Chassis Building 101. Ride Height

[RobNoLimit]

For most builds, the front suspension is purchased or re-used, so the shock length is somewhat set. The shock mounting for the rear suspension usually has a bit more freedom. I have two general shock rules that I try to follow.

1) Try to use the longest shock possible (within reason) - but never let the shock be the limiting factor of suspension travel (this is more important in compression, we sometimes allow the shock to be the limiter in 'droop')

2) Try to keep the shock at appx 50% travel when the suspension is at RH. (+/- 5% is well within reason)

I have seen many front suspensions that use short - 3 1/2" stroke - coil-overs, and the coil-over gets the blame for the harsh ride. Here's the quick math. At RH, a 3 1/2" stroke shock has 1.75" of compression travel. It also uses a short spring, usually 7" or so. If there is 850 lbs of the left front corner, and the LCA has a .7 to 1 motion ratio, then there is appx 1200 lbs of force compressing the coil-over at RH. If the spring was pre-loaded onto the shock 1/4", or 150 lbs., then you need a 600 lb spring to hold the car up. And, it rides pretty stiff. If you had a 5" stroke shock with a 12" spring, with 150 lbs pre-load, you would use a 450 spring. These concepts are for comfort and ride quality, But, as we lean more towards a 'soft spring - big bar' set up for performance, this is a winner in for both comfort and performance.



Stroke Compressed Extended Suggested Spring Length Price
2.9" 8.725" 11.625" 8" $250 [Purchase]
3.6 9.425 13.025 8 $250 [Purchase]
4.1 10.125 14.225 10 $250 [Purchase]
5.2 11.225 16.425 12 $250 [Purchase]
6.3 12.325 18.625 14 $250 [Purchase]
6.9 13.125 20.025 14 $250 [Purchase]

I copied this from ridetech's web site. Most shock companies will list the open and closed lengths of their shocks. I like to use their 5.2" stroke coil-over on our WideRide IFS fronts, and the 5.2" or 6.3" in the rear on most trucks. Looking at the 5.2" shock, it has a closed length of 11.225", and an open length of 16.425". The suggested RH for this shock would be 13.75 to 14" (13.825" is 1/2 of the stroke.)

Up front, if you were really going for it, starting with tire height and spindle CL, you could set the RC and crossmember height, then build the rail shape to match the crossmember up front while keeping the body at it's desired RH. Then, choose the desired shock, and fab the upper coil-over mount to set the shock in it's spec RH range.

?? does that make sense?

[Dr G]

Rob - I have read a few times now where you state not to use the shock to limit suspension travel, especially in bump/compression. Yet that is exactly how most road race cars are set up. With bump stops to tune the end of travel. I cannot speak for dirt ovals, Nascar or off road, they use different methods. Can you explain your thoughts on travel limitation? Gustave