Is Lower Tire Pressure Faster?- Mtbr.com
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  1. #1
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    Is Lower Tire Pressure Faster?

    I'd like to pose a slightly different take on the "what tire pressure do you run" question to all you XC race types---is the lowest non-flating pressure you can run generaly the fastest for XC racing?

    It seems the general trend, especialy from the no-tubes folks, is go-low. But in Ned Overend's book he talked about runing 45+psi, which would seem to result in lower rolling resistance. I've done some experiementation with my Paragon HT at 30,35,40, 45psi eventualy settling in at the lower end of that range becuase the ride was more comfortable and I could hold a line better as the bike wasn't jumping around as much, but I can't really objectively say that one setting was really slower or faster than another.

    Anyone done any real testing on tire pressure vs. speed?

  2. #2
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    Quote Originally Posted by Red29
    But in Ned Overend's book he talked about runing 45+psi, which would seem to result in lower rolling resistance.
    = "Old school" ; not that Ned isn't a legendary racing stud of epic proportions

    Quote Originally Posted by Red29
    I'd like to pose a slightly different take on the "what tire pressure do you run" question to all you XC race types---is the lowest non-flating pressure you can run generaly the fastest for XC racing?)
    = "New school" ; I'd guess almost every current World Cup racer is running in in the 20's almost every race. Naturally, that depends on your weight, conditions, tire choice, and riding style, but I'm sure you get the idea. The basic concept is that higher pressure is faster on smooth surfaces, but on any off road surface (even relatively smooth fireroads) all the micro-variations cause the total system of rider + bike to lose energy, instead of rolling along smoothly with the tire conforming to the surface (uses up some energy, but not as much?) Also, better traction for climbing, braking, cornering, etc. due to larger footprint. As low as you can go without flatting? Maybe not quite (preferences vary) but a lot lower than 45psi for most.

  3. #3
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    Depends on what tires and what trail you are riding. I don't thnk there is one specific answer. Some tires are designed to run on smooth hardpack dirt without a lot of rocks which is like a pure XC trail. In this case that tire is going to have a thin casing and low tread and to prevent pinch flats etc you are going to run 38-42PSI. As well the sidewall will be thin so if you don't run enough PSI it will fold and not work correctly. As well these tires are designed for smooth terrain and will roll very efficiently (fast) due to the tread design and higher PSI (and in turn lower weight)

    Where you start to need lower pressure is when you are on a trail with more rough obstacles like roots and rocks. YOu want your tire to conform to each thing which keeps it rolling instead of bouncing and skipping. Then you would want a tire that has more volume (bigger) so you can run less air pressure. THis does two things: allows your tire to contour to the shape of the ground and keep it rolling over things instead of hitting them and bouncing upwards losing momentum. As well it will help with traction. Another bonus is that with a higher volume tire not only can you run lower pressure for better traction but you also get a second suspension. The bigger volume tire is going to have more of a cush too it like riding on a big soft balloon. I'd run a higher volume tire at around 28-32 even with tubes

    So really you need to take into consideration what tire you are using and what kind of trail you will be riding.

    At least these are my findings. I'm sure there are some other variables

  4. #4
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    Running tubed tires? gotta run a minimum pressure for pinch flats. Running tubeless, which is the way I think is the way to go, roll over chatter bumps like nothing (FASTER) and in more control.

    In my book, lower is better (to a point - for me about 23-25 lbs - I weight 145#)... I read somewhere rolling resistance vs suppelness (suppelness helps forward momentum) is good down to like 20-25 lbs. I'd like to see real tests though, just for peace of mind.

    I run my tubless just to the point of feeling squishy. I think I'm faster for most of what I ride and I KNOW I'm faster over the nasties...

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    Casing and sidewall structure make a difference too, some sidewalls don't support themselves very well with lower pressures and the tire performance will suffer, like the specialized captains and the sb8's. Even with tubeless I wouldn't run some tires too low.

  6. #6
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    Tire tech. changes very quickly. With duel compound, ultra tough, super light, or crazy sticky rubber compounds what was considered the norm/most efficient last year could be old news and very dated.

