I'm wondering which criteria of steering design I should use.
Assuming that 100 % of Ackermann is the theoritical figure with each wheel tangent to the trajectory; and that 0% is the figure where both front weel have equal steering angle.
I know the good solution is to be somewhere in between these two (for tire drift compensation) but where??
Anyone knows?

[Edited on 11/12/04 by Aloupol]

This is probably the most debated issue of car design, how much to Ackermann to have. Carrol Smith, in his books on race car design, had many engineering reasons why you want little to none, if not anti-Achermann. Then in his last book, Engineer To Win, he said they tried different settings during one testing session, and he was blown away to find that the more Ackermann they dialed in, the better the lap times were.

Of course this is for hard-core race cars, so who knows. He words though made enough of an impression on me that I dialed in all I could (on my Mini.)

And FWIW, Staniforth likes Achermann.

On top of all these vagueries(sp?) keep in mind that if you have no Ackermann it makes it very hard to push the car when turning the wheel (if that's a concern for you.)

I've tried to follow this before. I read this http://www.cadvision.com/blanchas/54pontiac/steering.htmlbut it didn't make any more sense but this one http://ingallseng.com/performance/primer.html
explained it a bit better. I now believe that the angle on the wheel on the inside of the corner acts as a pivot for the car to turn on and increases cornering grip till it starts to slide. Then watch out as it goes straight on.

Chapman tried anti-ackerman on the Seven S1 and some of the Team Lotus racer circa 1958.
It was reported to give more grip on high speed constant radius curves but the drivers hated it because the steering response was unpredictable. That bit is fact or as close as we will ever get to the fact where Chunky was concerned.

My view is that due to tyre friction characteristics an ackerman angle is require when the weight transfer is low but as the load on the inside front wheel is transfered from it it becomes excessive and imposes an excessive slip angle on the inside front tyre.
This would sugest ackerman could help turn in but would be of less benefit towards the the apex of the corner and beyond.

Taking this idea a stage further roll couple distribution becomes important. I would surmise a front heavy car that was naturally inclinded to understeer would be set up soft at the front and would benefit a lot from a ackerman.
On the other hand on a tail heavy naturally oversteering car which would be set up with a lot of roll stiffness at the front so that the inside front wheel would have little or no download should work better without ackerman .
A Locost is very slightly tail heavy so my take is some Ackerman effect will be of benefit but 100% would be bad.

In any case with a rack and pinnion geometry classical ackerman is out the window -- the fore-aft position of the steering rack dictates the wheel angles.



[Edited on 11/12/04 by britishtrident]

[Edited on 11/12/04 by britishtrident]

Cool forum..
Well documented guys (with astounding built skills) interesting links, thanks for info, once again..

Forget it. It's not worth worrying about. Ackerman is only important if you need a few tenths of a second at LeMans, or if you push your car around alot. If you drive your car properly your inner wheel probably won't be on the ground anyway!

quote:

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Originally posted by britishtrident
Chapman tried anti-ackerman on the Seven S1 and some of the Team Lotus racer circa 1958.
It was reported to give more grip on high speed constant radius curves but the drivers hated it because the steering response was unpredictable. That bit is fact or as close as we will ever get to the fact where Chunky was concerned.

My view is that due to tyre friction characteristics an ackerman angle is require when the weight transfer is low but as the load on the inside front wheel is transfered from it it becomes excessive and imposes an excessive slip angle on the inside front tyre.
This would sugest ackerman could help turn in but would be of less benefit towards the the apex of the corner and beyond.

Taking this idea a stage further roll couple distribution becomes important. I would surmise a front heavy car that was naturally inclinded to understeer would be set up soft at the front and would benefit a lot from a ackerman.
On the other hand on a tail heavy naturally oversteering car which would be set up with a lot of roll stiffness at the front so that the inside front wheel would have little or no download should work better without ackerman .
A Locost is very slightly tail heavy so my take is some Ackerman effect will be of benefit but 100% would be bad.

In any case with a rack and pinnion geometry classical ackerman is out the window -- the fore-aft position of the steering rack dictates the wheel angles.

---

FWIW:

The LMP runs with no Ackerman.

It is documented that turn-in is brilliant (at all speeds).

It is a bitch to push around!!

According to Autocar the original Elise had 38% Ackerman.

I assume this means that the steering arm lines intersect at about two and a half wheelbase lengths behind the front axle line.

The Elise Mk1 front tyres were too wide, 185 as opposed to 205 at the rear. Note that Lotus didn't increase them for the Elise Sport which went up to 215 at the rear and actually decreased them to 175 for the faster Elise Mk2 though the rears went up to 225.

A low Ackerman value might be an attempt to reduce the ultimate grip of the front to compensate for the width.

I don't suppose anyone knows what the Mk2 uses?

ackerman angles suit a car going slowly. the faster you corner, the more the tyres slip. The slip angle is the difference between the direction the tyre is pointing and the direction that it is going. To compensate for tyre slip angle, you want to turn the outer front wheel a bit more than Mr Ackerman suggested. How much more depends on the relative slip angles of all 4 wheels, which depends on the tyres that you chose and on the weight distribution of your car, amongst other things.

The best way to tune it is, unfortunatly, to suck it and see. A bit difficult in practice. The best result will probably be part way between full Ackerman and none!

Thanks for advices, well it seems not to be THE key aspect of good chassis, some kings as Chapman and Dallara have choosen oposite ways.
Yes the slip angle of each wheel is related to its cornering force and vertical load, but conversely the centripetal force of the inner wheel depends on its drift, which is imposed since the inner wheel is not strong enough to drive the car. Then with not enough of Ackerman the inner wheel could push the wrong direction...
I will use between 100% and 80% Ackerman (depending on what allow my uprights) and see what happends.

FWIW I'm using about 50% on the mid-engine Mini. That is, the projected intersection of the steering arms cross behind the car at about twice its wheelbase.

[Edited on 1/4/05 by kb58]