so I know what it does, but I don't get why you would not want perfect ackerman-steering in your car... more so in FWD.

I found some other threads about it but couldn't find anything on benefits/drawbacks.

To me, ackerman makes most sense and even feels a little better than parallel... but then again I see other setups, which mostly use parallel.

So what do I lose when using ackerman?

Quote from Bandit77 :

So what do I lose when using ackerman?

Quote :

If both the wheels were turned by the same amount, the inside wheel would scrub (effectively sliding sideways) and lessen the effectiveness of the steering. This tyre scrubbing, which also creates unwanted heat and wear in the tyre, can be eliminated by turning the inside wheel at a greater angle than the outside one.

http://www.rctek.com/technical ... n_steering_principle.html
http://www.racing-car-technolo ... /Steering%20Ackerman4.doc

So basically, setting the Ackermann to either too little or too much will cause extra tyre wear and heating. I think in LFS the Ackerman% is the value of how parallel the wheels turn, so 100% is parallel and less % means the inside wheel turns more.

In LFS I don't think you lose anything by using Ackerman because it's already built into the geometry. If you had a choice of a geometry with/out it, it might have some downsides such as complexity or weight

At very low speeds, you want to run Ackermann steering (0% parallel in LFS), but at higher speeds when you are nearer (or at) the limit of grip, you will likely have substantially different loads between the inside and outside tyres. This means the peak slip angles are going to be different, so you want a steering geometry that gets both inside and outside tyres as close to their own peaks as possible. So this ends up somewhere between parallel and Ackermann steering.

If you find that your outside front wheel is slipping more (or sooner) than the inside front, you will probably benefit from a little bit of Ackermann steering. I think this is generally a rare situation in racing, though.

Over the years I've tended to hone in on about 60% parallel, pretty much in all cars in all conditions. It gives a nice compromise between turn-in, stability and balance. Mostly tweaked in conjuction with toe, it's one of the first settings I tend to optimise in a setup.

Interesting. I always tended to use one very simple steering setup.

Steering Lock: max (or a little less in some rare cases, with 540 degree wheel rotation)
Caster: max (or a little less in some rare cases)
Front Toe: 0
Rear Toe: 0
Parallel Steer: 100%

Quote from Bob Smith :

At very low speeds, you want to run Ackermann steering (0% parallel in LFS), but at higher speeds when you are nearer (or at) the limit of grip, you will likely have substantially different loads between the inside and outside tyres. This means the peak slip angles are going to be different, so you want a steering geometry that gets both inside and outside tyres as close to their own peaks as possible. So this ends up somewhere between parallel and Ackermann steering.

hmmm... ok, that makes sense to me. thanks.

so basically: the outer wheel is at its grip-limit (100% ackerman), the inner wheel has less grip anyway because of weight-transition. now you reduce the inner wheel's steering angle (by increasing parallel) and get more grip, because it's going "less sideways". is that it?
if no: I need some more explanation.
and if yes: wouldn't the inner wheel now work against the outer, leading to worse cornering? maybe not at the limits... hmm... I think i'm getting it.


@tristancliffe: what about your real race car?

Quote from Bandit77 :

so basically: the outer wheel is at its grip-limit (100% ackerman), the inner wheel has less grip anyway because of weight-transition. now you reduce the inner wheel's steering angle (by increasing parallel) and get more grip, because it's going "less sideways". is that it?

Assuming that cornering stiffness does not change with vertical load, then the optimum slip angle (for maximum lateral force) would move with load, so the inner wheel would need less slip angle to be at its own optimum. Using less than true-Ackermann steering would achieve this, so you are correct. Now the trouble is, cornering stiffness does change with load, so that complicates matters, depending on which effect is dominant from one moment to another.

Personally, I've found 30% to 50% works nicely for me, this really helps turn in for me and makes the car feel more darty and responsive. Tristan's 60% value may end up working better for outright cornering grip, but that's not so easy to feel for, as the difference there is much smaller. The only thing that is clear is that neither parallel (100%) or true Ackermann (0%) are ideal settings.

