Cobra Vibration Problem

[Cobra #3170]

Originally Posted by Richard Hudgins

Bruce,

Unless you have a spool rear end, the half-shaft phasing is a moot point as the relationship will change as you drive the car.

Of course I could be missing something here......

I believe you are missing something, the joints are normally set up so that both of them have the inboard ends parallel ( these are splined Hardy Spicer half shafts that can be indexed any where radially on the splines. I had one shaft assembled so that the out board side of the half shaft joint was lined up with the inboard side on the other end rather the inboard side being lined up with inboard end of the half shaft. Although rotational velocity will be the same the effective torsional length is slightly longer side to side. The rear end locks up very well under hard acceleration and thus a harmonic could be generated. I am not saying it will, but could, I am looking for a needle in a haystack here so I am making everything as equal as possible. When I was with Ford we had many vehicle vibrations that were related to drive line harmonics that in some cases (Granada for example had huge weights on the tail shaft of the transmission) had to have major fixes applied because we did not understand the source. I have no test time between now and the Reunion so every possibility has to be addressed. This car has sheared several stub shafts, one half shaft joint and one drive shaft joint so the drive line stresses are quite large even though I have never drag raced it. The engine makes 585 LB/ft at 7250 rpm. The half shafts could theoretically see
over 1000 foot pounds at 130 MPH assuming 18% drive train loss and dividing available torque by two. Who knows what that will do to components rotating at 1600 rpm not to mention tires .

[mickmate]

Hey Bruce your diff runs a BPL (British Posi Lock) Jag doesn't it? The (Belleville washer) clutch lets the halfshafts rotate to where they need to independently of each other. That was Richards point. They do need to have the u-joints phased on each half shafts to cancel out the velocity changes of spinning in an elliptical path. Look at your half shafts, I think you may have found your problem. The yokes on each half shaft should be positioned so that their center lines are on the same axis. You can't change the phase relationship from one side to the other, each axle is done separately.

[mickmate]

Driveshafts and U-Joints - Tom Sotomayor

[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 mickmate

Hey Bruce your diff runs a BPL (British Posi Lock) Jag doesn't it? The (Belleville washer) clutch lets the halfshafts rotate to where they need to independently of each other. That was Richards point. They do need to have the u-joints phased on each half shafts to cancel out the velocity changes of spinning in an elliptical path. Look at your half shafts, I think you may have found your problem. The yokes on each half shaft should be positioned so that their center lines are on the same axis. You can't change the phase relationship from one side to the other, each axle is done separately.

I know that the shafts cannot be phased side to side, I was talking about the load path on the right shaft being different than the load path on the opposite shaft. The u-joints are phased side to side on each shaft but differently yoke to yoke. I am making them identical today to eliminate that situation. If they were not phased on either shaft the car would shake itself to death, it is not that bad.

[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.

[ERA Chas]

Don't stop guys-lovin' every word and snap of this!!
-and you're not hi-jackin' this thread-it's Bruce's!

[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.

[Jac Mac]

Originally Posted by Cobra #3170

I believe you are missing something, the joints are normally set up so that both of them have the inboard ends parallel ( these are splined Hardy Spicer half shafts that can be indexed any where radially on the splines. I had one shaft assembled so that the out board side of the half shaft joint was lined up with the inboard side on the other end rather the inboard side being lined up with inboard end of the half shaft. Although rotational velocity will be the same the effective torsional length is slightly longer side to side. The rear end locks up very well under hard acceleration and thus a harmonic could be generated. I am not saying it will, but could, I am looking for a needle in a haystack here so I am making everything as equal as possible. When I was with Ford we had many vehicle vibrations that were related to drive line harmonics that in some cases (Granada for example had huge weights on the tail shaft of the transmission) had to have major fixes applied because we did not understand the source. I have no test time between now and the Reunion so every possibility has to be addressed. This car has sheared several stub shafts, one half shaft joint and one drive shaft joint so the drive line stresses are quite large even though I have never drag raced it. The engine makes 585 LB/ft at 7250 rpm. The half shafts could theoretically see
over 1000 foot pounds at 130 MPH assuming 18% drive train loss and dividing available torque by two. Who knows what that will do to components rotating at 1600 rpm not to mention tires .

WE run a fairly potent TVR/SBF here in NZ & have had similar issues , you must run the half shafts as per the middle item in attached drawing. You should also strive to mount the diff head assy in the optimum position to minimise the amount of plunge in the sliding spline area of the halfshaft throughout the full range of suspension travel along with the amont of angle difference on inner/outer u-joints throughout that range. Tire sidewall height also has an effect on this, 15" rims with higher sidewalls will 'soften' the effect, 17"+ with low profile tires will make it worse.
Richards experience with the 5000/FA cars is the same, CV joints would cure the problem, but that will require quite a bit of rework.

[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.

[Cobra #3170]

Originally Posted by Richard Hudgins

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.

Richard,
I am amazed at what old race cars are worth now, you are right, you gave away a fortune in cars. I once bought a BT-29 for the FT-200 and sold the rest of it for $500 if only we had known. Speaking of Carl Haase the company sent me to England once and I came back with a suit case full of FT200 gears and dog rings for 1/2 what Carl was charging, he was the only game in town so it was rough until Brits came along.

I have finished the back of the car, all I have to do now is put in the clutch etc., change brake and clutch fluids and check alinement/corner weights.
I should be ready in plenty of time, I will drive conservatively so if it vibrates I probably won't know it, I just hate having something wrong and not really finding anything definitive, thanks for the thought. I am inside resting today after a real thrash yesterday, it will be 105 degrees today so I will wait till tomorrow morning to hit it again.

[ERA Chas]

Bruce,
All buttoned-up and testing yet??

[ERA Chas]

Can somebody who's in contact Bruce reveal the outcome or ask him to post?
There's been great tech presented here and it's instructional for many of us who care.