Rim Width to Contact patch width changes "effective" camber?

[adambrouillard]

I had some thoughts on this and wanted to get some input. For example if you ran a wider wheel than your contact patch, so that your sidewalls sloped inward toward the contact patch. In this case under cornering load it would seem that as the tire deflected inward the inner sidewall would "stand up" and become more perpendicular to the contact patch and the outer sidewall would become even more parallel to the contact patch. I would think this would cause the tire to effectively become somewhat conical and create a negative camber.

Conversely sidewalls angled in toward the wheel would create positive camber under load.

What made me think of this was that the inside of a rear kart tire often wears faster despite there being no negative camber and no camber recovery in roll. Kart wheels are often sometimes significantly wider than the tires however.

Thoughts?

[Ockham]

What you're describing is actually a drifter's trick. Of course, it's also a style thing for them, and they're not after absolute grip.

That being said, there's some merit to this, but it's probably more useful for street tires than their stiff-sidewalled racing counterparts. If you spread apart the tire's walls, bear in mind that the contact patch's center will be lifted, but maybe not enough to matter.

My feeling is that if wider rims helped with grip, the manufacturers would recommend their use. I suspect they know more than many of us.

[murpia]

Apparently, back in the cross-ply days, this did work.

Teams would mount tyres on wider wheels to get more grip. Then the tyre manufacturers made them wider tyres. So they put the wider tyres on even wider rims. And so on...

At some point this whole business stopped, and radials came along as well... So I don't know if it's worth trying today.

Regards, Ian

[exFSAE]

Camber is purely a measurement of wheel orientation, not sidewall deflection. Camber is used mainly to work with some effects of sidewall deflection, but the two are separate ideas. I wouldn't even bother thinking of "effective" camber as you're not going to have much look intuitively figuring out what's going on in the contact patch between the sidewalls, belts and breakers, and anything else.

Ultimately regardless of what you do, a tire's going to deflect laterally regardless of what size rim it's on. Are there some advantages to going with a wide wheel? Sometimes. If you want to see what they are specifically, go ahead and open up some TTC data (if you haven't bought into it yet you should) and take a look.

[Charlie]

My experience is that a wider wheel increases the cornering stiffness of a tire (via additional sidewall stiffness), which is a similar effect as adding negative camber. But I think it needs to be taken on a case-by-case basis because it is very reliant on the tire construction.

[exFSAE]

Agree with the cornering stiffness bit, Charlie.

Don't think I agree that negative camber is similar to stiffening a sidewall though... my experience has been that cornering stiffness is generally fairly insensitive to camber - certainly in relation to load, inflation, and temperature.

But nothing is gospel in this business...

[BillCobb]

The effect of rim width on tire transient performance is a more interesting study (as it pertains to "feel", but you need to run a transient test on the tires. As usual, there is often an optimum value, established by the tire designers or just plain dumb luck.

A more interesting study is to plot the ratio of tire section (mm) to tire useage load (kg) vs. tire stiffness for the cars that do well for grip and feel. You will find that there is method in the madness.

[Charlie]

Originally posted by exFSAE:
Don't think I agree that negative camber is similar to stiffening a sidewall though... my experience has been that cornering stiffness is generally fairly insensitive to camber - certainly in relation to load, inflation, and temperature.

I don't think camber affects cornering stiffness measurably, but the result of adding negative camber is somewhat similar (though not exactly the same)