you're saying, and as I alluded to, I understand that pneumatic effect is separate from rolling resistance as it is traditionally understood and analyzed, but I think the problem of translating those results for the general public gets more complicated if you introduce a new concept, rather than saying, "your energy input will go down with pressure on rough surfaces," which is essentially what the Schwalbe study said, even though they labeled it rolling resistance.
    The bottom line is that there is some new and broader acceptance of the energy benefits of the pneumatic effect, even if it is erroneously labeled rolling resistance.
    I appreciate your input and I'm going to adjust my treatment of these issues in these discussions in the interest of being more accurate. Whether that will help or hurt the broadening of general understanding, I don't know.
    Quote Originally Posted by pvd
    Time to stop believing the hype and start doing some science.
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  29. #29
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    Quote Originally Posted by meltingfeather View Post
    I see what you're saying. My counterpoint would be that prior to seeing these results, many trail riders ignored or didn't consider pneumatic effect and ran higher pressure to reduce "rolling resistance," without accounting for the fact that their energy input might be higher due to pneumatic effect. I consider that a service, even if the methods were crude.
    Yes good point.

    Long ago I rode skinny tires at high pressures. I tried wider, heavier tires but they always rolled poorly. Back then I assumed wrongly that was due to weight. Progress requires good information AND good equipment choices. The Schwalbe article certainly helps.

    There is a tradeoff between being precise but too complicated or being simple but not really accurate. I don't know how to avoid that.

    As far as the range of pressures go, I'd also find interesting the difference in pressure front to rear that people run but you'd need to know that as a percentage. Then you'd need to know if they use different tires. The possibilities are so complex...

  30. #30
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    This article, although focusing on road tyres is very similar to the Schwalbe article. The diagrams, however, do a better job of conveying the competing performance factors that affect what is the best tyre setup for "you".


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