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    I dunno, after fiddling around with tyre pressures, I seem to have come up with a sort of medium pressure as being best (duh). Hard tyres can get you up and down nice smooth stuff - higher pressure means less tyre surface in contact with the ground, it's obvious. But as soon as you get onto anything bouncy (especially going downhill with some momentum) things can get really uncontrollable. I have had some hairy moments caused solely by high tyre pressure.

    So as somebody said, really depends on the terrain, but if it's varied then you end up with somewhere in the middle being your best bet.

    Though I am not too good on the scary downhill technical stuff, so I am thinking maybe I should just cut my losses on that stuff, go for a harder pressure and just get off and push when things get bouncy - I'll make it up on the smooth stuff. We'll see, but I don't think anyone can prescribe this stuff for you - just see what works for you.
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  8. #8
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    I weigh 135 and am running about 15 psi in 29X2.3s. If I have the pressure any higher I can't stand to ride the bike. It beats me up, and is really hard to control. So while my tires may roll slower on pavement, they are defiantly faster on any sort of rough surface. I think you will get the best performance out of your tires by running them at the lowest pressure possible without flatting or rolling a tire off the rim.
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  10. #10
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    Just above "squirmy feeling" is about the right pressure for me. Right now this translates into about 26-28# front 30-32# rear on my pump. 29x2.2/2.1 tires, no tubes. I go about 185#

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  11. #11
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    Quote Originally Posted by Yody
    Casing and sidewall structure make a difference too, some sidewalls don't support themselves very well with lower pressures and the tire performance will suffer, like the specialized captains and the sb8's. Even with tubeless I wouldn't run some tires too low.
    I think I can vouch for that... I've tried running my Captians low (still with tubes) and they feel squirmy yet not real supple like my WTB Exiwolfs.

    I just went tubeless on the Captians (on my new Stans Olympic's) and will ride them tonight... it will be interesting to see if they feel much different that with tubes at the same pressure.

    If I'm not liking them (the Captains) after going tubeless I'll go back to my favorite: The Exiwolf's...
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  12. #12
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    Lower is faster!

    Here is a more definitive study:

    http://www.bicicletta.co.za/Download...llustrated.pdf

    Basically if you aren't on the road, then lower is faster. Those wattage savings from low pressure are way better than from losing a few grams in the tire.

  13. #13
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    Quote Originally Posted by pbayne
    Here is a more definitive study:

    http://www.bicicletta.co.za/Download...llustrated.pdf

    Basically if you aren't on the road, then lower is faster. Those wattage savings from low pressure are way better than from losing a few grams in the tire.
    Thanks for the link!

    I'd be curious to hear any criticisms of that Schwalbe study or if there are any published studies to the contrary. Anyone?

    My first thought was: Schwalbe sells wide, supple tires, and thus, the study seems very self-serving. Don't get me wrong--it could still be a good study with good results, but it makes me wonder.
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  14. #14
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    Lower tire pressure is more forgiving in high speed cornering. Pump your tires up to 65psi and go ride a trail and then try the same trail at 20 psi. You will be able to tell which psi feels faster and better.
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    Thanks for all the info----I look forward to doing some more experiments when the dirt shows back up here in Chicago---

  16. #16
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    here's the basics of low preassure ........

    imagine your tire running into a 3" high root while inflated to maximum psi. your forward momentum is now "bouncing" backwards off the root and your bike loses momentum.

    now imagine the same scenario with low psi....your tire is softer, absorbs the root, and forms around it. allowing you to carry the momentum through and over.
    pretty simple stuff.

    now for a 40k TT on the road......the higher psi the better.
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    the 5th poster, ganginwoods, is correct

  17. #17
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    I am now a believer that lower air pressure creates lower rolling resistance and better traction, which in turn allows me to run any given trail faster. Of course there is such a thing as having your air pressure too low where you will get diminishing returns. I converted my tires to tubeless with Stan's at the end of last summer and noticed a huge improvement in cornering traction, and the tires seem to roll easier over rocky sections. I think the improvement is not because I am now tubeless, I believe its because I'm now able to run lower air pressure. With tubes I was running 32/35 lbs f/r, now I run 22/26 lbs f/r. I don't think they roll any faster on smooth sections or pavement, but most of the trails I ride are not smooth anyway.

