65 F-40 spring free length

[Jerry G.]

Anyone know the spring rate and free length( no load) of the heavy duty F-40 front spring fora 65.

[Joe L.]

Re: 65 F-40 spring free length

Anyone know the spring rate and free length( no load) of the heavy duty F-40 front spring fora 65.

Jerry------


I measure the free length at 12" but GM says that it was 10-27/32". Rated at 2344 lbs.

[Michael H.]

[Jerry G.]

Re: 65 F-40 spring free length

Thank you Mike. I'm sorting out the front end geometry on the race car this winter. When i went to the wet sump set up I had to raise the nose to get clearance. As you know, one thing leads to another. I'm trying to put softer springs in the front than what was in there (800#/inch) but I have an oversteer going on which will be agravated by going softer in front. So i'm stepping back and looking at roll centers and what i've done to the geometry over the years and how to balance the car a little better. I'm now trying to figure out what chevy engineers had in mind when they designed the performance suspension setup. I try not to underestimate the work they did 50 years ago. They were smart and had resources to get it right. any thoughs or insight is appreciated. Jerry

[Duke W.]

Re: 65 F-40 spring free length

Back then the philosophy was "spring it like a go-kart". That's changed. My recommendation would be to use base springs along with the 1 1/8" and 7/16" bars from the FE7 package as a starting point, but at least one adjustable bar will allow you to tune for the specific track. The target should be to limit roll to the range to 2-3 deg./g with proper balance.

Another change should be to install the C3 rear strut rod bracket. This will lower the rear roll center a couple of inches, which will reduce camber and toe change with suspension movement. (The C2's high roll centers were a good idea in the narrow tire era. This yields good roll stiffness contribution from the springs, but causes large camber and toe changes, which don't work well with wide tires. Modern cars have roll centers near ground level to minimize track and toe changes and achieve target total roll stiffness with larger anti-roll bars. Anti-roll bar contribution to roll stiffness varies with the fourth power of bar diameter, so a 1/16" change will make a noticeable difference.)

Front and rear camber at ride height should be set in the range of negative one to two degrees on both ends, with at least two degrees positive caster.

The most critical component is shocks. They should be double adjustable, if available, or at least rebound adjustable.

A few years ago I read a Bob Riley interview, and he said: "Ninety percent of setting up a race car is shocks."

Give us more information about your current springs, bars, and shocks, or if you do not want to reveal details, contact me, privately.

Duke