Well, I finally got the time to TEST the XR chassis stiffness. You've heard the "rear shock bar" controversey, theories, and testomonials. Now there is data......
But first, the car. I have hard rubber (BAT) upper strut mount, poly inner control arm bush and Rapido poly control arm to sway bar, Rapido poly sway bar bush , and NEW Rapido big sway bar.
The test: Jim (sprite racer) and I disconnected my rear shock brace on one end and put a dial indicator between it and the top of the shock tower. We then applied about 200 lbs to the rear bumper (left and right) and front seats.
Data:
location weight shock tower
rear bumper 200 lbs .013"
front seat 200 lbs .004
Test: measure the deflection at the front strut tower. First we tried standing on the bumper (at the edges) , then we pulled the tower horizontally away from the other one.
The dimensions are from the strut bolt to strut bolt.
location weight shock tower
front bumper 200 lbs .017"
front bumper 400 lbs .033"
The dimensions are from fender to fender.
front bumper 200 lbs .009"
Notice that measuring strut to strut deflection is more than fender to fender. This is because the rubber in the strut tower deflects ( .008" ) This means that the rubber is as soft as the chassis !!!
We then pulled the strut towers apart.
location weight shock tower
pull strut about 75lbs .010"
Comments:
1. The rear of the car deflects with as little as 200lbs, therefore
the rear bar WORKS.
2. The front of the car deflects less at the tower, BUT even the
hard rubber deflects ALOT. We need a real front bar. (Idea Factory
is hard at work on one)
3. The rear chassis spring rate is 15,400 (200/ 0.013) lbs/inch.
If during cornering, we pick up the inside tires (1500lbs) then
the additional load on that corner of the chassis is 750 lbs which
gives 0.049" deflection.
4. The front chassis spring rate is 11,800 (200/.017) lbs/inch
.If during cornering, we pick up the inside tires (1500lbs) then
the additional load on that corner of the chassis is 750 lbs which
gives 0.064" deflection.
One final test, a Camaro Z28 (1994)
Shock tower to shock tower
location weight shock tower
front bumper 200 lbs .000"
pull strut about 75lbs .003"
The Z is much stiffer than the XR, doh.