  18. #18
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    Quote Originally Posted by ganginwood
    here's the basics of low preassure ........

    imagine your tire running into a 3" high root while inflated to maximum psi. your forward momentum is now "bouncing" backwards off the root and your bike loses momentum.

    now imagine the same scenario with low psi....your tire is softer, absorbs the root, and forms around it. allowing you to carry the momentum through and over.
    pretty simple stuff.

    now for a 40k TT on the road......the higher psi the better.
    Well, sort of. The same principals still apply. So you have to determine what is the optimal tire pressure for the tires you use and the roads you'll be riding.

    I have 19mm tubulars on my track wheels. Never see anything but glossy concrete and hopefully wood this year. Best served at 130psi+.

    I have a set of "Classics" wheels; Wolber Profil 18s with 24mm Vittoria Corsa Evo Pave tubulars. Nice, cushy ride for crappy roads and gravel. Fast over the rough stuff. 80-90psi is normal. Like a magic carpet.

    My "normal" race wheels: 22mm tubulars on 50mm deep dish carbon. 100psi front, 105psi rear.

    All things being the same, if I were to race my state TT, which sees some chip seal roads for good stretches, I'd go 22mm, 24mm, 19mm. The super skinny tires, with the pressure they require, would be slower than their bigger cousins. But on a super smooth surface, they're the fastest, because they don't have to deform around anything, they aren't going up and down literally every bump in the road.

  19. #19
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    How is it that lower psi creates lower rolling resistance, this part about lower tire pressure being faster makes the least sense. Can anyone explain this part about lower pressures being better? It seems that the lower pressure would force the tyre to 'bend' around the terrain more at lower pressure, which would add resistance to the rotation of the wheel against the ground, like rolling a sand filled bag on the ground vs. rolling a bowling ball, and that higher pressure would give a lower rolling resistance because the tyre has to 'bend' less.
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  20. #20
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    Quote Originally Posted by lampy29
    How is it that lower psi creates lower rolling resistance, this part about lower tire pressure being faster makes the least sense. Can anyone explain this part about lower pressures being better? It seems that the lower pressure would force the tyre to 'bend' around the terrain more at lower pressure, which would add resistance to the rotation of the wheel against the ground, like rolling a sand filled bag on the ground vs. rolling a bowling ball, and that higher pressure would give a lower rolling resistance because the tyre has to 'bend' less.
    As has been stated by others in this thread it's because it soaks up the smaller bumps and let you carry your momentum. By absorbing these bumps it keeps your tires on the ground and lets you roll over them rather than hit each of these little bumps.

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    Quote Originally Posted by lampy29
    How is it that lower psi creates lower rolling resistance, this part about lower tire pressure being faster makes the least sense. Can anyone explain this part about lower pressures being better? It seems that the lower pressure would force the tyre to 'bend' around the terrain more at lower pressure, which would add resistance to the rotation of the wheel against the ground, like rolling a sand filled bag on the ground vs. rolling a bowling ball, and that higher pressure would give a lower rolling resistance because the tyre has to 'bend' less.
    Think of it this way:

    Rolling resistance is anything that prevents or slows the travel of your wheel/tire on a plane.

    With a lower pressure, your tire bends around the obstacle. With a higher pressure, your tire bounces upwards. Instead of pushing yourself forward, you are pushing yourself upward.

    Now, think of the thousands of tiny bumps over the course of a ride. Every time you hit a small bump or a root with over-inflated tires, you slow down.

  22. #22
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    Rolling resistance is the wrong term in this context. Rolling resistance increases as you lower the PSI a tire. Consequently there is more friction and consequently the tire is slower (but has better grip against the surface).

    The reason a less inflated tire is faster over rough surfaces has already been stated above. Basically less PSI keeps forward momentum better than more PSI. This makes perfect sense, but it does not mean that the rolling resistance coefficient of the tire is lower when you deflate it more.
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    I tried this on my last ride. I weigh 215 and usually pump my front to 40+ and the back to 45+. I was always under the impression that more psi = less rolling resistance. I lowered 7 psi front and rear and made a heck of difference. The trails that I ride are very technical with plenty of climbs and descents. Lower was better hands down(climbing and descending). I will probably go a little lower to experiment, but pinch flats could be a problem. Thanks for the info. I would've never thunk!

