Posted on: March 30, 2009

by Jim Morrison

After rebuilding my front suspension, I installed a set of the CAT Club 335 pound competition front springs. The front-end ride height was greatly increased from stock and looked funny, too. I decided to cut off the springs to lower the car by 55mm to my desired height (slightly lower than the stock height).

Now the problem was “How much to cut off the springs?”

I came up with two methods (one courtesy of Jim Burruss here in Huntsville and one of my own doing). Fortunately, they matched very closely. Here they are:

Jim Burruss’s way was to measure the distance between a point on the upper portion of the cross member and the A-arm at points directly forward of the centerline of the spring (i.e. points that are the same distance from the lower A-arm pivot point as the center of the spring). This measurement is made at the ride height with the new springs installed (in my case this was 103mm) and again with the springs removed using a floor jack to adjust the ride height to the desired level (resulting in an 81mm measurement in my case).

The actual points you use are not relevant, only the difference in the two measurements. Don’t forget to have the spring insulators installed for these measurements. The difference in the two measurements was 22mm. This is the amount to lower the height of the springs.

My method is a little more complicated. I measured the distance from the A-arm pivot to the center of the spring along the A-arm (185mm) and the distance from the A-arm pivot to the hub face (that the wheel fits against), which was 483mm.

To calculate the amount to lower the spring height, I then multiplied the amount I wanted to lower the ride height (55mm) by the ratio of the two measurements above resulting in: 55mm X 185mm / 483mm = 21mm.

Both techniques assume that a change in the freestanding spring height results in the same change in the compressed spring height. While not strictly true, the relationships very close to linear for the relatively small changes needed.

To mark the spring for cutting, set the spring vertically on a flat surface and measure the free height of the spring. Subtract from this the amount to lower the spring height calculated above, resulting in the desired spring height.

Measure up from the bottom of the spring to the desired spring height. Find the point on the top of one of the coils that corresponds to this height and mark it.

Now move up the spring one-half coil (one half turn) higher than the mark. This is the point to cut the spring. A torch or band saw should do it (I took mine to a machine shop for this). While you are there, have them bend that last half coil on top down (bending at the desired spring height mark until the top end touches the coil below it) to provide a fairly flat surface for the top of the spring.

Either procedure is not exacting, but I did get my ride height to within 4mm of my desired height. Some fine-tuning can be done by using different spring insulators.

One type (I believe this is the original style and is sold by the CAT Club) is approximately 0.6 inches high and fits on top of and outside of the coil.

The other style is like that used on a 1974 Mustang II (Ford part number D4AA-5415-A). It is about 0.25 inches thick and fits on top of and inside the coil. Replacing the Mustang II unit with the CAT unit increased my ride height about 0.5 inches to compensate for some spring sag since installation. I have even heard of some Tiger owners using the spring insulators on the top and on the bottom of the springs. While not designed to be that way, I have not heard of any problems doing this.

Editors Note: For the proper ratio of the lower arm, the distance from the A-arm pivot to the lower ball join should be used instead of to the hub face. If the spring is cut with a torch, shield the spring so that sparks cannot damage the wire surface.

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