Background
CMC used a 1983 RX-7 GSL as the donor for my Locost 7. They converted the front spindles from McPherson to double A arm suspension by cutting the spindle tube to be ~5-1/2" long from the lower ball joint to point where the tube was cut. A 1" long machined "cap" was inserted into the top of the tube. The cap has a 1/8" thick flange which rests on the top of the cut off spindle. The flange is 2" in diameter. The lower edge of the flange is welded to the spindle tube. The center of the cap is threaded to accept the 5/8"-18 thread for the studded Heim rod end which was used as the upper ball joint.
Apparently CMC modified various front McPherson spindles as a "one size fits all" since another CMC7 in town uses Toyota spindles and the dimensions are very similar to my RX-7 based unit. Both our front ends have extreme bump steer. Our toe in changed ~5/8" when the chassis was moved 5/8" up or down!
I have documented how I corrected bump steer in another article, "Correcting CMC designed steering geometry, bump steer". Another article documents how I reduced the steering wheel lock to lock turns from 3 turns to two turns, "Rack and pinion steering limiter".
These three articles are a complete description of the modifications I made to correct my suspension geometry.
To see an
enlarged view of most pictures, left click on a picture or right click
and select "View Image".
Notice that much of the Heim
rod end movement is already used in the resting
position. After the modification the Heim is not at as great an angle.
Problem
Unfortunately the mounting points of the upper and lower ball joints in conjunction with the inner mounting points of the upper and lower A arms resulted in badly designed front suspension for camber correction in dive and bounce.
An even worse problem is that by using the stock RX-7 steering arms, with a different track and wheelbase and the placement and width of the rack & pinion resulted in VERY bad bump steer and incorrect Ackermann affect.
I have a fairly simple method to correct the bump steer, Ackermann and to also speed up my steering ratio. I have documented that change in another article "Modifying the CMC designed suspension to correct bump steer".
Do you really need to have the suspension geometry correct?
The front suspension design is probably the most important and complicated part of building your own car. There are several methods you can use to have a correct suspension layout. Here are a few suggestions;
1. Buy a chassis that is known to be correct. Not low cost though.
2. Pick a donor car with truly good suspension and build your frame around the parts.
3. Design your own suspension using computer CAD software etc.
4. Copy another Locost that has correct suspension geometry.
Unless you use the exact same components, dimensions, layout and design of another car that has correct geometry you are just taking a crap shot at having your suspension ending up being truly "correct".
Truthfully, even though I could see that my bump steer and camber change was all wrong, I really didn't notice much adverse affect when driving the car around town. The only indication that bump steer was a problem is while cornering hard and hitting a bump. The car would dart slightly left or right as you hit the bump. Of course this is one classic result of bump steer. A hidden problem is that tire wear can be excessive with an incorrectly designed suspension.
You just can't/shouldn't push a Locost anywhere near it's limit on normal city streets. The more you push the limit, the more troublesome bump steer etc will become.
I decided that as long as I have put all this effort into building the car I wanted my suspension to be correct.
So how do you correct the suspension geometry?
I used the suspension program "Wishbone.exe" and a demo program called "Suspension_Analyzer v20_w98.exe" to redesign my suspension geometry. I basically redesigned the front suspension in Wishbone and then tweaked and finalized the design with Suspension Analyzer before the demo period ran out.
At first I took the normal line of thinking and found that I needed to lower the upper A arm chassis mounting points to induce negative camber change during dive (when the brakes would be applied on entering a turn) and to raise the roll center. I found that if I lowered the inner mounting points ~1" that I could obtain the desired camber and roll center change.
After looking at my chassis and considering the work involved in moving the upper A arm mounting brackets etc I went back to the suspension programs and tried raising the upper ball joint instead of lowering the inner A arm mounts. The result was the same AND it didn't involve rebuilding the mounts! I liked that idea. Maximum affect with minimum change.