  24. #24
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    Quote Originally Posted by serious
    Consequently there is more friction and consequently the tire is slower (but has better grip against the surface).
    Devil's Advocate, Part 1 Grip is created by a combination of friction and mechanical traction. It is absolutely required to have a sufficient friction + traction to provide adequate resistance against forces such as pedaling forces and steering, or else you ain't goin' nowhere (i.e. slower)

    Quote Originally Posted by serious
    Rolling resistance is the wrong term in this context. Rolling resistance increases as you lower the PSI a tire...it does not mean that the rolling resistance coefficient of the tire is lower when you deflate it more.
    Devils' Advocate, Part 2 ; I'd argue that rolling resistance has to be assessed and measured within the context of the intended environment. If the specific context is a non-smooth, irregular surface, then that is the valid measure rolling resistance for the application.

    Personal anecdote, I've ridden on knobby tires that actually measured less wattage consumed (as measured by a power meter) with slightly lower pressure on smooth pavement. I made a bit of a leap and theorized it was because the casing deformed more, recruiting additional rows of tread blocks toward the edges, which altogether formed a more continuous band of unbroken tread for the tire to roll on, as opposed to bouncing along on a very widely spaced middle row of treads only, offsetting/exceeding the extra energy consumed to deform the carcass more from the lower pressure. Take that with a grain of salt though, as it's purely speculation.

  25. #25
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    I ride tubes, weigh about 165-170, and run about 35 rear/ 30 front for average sized 2.1 tires.

    More than the pressure, I think it is helpful to think of the size of the contact patch for a given tire. I was running a pretty narrow 1.95 for a few months that took upwards of 45 psi for a contact patch concomitant to what I look for with a 2.1.

    That 1.95, on the rear of a 4 inch FS bike, got me thinking. It seems like a good suspension setup would allow you to use higher pressures for less friction while still negating a good bit of trail irregularity. Maybe there isn't a suspension that is that responsive?

    I saw a photo of Dave Wiens and Lance Armstrong in Leadville this past year. Looked like a smooth course and they were both doing the Stan's Notubes thing with the Crow tire with huge amounts of squat in the rear tire. God knows they are fast and their mechanics know what they are doing, but it still seemed a bit excessive to me.

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    Repost, but it explains it all

    http://www.bicicletta.co.za/Download...llustrated.pdf

    I posted this above, I know its longish with techno babble but it addresses this discussion directly. I highly suggest giving it a full read. The last paragraph on page 3 says it all.

  27. #27
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    Quote Originally Posted by GrantB
    I ride tubes, weigh about 165-170, and run about 35 rear/ 30 front for average sized 2.1 tires.

    What do I know?
    not much. you're still running tubes and 2.1's.

    edit: just kidding.
    Quote Originally Posted by Mike Aswell
    the 5th poster, ganginwoods, is correct

  28. #28
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    Quote Originally Posted by Circlip
    Devil's Advocate, Part 1 Grip is created by a combination of friction and mechanical traction. It is absolutely required to have a sufficient friction + traction to provide adequate resistance against forces such as pedaling forces and steering, or else you ain't goin' nowhere (i.e. slower)



    Devils' Advocate, Part 2 ; I'd argue that rolling resistance has to be assessed and measured within the context of the intended environment. If the specific context is a non-smooth, irregular surface, then that is the valid measure rolling resistance for the application.

    .
    You lower the pressure to increase surface area which allows more float over sand and a bigger traction area. No different than with a jeep. The lower pressure of the roughest surfaces (grass meadow in schwalbe's report) allows the tire to act like a shock so you're not wasting energy bouncing up. In the purest sense this probably isn't rolling resistance of the tire. But that's semantics...

    However, in schwalbe's report, realistically there isn't a big difference between any of the factors between road and gravel. I'm also not sure how they define "gravel?" I don't ride over grass that much! The report also doesn't mention turning. Too low of pressure (one must consider rider weight and speed) and the tire will roll and will be on the sidewall where there is no tread for traction.... Same is true in a jeep.

    For racing situations, riders should pick a tire and choose a pressure that they feel is the most appropriate for the course/terrain.

