Cobra Vibration Problem

[Richard Hudgins]

Originally Posted by Cobra #3170

I believe you are missing something, .

Indeed I had. I did not know that you had that type of half-shaft.

However, just a quick thought relative to your half-shafts.

We used to have a very similar problem with the splined shafts in the old FA (F5000) cars when Goodyear came out with the 15 inch slick. The loads on the splines were such that they would lock up (Not slide/change lenght) under acceleration and produce a very severe vibration. There were many tries at fixing this, including ball bearings in the splines to allow sliding under the loads. (The real problem was that the splines were not very good and too soft as well and would bind after a little wear.)

The fix was first Metalastic joints and then CV joints.

Maybe you have a similar condition and the splines are worn.

[Cobra #3170]

Originally Posted by Richard Hudgins

Indeed I had. I did not know that you had that type of half-shaft.

However, just a quick thought relative to your half-shafts.

We used to have a very similar problem with the splined shafts in the old FA (F5000) cars when Goodyear came out with the 15 inch slick. The loads on the splines were such that they would lock up (Not slide/change lenght) under acceleration and produce a very severe vibration. There were many tries at fixing this, including ball bearings in the splines to allow sliding under the loads. (The real problem was that the splines were not very good and too soft as well and would bind after a little wear.)

The fix was first Metalastic joints and then CV joints.

Maybe you have a similar condition and the splines are worn.

That very well could be an issue, I was going to put Kirkhams conversion using Porsche 930 joints in, but my days of running this car are coming to an end so I did not convert it. The geometry is such that they move a small distance but that could still happen. Interestingly a F5000 car that ran in the 60's used Cobra half shafts and joints, I think the driver was Ron Grable and the car was built in the bay area. He liked the design of the English Hardy Spicer joints because they had extra space in the needle bearing cup.
They looked like there was a missing bearing only there was not enough space for another needle. I guess the thought was reduced friction at high loads , I never did find out why they made them like that. I had a 1972 March that used the metalastic joints on the Hewland FT-200, it had the larger Formula 2 joints but I still tore them up on a regular basis, they worked but were a weak point in the car.

[Richard Hudgins]

This may be the car you are speaking of:

American Mk1. Circa 1971 Pranged at Donnybrook and not raced again to my knowledge.



I had a 712 and a 722.

On the 712 even with the underpowered Lotus twin cam (relative to the F2 engines), the joints were good for a weekend. (The twin cam was built by Gus Hutchinson which made it a very good twin cam compared to the English built units.)

The 722's inboard brakes used to cook the CV boots and joints so they would fail in 2 weekends.

Quite expensive cars to run both in dollars and preparation time in any case.

But great sport to drive.

[Cobra #3170]

Originally Posted by Richard Hudgins

This may be the car you are speaking of:

American Mk1. Circa 1971 Pranged at Donnybrook and not raced again to my knowledge.



I had a 712 and a 722.

On the 712 even with the underpowered Lotus twin cam (relative to the F2 engines), the joints were good for a weekend. (The twin cam was built by Gus Hutchinson which made it a very good twin cam compared to the English built units.)

The 722's inboard brakes used to cook the CV boots and joints so they would fail in 2 weekends.

Quite expensive cars to run both in dollars and preparation time in any case.

But great sport to drive.

I lied, I had a March 712 formerly Carlos Pace F2 car I was thinking F2 when I said 722. I had a Vagley tuned lotus and then a Grimaldi built injected big valve Hart copy. When they allowed BDA's I dropped out because I could not afford to buy all new engine stuff then, good old SCCA strikes again. Later on I had a March 78B that I ran for 1 season, at that point finances could not support an Atlantic car anymore. 722 were stones down the straight because they were so wide, the 712's used to eat tub rivets especially at the rear bulk head but they handled well. My car had floating front rotors which was pretty neat in 1971,the brakes were fabulous, you could go in deeper than just about any car out there lap after lap.

[Richard Hudgins]

I bought a B18 from Joe G. and then traded him for the 712. Here is photo of one if you are not familiar with them The one in the photo may be Siffert's old chassis.





He had another buyer for the B18 who was willing to pay enough that I got the March straight across with no extra money. (Very unusual for any deal with Joe G.)

The 722 came with a Hart motor as well.

And if you remember the BDD kits were 5.5 K. Way rich for my budget back in the day.

[Cobra #3170]

Originally Posted by Richard Hudgins

I bought a B18 from Joe G. and then traded him for the 712. Here is photo of one if you are not familiar with them The one in the photo may be Siffert's old chassis.





He had another buyer for the B18 who was willing to pay enough that I got the March straight across with no extra money. (Very unusual for any deal with Joe G.)

The 722 came with a Hart motor as well.

And if you remember the BDD kits were 5.5 K. Way rich for my budget back in the day.

Boy, you have me beat, I never got a deal on anything from Joey. I bought my 712 from Fred Opert through Doug Shearson so they also gave me a lesson on how not to buy a used race car. The Vegantune was junk and I got a worn out clutch to boot. I learned everything the hard way making mistake after mistake because everybody I knew was into big production cars and didn't know about Formula car set up, especially aero. When I finally got to know a little bit I was too old and slow to be competitive in a formula car. They are truly amazing though, after driving an Atlantic car everything else feels like a pig on wheels, it spoils you for life. I tried to trade Joey my Cobra for a March 74B in 1974 and he would not do it, wasn't that a lucky break. In those days all I cared about was racing so it didn't matter that I was going to lose my Cobra, I just wanted to go fast.
Was the B18 a Chevron? They made a very nice car in terms of workmanship and design. Dan Carmichael had one as did Chip Mead both were pretty fast.

[Richard Hudgins]

Originally Posted by Cobra #3170

I tried to trade Joey my Cobra for a March 74B in 1974 and he would not do it, wasn't that a lucky break. In those days all I cared about was racing so it didn't matter that I was going to lose my Cobra, I just wanted to go fast.
Was the B18 a Chevron?

Yes it was a Chevron.

You were very lucky that Joe G. did not trade you.

Fred Opert-Doug Shearson. Now that was a scary crew there. Had to hold on to your wallet almost as hard as when dealing with Mr. Haas for Hewland parts.

As we both know, back then a race car was only good for a couple of seasons.

Hell, I virtually gave away both of my March's, a number of FF's, various production based sports cars, and in a moment true insanity, traded a B6/8 Chevron (with a blown up BMW M10 engine) for a small transporter.




I certainly wish that I had kept my old race cars. I would be able to retire in style if I had.


Hope you find your vibration problem and can cure it rapidly.

It would be a real pity to miss Laguna.