Anti-ackerman steering

[autogyro]

I always found a rear wheel drive race car more pointy and easier to steer with slight toe in. How does that play out with negative camber, into this ackerman variable?

[Jersey Tom]

That's a really in-depth topic...

Just think of Ackermann as dynamic toe. It does affect a variety of things... but Ackermann has an increasingly noticeable effect the more steering angle you put in. Around zero and small steering inputs, it's not going to really do much in comparison to static alignment.

[autogyro]

OK JT, to keep it simple.
Is it always best to start from the tyre spec and work up the suspension design from that point.

[Jersey Tom]

I would certainly say so, yes. Without having a good knowledge of tire properties in advance, I don't see how one could have a good prediction of vehicle balance, transient performance, required front-to-rear stiffness split, dynamic steering settings, etc.

Obviously that's not a luxury everyone has, but such is life.

Other way to think of it... is to strip a car down to the most basic form of a bicycle or rigid body model. All the vehicle response is then purely a function of the tire properties and vehicle inertia. Adding kinematics, downforce, steering settings and whatever are just modifiers on what's fundamentally tire-driven.

[marcush.]

But you can get an idea by experimenting with toe settings .the change in behaviour should tell you if your dynamic toe is close or way off.If you need toe in in quick corners and toe out in the slow ones you do not need tyre data to decide on changing ackerman charcteristic.

[Jersey Tom]

Sure. Build-and-test (tune-and-test, guess-and-check) is always an option. But for prediction - the key word in my previous post - you're sunk without it.

With how limited testing time is in some pro series (F1 is a good example), the more work you can do in advance the better.

[marcush.]

JT ,I´m not debating on this and you are spot on to better be prepared before wading through endless minute adjustments.
I have to say that very rarely my teams had the luxury of more practise time let alone testing opportunities or the money to buy another 3 sets of tyres or another 20 laps on the expensive engine so you need to evaluate these things on the fly...Being prepared always pays off

[raymondu999]

Hi. Just wondering. Would there ever be any circuits where the vertical load transfer between inside/outside wheels are not big enough, and would maybe go less anti-ackerman or even pro ackerman? I was thinking maybe Monaco or Monza should be faster that way? No?

And how does having pro/anti-ackerman change camber requirements in the setup?

[olefud]

It seems that the F-1 tire loading on an "axle" would not be much different side to side given the substantial aero down force and low Cg. This would suggest an unbiased Ackerman setting. I've heard that NASCAR uses substantial pro-Ackerman on flat tracks and just a touch of pro-Ackermanon on fast highbanked tracks, the latter due to the high aero and centrifugal force. I have a bit of trouble envisioning the advantage of anti-Ackerman in any event.

My rather crude testing on production race cars suggests that Ackerman helps turn-in up to the point of incipient tire "wash", ie loss of drift angle with scuffing. This tends to occur earlier on equally-loaded tires. But there were always too many uncontrolled variables to find anything conclusive.

[Jersey Tom]

It seems that the F-1 tire loading on an "axle" would not be much different side to side given the substantial aero down force and low Cg.

F1 car... 1400 lbm, 4-5G's of cornering, 55" track width. CG estimate isn't easy, but let's go with axle height or lower, maybe 10-13". Non-trivial amount of load transfer, and there are a lot of slower speed corners that don't have the outrageous downforce of a "right foot down" sweeper.

I have a bit of trouble envisioning the advantage of anti-Ackerman in any event.

Then it's time to start looking at tire data.

The whole "helping turn in" thing is a bit vague. Depends what you consider "helping" in vehicle dynamics terms, but in any event since initial turn-in is going to be more of an on-center response I'd lean more toward alignment settings for that one.

Tell ya one thing that's for sure, I could go for some Half Fast Subs or Abo's Pizza right about now.

[olefud]

Jersey Tom raises good points about my post that don't lend themselves to a quote. My point is that the percent difference between inner/outer tires would be minimal. True, much less so on slow sweepers; but the setup is primarily for the high speed work where tire damage- or worse- will chew tires in a hurry. My sense is that most F-1 cars push rather much in slow corners anyway.

Could of been more analytical on turn in. On the flatter circle track, the vehicles I've worked with need a handful of wheel to initiate turn in. With proAckerman, the inner tire is turned a bit beyond the instantanous turn center which generates a turning force vector that resolves into pure turning force and a drag component. The drag component is a rearward thrust inboard of the Cg
analogous to the polar moment turning thrust that differing OD rear tires produce under power. With the inside front tire dragging a bit and the outside rear tire pushing more than the inside rear, the car tends to rotate. With anti-Ackerman, the same drag vector can be produced, but with much diminished pure turning thrust.

At least that's how it works in my head.

[Jersey Tom]

I've heard the theory of the slip angle drag contributing to yaw, but I just don't buy it. Longitudinal distance from the inside front tire to the CG is going to be greater than the lateral distance from the inside front to the CG in just about any situation. That being the case, you get more bang for the buck with every pound of pure lateral force than longitudinal (e.g. slip angle drag) force, when you multiply it against the appropriate moment arm.

There are plenty of reasons for running pro-Ack, and plenty for running anti-Ack.. in stock car, sports car, open wheel, you name it.

Pretty sure there was a picture on F1T maybe a couple years ago.. Ferrari, at Monaco? Showing a wild amount of anti-ack (and/or toe-in) steering, for what it's worth.

Out of curiosity.. do you work for one of the handful of race shops in and around the Boulder / Denver / FoCo area?

[raymondu999]

When I think of all this though, something doesn't add up (for me). Geometrically speaking, Ackerman (in the absence of load transfer between inner/outer wheels) would make perfect sense (to me, at least). With anti ackerman, the two fronts would be turned in at different steering angles, hence inducing different turning radii (if they were on separate bodies). But obviously, the car being one single body, it has one turning angle and one turning radius.

Wouldn't anti-ackerman, by definition, mean that either the outside tyre is sliding outwards, the inner tyre is sliding inwards, or a bit of both? I hope my question makes sense to you

[olefud]

[quote="Jersey Tom"] Longitudinal distance from the inside front tire to the CG is going to be greater than the lateral distance from the inside front to the CG in just about any situation. That being the case, you get more bang for the buck with every pound of pure lateral force than longitudinal.

Very good point. Had to think on it for awhile. My take is a bit qualitative, but on initial turn in the car has to start the arc around the turn center and also initiate the yaw rotation. These are distinct and cumulative dynamics that make turn in more demanding than steady state turning.

The force couple between inside front and CG is pure rotation while the lateral force is orthangular to the CG force {assuming trailing brake). This seems more effective in getting the car to rotate.

I work outside of racing but I'm doing some interesting IR&D on brake cooling.

[Caito]

When I think of all this though, something doesn't add up (for me). Geometrically speaking, Ackerman (in the absence of load transfer between inner/outer wheels) would make perfect sense (to me, at least). With anti ackerman, the two fronts would be turned in at different steering angles, hence inducing different turning radii (if they were on separate bodies). But obviously, the car being one single body, it has one turning angle and one turning radius.

Wouldn't anti-ackerman, by definition, mean that either the outside tyre is sliding outwards, the inner tyre is sliding inwards, or a bit of both? I hope my question makes sense to you

Ackerman, as first thought(geometrically) doesn't take account for slip angles. Tires are always slipping, else you'd get no force at all.


Here you have a pic of it considering/not considering slip angles: