Tyre pressures for those with 'big bones' – quick poll

Scienceofficer, I think you're confusing my two arguments.

1. Higher pressures reduce the likelihood of pinch flats. I think we're all agreed on that one.
2. I have found higher pressures do not reduce grip.

If I found that higher pressures reduced grip, I would look for a compromise between losing time riding slower round corners and losing time mending punctures.
I haven't found that, so why compromise. Pump 'em up, avoid punctures, roll easier and go round corners the same.

"How very scientific and objective."

I think this is the key point.
Like most things in mountain biking, it's all marketing, personal preference and repeated folklore.
Has any scientific and objective testing ever been published on MTB tyre pressures ?

16.5-17 stone and around 25 PSI tubeless on 2.5/2.3s

6'3 and 17 stone and role on some fat Nobby Nic's (2.4) or Racing Ralphs (2.35) at between 35-45 PSI dependant on the trail conditions.

If I found that higher pressures reduced grip, I would look for a compromise between losing time riding slower round corners and losing time mending punctures.

You know it is possible for this to be true; it's basically about cornering speed. I think what people are saying (including me) is that tyre pressure affects the contact patch, which will 'change' the level of grip and therefore be a factor in determining cornering speed.

There are a number of factors, like weight distribution, balancing the bike font and rear, use of the brakes, suspension performance, that determine how fast you're going to be able to go around a corner; the contact patch on the ground is only one of them. If these other factors are far from optimal, then they will be the limiting factor long before tyre pressue is hence why you might not see any benefit from lowering that pressure.

Has any scientific and objective testing ever been published on MTB tyre pressures ?

I believe Schwalbe has done some work on this.

back on topic…
16 stone spesh purgatory tyres tubeless on flow rims @ 30/35psi

the higher pressures : grip thing is very dependant on what you ride..

smooth trails aren't that dependant on tyre pressure – as the contact patch is always fairly constant

rooty/rocky/rough trails, particularly in the wet, tend to need lower pressure in order to allow the tyre to form to the trail and get more grip. The wet comment is because normally, in the dry, 90% of bikers aren't anywhere near the limits of grip on a tyre.

beyond a point, lower pressures don't help with anything, and can make the bike a little squirmy. I found my jumping accuracy on larger jumps (>20') improved when I increased from 15 to around 25 psi, at the cost of increased trail buzz – essentially at the lower pressure the compression of the jump was blowing through all of the cusion in the tyre and I was running just rubber on rim

German Bike Mag tyre test results
http://forums.mtbr.com/showthread.php?t=419392
Lower pressure gives better traction over uneven ground & was faster (for same tyre with higher pressure).

17st, tubeless, (f) Fat Albert 2.35 26-28psi (r) Bonty Mud X 2.0 32psi;proper grip for off road, negligible additional drag for climbing

17st, tubeless, (f) Fat Albert 2.35 26-28psi (r) Bonty Mud X 2.0 32psi;proper grip for off road, negligible additional drag for climbing

German Bike Mag tyre test results
http://forums.mtbr.com/showthread.php?t=419392
Lower pressure gives better traction over uneven ground & was faster (for same tyre with higher pressure).

(Edit)
Schwalbe (Here comes the Science bit)
Why do wide tires roll better than narrow ones?
The answer to this question lies in tire deflection. Each tire is flattened a little under load. This creates a flat contact area.

At the same tire pressure, a wide and a narrow tire have the same contact area. A wide tire is flattened over its width whereas a narrow tire has a slimmer but longer contact area.

The flattened area can be considered as a counterweight to tire rotation. Because of the longer flattened area of the narrow tire, the wheel loses more of its roundness and produces more deformation during rotation. However, in the wide tire, the radial length of the flattened area is shorter, making the tire rounder and so it rolls better.

Rolling resistance:
At 2 bar a 60mm wide tire rolls as well as a 37 mm tire at 4 bar.

It's pretty obvious that a 10 stone rider directly comparing tyre pressures with a 16 stone rider is pretty meaningless.

I think the important thing is the ratio of weight divided by tyre pressure.

If you do this with consistent units, you end up with (approximately) the contact patch of your tyre, e.g if I weigh 187lbs and have my tyres inflated to 35psi the tyre contact patch will be ~5.34sq.inches.

Hence tyre pressure should be scaled with rider weight, i.e. a 16 stone rider would need ~47 psi to get the same contact patch as a 12 stone rider at 35psi, and as a 10 stone rider at ~29 psi

Lower tyre pressures giving better grip is just a myth.

No it isn't.

Don't agree with the low pressure/better grip idea. There is an optimum pressure band which can be adjusted for suspended weight. Either side of that will cause either a reduction in grip due to decreased contact area or a reduction in grip caused by tyre deformity.

Michelin do not produce charts like the one linked for fun. They want their tyres to perform at the optimum level. The importance of this is increased due to knobbly tyres giving dven less contact than say slicks.

It will probably feel like going from a hard tail to say a 100mm FS bike

0_0

I don't actually know whether I'd like that, haha. I like the solid response my bike has, doesn't feel "mushy" or slow for lack of a better description. Most of the stuff I ride is quite easy enough tbh, just trail centres like Cannock and bridleways.

I got slated for starting a 'what tyre pressures' thread on the old forum. Gets!! 😡

I don't actually know whether I'd like that, haha.

You know as a recent convert back to a hardtail (not exclusively mind, I still date other bikes) I can emapthise.

I meant more in terms of how much grip you get, not how musy the back feels. Just a matter of how long before the tyres let go.

Don't agree with the low pressure/better grip idea. There is an optimum pressure band which can be adjusted for suspended weight. Either side of that will cause either a reduction in grip due to decreased contact area or a reduction in grip caused by tyre deformity.

Michelin do not produce charts like the one linked for fun. They want their tyres to perform at the optimum level. The importance of this is increased due to knobbly tyres giving dven less contact than say slicks.

There is more to it than just contact patch – every time you go over a small bump on your bike, some of your forwards momentum is redirected upwards and lost. Tyres with lower pressures tend to deform around these bumps which lessens the overall effect of this energy loss.