toe set up

[Thailand]

How about your toe set up in front and rear tyre
for
-acceleration event
-autocross&endurance event
and why? (e.g. toe-in or toe-out)

[sbrenaman]

What slip angle and slip ratio produce max longitudinal force? That's the slip angle (toe) the tires should operate at for the acceleration event.

What slip angle at what normal forces produce max lateral grip? Look at what normal forces the outside and inside tires operate.

You're going to need the Tire Test Consortium data to determine this, and it won't be painfully obvious. You'll end up making compromises.

[RollingCamel]

I wouldn't recommend toe out for the following reason.

h t t p : / /tinyurl. c o m/2883hl6

[exFSAE]

Or, just track test it.

[Mike Cook]

toe out in the front,
toe in in the rear.

[adambrouillard]

I've been questioning the whole toe out in front to improve turn in lately. It seems like the outside tire would build forces faster if you started with toe-in. Here's a theory though that could explain the traditional idea. Maybe with a fast turn in, the wheel height changes due to scrub/caster would create a temporary greater force at the inside tire as it moved down in relation to the chassis. In that case the inside tire might provide the majority of initial turning force. Thoughts?

[sbrenaman]

Originally posted by exFSAE:
Or, just track test it.

That's funny.

[EHog]

Originally posted by sbrenaman:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by exFSAE:
Or, just track test it.

That's funny. </div></BLOCKQUOTE>

Sure is.

[Neil_Roberts]

Every competitive road course setup includes front toe out and rear toe in, but the reasons for that are not widely known. Hopefully what I said about it in Think Fast will help. The main reason is a simple consequence of simple geometry.

Especially in FSAE, where the tightest turn radius is not very many times larger than the track or wheelbase, the fact that each tire is in a different place relative to the turn center causes the slip angle of each inside tire to be significantly different from the slip angle of its neighboring outside tire. A top-view CAD wireframe model is a great way to sort out the geometric consequences of tight turn radii. Figure 48 in Think Fast is a nifty illustration of the differences in slip angles for 4 different turn radii.

There are two additional reasons to use the conventional toe settings.

At the front: Low speed understeer is a universal fact of life because velocity shows up in the automotive cornering stability equation. Because the inside front tire is usually lightly loaded, it can't produce much force in any direction, so the drag that it can produce at a large slip angle is nearly as useful and more consistent than the lateral force it can produce at a smaller one.

At the rear: Particularly with an open differential, rear toe-in is the adjustment to use to optimize the tradeoff between inside wheelspin and power oversteer. If you have inside wheelspin, more toe in will reduce that because the inside rear tire will have more of its longitudinal force capability available. Too much rear toe in results in power oversteer.

[exFSAE]

Especially in FSAE, where the tightest turn radius is not very many times larger than the track or wheelbase, the fact that each tire is in a different place relative to the turn center causes the slip angle of each inside tire to be significantly different from the slip angle of its neighboring outside tire. A top-view CAD wireframe model is a great way to sort out the geometric consequences of tight turn radii. Figure 48 in Think Fast is a nifty illustration of the differences in slip angles for 4 different turn radii.

I'd be curious to see the referenced illustration (no, not buying the book just yet http://fsae.com/groupee_common/emoticons/icon_smile.gif ). In any event though.. the old wives tale that most go off is that toe-out makes for responsive initial turn-in, ie before the car can build much lateral acceleration. Does that make the "tight turn radius" thing less relevant? I'm not sure.

I've heard some alternate explanation regarding stability, with a toe out setup being "less stable" with the forces pulling out, rather than "pushing in" with toe-in. I say that's complete BS since you have a left and a right force either way. I'd have to sit down and think about it.. if when given a small (external?) input, if with load transfer the result is a stabilizing or de-stabilizing force.

