Tubular Control arms?

[Tom Kirkham]

I hope I can clear up some of the confusion.

The two billet chassis cars we have built have almost nothing in common with any other car. Other than bearings, the suspension is not interchangeable with anything else.

All of our 4 inch round tube frames (427, 289 hybrid) have the suspension points in the original 427 coil over suspension location. This means that if you have a complete suspension corner from a CSX3000, a CSX4000, a AC MKIV or a Kirkham and I also believe Hi-Tech, you can bolt the complete suspension corner into any of the other cars. Please note that I said complete suspension corner, as parts between the various manufactures are not completely interchangeable.

The 3 inch round tubular chassis also known as a 289 leaf spring has a very different, yet similar suspension. Our leaf spring suspension frames have the suspension pick up point in the same location as the original CSX2000 leaf spring cars.

Now a little history of the 427 suspension. According to my discussions with Pete Brock and others, as well as what has been published, Negstead (he also designed the GT40 suspension) designed the 427 Cobra suspension on Ford's computer. And if you look at the rear lower control arm you can see the similarity. The problem was AC had according to Peter had "rooms" full of parts and were not about to discard all of these parts. The Hurlocks had also already bought the 4 inch tubes cut to length to work on a 90 inch wheel base car. So Negstead's suspension was not going to work as designed. For example the trailing link suspension point would have ended up in the driver's hip pocket. Remington was sent over to help sort out the mess. That is why the 427 suspension uses several modified components of a 289 suspension. The 427 front lower control arm is a modified 289 front lower control arm. The only difference between the two is the 427 has two ears welded on to hold the shock. This sounds like a simple change but.... The 289 loads the suspension through the top of the upright due to the leaf spring and the 427 puts almost the entire suspension loads through the lower control arm. The problem is the lower ball joint. On a 289 Cobra the suspension ball cups (part of the ball joint) almost never wear out, and on an original 427 Cobra it is a very well known problem area.

Now many years later with 20/20 hindsight and lots of river having passed over the dam... There is a market for original style suspension cars. What to do about the problematic front lower ball joint? There is also another less known problem with the original ball joint. It is very small. The original A-arms have the ball joint housing very, very, and let me repeat, very close to the brake rotor. It is very common for original cars that are driven on the track to have a grove in the rotor where the A-arm (ball joint housing) rubs into the rotor when the car is cornering. This is caused by the front spindle flexing, allowing the rotor and the A-arm to contact each other. So what we have here is a double whammy. The rotor is too close and the ball joint is smaller than any off the shelf ball joint from a major manufacturer. If you make the ball joint bigger you have to make the lower control arm shorter. If you make the lower control arm shorter, you change the king pin inclination angle, scrub radius (which already is not great), etc., which forces other changes... And this is where the various manufacturers diverged on the front suspension.

[4pipes]

Tom, after reading your post I got a little concerned. I have 15,000 miles with some track time on Kirkham #185, what inspection technique should I use to check my ball joints and what would you expect the ball joint life to be?