Tuesday, April 15, 2014

MTB Flats and Tire Pressure

I've been thinking a lot lately about flats. My last two MTB races have ended in flats and DNF's. I hate DNF's -- at least mine, I'm not as upset about yours. I've riding mountain bikes for almost 30 years, but one thing I never really got into was the geeky/nerdy (I use these words in the nicest possible way) aspects of bikes, gear, and equipment. But as I've transitioned into cyclocross and realized the importance of tire pressure there, now that I'm getting back into mountain bike racing I'm starting to see the importance of tire pressure there as well. I was talking to my coach the other day after our race -- which, BTW, he won and I DNF'ed -- and he revealed that he was running 18 psi in his tires, while I had pumped my up to my 1990's influenced pressure of 42 psi. I'd always assumed that higher pressure allowed the tires roll faster AND helped reduce the risk of punctures. So while this higher pressure line of thinking is valid for pinch flats, the more I think about research this, I probably couldn't have been more wrong.

First, the rolling resistance issue. I found an interesting article on the African website mtbonline (article here: http://bit.ly/1hFDnLe) that cited a study done at the German College of Physical Education, Cologne. Researchers there found that when riding off-road, specifically in a grassy meadow, a power savings of 18 watts could be realized by reducing pressure from 57 psi to 21 psi. The finding is explained well here:

"A tyre with less inflation can adapt to unevenness more easily. The total system needs to be lifted to a lesser degree and less frequently. Resistance is reduced, less power is required. Off-road a reduction of tyre pressure reduces rolling resistance."

Regarding the puncture issue, it's interesting because in my recent Google searches I've stumbled across several sites that encourage you to increase your pressure if you ride on jagged rocks and other menacingly sharp terrain (see: http://bit.ly/Q9ZHlw or elsewhere). Again, as I think about this it seems out of balance with reality. I've started thinking about this as analogous to popping a balloon. A balloon that is highly inflated and who's surface is under greater tension can be popped relatively easily even with an object that isn't necessarily that sharp, while an underinflated balloon is very difficult to pop as its surface conforms much better around the object that is trying to puncture it.

All this to say that I am FINALLY coming around to the idea of low tire pressures for tubeless mountain bike tires. If I was able to keep up pretty well before despite a 20W handicap, look out once I get my pressures dialed!

The other issue is being able to fix the tire on the go. Even with lowering my pressures, I imagine I'll still get the occasional flat. We all will. For me, during a race the key is to stay calm and go through a process of evaluating what caused the flat and what it will take to fix. In this last race, I'd drank so much of the tubeless Kool-Aid that I assumed with fresh sealant I should be able to just put more air in the tire and I'd be back in business. Unfortunately, no amount of added air was likely to fix the 1/4" gash in the casing just behind one of the side knobs. After using up my one and only CO2 (mistake #1: bringing just one CO2 and no pump) with half of the air escaping as tried to fit the unfamiliar CO2 adapter (mistake #2: nothing new on raceday), I was done. Race over. In the days since, I found this article which should be good to review a process for evaluating and repairing flats: http://bit.ly/Q9ZHlw Even though I worked in a shop as a wrench for years and having been building up and fixing my own bikes for many more years, in the heat of a race, all that experience tends to go out the window. I figure reviewing the page above can't hurt.

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