Tires with stiff sidwall vs soft sidewall and more pressure?

[John Milmont]

Whats the advantage of having a tire with a stiff sidewall when you can add more pressure to a tire with a softer one? The hoosier's i last ran seemed to have way stiffer sidewalls than kumho victoracers; and when i ran the hoosiers at the same pressure that i use to run the victoracers at, it was way to stiff with the tires bouncing over every imperfection.
So, the solution was to just lower the air pressure. But then, its seems like its an equation where the 2 sides just cancel each other out: Stiff sidewall? take out air. Soft sidewall? add air!

I know they say that your supposed to get more responsive handling and better turn in with a stiff carcass, but it seems like i can get that by adding more air. Again, its back to the equation.

The only things i can think of is that a stiff tire sidewall has a different springrate than air, and could possibly control the contact patch better than a soft sidewall with lots of air, and maybe require less camber? Or, maybe stiff sidewalls have some sort of damping property to them; so its like having a tire with lots of air, but without the uncontrolled bounciness that would require more damping from the shock?

Anyone have any thoughts on this?:huh:

 

[Cliff Beer]

John, I'm no tire expert (so this is probably worthless) but I have played a bit with what you're thinking here, and done a little consulting with some tuning "experts" who live/eat/breath this stuff. In a nut shell, your conclusions in the last para are spot on - the stiffer sidewall tires tend to have (amongst other properties) the ability to control the contact patch better both in lateral loading and vertical movement. That is, in fact, the basic reason for having a stiff sidewall (but again, amongst other reasons such as high speed capability/thermal durability). Adding psi does indeed make the tire more "stiff" but only marginally improves road handling. Better to have a tire that is generally matched to springs and shock rates all working in unison. Just my $.02.

 

[Jac Mac]

No simple amswers to this one as by increasing/decreasing pressure your changing several things at once;
1. Tire contact patch.
2. Effective spring rate.
3. Roll rate.
4. Effective tire dia

Its all a big balancing act & even the best teams in the world struggle with it at times especially when mother nature thows in a wild card.

 

[Gord Zonailo]

Another key factor is the wheel width vs tire width. Assuming that the wheel is matched correctly to the tire, the tread width will be about the same as the wheel width. If this is the case, the centre of the tire bulges more with soft sidewalls and higher pressure and this reduces the contact patch.

For track use I like to run a wheel width that is even a bit wider than the tread width, about half way between tread width and section width. This supports the tire shoulders much better, does not roll under as much during hard cornering, and does not bulge the center of the tread as much. It then enables running a higher pressure, or having the tire be less sensitive to pressure and also be much more responsive to steering and more stable in the corners if you can run a bit more pressure. And of course the tire contact profile, and camber (or lack of) can be checked by taking temperatures at the outer third, middle and inner third of the tires and the pressures right after coming in off the track. Plus observing the tire wear patterns.

On the street the wider wheel enables use of a bit lower tire pressure. The downside is if too wide, the wheel rim is not protected as well by the tire sidewall bulge so it becomes more susceptable to "curb rash" if you are not careful.

Gord