Bump Steer

[Dbotelho]

I need to purchase a new power rack for my project car.  I am wondering if anyone knows the equation for measuring the mounting points on upper and lower arms  to figure out the correct width of spread on the rack any info would help thx.

 

[Mike E]

I'm not sure there is one equation as an answer for this. There are several factors involved in the proper placement of the inner and outer tie rod ends as well as the height of the rack and the spread between the inner joints. I'm a little redneck in the engineering department and I just go with trial and error until I get the bump as close to zero as I can. Some will argue it doesn't have to be that close, but if you take the time why not take a little more to make it as close as possible to zero. 

What is the spread on the rack you are replacing?

Does your car have bump steer now that you are trying to correct?

Is it a true "A-arm" car or a "Play-arm" (radius arm) car? It makes a difference on the angles of pivot between the arms.  

 

[L.R.S.]

Trial and error, repeat. When you get it close to where you want it you can use toe plates to fine tune. Tedious process but absolutely worth it.

 

[Dbotelho]

Thanks for the insight. I like the toe plate idea and cycle the suspension.
my car is a desert dynamics chassis the a arm configuration is not a traditional   A arm style 

The mounting point for the rear of the arms are further back on the chassis. 
 I’m not sure what you would call that style. 
 
As far as I know it had either an 11” spread rack or possibly a 15” wide spread rack in it before when it was assembled.   
 

 

[Rockwood]

You’ll get into the ballpark by putting a string between the upper/lower mount on the chassis, and the same on the spindle, then measure the offset from the string to the tie rod mount on the spindle to get the offset on the rack. Measure between the strings on the chassis, add/subtract the offset, and you’ll have a start. After that, it’s a matter of moving the rack around to get it right. 

Toe obviously has to be zero before starting.