Higher Tire Pressure is NOT Faster

With the advent of tubeless tires pressures started to fall, at least as I recall. It was hard for me to change, or believe it was beneficial except for turning perhaps. I recall hitting my rims on a few hard hits when I first went tubeless lowering my pressure from mid 30 psi to mid/high 20s. I quickly decided it was not worth it for my riding, and went back to low mid 30s. I didn’t think about it much, and felt fairly confident it was harder to pedal and slower overall with less pressure. I haven’t given it much thought and figured “firm” with a little give, not nearly low enough to pinch, is where I would stay, and little extra would be worked out by the suspension, or make my suspension work.

Perhaps my assumptions are wrong, perhaps not: “the tire pinch-flats before its performance deteriorates” according to this:

https://www.renehersecycles.com/myth-16-higher-tire-pressure-is-faster/

The line between pinch flat or dinged rim or not seems small. I’m not quite sure how the data in the article above deals with MTB tires and trail riding, also it seems the pressures they looked at were all much higher than most people use for MTB. I’m curious if there are “diminishing returns” as they said on the low end of pressure just as they seemed to show on the high end of tire pressure. My assumption is that the roll down tests did not look at all the many different knobby choices, which to me will have the greatest influence over friction and roll down as the friction coefficient is a multiplier(or mathematical “factor”) in the roll down. See this page for doing your own roll down test: trail-turn-design. Lastly, as the wider tire studies show below, rolling resistance is largely a function of contact patch, which will change with size and pressure. What combination of pressure, tires size, and tread pattern triggers diminishing returns?

What do I do on my pump track bike? I definitely noticed a BIG difference going from a small block 8, which has a VERY high friction coefficient, versus a nearly slick tire. The small study I link to at the top of the page is hard for me to believe because I know, or thought I knew, that harder skateboard wheels seemed significantly faster on wooden half pipes and concrete. I’m not sure that assumption is right either now, but it felt like it, or so I believed. I went into mountain biking in the 80’s thinking the same thing, and went with feelings as I didn’t like the feel of riding at high pressure, nor low pressure- plus I didn’t want to fix a flat (or rim) because I was few psi shy of just enough. I also thought that at “too low” imo, or near pinch flat range, the friction coefficient and roll down would be making me work a lot harder pedaling. So much for feelings I suppose, or for now anyway. I need to do some roll down tests of my own, which has been high on the bucket list of nerdy trail science things to do because it has big implications for trail design and speed control…should the next roller be two or three feet high, and how far away, what abut that jump?

A case in point in assumptions and conjecture was when 29 inch wheels became a thing and there was rumbling in forums and comments sections all over the MTB space, hot air. Anyway, it was a relief to learn that 26 inch tires aren’t necessarily slower or faster than a 29 inch tire downhill.

In spite of the small study and findings about pressure above, I’ll probably still add more air than I “need,” but I’m not sure I have found that number yet, or care to tweak that much, backpack be damned. I feel no pressure to change, yet. However, now I don’t have much of an excuse for being lazy: M18™ 18V Cordless Tire Inflator. What we do know is that rolling resistance increases slightly with tubes (negligible with latex) so tubeless tires are faster in that regard.

BUT, HOWEVER…

This seems to say more pressure is faster from a Watts standpoint, the Crr goes down with psi:

Rolling Resistance Test Results (Speed: 29 kmh / 18 mph / 8 m/s, Load: 42.5 kg / 417 N)
Rolling Resistance 120 psi/8.3 Bar7.7 Watts
CRR: 0.00231
Rolling Resistance 100 psi/6.9 Bar8.3 Watts
CRR: 0.00249
Rolling Resistance 80 psi/5.5 Bar9.2 Watts
CRR: 0.00276
Rolling Resistance 60 psi/4.1 Bar10.9 Watts
CRR: 0.00327
source

Wider tires? FaSter

at the equal pressure, the casing tension of the wider tyre is higher and the contact patch is shorter and wider. The first of these factors means less energy is lost to casing flex, and the second reduces rolling resistance because the shorter, wider contact patch reduces frictional losses….the amount of rolling resistance a tyre generates is largely down to its construction and rubber compound (assuming it’s inflated to the optimum pressure).

Simon von Bromley

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