[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?

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?

 

[Circlip]

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

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.

 

[Yody]

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

 

[CactusJackSlade]

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...

CJS

 

[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.

 

[serottak]

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.

 

[markowe]

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.

 

[the munts]

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.

 

[2WD]

http://www.schwalbetires.com/tech_info/rolling_resistance
http://www.schwalbetires.com/tech_info/inflation_pressure

Some good info

 

[G-Live]

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#

G

 

[CactusJackSlade]

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...

 

[pbayne]

Lower is faster!

Here is a more definitive study:

http://www.bicicletta.co.za/Downloadable docs/Rolling Resistance Eng illustrated.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.

 

[millennium]

Here is a more definitive study:

http://www.bicicletta.co.za/Downloadable docs/Rolling Resistance Eng illustrated.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.

 

[michaeldmanthey]

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.

 

[Red29]

Thanks for all the info----I look forward to doing some more experiments when the dirt shows back up here in Chicago---

 

[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.

 

[MikeU]

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.

 

[duke of kent]

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.

 

[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.

 

[agm2]

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.