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Discussion Starter · #1 ·
Guys I could use your help,

I am building a scratch built kit car. It is a Mercedes Gullwing replica however its design is very similar to a Factory Five Cobra. I have a Heidt Mustang II front crossover with their tube control arms, front strut eliminated, Mustang II front spindles. I am running a Desoto hemi. Figure that it weighs in the same neighborhood of a cast iron Chevy Big block. Heidt offers all sorts of choices for coilover shocks. Was wondering if someone could make a recommendation for both shock and spring?

The rear has a Thunderbird SC IRS, Factory Five control arms through QA1 Heim joints. I bought some used shocks many years ago to help me mock things up but it is time to get serious and I could use some help. I havn't finalized the shock tower yet ( well I have but can still cut it and move it)
Am looking for shocks that will fit this set-up. Can't help notice how the threaded lower section is very close as it dives below LCAs. I am have all sorts of range in both full bump and full droop (without the shocks on) so was wondering realistically how much range should I shoot for? What would be a good length shock to shoot for? with how much subtracted for bump stop? and what spring rate?

thanks in advance

469 Posts
There's some good info available at the Ride Tech website. If the Heidt's IFS that you have takes the short 3.6" stroke shock that uses 8" springs, it may not provide as good a ride as a setup with the next longer shock that uses 10" springs. If you go into the ride tech spring rate calculator, for a given corner weight, the shorter shock will require springs that are about 100 lb/in stiffer.

StreetRods - RideTech.com - Suspension Specialist - Online Store

Coil-Overs - RideTech.com - Suspension Specialist - Online Store

The ride tech spring rate calculator can be used for both IFS and IRS, as long as you can measure the motion ratio for your setup.

Spring Rate Calculator « Ridetech ? Tech

The Mustang IFS is usually setup so the upper control arms are close to horizontal at ride height. My '37 Oze has control arms are made by ride tech, and function like a Heidt's super ride II. I ended up cutting off my upper spring mounts and fabricating new mounts, so the supplied 4.1 stroke shocks with 10" springs would fit properly and allow the shock to have it's full rebound stroke.

To figure all of this out, I used adjustable coil-over mock-up bars from Speedway. These bars had to be modified to go as short as I needed, but they were valuable during the build because they allow the suspension to function normally, regardless of the weight of the car.

Coil-Over Shock Mock-up Tool

These mock-up bars can also be used for the IRS.

Inexpensive, but nonadjustable mock-up bars can also be valuable once a specific shock has been chosen. The ideal length for normal ride height is 50-60% of the shock's travel in compression. I made bars of this length to use during most of the build process on my car and even used them while I did my own front-end alignment. Bars can be made from 1" square steel tubing, with holes of the proper size, drilled with the proper spacing.

You will find that most brands of coil-over shocks sell models with very similar nominal length and stroke. For the best ride, get as much stroke and spring length as possible.
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