Quote:
Originally Posted by tracer bullet
Yeah so if you see the contact patch thing, you can logically follow that the tire width will keep it the same just change its size. As you go wider it gets more narrow and vice-versa. Traction benefits in whichever direction it grows, and vice-versa.
Shorter sidewalls tend to stiffen the tires, sure, but in that direction, the bump absorbing direction. The actual tread of the tire which touches the road doesn't change much if at all. There's sort of a certain amount of tread "blocks" on the ground and they don't change unless of course the tire's like actually flat.
True drag tires are sort of a different animal. They go wide of course, but they're also really low on pressure (15psi???) so that they get that front to back grip as well. Ever seen the slow motion take-offs and watch the wheel turn before the tire does and how the entire sidewall twists and buckles?
I'd love to see some resources showing otherwise, if you find them. Not to prove any points or right / wrongs just because I think it's pretty interesting.
I kind of wish cars would have square setups, I know big rear wheels look cool but as I get older I'm less interested in expensive tires due to their sizes and more happy to actually rotate them and make them last longer. Maybe my front lefts from the on / off ramps wouldn't wear so fast if I could rotate them, LOL. I know a lot has to do with balance, over and understeer, etc. and I know a little understeer is safer, but I wish they'd have square setups and then do the rest with suspension tuning. It's certainly far easier to fit big tires int he rear, there's no engine or steering just eating up trunk space.
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Like I said, I’ll look for the sources.
Tire contact area is not as simple as corner weight divided by inflation pressure. They are the primary variables but not the only two variables that affect contact area. Tire construction/stiffness of tread and sidewall do alter the static, and dynamic, contact area. Inflation pressure contact length and width do NOT change linearly with varying pressure. As the pressure is increased, there’s a pressure above which the circumferential length no longer decreases as the pressure continually increased. Similarly, as you reduce tire pressure, the contact width will stop decreasing at a certain pressure. This has been demonstrated by many tests and these data/results are out there. Contact patch area and shape vary with pressure stiffness and structural tread/sidewall stiffness. Data shows very highly non-linear relationships in circumferential contact patch length vs. inflation pressure. For example, a 20” tire will have a higher structural stiffness than a 19” tire of the same width but they’ll have different contact patch length-pressure relationships as well as a different transition pressure where the length stops decreasing with increasing pressure. So for a given pressure it’s possible for a 20” tire to have a longer or shorter contact length than a 19” tire. Tire design is very complex and tires have highly nonlinear relationships between lateral grip and slip angle, grip and normal load, grip and temperature, etc. Nonlinear relationships also exist for contact patch length and width (bilinear) vs. pressure.
So two 285 mm tires, on the same or different wheel diameters, from two different tire manufacturers will have different contact areas, shapes, widths and lengths.
I’m talking about drag tires used on street cars - people put on 17” wheels (along with smaller calipers and diameter brake rotors!) so the can maximize sidewall height and flexibility to “wind up” the sidewall at launch. Yes, lower pressures than street tire pressures are used as well to maximize grip.
In the E generation M3s I ran square tire setups but with the F and G generations I find a staggered setup works better because of the massive torque these cars have. I still run a 285-295 mm front tire but the rear is at least a 305 mm tire. It sucks not being able to rotate but the staggered setup yields better times.