ackermann percentage

[fora]

hi
i wanted to know how do the teams decide on the percentage of ackermann they require for their steering system and what could be the reasonable value of ackermann percentage

looking forward for your suggestions
thanks

[fora]

hi
i wanted to know how do the teams decide on the percentage of ackermann they require for their steering system and what could be the reasonable value of ackermann percentage

looking forward for your suggestions
thanks

[ben]

Having spoken to a number of race engineers, there are differing opinions on how to determine it. Claude Rouelle says that you should look at the trend in slip angle for peak lateral force.

In this case if the slip angle for peak lateral force increases with load you need anti-ackermann because the more heavily loaded outer wheel will need more steer angle to reach its peak and vice versa if the slip angle for peak lateral force decreases with load.

The other view is that pro-ackermann increases initial yaw rate in the same way as toe-out. Pro-ackermann is often referred to as dynamic toe-out. Toe out increases initial yaw rate because if you have half a degree of toe out (for example) then if you steer the wheels by half a degree the outside wheel has zero net steer angle whilst the inner wheel has 1 degree. At this point you have a larger longitudinal force on the inside wheel giving you a yaw moment that wants to turn the car in.

Having built one car with fairly parallel steer and one with pro-ackermann I tend to think that the initial turn-in benefit due to ackermann is more important for FSAE than running the tyre at its optimum slip angle. If I was doing NASCAR I'd probably reverse that priority order.

I've spoken to a number of race engineers who have had completely opposite preferences on pro or anti ackermann depending on the driver's feel. With this is mind you ain't going to find an optimum in a vehicle simulation package. Especially if you haven't got a track validated tyre model.

If I was doing an FSAE car tomorrow (even if I had tyre data) I'd probably design as much pro-ackermann as I could - I suspect packaging will impose an upper limit. Then I'd make steering arms up to allow this full level of ackermann plus half this value, parallel steer and anti-ackermann of similar magnitude to the medium level of pro-ackermann and track test all four.

Comparing yaw response and lateral force levels with driver comments should quickly determine which level is the best.

At this point your ADAMS guy will tell you that the anti-ackermann should be best despite the track test saying not and you can lock him is the workshop with the driver and let them fight it out :-)

Ben

[Brian Evans]

my view is that if ackermann is within a reasonable envelope it should be viewed not so much as a design specific but more as a tuning tool after the car is in test. Pick something, design to it, design in a way to change it plus and minus, and use it along with other variables to see what the car wants to make it fast. Since so many cars have been successful with such different ideas on ackermann, a reasonable conclusion might be that it really doesn't matter all that much...or that all the other design choices in the front suspension end up requiring different ackermann settings.

On my personal cars I like some positive ackermann because it makes the car easier to push around the paddock and the shop. Everybody's goals are prioritized differently... http://fsae.com/groupee_common/emoticons/icon_smile.gif

Brian