At 100% parallel both front wheels steer as much, thus the inside wheel will work against the outer. Not enough Ackerman.

At 0% parallel the inside wheel will steer much more than the outer. Too much in fact, because it's pointing slightly sideway to the direction of travel. Scrubbing speed, increasing heat, too much Ackerman.

The optimal balance between parallel and Ackerman steering is somewhere between 0 and 100%, and it depends on other factors such as toe and what kind of corners you need to negotiate.

Or whatever Bob and Tristan say

Quote from Bandit77 :

@tristancliffe: what about your real race car?

Ummm

*gets car manual out (yes, they come with owners manuals too )*

The '98 car (mine) has 43.5% Ackermann (56.5% parallel), whilst the '02 car went down to 29% (71% parallel). I've got other figures from other years at work, but they won't be that far off the above.

hmm, this is an interesting topic, i've always used settings like forbin but normally abit of rear toe to keep it stable, i always thought 100% parrallel steer would keep things nice and predictable between the fast and slow corners, but may start using some ackerman if it help.

learns new thing everyday

One sets some offset with toe-settings?

With TOE-OUT applied, the inner wheel has a LOT of the effect ackerman would give already ...

Quote from Riel :

With TOE-OUT applied, the inner wheel has a LOT of the effect ackerman would give already ...

At high speeds, where very limited steering input is required, I would agree.

On tighter corners, full Ackermann would typically be adding about a degree of toe out, which is more than you would want to run as static toe.

Once you're down to FSAE machines like the MRT5, at an autocross track, then Ackermann could add up to 2½ degrees of toe out. Also note that Ackermann does not add toe out equally to each wheel either, as the inside wheel gets more toe out than the outside wheel. Hence why Ackermann is something of a hot topic at FSAE level, but GTR / high level Formula machines don't bother with it and just use a little static toe.

Quote from Bob Smith :

Hence why Ackermann is something of a hot topic at FSAE level, but GTR / high level Formula machines don't bother with it and just use a little static toe.

I just found this interesting tidbit:

Quote :

Some race cars use reverse Ackermann geometry to compensate for the large difference in slip angle between the inner and outer front tires while cornering at high speed. The use of such geometry helps reduce tire temperatures during high-speed cornering but compromises performance in low speed manoeuvres.

Which could be partly reproduced in LFS through toe in

Yep, a little toe in for high speeds, counteracted at low speeds by using slightly more Ackermann.

wow...

what about dynamic ackerman and toe?


[off topic]

hey bob, your avatar kind of indicates what I think my recently obtained G25 misses: some more buttons on the wheel itself.
I mean, one of my two buttons is set to "accept incoming call" and the other one to "activate NOS", so now there's none left for the control of my hifi-system.
no, seriously - two or four more wouldn't have been bad... and the horn in the middle.

[/off topic]

I just have a quick question for you
My game is in French, so the traduction of the Parallel steer field is litterally 'Ackerman Effect'.

So in LFS, if I put this value to 100% do I set Parallel steer to 100% or Ackerman effect to 100% ? Because I usually set it to 100% assuming I was setting Parallel steer to 0%
If I have set parallel steer to 100% it might have explained some of my entry understeer issues...

That is a problem with the translation. 100% is fully parallel steering regardless of the language you play in.

Quote from Zen321 :

what you said...

I've set the language to english anyway... exactly for the reason of terminology.

Quote from Bob Smith :

Yep, a little toe in for high speeds, counteracted at low speeds by using slightly more Ackermann.

If I get this right, your suggestion would be even better than pure Rev Ackermann.

It looks too good, why would they apply Rev Ackermann then?

Quote from NightShift :

It looks too good, why would they apply Rev Ackermann then?

Crazy fools.

That and if slow corners are very rare, you get the bonus of less tyre scrubbing in a straight line.