Ackermann Article

[260DET]

Here is a comprehensive article on Ackermann, the comments at the end have some practical value which is nice. One thing I found interesting was the turn in assistance given by dragging the inside front wheel as a result of a large slip angle induced by static toe out and Ackermann.

 

Anyway, enjoy http://www.clubracer.com.au/articles/ackerman.asp

[JMortensen]

Interesting little note in there about raising roll center and Ackermann together to get better turn in... thanks for the link. Curious to see what Cary has to say. Any more feedback on your new steer knuckles yet Richard?

[rudypoochris]

good read thanks

[260DET]

Yeh Jon, noticed that comment about the roll center, interesting.

 

On mine, taking account of the physical limitation imposed by a front mounted rack, there is now as much Ackermann as is possible. With that plus 3mm toe out the car now turns in beautifully, its really big fun to drive Too nice in fact, to get better drive out of corners the rear end is being softened a bit. To be really quick I figure its got to be a bit of a pushing pig, by imposing more load on the front, which I now know is working well, so the rear wheels can use more of their grip for forward traction.

[tube80z]

Yeh Jon' date=' noticed that comment about the roll center, interesting.

 

On mine, taking account of the physical limitation imposed by a front mounted rack, there is now as much Ackermann as is possible. With that plus 3mm toe out the car now turns in beautifully, its really big fun to drive Too nice in fact, to get better drive out of corners the rear end is being softened a bit. To be really quick I figure its got to be a bit of a pushing pig, by imposing more load on the front, which I now know is working well, so the rear wheels can use more of their grip for forward traction.[/quote']

 

Thanks for the link Richard, it was a good article. I had read most of that but was very interested in what Calude Roulle had to say.

 

I've ran similar setups to what you've done with your car (new knuckles) and moved the rack as far back as possible, even thought about making a new crossmember to get a little more here. As you've indicated that car does turn in better. For my tube frame project I've looked at this quite a lot and finally came to the conclusion that I can't use the rack mounted at the bottom of the control arms. What I'm doing there is raising the rack and the steering arms up on the strut to allow them to get deeper in my small 13 inch wheels and be more outward in relation to the steering axis. If my calcs are right I can get around 100 percent from the classic definition. If that's not enough then I may think about changing this to rear steer.

 

In a race engineering class I took there was a lot of discussion around Ackerman and what we were told is that you can pretty much set it at 100% and forget about it for the typical club racer/autoxer. Some time might be gained but it would be better to play with static toe. No real explanation was given. They did share it is possible to use strain guages on the tie-rods to determine the optimium Ackerman or toe setting. No real mention was made of dynamic toe curves. When I originally read this I was interested but no longer had the car I had played with to do any measurements.

 

I'm not sure I completey agree with the statement about how raising the RC works. I will agree that it will make the car react quicker but I think that has more to do with the rate of slip angle generation than the amount of roll allowed. An underground front RC has an initial negative jacking component when steering starts and may help with toe out and pro-Ackerman.

 

As I've worked on my car getting the front to stick better I've had a similar issue with the rear struggling for grip. I've been running what I thought was too much rear camber to compensate and dealing with tire temps that were 15 degrees or more hot on the inside edges. At my last race I used droop limiters on the rear (up to that point only on the front) and found something very interesting. I wasn't having a problem with rear negative camber but it was caused by overusing the inside edge powering out of the corner as the car rolled and extended the inside. The limiter no longer allows this and the temps were suddenly normal.

 

On the front, as with toe-out (or Ackerman) you'll find using limited droop can really quicken turn in.

 

thanks again for the link,

 

Cary

[JMortensen]

I'm not sure I completey agree with the statement about how raising the RC works. I will agree that it will make the car react quicker but I think that has more to do with the rate of slip angle generation than the amount of roll allowed. An underground front RC has an initial negative jacking component when steering starts and may help with toe out and pro-Ackerman.

Interesting. I had always taken this argument the way the author of the article did. Before the weight transfers the Ackermann pulls the inside tire harder and that's what gets the car turning faster. Once the weight transfers the effect of the Ackerman is more or less over. So it would seem to me that the higher roll center makes the car more resistant to the weight transfer enhancing the Ackermann effect. That's the way I had always thought about it anyway, and it really seems to be the case in my friend's 510.

[260DET]

Interesting. I had always taken this argument the way the author of the article did. Before the weight transfers the Ackermann pulls the inside tire harder and that's what gets the car turning faster. Once the weight transfers the effect of the Ackerman is more or less over. So it would seem to me that the higher roll center makes the car more resistant to the weight transfer enhancing the Ackermann effect. That's the way I had always thought about it anyway, and it really seems to be the case in my friend's 510.

 

Hanley in The Racing & High Performance Tire talks about the delay in weight transfer. My guess is that if the car is set up to get maximum drive out of corners resulting in some understeer in other situations, you may be able to use that delay with Ackermann assisted quick turn in (plus some trail braking?) to drive around any such turn in push problem. Going to try it anyway.

[johnc]

Myself and Erik Messley spent a lot of time (more then we should of in hindsight) trying to determine if Ackerman would help the handling of my 240Z. Initially in setting up the car for autocross (low speed, tight corners) we felt that there would be an advantage. We did a little bit of testing at two different practice events and couldn't discern a difference. Suprisingly we found that shock tuning had a much bigger impact on transitional steering behavior (turn-in) then toe changes. Thinking about it we determined that how quickly the outside tire got loaded was the biggest determining factor in transitional response (at least in my specific example).

 

So, we tried a test where we put a 225 width tire on the inside front and a 275 width tire on the outside front and tried a few turns. Although there was an obvious difference in transient (turn-in) response, it wasn't as big as we expected. That stupid little test convinced us that a decision about Ackerman wasn't as important as the time and effort we ended up spending on it, so we went on to other things.

 

IMHO, Ackerman is one of those interesting theoretical engineering discussions that is of little value on production based racing sedans. 95% of the benefits supposedly available to the sedan racer through the use of Ackerman can be achieved through static toe settings. Now, in the world of purpose build race cars (Formula Fords, F1, etc.) Ackerman is of more importance.

[JMortensen]

John I think you just set the toe way out instead of using Ackerman. Nothing wrong with that I suppose, but that's probably why it didn't make too much of a difference in your case. If you had the Ackerman going you probably could have reduced the toe and had the same basic effect when autoxing.

 

Ackerman is a splitting hairs type of thing, but to paraphrase YOU regarding chassis setup "It's a bunch of little things, not one big thing".

[JMortensen]

I guess what I just said is basically the last paragraph of your last post... guess I don't read so good sometimes...

 

I still think there are times when it would be advantageous to use one over the other. Like under heavy braking. A more neutral toe setting would make the car more stable under braking than a big toe out setting. Also at autox I think the difference might be much more dramatic since the steering angles are so much greater.

[johnc]

BTW... I'm not saying Ackerman is of no value on our cars, but IMHO, its ROI is much less then other things you can do to improve steering (bumpsteer, front track, etc.) Given a limited amount of testing time and budget, I would rather do things like build splitters that generate front end lift...