Below is an excerpt from a thread on The Cobra Forum in which Doug Arnao introduced his Bilstein shock package for the Cobra.
I want to add some comments about the suspension modifications that Doug recommended, and the results.
First of all, a thousand thank-yous to Doug Arnao, who has been tremendously helpful to Nate and I during this project. Doug has been consistently informative, helpful, and supportive as I've struggled to understand new concepts and to sort out the practical details of our suspension mods.
Also, thanks to James Creasy for his kind compliments about my driving! I'm afraid he vastly overstates my level of skill, but I am working hard at it.
Also, thanks to others who shared their knowledge with me during my quest for information, including Jesper Ingerslev, Jim Schenck, Jim Riddell, Gordon Levy, James Creasy, Wayne Presley, David Borden, and the tech support people at Pro Shocks and Speedway Engineering.
This has been a large project, because we've essentially re-engineered the car's handling. We've changed just about everything short of altering the suspension geometery, including new springs, shocks, front upper and lower control arms, a bump steer kit, Flaming River steering rack, and a custom anti-roll bar installation. The only "stock" parts left are the front and rear spindles, and the FFR IRS control arms.
We did the project in a rather piecemeal fashion, out of necessity rather than by choice. Money and time constraints dictated that we do things one step at a time. Therefore, we got a fairly good idea of the effects of each change.
Now that we're near the end of the project, I feel pretty strongly that it's important to incorporate the entire package. We did get a very significant improvement just by going to 800 lb front springs, and getting rid of the bump steer also made a noticeable improvement. The hefty Bilstein shocks dramatically changed the way the car behaves - entirely for the better, in my opinion. And the anti-roll bars are a crucial upgrade, because they help to control camber, they keep the car from acquring too much rolling momentum during transitions, and they allow the car to be balanced properly.
When we started, the car - built to FFR's specs for an IRS car - was extremely soft in both pitch and roll, particularly at the front. Getting on the brakes caused the nose to drop sharply, and getting on the power caused it to lunge for the sky. Turning the wheel caused it to roll over dramatically, and wallow, too, since it was very under-damped. (To understand why this was happening, read my Motion Ratio page.)
To put things in perspective, last winter I was driving around over all the new frost heaves in my (bone stock) Ford Probe GT, and as I hammered over the bumps, I thought, "these are going to be really nasty in the Cobra". But when I actually drove over the same roads in the Cobra a few weeks later, I didn't even notice the bumps! The Cobra's suspension was far softer than the Probe's.
Then we took the Cobra to Loudon in April for a Thursday test day. It was our first time there with race tires, and we found that the rear suspension was bottoming severely in turns 4 and 10. There was actually a dent in the 3/4" square frame tube where the upper control arm had been slamming into it.
That started the project. After talking to a number of people who race and autocross FFR Cobras, I concluded that stiffer springs, shocks, and anti-roll bars were needed. But it was only after I talked to Doug that everything gelled, and I became confident that we had enough information to proceed.
Tire tempuratures told an interesting tale. When Doug first saw some temps we'd taken with the stock setup, he immediately identified several problems. The rear temps were over 20 degrees hotter than the fronts, and the inside edges were much cooler than the outside edges, especially at the front. And all the temps were much too low. Doug said, "because of the setup, the driver doesn't have enough confidence to take the car to the limits of the tires."
Because of the soft rubber bushings in the lower control arms, and the extremely soft front springs, front camber was going to hell. As a result, the car had severe understeer on corner exit - but in corner entry and midcorner, due to in appropriate wheel rates and roll couple, it had extreme oversteer. Not a happy combination!
Through the course of the summer, as we made our upgrades step by step, the car felt better and better with each new change. Our lap times came down, too, and we found ourselves consistently bringing home trophies, from COMSCC events as well as the New England Replica Challenge and an EMRA time trial at Lime Rock. Perhaps the most dramatic improvement was a 1:21 at Loudon on the chicane-chicane configuration in practice for the COM event in August. This time was about six seconds better than our previous best.
With the total package, the car is transformed. No longer a Cadillac in Cobra clothing, it is now quite stable in transitions. No more bobbing and wallowing. It's stable in midcorner, and the corner exit understeer is gone. It responds well to anti-roll bar changes. We made the rear anti-roll bar adjustable, as Doug recommended, and when we wanted to dial in a bit more understeer for the South Oval configuration at NHIS, moving the rear bar to a softer setting did the trick.
Essentially, the car talks to you, tells you what it's doing - and it lets you talk to it. You can make a change with the steering, throttle, or brake, and the car responds as it should, smoothly, precisely, in a way that gives you confidence that the car will always do exactly what you ask it to.
At the NERC, I gave another FFR Cobra owner a ride around Loudon. He had run his Cobra there the previous year, but didn't run this year because he wasn't comfortable with the way the car handled.
Three corners out of the pits, he was jumping up and down in his seat, shouting in my ear about how much better my car felt than his. Even though he was the passenger, he could feel what the car was doing and what I was able to do with it. When we came in after three laps, he said he was shocked at how much better it felt than his.
We still drive the car on the street. Because of the large increase in spring and shock rates, I was concerned that the car would be so harsh that it wouldn't be much fun to drive on the street any more. But this concern has proved to be unfounded. While it is noticably stiffer over bumps, it's still well within the range of acceptable street behavior. Nate feels it's more comfortable than a traditional British sports car, like an MGB. I find myself thinking about the ride comfort only when I stop to wonder about it. To me, it feels exactly right for a high performance sports car.
Both Doug and Nate feel we could go even stiffer, particularly if we were to use it only on the track. 800 lb front springs are actually quite a bit softer than what Doug calculated as optimum for the car for track use.
Anyway, thanks again to Doug, and a thousand thanks to my brother Nate, who put in so much hard work on this project, particularly fabricating the anti-roll bar mounts and other parts.
If anyone's interested in making your FFR Cobra really handle, I strongly suggest you give Doug a call!
For more information about the Bilstein spring and shock package, contact Doug Arnao at Vehicle Craft, 973-366-6577.
If you are interested in adding anti-roll bars to your car, contact Mark Reynolds at Breeze Automotive, 508-612-0393.
To better understand the relationship between spring rate, wheel rate, and their impact on ride and handling, read my Motion Ratio page.