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Street Race Truck More... The Toolbox

Coil-Over Mounting Guide

10/8/2018
How to Determine Your Spring Rate

To get started, we’ve assembled a list of street rods from the Model T era, up to the late ’40’s. And before you start comparing coil-overs, you need to know the front and rear weight of your vehicle. Our guide is taken from an average (custom applications may vary) and the rates were determined with a driver in the car, using an automatic transmission, a stock small block Chevy engine, and normal street equipment like a spare tire and gas tank. The difference in weight between a fiberglass body and steel body is not enough to affect the overall weight.

If you have accessories installed like air conditioning or you’ve removed the fenders, be sure to add or subtract that weight accordingly. Larger vehicles like four door sedans and deliveries will of course weigh more. If you're using a big block, be sure to add about 200lbs to the overall weight.

The chart above is an example of a '32 Ford street rod, with air conditioning, a stock small block Chevy engine, an automatic transmission, and the fenders removed.

Compensate for Mounted Angle

Vertically mounted shocks will control suspension movement, but on a street rod, most shock manufacturers recommend mounting shocks at a slight angle (viewed from the front or rear). This increases handling by limiting side to side movement, which will decrease body roll during cornering. On most street rod applications a mounted angle of 15-25 degrees is preferable for great handling.

However, a spring rated at 200lbs will offer 200lbs per inch if it is mounted completely vertically. Which means the effective spring rate will decrease as the mounted angle increases. To compensate for the loss, use the chart below and take the mounted angle divided by the correction factor.

Example:

  • If the desired spring rate is 300lbs per inch
  • Take the mounted angle, in this case 20 degrees, and divide it by the correction factor of .88
  • 300 ÷ .88 = 340.9
  • The desired rate becomes 340.9lbs. For this example, moving to a 350 rate spring would be preferable
Shock Mounted Length

To get the most accurate measurement for a replacement shock, you can measure the compressed and extended length of the existing shock. Then determine the desired mounted length. To do this, set the vehicle to the desired ride height and measure center to center between the upper and lower shock mounts.

On a street rod, you should allow for about 60% of the shock rod to be showing. This allows for 60% of the stroke to absorb road vibrations and bumps and the other 40% for rebound.

For more information, check out this article on Picking the Right Shocks for Your Projects

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