anti or regular ackerman steering?

21158 Views 10 Replies 6 Participants Last post by DesignIt, Jul 11, 2005

00 scrub

Discussion Starter · Aug 22, 2004

I was wondering what kind of steering setup does the Elise have? Does it utilize reverse/anti-ackerman or standard ackerman? Any thoughts as to why one or the other would be better? I've been thinking about it and I've argued both sides in my head and can't come up with a conclusion that would say one is better than the other (for most road vehicles).

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Mark A

· #2 · Sep 11, 2004

The Elise, along with almost all road vehicles in the world uses Std Ackerman.

Reverse/anti-ackerman is used on race cars only, and even then it is usually open wheeled cars.

Anti-ackerman is not a good idea on a road car.

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00 scrub

Discussion Starter · #3 · Sep 13, 2004

I recently had a discussion of ackerman with a Formula Atlantic racer and he said that it can relaly aid turn in even if you "drag" the inside tire by applying too much ackerman. He said that the car pivots somewhat about that tire.

What I'm not understanding is how the slpi angle requirment of the inside tire differes from teh outside(loaded) tire. I would think that it would be safer to have less slip angle on the inside tire as going over the optimum slip angle for a tire really drops the ability of teh tire to provide grip/traction force.

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z06fun

· #4 · Sep 14, 2004

I would imagine that the inside tire may be optimum at a higher slip angle compared to the outisde tire because it has less weight on it. With less weight it has a higher coefficient of friction (not higher peak frictional force, just higher coefficient).

So real life evidence of the inside tire giving more grip at a higher slip angle is that many racers tend to add front toe out to get a bit more peak grip from the front of a car.

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Stan

· #5 · Sep 14, 2004

>>>So real life evidence of the inside tire giving more grip at a higher slip angle is that many racers tend to add front toe out to get a bit more peak grip from the front of a car.<<<

Exactly! An a car with lots of ackermann / toe out can be *much* easier to push around the paddock too, another clue.

Ackermann is sorta like toe out that goes away for the straights so you cut tire drag. Mostly it helps the early part of a turn.

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Stan

· #6 · Jul 7, 2005

We have 30% Ackermann on the US Elise.

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DesignIt

· #7 · Jul 8, 2005

Hmmmmm...

Okay, I know what Ackermann steering geometry is and how it should be set up. (The intersection of the lines projected by the steering arms should intersect on the axis of the rear axle.) I've even seen a set up that I would call "anti-Ackermann" (or maybe "reverse Ackermann") where the steering rack was mounted in front of the axle and the intersection of the lines projected by the steering arms was 8 feet in front of the axle. This setup was a complete disaster and dangerous for street driving, so it must not be what you're referring to.

Here's what I'm confused about:
1. What are you referring to as "anti-Ackermann"? Is it any steering geometry that does not conform exactly to the Ackermann design? (e.g. parallel steering arms.)
2. What do you mean by 30% Ackerman? Is this is a percentage of the actual distance from the front axle to the intersection point of the steering arms divided by the wheelbase? (actual Ackermann point).

Help me out here, this is the first time I've seen the term anti-Ackermann.

Thanks,
John

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Ruediger

· #8 · Jul 9, 2005

John

See:

http://www.eng-tips.com/viewthread.cfm?qid=117106

Does it help?

Rüdiger

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DesignIt

· #9 · Jul 9, 2005

Ruediger,

Thanks! I never considered the effect of lateral offset of the steering arms as Norm Peterson pointed out on the eng-tips link. I understand the reason for non-Ackermann now in regard to high lateral forces and slip angles. The post made by Greg Locock on the eng-tips link further illustrates how complex it is to accurately determine the correct percentage of Ackermann to use. It also appears that the percentage of Ackermann can only be perfectly correct at one point in the lateral g range and slip angle. Further complicated by different slip angles for different tires, I would think. Suspension geometry is a very cool subject. (Yeah, I know, I'm a geek. :sad: )

I got a bit tripped up on the nomenclature of "anti-Ackermann". After reading your link and a bit more on the subject, I think the two assumptions I made in my last post must be correct. If anyone knows differently, please school me.

Thanks,
John

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Ruediger

· #10 · Jul 9, 2005

John
Suspension geometry is a very not only a very cool but also a rather complex subject. If you are interested to learn more about it you should also read this paper.
http://campus.umr.edu/fsae/library/sae_paper/paper.html
May help you to build your next yourself .

Regarding Ackermann you read there :
>>> Ackermann steering occurs when the outside wheel turns less than the inside wheel. This is possible since the amount of steering angle for each wheel is determined by the steering geometry. Reverse or anti-Ackermann occurs when the outside wheel turns more than the inside wheel during cornering [3].
During a turn, the inside wheel travels around a smaller geometric radius than the outside wheel. Ackermann steering can be used so that the wheels travel about their corresponding radii, theoretically, eliminating tire scrub. However, designing for precise Ackermann steering might not provide the best handling since tire slip angles influence the actual turning radius [9]. The designer must decide, based on the requirements, if the steering system design will include Ackermann geometry.<<<

Rüdiger

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DesignIt

· #11 · Jul 11, 2005

Ruediger said:
May help you to build your next yourself . Rüdiger /QUOTE]

Ruedigner,

Thanks again! Great paper on suspension and chassis design. BTW, I do have a TIG welder and a tube notcher, so who knows what might happen...

John

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