Anti ackerman design problem

[coaster]

Hello, i am trying to figure out my steering rack position, the rack is behind the front axle in the range of 100mm to 135mm.
Every position i try for the rack results in anti ackerman, towards the axle i get more and away i get less which points to a fundamental flaw.
Are there any tricks to remedy this?
Rack length and height only seem to effect bump steer.
Is it simply a fact the rack must be in front of the axle?

[DiogoBrand]

I hope I'm not talking crap here but I think you can get ackerman steering regardless of the steering rack being in front or behind the front axle. What matters is if the track rod arms are angled inwards in this case (or outwards of the rack is in front of the axle).
Also, you're probably gonna get more predictable results if the rack is in line with the track rod joints.
Hopefully someone will explain it correctly for you but I believe that's it.

[Greg Locock]

Yes, it's the relationship between OTR, ITR, and kingpin in plan view. Try moving OTR in Y. What ackerman curve are you aiming at?

[coaster]

Its jza80/soarer upright with qa1 chevy ball joints and a shortened nb miata steering rack.
As in the jza80, the rack is behind the axle. Reasons were for packaging, its a clubman lotus style frame with a narrowed frame to suit a wolsely grille and the front axles in line with the grilles front edge.
Similar to a 1930's racing special in terms of looks, i was aiming for a 2200mm wheelbase for racing around witches hats. Short corners are more suited to ackerman instead of anti ackerman.
Backed myself into a corner with the design.

[Greg Locock]

So low speed high steering angle maybe 70% ackerman at full lock on the inner wheel? or more? usually your inner wheel cut angle is set by packaging, and then you wonder about how much you should be between parallel steer and ackerman on the outer.

[coaster]

https://freeimage.host/i/RS9gSa https://freeimage.host/i/RS9PNR https://freeimage.host/i/RS9LoN
The soarer upright has a 12mm trail offset from the kingpin line, i chose a 5 degree trail to match a caterham, 1.5 degree neg camber to match caterham, the steering arm is 132.9, racks stroke 62.5 each way.
For each 25mm fore or aft movement of rack behind the axle centreline i get an increase or decrease of 1.1 degree of anti ackerman (per side) , never does it reach neutral or toe out ackerman.
My rack position ranged from 70mm to 145mm from behind axle centre, at 145mm the control arm angles started to get a bit skewed.
Rack length knuckle centre is 482 which is in line with the proposed inner wishbone centres.

Id try locostbuilders website but its all fluff and guesswork over there, there are alot of real engineers who frequent this forum.

[Tim.Wright]

Move the toe link outer hoint in the lateral direction like Greg said. This sets your Ackermann.

[coaster]

So correct, i used the old kingpin/kingpin/rear axle centre triangle for the toe link position, moved the rack to minus 70mm, 60 or 50 might be better for toe rod angles at full lock.
At full lock ive got about 5 degrees toe out, i must weld some tabs on the uprights.

[coaster]

Does anybody have a ratio or formula for ideal ackerman settings in terms of slip angle?

[Greg Locock]

It's tire dependent, load transfer dependent, you name it. This sort of implies you are going for max latacc.

[coaster]

Thanks for your input, i will try for the 70% inside wheel but i cant give feedback for a good 18 months as no metal is yet cut except for rack shortening.
Possibly longer.

[Greg Locock]

Ouch, don't go for 70% because I mentioned it! That's a good figure for reducing parking loads at maximum lock on a truck, not max latacc through the witches hats in a sports car!

The best discussion of ackerman vs turning circle I have come across is in Reimpell and Stoll. The point there is that for a given inner wheel cut angle, MORE steer on the outer wheel, ie moving towards parallel steer, will always reduce your turning circle at low speeds.

But... with load transfer and so on, and away from full lock, the outer wheel becomes far more important.

[Stu]

Anti-Ackerman can be used as a ‘good’ thing, you just need to have your tyre loads/suspension loads really ‘down’ to use it!

[Greg Locock]

Bear in mind that static toe is fake ackerman. I knew one racer who set his car up for parallel steer and then adjusted the toe for each track.