Ackermann ?

[mignum]

Do I need to set caster with the upper control arm or the lower? I am thinking that this could have an affect on my bump steer and ackermann, after more investigation I am quiet sure that I need a 19 3/4 steering rack, per Joe's recommendation.

[DLM06]

make sure you check ackerman and bump together,and the best way is mark your tires around the center, set your toe, roll your car to what travel on RF you want, re measure your toe front and back, then steer your tires at different amounts and re check, make sure to measure as low as possible to the contact patch front and back, this will tell you exactly what is happening with your front end.

I tried this and finally gave up on our '11 Rocket. The bump went all over the place when we steered from straight ahead and every expert I called including Rocket told me to just set the bump straight ahead and forget everything else. What kind of bump readings do you get when you turn the wheels from straight ahead? Do they change significantly left vs right? What do you do if the bump is fine straight ahead but changes when turned?

[MasterSbilt_Racer]

Do I need to set caster with the upper control arm or the lower? I am thinking that this could have an affect on my bump steer and ackermann, after more investigation I am quiet sure that I need a 19 3/4 steering rack, per Joe's recommendation.

Chassis manufacturer recommendations vary, but it will affect the ackerman. I think few cars have it right when straight ahead, turned left, and turned right.

[hucktyson]

Chassis manufacturer recommendations vary, but it will affect the ackerman. I think few cars have it right when straight ahead, turned left, and turned right.

With the geometry required to have correct bump steer at all 2 locations it would be impossible ..

[mignum]

So I need to be concerned with the wheels turned like I'm headed off into the corner instead of straight mostly right?

[hucktyson]

So I need to be concerned with the wheels turned like I'm headed off into the corner instead of straight mostly right?

When the car is handling their not turned very far anyway ..

[let-r-eat]

Draw a straight line through your lower control arm inner pivot and your upper control arm inner pivot. Your rack inner tie rod position should fall along that line. If your rack and pinion is too short the inner tie rod pivots will fall inside that line and the rack would likely need LOWERED in the car to make it work. That is why you shim a rack and pinon up and down, side to side to correct bump. What you are trying to accomplish is get that inner tie rod pivot in line with the lower inner and the upper control arm inner pivot.Think about this. Looking from straight ahead at the steering the pivots need to lie on the same plane. To illustrate this you would take a string and put on your lower control arm inner pivot and stretch it to your upper control arm inner pivot. Stand back and look at the inner pivot of your rack.............inside that line or outside that line with the wheels pointing forward. If your pivot is outside that line you need to shorten or inside you need to lengthen.Ask yourself this:What determines the length of rack I need? Is it the distance between the lower control arm pivots? Yes if the rack is level with the lower control arm pivot points. No, if the rack is higher. The rack must then lengthen.Here is something else that might help.How do you post pictures here:

[7uptruckracer]

Thanks Let-Er-Eat Ive always wondered what determined the rack width and I'm about to use this on a car. Really great info!

[mignum]

Yep, thanks that helps clear things up for me.

[let-r-eat]

Glad that helps you. Sometimes its easier to get a piece of graph paper and map out all the front end points. A design software is much easier, of course. You can use one square on the graph paper for one inch and get the basic suspension mapped out in front view and side view. Things like anti dive and caster etc can all be figured using mathematics. There wasn't all this computer stuff back in the day, we just used pens and papers.

[mignum]

Took the tie rod ends loose and put the inner pivots on the string line and it looks pretty close with the tie rod ends out of the way. With the tie rod ends on the rack it appears that it may be a little inside the string line which would indicate that a longer rack would be needed. But... its pretty close... Joe said that front takes a 19 3/4 rack, if I go that route it appears that it might be a little long.... but that may help me with my ackermann and bump steer problem, I am sure Joe knows a lot that I dont, they may have found that after more testing that the 19 3/4 worked better... heck I dont know!

[let-r-eat]

This is very important because that is what determines the ability to control bump steer. We can talk about the relationship between the LCA lengths and tie rod lengths, steering arm position another time.

[let-r-eat]

Took the tie rod ends loose and put the inner pivots on the string line and it looks pretty close with the tie rod ends out of the way. With the tie rod ends on the rack it appears that it may be a little inside the string line which would indicate that a longer rack would be needed. But... its pretty close... Joe said that front takes a 19 3/4 rack, if I go that route it appears that it might be a little long.... but that may help me with my ackermann and bump steer problem, I am sure Joe knows a lot that I dont, they may have found that after more testing that the 19 3/4 worked better... heck I dont know!

A 1/4 of an inch on each side may not seem like much but will make a ton of difference when looking at bump. There's a ton of difference between 19 1/4 and 19 3/4. That's a 1/4 of an inch per side and depending on suspension travel could amount to 4 or 5 times that in bump.