78 GS750 suspension upgrade help

[rwcfrank]

Hi everyone, I am building a Cooley copy from my 78 GS, I have done some research and found out that the rear shocks were angled at 54.5 degrees. I have 2 questions. Firstly the bike I have came with Progressive shocks and seemed super stiff, probably the heavy duty spring kit. Secondly, on the Cooley race bike it looks like Pops moved the lower shock mount forward on the swingarm and leaned the shocks forward at the top after bracing the arm. I can't understand why he would move the shocks away from the axle so I would love to hear theories. Secondly, does anyone know how I can calculate spring rate for the shocks? Not only does leaning the shocks call for a lighter spring rate but I have knocked about 40 pounds or more off the bike.

[shinzon1]

Most of the times the progressive springs have the rate stamped or printed on them. Unless it has worn off or it could just be in a place that is difficult to see. Can't help you with the rest though. sorry.

[Nessism]

Google search on "spring rate calculation"...first hit = http://www.engineersedge.com/spring_comp_calc_k.htm

You can use the formula from that link to calculate spring rate.

Moving the lower shock mount forward is an easy way to increase ride height in the back, and speeding up steering. Beyond this, I'm not sure all the reasoning Yoshimura employed when modifying the chassis.

[rwcfrank]

The springs are the older style with 2 parts versus the newer progressive wound single spring. The only markings are the multi color paint dots on the spring which I have had no luck deciphering. Nessism, thanks for the link, if I werent such a math dummy it might be asier to use but I can get some help with it.

[Nessism]

If your shocks use two different stacked springs, there are more calculations you need to make. For example, calculate the spring rate of each spring individually, then plug those numbers into another calculation to get the combined spring rate. Look around for the secondary formula. It shouldn't be hard to find.

BTW, that link I provided has a Compression Spring Constant (k) Calculator. Open up the link as provided and take a look. It doesn't get any easier than that.

[salty_monk]

Useful link http://www.pontiacracing.net/js_coil_spring_rate.htm

1 pound = 0.45359237 kilogram
1 inch = 25.4 millimeter
0.45359237/25.4 = 0.0178579673228346

The conversion factor is 0.0178579673228346