    While the report also talks about pinch flats, they don't mention rim damage. For general going out and pounding the local trails, once I'm in the low 30/upper 20's (with tubeless maxxis and I weigh 158lbs) I'll start feeling the tire bottom out on the rim too often. So for the local terrain/training rides, I'll try and keep about 35psi and I'll still bottom out occasionally. At least I feel a little better about not damaging my rims.

  29. #29
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    I'm not clear on how your post below correlates to the post you quoted from me, aside from perhaps from the ambiguous semantics of "rolling resistance". However, I will say that I really liked another post further above which seemed to cut through all the tech mumbo jumbo and suggested to simply run the lowest pressures that don't feel squirmy (given a rider's own tire selection, weight, and riding style/terrain).

    Quote Originally Posted by nm_gunslinger
    You lower the pressure to increase surface area which allows more float over sand and a bigger traction area. No different than with a jeep. The lower pressure of the roughest surfaces (grass meadow in schwalbe's report) allows the tire to act like a shock so you're not wasting energy bouncing up. In the purest sense this probably isn't rolling resistance of the tire. But that's semantics...

    However, in schwalbe's report, realistically there isn't a big difference between any of the factors between road and gravel. I'm also not sure how they define "gravel?" I don't ride over grass that much! The report also doesn't mention turning. Too low of pressure (one must consider rider weight and speed) and the tire will roll and will be on the sidewall where there is no tread for traction.... Same is true in a jeep.

    For racing situations, riders should pick a tire and choose a pressure that they feel is the most appropriate for the course/terrain.

    While the report also talks about pinch flats, they don't mention rim damage. For general going out and pounding the local trails, once I'm in the low 30/upper 20's (with tubeless maxxis and I weigh 158lbs) I'll start feeling the tire bottom out on the rim too often. So for the local terrain/training rides, I'll try and keep about 35psi and I'll still bottom out occasionally. At least I feel a little better about not damaging my rims.

  30. #30
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    cx magazine did a test (with cross tires) rolling down a section of grassy hill. They found lower pressure was faster in these conditions, the article is not online but the difference was about 10% between 25 and 70 psi

  31. #31
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    I should add that when I talk about rolling resistance I specifically refer to a tire that is rolling freely (no pedaling, no worries about traction). In that context all that matters is friction and the ability of the tire to deform against irregularities. As the tire pressure decreases, friction increases and so does the ability to deform around irregularities. In rough terrain deforming is more important, in smooth terrain low friction is more important. In real life we need to compromise between these two.
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  32. #32
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    Quote Originally Posted by serious
    As the tire pressure decreases, friction increases and so does the ability to deform around irregularities.
    I'll assume you are speaking of "internal friction" related to the greater deformation of the material(s)/tire casing, as opposed to friction related to the increased size of contact patch at lower pressures (which does not change to any noticeable degree as pressures decrease).

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    Quote Originally Posted by GrantB
    More than the pressure, I think it is helpful to think of the size of the contact patch for a given tire. I was running a pretty narrow 1.95 for a few months that took upwards of 45 psi for a contact patch concomitant to what I look for with a 2.1.



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  34. #34
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    Circlip, I mean friction due to increased size of contact patch. Yes, small variations in PSI won't affect friction much, but the friction between a 1.95 and 2.4 tire is different. The effects are far more obvious on a car, where an under-inflated tire will roll far worse than an over-inflated tire.
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  35. #35
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    "What do I know?"

    Quote Originally Posted by ganginwood
    not much. you're still running tubes and 2.1's.

    edit: just kidding.

    I realize tubes are like the acid wash jeans of wheel tech.
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  36. #36
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    Quote Originally Posted by serious
    Circlip, I mean friction due to increased size of contact patch. Yes, small variations in PSI won't affect friction much, but the friction between a 1.95 and 2.4 tire is different. The effects are far more obvious on a car, where an under-inflated tire will roll far worse than an over-inflated tire.
    Not that Wikipedia is to be taken as a 100% reference, but you might want to at least check this link out and do a bit more research into your statement above using more formal sources of information;

    http://en.wikipedia.org/wiki/Friction

    In particular;

    Friction is distinct from traction. Surface area does not affect friction significantly because as contact area increases, force per unit area decreases. In traction, however, surface area is important.

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