rack position vs ackerman

[Brian Evans]

Can someone direct me to an on-line discussion of how steering rack position influences ackerman?

Thanks, Brian

[fade]

enjoy (http://aleph0.clarku.edu/%7Edjoyce/java/trig/)

[Marshall Grice]

damn! I fell for the crappy link.

try THIS (http://fsae.com/eve/forums/a/tpc/f/125607348/m/72010810921?r=72010810921#72010810921) one.

BTW the FIND feature works very well...

[kwancho]

Uhm, I'm pretty sure that article only discusses the steer-axis, aka position of UBJ and LBJ. Brian, I assume you mean fore and aft in the car. First, I advise making some sketches in your solid modeling program and see where you get.
Then, read this:
http://fsae.com/eve/forums/a/tpc/f/125607348/m/19610317231/p/2
Basically, the important parameter is the angle between the tierod and the steering arm. 90deg. is parallel steer. More acute = more ackermann. But, as you can read, I had a tough time making geometry to my liking.

[Jersey Tom]

You will probably find its much easier to adjust Ackermann via the steering pickup on the upright than it is with rack location.

[Brian Evans]

thank you, Alex, I'm now embarassed that I didn't come up with that simple explanation myself. I have a car with parallel steer and I am looking at moving the front hubs forward 4" to extend the wheel base. I need to decide if I leave the steering rack where it is, rear of the axle axis, or move it in front of the axle axis and make this a front steer car (with parallel steering this option isn't too hard). While I don't particularly mind not having any Ackerman, I don't think that negative ackerman is a good choice, so I will look at placing the rack to optimise positive ackerman.

It's a 1977 Lola T440, BTW. You guys play about with engineering new cars, I play about with engineering old cars. What the hey, I'm old too...

Thanks, Brian

[Brian Evans]

BTW, I assume by "solid modelling program" you mean a pad of graph paper and a pencil, because that's what I used...

cheers, Brian

[kwancho]

Haha, classy. Yeah, we were actually going in the opposite direction, but we're front steer, so moving the hubs backwards also gives us less ackermann.

[J. Vinella]

Originally posted by Jersey Tom:
You will probably find its much easier to adjust Ackermann via the steering pickup on the upright than it is with rack location.

Ahem, the trouble we have here in the great northwest is we grow people like trees. Tall drivers don't like the rack hitting there legs so this forces the rack to move forward, thus making it hard to achieve parallel steer(which can be debated as "ideal"). Tom sugested the solution we use.

One could also use a lower mounted rack but then that opens up another can of worms with adding links and more slop, friction...

[Marshall Grice]

Uhm, I'm pretty sure that article only discusses the steer-axis
if you click on the steer-axis folder, yes it only talks about the steer-axis. however if you click on the toe folder, you'll find a very detailed explaniation of ackermann.

[Sathersc]

Being a team member "built like a tree" or as my team refers to it, "sasquatch", and also ultimately in charge of steering rack location, I can't find a good spot ergonomically on our car period. With that in mind, we're going low, but we're doing it far enough foreward that we shouldn't have too much, if any slop (1 link). With that in mind, I'm finding that putting it low is letting me set how responsive and with how much ackerman I want the steering to act with, without introducing any significant change in bump steer characteristics.

[Conor]

I've received permission from the guys at Race Data Power to post a copy of one of their work books that should be able to help you guys out on the this topic. Something that hasn't yet been mentioned is instant toe changes with steer angle, so you guys can play with that as you see fit in the work book. Also, the workbork I will be posting is SINGLE PLANE ANALYSIS, so you'll have to use vector projections of link lengths to better approximate a 3D analysis. Give me a day or so to put it up.