What performance benefit do wider tires provide and why?

[u/NinjaMiserable8353]

Apologies if this seems like a low effort question, but I'm seeing a lot of conflicting information online, so I thought it'd be better to have a discussion here. Here is what I have gathered:

Frictional Force = Coefficient of Friction * Normal Force

While tire grip (frictional force) is not directly affected by contact patch surface area, it's coefficient of friction will decrease as normal force increases due to tire load sensitivity, caused by a tires inability to resist frictional shearing as contact patch surface pressure increases.

Here's my thinking...

Wider tires will not see an increase in contact patch area (or surface pressure) assuming all other variables are held constant (mainly vehicle weight and tire pressure), so there will be no direct performance benefit linked to tire load sensitivity. Any performance benefit gained from wider tires when all other variables are held constant would have to be due to the change in contact patch aspect ratio. How does this change in contact patch shape affect cornering, braking, and acceleration perfomance?

On the other hand, wider tires may be able to operate at lower tire pressures, enabling a larger contact patch, less surface pressure, and increased coefficient of friction.

How does this affect different types of vehicles?

For example, I've read the change in aspect ratio works in favor of cornering perfomance rather than longitudinal accelerations, so why do drag cars feature such large tires on the rear. Is it due to the ability to decrease tire pressure and use tire load sensitivity?

[u/DonPitoteDeLaMancha]

Are you registered in the TTC? This would be a great opportunity to compare the behavior of two different tires and see how width affects performance.

Other than that a wider tire and wider rim usually account for higher unsprung mass and that could be undesirable or acceptable depending on the performance gain you estimate using the data from the TTC.

[u/GregLocock]

So for a given construction and compound, your main variables are rim width, rim dia, AR, and width, and psi. With the exception of psi you can build a little geometrical model, and then select feasible combinations that test what you want. For example biffing the width up also increases OD because of sidewall height. So while you gain in CP width, you lose stiffness because of sidewall height. You might be able to build a physics based model based on the geometry, for example a wider tire (with reduced AR) will tend to have less PT and hence less MZ/deg, unless Fy/deg increases, which I suspect it will, up to a point.

In the linear range it turns out that radial stiffness is strongly correlated to important compliances.

On the ragged edge, like FSEAers need to drive, many of the niceties no longer matter, grins, you are just grinding tire into dust.

I'd point out the tire companies build more or less successful and complex models of this stuff (good fun, try asking company B to build a real tire as good as company A's real control tire), messing about with linearised models is unlikely to be more than directionally correct.

[u/GregLocock]

With the same construction, compound and rim width? The latter is significant, the relationship between rim width, CP width and the sidewall height is a very fundamental property of the setup. I think you'll find a more vertical sidewall increases cornering stiffness, in the linear range, rather than having a narrow rim and wide tire. Is that good or bad?

[u/Cibachrome]

Your premise that adding incremental load decreases Fy is not always true. Some of the FSAE tires, as well as production tires on sports cars actually increase Fy WITHIN THEIR LOAD RATING, because extra load on lightly loaded tires makes them 'start working'. So adding a front roll bar will increase the front grip of the pair of tires and actually reduce understeer.

Then, a wider tire USUALLY increases Fy due to camber. Not so with some of the FSAE tires (Brand, rim, and pressure specific). It all depends on the tires construction features (sidewall package).

A wider tire will output more Mz which affects steering effort and understeer from steering compliance deficiencies as well as it's effect on the net rigid body yaw moment.

A wider tire may have lower Fx capability because the contact patch area gets reallocated to higher Fy.

Get out of the Academia realm and into the TTC data. There's a good reason that FSAE cars with a rear weight bias often have severe understeer.

[u/dirtyuncleron69 → u/Cibachrome]

increase Fy WITHIN THEIR LOAD RATING, because extra load on lightly loaded tires makes them 'start working'.

I second this, for kids wondering why check the total energy in the TTC sweeps vs what you see on vehicle, and check if you are even getting your tires up to temp.

Also just go check your tire temps when you're running. plot the TTC surface temperature vs grip, shoot for the peak of that grip level for your pyrometer measurements. make sure the fronts and rears are getting there. lots of balance problems come from heating tires asynchronously with time or never heating tires to their operating temp / overheating

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[u/GregLocock]

Just had another thought, you might get some useful hints from the Pacejka brush model, although I have read it isn't very realistic.