Steering

[boxsterb1]

I have searched steering extensivly and can not seem to find the answers that I am looking for. Are most teams running ackermann steering? if so, we are having problems with how to best achieve the geometrey for ackermann steering. the steering rack seems to be in the way packaging wise. we need help!!

[boxsterb1]

I have searched steering extensivly and can not seem to find the answers that I am looking for. Are most teams running ackermann steering? if so, we are having problems with how to best achieve the geometrey for ackermann steering. the steering rack seems to be in the way packaging wise. we need help!!

[Vinnie]

do you have tire data? otherwise you need to test to find out the best ackermann geometrie. theoretically you can come close to perfect ackermann (ADAMS can make a good approximation on this), but perfect ackermann at every steering angle is nearly impossible to achieve.

the more lateral force and normal force aplied on the tire, the bigger the slip angle of the tire will be and the closer you get to paralell steer. Formula 1 cars even run reverse ackermann because of this.

too much ackermann will cause the inner wheel to be dragged by the outer wheel, which will cause the tire to heat more rapidly, but excessive wear of the inside tire will be a result and it is also not a nice way to go around a corner

you can put the steering rack at 4 places: above or under the drivers legs and in front of the front axle or behind. Never go below your lower pick-up points or above your upper pick-up points, this will certainly cause bump steer! check where the driver legs will be and see where it is not possible to place the rack. than check packaging at the inner wheel, because if you're racing with a lot of ackermann, placing the rack in front of the front axle can limit your maximum turning angle of your inside wheel.


by the way: I know a lot of teams run with relatively much ackermann, because its a low speed track with a lot of turns, therefor the slip angle of the tire will not be that high.

[Vinnie]

sorry forgot to say cheers

Signed: Mr Ackermann

[Kevin Hayward]

vgoor,

Given that it is low speed the slip angles are quite high. Go to a race like the Indy 500 to see low slip angles.

I think the biggest advantage of running lots of ackermann (i.e. inside wheel turning considerably more than the outside) is to induce more drag on the inside wheel.

Increased drag on the inside wheel will create a moment that will help turn the car into a corner. The cost is increased heat generated in the tire. However given that it is often difficult to get the tires up to proper running temperatures this is also a bonus and should not result in excessive wear.

Kev

[D-Train]

Yeah, I'd agree with Kevin on this one. During all of our testing sessions last year we ran excessive ackerman (sorry, don't know the numbers) to induce the yaw moment Kevin mentioned, and on a circuit we never had problems with the inside tyre wearing faster than(or even close to as much as) the outside tyre.

As for rack packaging issues, take a look at pics from the event(s) or build pics of similar cars (monocoque or spaceframe?). You should be able to get a really good idea of what you can and can't do with your own chassis design.

[Cement Legs]

I cant really see the purpose of dragging a wheel to induce yaw. Isnt that kinda like the cart leading the horse. I'm just trying to understand why you wouldnt just design the handling you are looking for into the suspension and steering geometries.

[Vinnie]

I knew many teams run with a lot of ackermann to create that yaw moment to go around a corner, but we have never actually test which ackermann angle suits us best. thats why this year we make the ackermann adjustable, so we can test it.

so I'am curious to see what will be the result,

too bad we don't have tire data.....

[Kevin Hayward]

Cement,

I would be interested to hear your ideas. However if the goal is to create fast turn in then increasing the drag on the inside wheel is one of the ways to do it. It is also a way that helps heat up the tires.

It is worth discussing some of the ideas.

I don't think putting toe-out on the rear is a great idea, though it should improve turn in. Except it will also improve turn-in along the straights.

Load dependency of the tires (roll moment distribution) will only get you so far.

Playing around at the speed at which the front generates grip with dampers and roll-centres again only gets you so far. High front roll centres also introduce other areas.

The damper changes also affect the car all over the track. Stiff dampers to improve response can be a nightmare for overall grip.

When designing these cars to get around a turn I found I was only really interested in turn-in response. Its not too hard to change the settings of the car to get it around a steady-state turn to hard, but apart from the skidpan (which is only mostly steady state) there are not many corners in FSAE that qualify.

One thing that has always interested me is that these cars (in testing) go faster with more ackermann than the traditional way of looking at the tire data would suggest is appropriate. I reason that the drag of the inside tire is what is helping. I may be wrong, but in the end you set the car up to what works.

Cheers,

Kev

[Cement Legs]

OK, so that answers my question. Basically you are using a more inclined ackerman to increase turn in while generating more heat. While I dont have any solutions to offer it still seems inefficient. Having said that, and after thinking about it again it would almost seem like a simple form of mechanical traction control. hmmmmmm

Anyway I was just looking for reasons behind the idea that eluded me.... Thanks for the enlightenment. http://fsae.com/groupee_common/emoticons/icon_smile.gif