Tire Spring Rate and its Affect on Vehicle Suspension. 1

[CapriRacer]

As you have no doubt noted, RunFlat tires are becoming more commonplace. They are reportedly much stiffer than conventional tires and that causes some ride complaints. How does the spring rate of a tire affect the Spring-Mass-Damper system we know as the vehicle's suspension. This question could also be posed with Plus-Sized tires as the tire type.

Here's what I know:

Tire stiffness is mostly a function of tire pressure.

The tire acts as its own Spring - Mass - Damper system with the spring rate of the tires being an order of magnitude higher than the spring rate of the suspension, and the damping being....uh....lower.

Any guidance in understanding this would be greatly appreciated.

The tire acts

 

[GregLocock]

Typical level of damping in a tire is 7% of critical, ie pretty much undamped. Easily estimated by bouncing the wheel and tire off the concrete.

Stiffer tires are bad for ride, shake and impact harshness, in most circumstances. You might occasionally find a surface where a stiffer tire is better for ride and shake, because it is stiff enough to break the stiction in the shock and so get the shock working.

The other thing you'll find is that the stiffer tire causes wheelhop frequency to increase. This will change how it interacts with the engine rigid body modes, which will usually make the car even shakier.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[CapriRacer]

Thanks, Greg.

But can the suspension be changed to compensate for these changes? What changes would be appropriate?

 

[GregLocock]

A long hard think about the shock absorbers, at a guess.

I'm serious about the engine mounts - that's what all the fun and games with mass dampers was about in F1 this year.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Yawing]

With the higher nat frequency of the non suspended mass, it will mean an even lower damping ratio unless you come up in the sprung mass nat frequency to match. either way I dont think it is possible to get back to the same levels of force transmissability (effectively how "harsh" the car feels over bumps).

on another effect of increasing the tyre spring rate versus the suspension wheel rate, if it is a race car, you would find the tyres take longer to reach operating temp (if everything else compound wise was equal). This is because the energy input into the contact patch will be taken more by the suspension spring, and less by the tyre.

 

[cibachrome]

Tire spring rate also depends on construction details AND size AND rim width. For example two constructions of the same size GDY or DUN tires in the 245/40R18 realm can be 275 N/mm rate or 325 N/mm rate at kerb, and equal rate at a bit more load. A smaller aspect can in fact be "softer" than a higher aspect for the same pressure and rim width in run-flat land. For the same size, etc., there can be a 20% difference in rate between brands. Damping is kept low for rolling resistance and structural integrity reasons. Don't want any glowing tires, now, do we ?

 

[apex944]

if it is a race car, you would find the tyres take longer to reach operating temp. This is because the energy input into the contact patch will be taken more by the suspension spring, and less by the tyre.

Not always. We have found that there are conditions where increased pressure will also cause the tire to heat through the contact patch sliding on the surface.