Moving the front shocks

[ERA2076]

I am setting the car up for open track. My car had been sitting for several years and the Spax shocks were locked up i.e. no rebound. Since I needed new shocks I decided on Penske 7500DA. These are a double adjustable coil over which allows for the body to be mounted top or bottom. I elected to mount them on top to save a little unsprung weight. By mounting them on the top it allowed me to look at a couple of other tuning modifications.

I have been reading Fred Puhns book for many years, but recently delved into Carroll Smiths stuff.

Since the FIA is such an easy car to work on and has so many fundamentals built in, e.g. Raising Rate Suspension, it is easy to apply Smiths logic to the FIA as far as tuning the suspension. Below are some calculations based on using a 350 lb front spring and moving the spring mounting locations in the front.

This first set of data represents no geometry changes and just adding the heavier spring. I did the math on the 250 and it seems it was around 166. Bob thought the track cars were built around 350 so that is where I am starting.

As the car rolls the shock angle opens.

Stock Shock Angle = 67 Degrees
Motion Ratio = .542
Spring Rate = 350 lb

Angle 67 68 69 70 71 72 73 74
Wheel Rate 174.5 175.8 177.0 178.2 179.3 180.3 181.3 182.3

By rotating the top of the shock outboard as far as we can without hitting the upper control arm in bump or droop we get the following

Shock Angle 72 Degrees with Stock Motion Ratio
Angle 72 73 74 75 76 77 78 79
Wheel Rate 180.3 181.3 182.37 183.25 184.0 184.8 185.5 186.1

By moving the bottom of the shock outboard 3/4" we gain some serious leverage.

Shock Angle 72 Degrees and move CA mounting point 3/4 outboard
Leverage = 0.651

Angle 72 73 74 75 76 77 78 79
Wheel Rate 216.5 217.7 218.9 220.0 221.0 221.9 222.8 223.6

40 lb of wheel rate - right at 24%

Mockup of moving lower mounting outboard to increase leverage. The back of spindle has had the gate reference removed and then radiused.





The top of the shocks are rotated to the intersect of the wheel house aluminum. Anymore and we risk kissing the upper control arm in droop. We will need to clearance the aluminum.



I am really getting excited about it. We had a tire on it today and it articulates so freely - it really brings a smile. I plan to run it with the 350 just to see how it is, but I may well be able to run a lighter spring.

Then again, I could be wrong