An early Mustang rear suspension is dead simple, right? Maybe on a street car, but if you want to build a reliable race car, Curt Vogt and his Cobra Automotive crew are as particular regarding the rear suspension as they are on the rest of the car.
[Who's the Boss? Cobra Automotive's Boss 302 Mustang]
Curt was quick to remind us that our goal is to build a modestly priced Group 3 car, and not a Group 5 terror. (Group 3 cars are supposed to remain closer to stock while Group 5 can get a bit radical.)
He also reminded us that we needed to get the car safely on track and then we could develop it from there. Speed will come later.
To that end, he started us with some used, heavy-duty Mustang rear springs from a company called Standen’s–part No. 42-331HD. We checked and this company is still in business. Last time we bought springs like this, they cost about $600 a pair.
Curt suggested that we start here and later move to sliding spring mounts and all the trick stuff that he does on his big-dollar cars. (Those competition-ready rear springs go for closer to a grand per pair.)
We mounted these springs using Cobra Automotive’s Aluminum Competition Front Eye Bushings: an aluminum center paired with Delrin end plates. They’re designed to eliminate deflection commonly encountered during track activities. The bushing kit retails for $92.95.
[How to make your own Delrin and Urethane bushings]
At the rear of the leaf springs, we used Cobra Automotive’s Street and High Performance Shackle Kit that combines polyurethane bushings along with beefy end plates. The intention is to keep the springs from moving back and forth when cornering, and retail is $116.19 for the pair.
We then used Cobra Automotive heavy-duty U-bolts (part No. 297-300) that sell for $54.77 and allow the use of lowering blocks under the springs. We also installed Cobra Automotive’s heavy-duty spring plates (part No. 100-5795A) that sell for $145.
We also needed to deal with the centering pin used to locate the stock-type leaf springs. The pin is a soft, weak 5/16-inch affair that’s just not up to racing.
We drilled out the stock pin with a hardened drill bit and then installed a hardened 3/8-inch hex-head screw. This setup will use the head of the faster as an alignment pin. If the head of that fastener is too large, however, you will need to machine it down to fit snugly into the leaf spring. It is very important that it is a tight fit as this pin keeps the rear end square in the car.
We then mounted Koni Classic rear shock absorbers as originally fitted to both street and race Shelbys. We will adjust the stiffness once we are out on track. (While popular in motorsports today, remote-reservoir shocks are not allowed in our Group 3 class.)
Our final trick: Replicate the original over-rider traction bars with a Cobra Automotive competition kit that uses Afco dampers hung with weld-on brackets.
We need to weld those brackets to our new rear end, so we used a wet rag to keep the axle housing cool. Curt had the struts used and sold us the build-it-yourself spring rod bracket kits for about $50.
If you are sensing a theme here, it is that the rear end of a stock Mustang is not up to the rigors of vintage racing and every bit of it has to be strengthened and prepped for racing. But now, after this work, we should be ready to head out on track.
My Datsun uses the same set up, the set up is what Nissan specified in their Competition Suspension Manual. Trevor Harris was the one who did the specifications.
Makes sense that wildly different leaf spring cars would end up with the same set up.
I'm also willing to bet that the car handles well enough in relation to the much fancier set ups that it won't spoil the fun of driving the car.
Our final trick: Replicate the original over-rider traction bars with a Cobra Automotive competition kit that uses Afco dampers hung with weld-on brackets.
I understand that this style of torque link can help manage shock loads, but is that the only advantage here or are there other benefits that I'm unaware of? A traditional solid bar seems like it'd be cheaper/simpler/lighter so I'm guessing there's a good reason to go this route. Does the extra travel help to avoid suspension bind or some other issue a solid bar might exhibit?
Keep up the good work, I'm digging the old tech!
Interesting, the setup looks a lot like a FOX quad shock to prevent axle windup. That's high tech, modern stuff. LOL.
The original Shelby pieces were sort of a poor man's 4 link in function. I would imagine that the flexible member allows a better range of motion and prevents binding.
Just one nit to pick, that's not a hex head, it's a socket head cap screw.........
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