Originally Posted by
thebulls
The other issue is rotational inertia, and is probably the main reason that racers moved to narrower (and lighter) tires. In a race, being able to hop onto a breakaway, and being a tick faster in a sprint, makes the difference between winning and losing. For recreational cyclists, being able to accelerate a tick faster is probably of much less concern than riding on comfortable tires.
I'm not entirely convinced by your model for pinch flats. Typically, these do not occur by having a whole section of tire pressed flat against the rim, but by hitting something sharp like the edge of a pothole. So the question is, if you took a 1/4" thick steel bar and put it across the tire (not along it or pointed into it) and then pushed it against the tire, how much force would be required to press it to the rim on a 700x23 pumped up to have the same rolling resistance as a 700x32 (e.g. 140 psi versus 95 psi)?
The one other comment that I'd add about the wide-tire, narrow-tire discussion is that the reason that a wide tire is faster at a given pressure is that the sidewall deflection required to make the contact patch is less. Narrow tires have to have more sidewall deflection because their contact patch is elongated. So to make a narrow tire have the same rolling resistance, you have to pump it up enough that the sidewall deflection is reduced. Of course, the smaller contact patch means that cornering ability is compromised.
Nick
Thanks for the reminder about rotational energy. So true.
I am aware of the failings of the pinch flat model, but it's all I can model without a test lab. I have run different tire sizes and pressures on my rollers and determined that my tire and pressure choice is more noticeably faster there than on the road, as the small rollers emphasize the deflection of low pressure tires. Otherwise I'm guessing, but I think the conclusion would be the same for small objects like a stone as for flat ones, like the road.
As you note, I calculated that the contact patch for a narrow high pressure tire is shorter than for a wide low pressure tire, not longer. Thus deflection is less and less energy is lost. However the difference in deflection is relatively small compared to the difference in deflected cross section, where the real difference lies. Thus if the deflections are approximately equal, the comfort will be also. The only way to get more comfort is to have more deflection.
The most interesting thing from all my calculations is how close a tandem is to getting a pinch flat, no matter what tires we run. We are limited by the strength of our rims, which is limited by our desire for light weight, especially in our rims. I never pinch flat my single.
AFAIK, cornering ability is better on a high pressure tire. You may recall from your Physics 101 that friction is between two surfaces is independent of contact area. So why do race cars have wide tires?
http://www.physicsforums.com/archive...p/t-56486.html
Similar for racing motorcycles. Their tire compounds are designed to run at about 185°F. Again, bicycle racers seem to have no problem cornering on skinny tires, especially since tire distortion is less in a high pressure tire.
It would be very interesting to take a pile of tires and a pump to an outdoor velodrome on a rainy day and see what different tires did at different pressures, just trying to stay upright while riding slowly on the banking. Bring a Michelin Man suit, of course. I know that my Tricomps just barely keep me upright when the painted surface of our local velodrome is wet.