When I started looking into this topic I thought it was gonna take me 10 minutes to find a definitive answer. Four hours later I’m neck deep in graphs, formulas, research papers, etc. It looks like a lot of cycling nut PhD students have done their thesis’ on researching the optimum tire pressure for a bike. Glad to see some of them are doing something useful.
Before I head out for a ride I inflate my clincher tires to 90psi (front) and 100psi (rear). I had no rational behind this except for that it feels good, it’s within the limits printed on my tires, and it’s what I’ve always done. I never really put too much thought into it.
There have been a few common themes throughout all the research I’ve been digging through that I’ll point out:
Tests have shown that rolling resistance is high at very low tire pressures. As pressures increase, tires roll faster, but the performance levels off at a certain pressure. Beyond this point, higher inflation offers negligible performance improvements. A properly inflated road tire (700x23C) at ~85 psi has nearly the same rolling resistance as the same tire at ~115 psi. This is only a marginal difference – depending on the weight of the rider. But at the higher pressure, the rider gives up some of the cornering traction and comfort necessary to cycling.
There’s a perception that running tires at high pressures of 115-140 psi somehow makes the bike roll faster. The thinking behind this is that it reduces rolling resistance because less surface area of the tread is touching the road. However, at higher pressures, internal losses due to flexing of the casing decrease, but suspension losses due to vibration and bouncing of the bike increase. The resulting vertical movement is robbing forward momentum. This cancels out most of the gains the increased pressure attempted to provide. What those high pressures really do is accelerate wear, compromise handling and give an extremely harsh and skittish ride.
Everybody knows what pressure their tires are rated for – it’s printed on the sidewall. However, not many people know what their rims are rated at. Rims change over time too. As your brake surface wears, high tire pressures will gradually cause your rim flanges to flare outwardly. Eventually, you will find the point at which the forces trying to blow your tire off of the rim will exceed the ability of your tire and rim to resist them. It usually happens in the middle of a turn and you’ll be on the ground before you know it. Also, heavy braking on descents will heat the rims and increase tire pressures beyond their tolerances.
Optimum tire pressure really is about rolling resistance. This is a whole other topic that depends on casing, durometer (hardness of the rubber), rim width, air temperature, load and other variables. This debate inevitably goes into clinchers vs tubulars. This is like discussing capitalism vs communism, Jesus vs Allah, Campy vs Shimano, Mac vs PC, Coke vs Pepsi. You’ll hit the same brick wall everytime.
What would I recommend for optimal tire pressure for your clinchers? I wish I could give you a chart and an easy answer. What I will say is that you can’t go wrong if you’re somewhere in-between the recommended pressure printed on the sidewalls of your tires. My Michelin Open Pro 3’s say 87-116psi. Be on the lower side of your tire’s tolerances if it is wet out. If you have a tire like the Vittoria EVO that’s rated for 115-140psi, make damn sure that your rim is rated for this as well.
Optimal tire pressure for tubulars (called singles here in Aus) is a completely different topic since both the tire and rim are made differently and can be run at higher pressures. More about this one later when I have a spare afternoon to look into it properly! Or can someone else offer to write that one for me?