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Toe Induced Scrub

Professional Motorsport Data Analysis

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Obviously a tire with toe induces a side force and associated induced scrub. Similarly a cambered tire induces a side force (camber thrust). Presumably there is an associated camber induced scrub.

Just as toe-in is stabilizing because the side forces are aimed inward, which generates a restoring force in response to a lateral disturbance, negative camber should be stabilizing for the same reason, although I don't think that is widely recognized.

Millikan provides this interesting nugget:

"Camber also works like steer: When a tire is cambered it tends to pull the car in the same direction in which the top of the tire is leaning. A simple way to think about this is camber-steer force equivalence. For many radial tires, 1.0° of camber produces about the same lateral force as 0.1° of steer (10:1). For bias-ply tires the effect is more pronounced: 1.0° of camber is equivalent to about 0.2° of steer (5:1). From this rule of thumb, it can be seen that the static negative camber will require toe-out to keep the wheels from fighting each other."

Of course, the "fighting each other" is what provides the stabilizing influence, and there is such a thing as too much stability.

My point: for minimum scrub (and presumably straight-line rolling resistance) some toe-out is needed to offset negative camber. Further a bit of toe out on a rear axle is not necessarily problematic on a circuit car in the presence of rear negative camber. While I have experimented with modest amounts of rear toe-out without going straight to hell, few authorities are willing commit that last point to writing - perhaps for understandable reasons given a whole host of uncontrolled variables, not least being compliance-induced alignment changes.

Hi James, I'll try pick through your points below 🙌

-A tyre will a non-zero slip angle will produce a side force. When a tyre a slip angle while a car travels straight, this is what people normally refer to a tyre "scrubbing" along the road which is what you're saying. In this case, in my mind, you end up with a tyre force component aligned with the longitudinal axis of the car - aka some induced drag.

-For the case of camber thrust, I'm not sure you and up with any force components from the tyre that result in a drag force? Not sure I am interpretting your point properly though. That is, apart from adding some extra rolling resistance from increasing the strain energy in the tyre because you'll be deflecting the carcass more now with extra camber.

-Adding negative camber to the rear of the car is certainly a common tool for adding rear stability, maybe not so much discussed as you say though. Digging a little deeper into that, sometimes it's more the dynamic negative camber we're focussed on rather than static, although clearly you can achieve similar things with static camber. What I mean is often you'll use rebound damping, some anti-lift or even stiffer rear spring rates to give you less negative camber loss as the rear end rises on braking and corner entry.

-Rear toe-out is again something that you'll see in some cases. But in a similar way to the camber, you're usually focussed more on the dynamic toe by tweaking the bump-steer curve and amount of heave you're getting to tune the corner-entry balance.

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