Then again there's the thing of camber thrust. Cranking 3-4 (or more) degrees of negative camber into the front tires creates "inward" lateral force while going down a straight. Small amounts of toe out, particularly on soft-treaded sportscar tires, might not even be enough to cancel out the camber thrust. In any event that's more relevant to bigger cars that can actually put load on the tires, rather than FSAE tires with pretty much no camber thrust to speak of.

It's a moot point of discussion without specific numbers in an example to go off of. I don't like slipping into hand waving!


At the front: Low speed understeer is a universal fact of life because velocity shows up in the automotive cornering stability equation. Because the inside front tire is usually lightly loaded, it can't produce much force in any direction, so the drag that it can produce at a large slip angle is nearly as useful and more consistent than the lateral force it can produce at a smaller one.

Mmmm, not sure I'm entirely with you on this... even with the impact of low velocity - while the yaw rate component will result in different steering and sideslip angles to get to a "balanced" set of slip angles, I'm not sure if the slip angles themselves will necessarily change. But I guess that comes down to how one makes a working definition of under/over-steer, which is not entirely straightforward IMO!

But, would you say that the peak lateral force of a tire is always greater than or equal to the "slide" force past the peak, when the tire is effectively a rubber brake? Would we also say that the distance from the CG to the front axle is pretty much always going to be greater than half the track width? If that's the case, then we should always be able to get more yaw moment by focusing that inside tire's force to be lateral, rather than longitudinal. Not to mention the benefit of reducing slip angle drag on low horsepower cars or through "momentum" corners. Just a thought.


At the rear: Particularly with an open differential, rear toe-in is the adjustment to use to optimize the tradeoff between inside wheelspin and power oversteer. If you have inside wheelspin, more toe in will reduce that because the inside rear tire will have more of its longitudinal force capability available. Too much rear toe in results in power oversteer.

Agreed.

It's an interesting point of discussion and one that I haven't been able to really work with in a while. There are some other interesting things going on with the combined effects of toe and camber on a tire. As with all this kinda thing, it's hard to sort out specific causes and effects!

[adambrouillard]

Thanks for the input Neil, I've read your book and I think it makes sense in that you want more toe out for tighter turns, and I would think some ackerman might be the solution for this, but I'm mainly curious about the initial turn in response that supposedly is improved by static toe-out. I've never really found a satisfactory answer to this.

[murpia]

Originally posted by adambrouillard:
I'm mainly curious about the initial turn in response that supposedly is improved by static toe-out. I've never really found a satisfactory answer to this.
Interesting.

Experience tells me that more front toe-out pushes a car towards turn-in stability and if you go too far it becomes 'lazy' (to quote the typical driver). Front toe-in seems to make a car nervous on turn in.

The explanation I have developed for this is that side-to-side variations in front tyre vertical load cause variations in front lateral forces & slip angles as soon as turn-in occurs. With toe-out these variations are understeer in direction, with toe-in they are oversteer in direction.

Anyone have any better ideas?

Regards, Ian

[exFSAE]

The straight forward thing to do would be to fire up ADAMS (or even some simple Matlab rigid body sim with some basic load transfer parameters) and do two step-steer runs... one with a degree toe in front, one with a degree toe out. Or better yet, 4-corner it with front AND rear toe:

In / in
Out / out
In / out
Out / in

And then look at the time evolution of yaw rate, sideslip angle, and maybe average front & rear axle slip angles. Maybe I'll do that tonight.

Intuitively you'd think for a slalom-heavy FSAE course you'd want the setup with high sideslip response rates and minimal overshoot.. and as a corollary the worst setup would have the most overshoot with slow response. Would then be a good thing to track test (ie actual engineering work... hypothesis, virtual work, confirmation test!).

Other thing to keep in mind is how front toe (and Ackermann) are going to affect your brake-in-turn capacity. Intuitively I'd expect toe in to have less inside wheel brake lockup than toe out... since you're forcing the outside tire to do more of the cornering work and giving a little more longitudinal headroom to the inside tire.