Steering design

[Deepak Jain]

Hello friends,
I'm working on steering design of our car.I'm using Lotus suspension Analysis(LSA) software for dynamic ackermann%.I've referred many books and is given that ackermann% has to between +70% and 100%. But I'm getting a negative ackermann%. Can the ackermann% be negative???

[BillCobb]

Yep.

[PatClarke]

It sure can Deepak,

It is not the 'Ackermann' that is important, it is the effect of the 'Ackermann'.
Positive effect is the front wheel toe-out increases (or the toe-in decreases) as the car is steered. Obviously, negative Ackermann is the opposite.
There are other issues, such as steering rack position, that can have an effect on this geometry.

But before you start designing steering geometry, you have to ask why you would want to change the toe angle as the car steers and, if you do, how much and whether it should be negative or positive.

Finally, in the Lotus software, why do you think the Ackermann HAS TO BE the percentage you quoted?

I think some more research is required Deepak!

Cheers

Pat

[Deepak Jain]

Thanks for the reply pat I think ackermann is represented in % only and even in LSA it shows in %.

[Deepak Jain]

Originally posted by BillCobb:
Yep.

Thanks for the reply.So,Won't it be a problem if it is negative as positive ackermann % is encouraged.It may be a basic question. But as a begineer,I'm quite confused.

[ed_pratt]

Deepak,

I think what Pat was asking is, why do you think the value must be between 70% and 100%?

What sort of vehicles had those values? Are they the same as yours?

As Pat suggested, try and read a little more about steering geometry. This might be a good place to begin

http://www.rctek.com/technical...ering_principle.html

Good luck!
Ed

[Warpspeed]

I suppose the whole thing depends upon your design concept.

Full hundred percent Ackermann is the ideal for zero slip angle, and great for minimum tire wear and rolling resistance when parking or at walking speed.

As you drive faster, something less than hundred percent Ackermann may (?) have an advantage.

The faster you go, the less Ackermann you probably need, as the outer front tire starts to suffer.

But then, what sort of maximum tire slip angles do you anticipate having at max load, max G ?
Tire data will be a clue here.

You can go to zero Ackermann, and even beyond zero into negative Ackermann territory, but at anything less than the speed of light, on a very low grip surface with cast iron tires, it may have more disadvantages than advantages.

Something between 70% and 100% would probably cover the sane range for a general purpose vehicle with typical steering angles, but how wild do you plan to go ?

[Deepak Jain]

Thanks Pratt and warpspeed for your suggestions.And Yeah I've actually misunderstood that statement of Clarke about the Ackermann %.

[acedeuce802]

Originally posted by Deepak Jain:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by BillCobb:
Yep.

Thanks for the reply.So,Won't it be a problem if it is negative as positive ackermann % is encouraged.It may be a basic question. But as a begineer,I'm quite confused. </div></BLOCKQUOTE>

Well, what do your tires desire?

[SteveHarasym]

Choosing appropriate ackermann is about getting the most lateral G out of your tires in a turn. Become familiar with your tire data and the solution will present itself.