Adjustable Steering?

[Keysmer]

I'm analyzing the possibility of making an adjustable steering system in which I could choose, by testing, the amount of anti-ackermann for each situation (skid pad for example), and/or adjust the steering ratio.
We don't have any tire data, so if we design a single configuration system, we'll be far from optimun.
The system wouldn't be so complicated I think, and I don't see strong disadvantage.
I've found a lot of posts in this forum about adjustable ackermann. It seems that the system is very used, and I'm almost sure it would be a great help on the car set up.
It's really necesary? How much steering adjustments can help the vehicle performance? I would like to know your recomendations, thanks.
Keysmer Damo

[Keysmer]

I'm analyzing the possibility of making an adjustable steering system in which I could choose, by testing, the amount of anti-ackermann for each situation (skid pad for example), and/or adjust the steering ratio.
We don't have any tire data, so if we design a single configuration system, we'll be far from optimun.
The system wouldn't be so complicated I think, and I don't see strong disadvantage.
I've found a lot of posts in this forum about adjustable ackermann. It seems that the system is very used, and I'm almost sure it would be a great help on the car set up.
It's really necesary? How much steering adjustments can help the vehicle performance? I would like to know your recomendations, thanks.
Keysmer Damo

[Cement Legs]

I think most people would tell you that once you find the right setting considering all other design specs set in stone you are probably not going to change things very much. Having said that, we are nowhere near finding the best steering setup for our vehicle because we are still working on finishing our first car. With that in mind, we fabricated our upright with a removable bracket for the top a arm and steering linkage to connect to. All we have to do is make a new bracket for each side and we have a new setup. I would think that would mean 30 minutes on the computer and 2 hours on a mill. We decided to go with paralell steering to start but we are definitely going to play around with both regular and reverse ackerman.

[D-Train]

I believe you can determine what ackerman you need to run (regular/reverse) from your tire data. Look at a Lateral Force vs Slip Angle plot, with multiple vertical (normal) load lines. Each line will have a maximum somewhere where you can get the highest lateral force (i.e sideways grip).

If the maximum point moves to the RIGHT as vertical load increases, you'll want the inside wheel to turn in LESS (regular ackerman) as maximum grip for the unloaded (inside) tyre occurs at a smaller slip angle; if the maximum point moves to the LEFT with increased vertical load, the inside wheel should turn in MORE (reverse ackerman).

[ben]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Dwayne:
If the maximum point moves to the RIGHT as vertical load increases, you'll want the inside wheel to turn in LESS (regular ackerman) as maximum grip for the unloaded (inside) tyre occurs at a smaller slip angle; if the maximum point moves to the LEFT with increased vertical load, the inside wheel should turn in MORE (reverse ackerman). </div></BLOCKQUOTE>

At the risk of commiting heresey at the alter of Optimum G http://fsae.com/groupee_common/emoticons/icon_smile.gif I think this argument oversimplifies things. In the case IRL cars or anything on a big oval I think the anti-ackermann argument has merit, but with the tight corners and lower speeds we see I think the yaw moment induced by the difference in longitudinal forces with ackermann geometry would be of greater benefit even if your tyre curves suggested anti-ackermann.

Ben

[Charlie]

I agree with Ben's comments, but besides, it was stated he didn't have tire data.

As for testing different ackerman settings, it's definitely something I would recommend, but you don't need fancy adjustable steering for simple tests, you can just do it with static toe! You'll have a limited range for full track testing as you don't want to scrub your tires off, but for skidpad testing it's virtually unlimited.

[D J Yates]

I was discusing ackermann with one of the judges (might have been Alan Staniforth, although i didn't realise at the time) during our design presenation at Formula Student this year. He suggested that our anti-ackermann would give us some steering problems, and it did. Manouvering around the padock is very difficult with the inside wheel fighting against the outside. The main problem with our design was that amount of anti-ackermann increased with steering angle, so although a small amount of anti-ackermann many have benifited us in high speed corners, in the slow speed tight corners where positive-ackermann would be usedful, our increasing anti-ackermann caused problems. To summerise, how the ackermann changes with steering angle is more important than the actual quantity. If you focus on this but make the ackermann adjustable you should be onto a winner.

As for the amount of ackermann required, Z made a good point in another ackermann related thread. Think about the tyre curves, but don't forget about the kinematic steer angle required for the corner. I.e. Define the ackermann for a tight corner assuming kinematic steer angles and no slip angles. Then think about load transfer, slip angles and the tyre forces, and modify the ackermann accordingly.

[Buckingham]

Charlie had it right. Test with static toe, but be mindful of how linear/non-linear your ackerman is. Also, bump steer your car for your entire range of toe adjustment before you do any steering tests, otherwise you could end with poor results.