Steering geometry and Ackermann

[Sergi]

Hi, I'm designing the steering in this years car in our team, and we don't know exactly how to achieve the best geometry.
As I've read almost everywhere, ther are very much and different theories about that, so I had to make my own one and decided to design the steering system in front of the fron axle and below the drivers feet (mainly because of package). The idea is having half degree toe-out (rear wheels will have half degree toe-in) and pro-ackermann geometry, as the turns are generally small.
However, I think the best is to make the steering adjustable, but how do yo achieve it? What points do you design adjustable to change the geometry so you can have less or more Ackermann?
Another question that I have is how much slip angle do yo assume you will have in the turns, so you can calculate an approximate curve of the ideal angle difference between inside and outside wheel? I made my calculations supposing 7 degrees slip angle in turns of radii of 6 - 12 meters, basically because the tire data we have said the biggest lateral force was achieved at slip angles of 7-10 degrees aproximately.

Thanks, looking forward for suggestions!

Cheers,

Sergi

[Sergi]

Hi, I'm designing the steering in this years car in our team, and we don't know exactly how to achieve the best geometry.
As I've read almost everywhere, ther are very much and different theories about that, so I had to make my own one and decided to design the steering system in front of the fron axle and below the drivers feet (mainly because of package). The idea is having half degree toe-out (rear wheels will have half degree toe-in) and pro-ackermann geometry, as the turns are generally small.
However, I think the best is to make the steering adjustable, but how do yo achieve it? What points do you design adjustable to change the geometry so you can have less or more Ackermann?
Another question that I have is how much slip angle do yo assume you will have in the turns, so you can calculate an approximate curve of the ideal angle difference between inside and outside wheel? I made my calculations supposing 7 degrees slip angle in turns of radii of 6 - 12 meters, basically because the tire data we have said the biggest lateral force was achieved at slip angles of 7-10 degrees aproximately.

Thanks, looking forward for suggestions!

Cheers,

Sergi

[Luniz]

To achieve an adjustable ackermann percentage, you would have to be able to slide the steering rack fore and aft for example. You have to be careful with the clearance of the steering rod and the rims at those extreme steering angles we are using at FS events.

[Jersey Tom]

Easier way to adjust Ackermann is to have multiple pickup points on the upright.

If I remember right it's really determined by the angle between the tie rod.. and the line in the top view that would connect the steering axis to the steering pickup point. For front steer.. 90 deg = parallel steer, <90 deg = pro-ackermann, >90 deg = anti-ackermann. I could be wrong. It's been a while.

There's also what I call 'Ackermann progression' or how much the gain changes through the steering. If I remember right I managed to be anti-ackermann at low steering angle and pro-ackermann at high steering angle. Inadvertently instead of by design..

How much Ackermann or toe you want, is a separate lengthy discussion...

[Scalesy]

What sized wheels are you using? Ackermann adjustment holes on the upright side aren't always possible with smaller rims, especially 10"...In that case you'd have to set the point on the upright where clearances let you, and set your ackermann characteristics with rack placement instead (with the angle of the tie rod mentioned above).

[The_Man]

A year ago when I was doing the steering geometry design I too had a packaging issue not allowing me to have multiple pick-up points on the upright so I decided I could change the geometry by having different length steering tie rods or ajustable length tie rods. This of course meant that the rack length had to be adjusted to maintain toe. This was archived by having a rodend at the ends of the rack. This turned out to be a disaster as this caused a huge compliance issue. Also it was nearly impossible to set both the toe and steering geometry right at the same time. So quick changes where impossible. Also if I get it wrong the toe is all wrong and the steering mechanism has a tendency to get stuck up at somepoints. Eventually we had to redo the entire steering just a month before competition.

Just think twice before you try something like this.

[alumasteel]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">If I remember right it's really determined by the angle between the tie rod.. and the line in the top view that would connect the steering axis to the steering pickup point. For front steer.. 90 deg = parallel steer, &lt;90 deg = pro-ackermann, &gt;90 deg = anti-ackermann. I could be wrong. It's been a while </div></BLOCKQUOTE>

Quick note... This is correct given that you've already stated you have the steering in front of the axle.

Looking at "ackerman progression" isn't difficult if you restrict the scope to the top view. Set your inner and outer tie rod locations as inputs, along with the kingpin "steering" axis location, and your static toe. Then move your rack (inside tie rod point) through it's linear travel and just solve for the wheel angles using some simple trig and your input values. You can then plot angle difference between the outside wheel and inside wheel. If your outside wheel is steered more than the inside then you have anti-ackerman, and vice versa for pro-ackerman.

If you're just starting out, I would start by playing around with those inputs and see what kind of effects they have on the output. That is, run a sweep of various settings for rack location fore and aft while keeping your steering arm constant, and plot your ackerman progression for each rack location on the same plot. Then do the same for different permuatations of steering arm length and lateral position of the outer tie rod point while keeping the rack in a fixed location. Look at the plots and see how the different options change your ackerman curves qualitatively. Based on that you can decide how you want to make your geometry adjustable. Of course, you should select your sweep/permutation ranges carefully so that the tie rod doesn't interfere with your wheel.

[Sergi]

Thanks for your recommendations, actually I was already making plots of the ackermann curves for different positions of the rack, the tie rod, etc.

I thought of making the rack postion adjustable, but as The_Man commented, that would change the static toe.

But I'm very interested in Jersey Tom's idea of having multiple pick up point on the upright. How do you manage to do this? Do you have any photo? I can't realise an easy way of doing it!

If you have any photos of an adjustable steering, I would appreciate very much if you could send them to me.
My email is sergisebas@gmail.com

Thanks!

Sergi

[Jersey Tom]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Sergi:
But I'm very interested in Jersey Tom's idea of having multiple pick up point on the upright. How do you manage to do this? Do you have any photo? I can't realise an easy way of doing it! </div></BLOCKQUOTE>

Where you would have drilled one hole... drill a few? 3 maybe?

[The_Man]

http://www.pbase.com/racingmaniac/image/114782184/original.jpg

3 holes choice of 3 pick-up points.

[Sergi]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> Where you would have drilled one hole... drill a few? 3 maybe? </div></BLOCKQUOTE>

Ok, I thought you meant a something like a groove or slot (I don't know if this word is correct), but then you would have the problem of adjusting both right and left pick up points equally.

Actually I thought of designing it with some "spacers" that will move that pick up point a few millimeters (the thickness of the spacer) towards the car in the Y direction.

What do you think abaout it?

[The_Man]

That should also do, however that may be a heavier option.Though, it'll allow you vary the steering rod continuously rather than to discrete holes.

[Sergi]

Ok, thank you all for the advices!

Cheers,

Sergi