When I was on my universities formula SAE team, our suspension guy was trying his hardest to optimize the steering of the car and was playing with the concept of reverse ackerman. I don’t recall all the specifics, but he gave the car quite a bit which tends to be better for higher speeds and high load transfer. This worked quite well; but of course the trade off was low speed maneuverability. This became apparent on the car’s first test drive which was down a narrow strip of concrete in front of our engineering building. Previous years cars were able to turn around on this strip, but the ’07 car couldn’t make it!
It is very difficult to set up good reverse ackerman geometry (RA) and virtually impossible to make it so that it works over a broad range of speeds and cornering forces. Formula One teams have it easy since they can swap different racks in and out and have multiple tie rod pickups on the upright. I can bet you when they race Monaco, their RA tuning is different than any other track; and that’s one of the reasons they have a whole different rack just for that track.
The reason why it’s difficult to set up is because its tire dependent and there’s quite a few variables. You have to know the slip curve of the tire at a given load (you can read a little about that in my next post Its all about grip, catch my drift?), then calculate when your tires are going to see that load. So, the diameter of a turn and the speed of the vehicle are going to be major factors in load transfer. The problem is, a high speed turn that has a large radius will have a large amount of load transfer with very little steering input, a low speed but tight radius corner may or may not transfer the same amount of load and require a lot more steering angle. High loads require more RA, and if your highest load is found in a high speed turn and your RA is optimized for that, your car may not even make it through the hairpin. To optimize the steering for both, you would need some crazy geometry that would decrease the amount of RA on tighter corners where there is a lot of steering input; the exact opposite of what normally happens.
I guess it would be possible to set up your geometry so that when the steering wheel is cranked all the way, you could make the inside wheel turn more, thus decreasing the amount of RA. I would also imagine that something like this would feel very unnatural for the driver and produce a lot of tire scrub.
Anyway, we showed up at competition with a car with a lot of RA and a course that was tight. I had never been to the F-SAE competition before that time and I was told by my teammates that year’s layout was the fastest they had seen. But to me, it looked like a slow auto-x course. I don’t know how things have changed since then, but I think it’s always safe to assume that the official F-SAE competition in this country will never have high speed sweepers or allow speeds that would put students at risk; they will probably always resemble a cone-course auto-x. Needless to say, it was a little tougher to get our car around the really tight parts of the track with all that reverse ackerman; but even so, we still finished the enduro and did very well.
4 Comments to "Easy on the reverse ackerman!"
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Will you please show the reverse ackerman geometry?
what should be the %ackermann for this geometry in static?
Even if I knew the percentage our suspension guy used (which was most likely wrong) it would not work on anything other than a FSAE type car with the Hoosier tires we were running at the time. Please refer to my article ” Its all about grip, catch my drift?” for more insight into calculating how much RA is needed for an application. You will need to have a reference of your tires slip curve and also calculate the load for the inside tire.
Hey! Thanks for your so knowledgeable exlpaination, you have given. Will you please tell me some formulae to calculate turning radius, inner and outer angles for reverse Ackerman mechanism. Pls do the kind. Thank You.
will it be a good idea to use RA in my BAJA SAE ATV? If yes, how do i design it?