steering angle/Ackerman/Unisteer quiz

[pav8427]

I just found out that I can get a Unisteer rack(Mustang power unit)in custom lengths to solve my problem of finding a unit to match my lower a-arm pivot points and reducing bump steer.
My question is, is there a good all around steering angle to 'shoot' for to result in a decent driving street car?
Also,from previous posts concerning Ackerman, I am using B-body spindles that have the outer tie rod mounts pretty much in line with the lower ball joints. Is there a good recommendation on how to set this up for a good useable amount of Ackerman.
I should be able to set up my rack 'in line' with the outer tie
rods.(maybe just forward, very little)

Any help would be greatly appreciated.

Thanks, Doug

[sinned]

Using your lower control pivots to determine steering rack length will have zero affect on reducing bump steer when used in the context you plan. You need to make accurate measurements of the entire front suspension geometry and plot them into a graph or program to determine where the steering arm pivots need to be. It is a huge and common misconception that as long as the steering arms match the LCA there will be no bump steer...that is incorrect. Steering arm length, height and even caster angle all come into play when determining bump steer.

[pav8427]

I understand that,What I meant was that I can get a rack built to what I need for length between inner pivot points,(at rack centerline) instead of trying to find a factory rack thats 'close'
At this point just with general Stanley measurements the rack centerline will be real close to the lower pivot points,maybe 1/4-1/2 inch lower and if I adjust my a-arm mount setup, I should be able to get the rack centerline at or a little forward of outer tie rod pivot points.
From futher reading, I figured that I probably can't change the Ackerman that I will end up with w/o changing the outer tie rod pivot point. What I have is what I get.
Going back, what I really should have asked is just the steering angle part of my quiz.
The reason I ask this is so I can make sure I have enough travel on the rack I intend to use.

Thanks, Doug

[Chevy350]

I won't go into the bump-steer issue, as Dennis has that covered, but I can mention Ackermann a little:

You said that you can position the rack at the centerline between the outer pivot points of the tie-rods, or slightly ahead/in front? What you get then is close to 0% ackermann or even anti-ackermann... Let me show you.

Here, I snapped a picture of a rolled-over '77 Caprice wagon:
Caprice LCAs (http://www.v8power.net/christian/images/caprice%20lca.jpg)
You can clearly see what you mean with the outer tie-rod ends being inline with the LBJs, this would give ~0% Ackermann IF the position of the centerlink and therefore the angles of the tie-rods hadn't been as you see in this picture. As the centerlink is pushed back into the crossmember, you see the angles between the steering arms and the tie-rods become LESS than 90degrees - this is what creates some ackermann here. (I thought the best example of the b-spindle ackermann, would be in it's own home, eventhough the 2.gen F-body and 3.gen A-body uses the same spindles)

My conclusion is that you want your rack mounted further back than planned, unless you want 0% ackermann for your intended usage. (I doubt it, unless you're building a "tall and narrow-tire" racer with 40s/50s technology)

[pav8427]

I see what you mean.
Heres the problem I have, my lower a-arms are symetrical and the way I have my front x-member designed I don't think I can move the rack back.(not having all parts in hand)
I have a handle on the bump steer issue and should be able to bring that down to near zero(I hope)
When Stanley was brought out again I found out that the outer tie rod ends are actually around 3/8" inboard/side of the lower ball joints.
Not being a suspension guru,is that going to help/hurt the Ackerman issue at all?
Is there a certain difference in steering angle/side a person should attempt to get to bring Ackerman into a real world usable amount?

Thanks, Doug

[High Plains Mopars]

On a stock chassis you may only be able to get 2-3 degrees of ackerman. If you can squeak this out of your layout, then this is not a bad amount for all around driving. If you going to be doing autocross stuff with lots of tight turns, then 3-5 would be better, if you can get it.

Now if your real familiar with your bump characteristics, you can design you ackerman into your bump measurements so that the more you roll over on the suspension, the more ackerman you generate. But I wouldn't recomend this until you've got a good solid baseline dialed in and you are looking for experiments to make minor improvements.