Cobra Vibration Problem

[slider701]

A good way to check to see if the clutch/flywheel is the culprit is to take off at medium to hard throttle in 1st and then shift straight into 4th and stand into the throttle hard. If the clutch is slipping it will show with this type of loading.

[Cobra #3170]

I am pretty sure the clutch isn't slipping, it is an 8" Magnum force twin disk with metallic facings, the car can and did spin the tires yesterday at over 100 mph in third so I think the tires would give up before the clutch under any circumstances.

What I have found so far:

The drivers side Goodyear has 3/16" of radial run, out the passenger side is about 1/8.

The LH halfshaft is 180 degrees out of phase along its' length with the RH halfshaft, I know that the radial acceleration and deceleration will cancel through the system but it will be different along the length of the halfshaft, side to side perhaps I am getting a vibration from that difference at high torque levels.

The drive shaft looks OK for phasing but I need to really check it with a straight edge.

The rear end cradle studs are loose on the drivers side, it is always the passenger side that comes loose first because of torque reaction, this is the first time the drivers side has come loose. I will clean them and use green Loctite on the stud threads this time on all 4 studs.

That seems like an excessive amount of tire run out, no passenger car company would ever allow a tire like that on any of their vehicles.
God only knows what the force variation under load is. Many years ago while in the military I bought a set of recaps from the PX, they cost $10 each and had similar run out, that car shook like hell too.

I will check the flywheel tomorrow, along with all mounts for engine trans and diff as suggested by loading them.

[MOTORHEAD]

3170: "only shows up under extreme torque"
Had a similar situation in a bdr. "go" pedal goes down, rearend goes down (squats), tire hits body/frame. Don't think tire would hit anything on an original/shelby/kirkham, but it's worth a look.
Ted

[Cobra #3170]

Originally Posted by MOTORHEAD

3170: "only shows up under extreme torque"
Had a similar situation in a bdr. "go" pedal goes down, rearend goes down (squats), tire hits body/frame. Don't think tire would hit anything on an original/shelby/kirkham, but it's worth a look.
Ted

I am going to check for that as soon as I get the wheels off, you never know what is going on when things get violent.

[Barnsnake]

Since the problem is primarily when significant torque is applied, I would take a very long look at the mounting of the rear-end center section. If it is rotating the pinion upward under acceleration the mis-matched driveshaft u-joint angles would cause exactly the symptoms you describe.

[mickmate]

Hey Bruce what did Brian use for a bushing material? That's been around for a minute or two and hasn't just done the church run on Sunday.

[Cobra #3170]

I agree that could be the issue, the cradle uses the same bushing that the original uses a rubber bushing with a threaded insert that takes two 5/8" bolts one on either side. If the bushing were completely gone (it isn't) the nose could go up a maximum of .6" before metal to metal contact. I am going to put a hydraulic jack under the the front of the rear end and try to force it up today to see how much movement I get. The bushing is a sleeve within a sleeve it has an outer sleeve that is approximately 1.75" in diameter and then the inner which is about .93" tapped for the two mounting bolts that come in from the sides of the double shear mount in the car.

[Cobra #3170]

Originally Posted by Cobra #3170

I agree that could be the issue, the cradle uses the same bushing that the original uses a rubber bushing with a threaded insert that takes two 5/8" bolts one on either side. If the bushing were completely gone (it isn't) the nose could go up a maximum of .6" before metal to metal contact. I am going to put a hydraulic jack under the the front of the rear end and try to force it up today to see how much movement I get. The bushing is a sleeve within a sleeve it has an outer sleeve that is approximately 1.75" in diameter and then the inner which is about .93" tapped for the two mounting bolts that come in from the sides of the double shear mount in the car.

I just remembered how the mount is configured, what I described is incorrect.
There are two metallastic bushings pressed in on each side of the mount, in the center is a floating threaded sleeve that is positioned via a 90 D. 1/4" threaded hole. The mount has a clearance hole that is used to position the sleeve with a 1/4" bolt to start both main mounting bolts, it is then used to keep the sleeve from rotating while torquing the main mount bolts, after torquing you remove the 1/4" bolt.

The transmission and rear end are not inline in a top view because the engine is offset to the right but is supposed to be flat in the side view, if the front of the diff moved up it could put the U-joints slightly out of phase. I thought about making the front mount solid but did not because I did not want to cause drive line failures from shock loading.

[Cobra #3170]

The bad news:
The flywheel has dings, cuts and scrapes all over it both front and back, I could not figure out what caused it because there was nothing in the bell housing. I then noticed that a piece broke off the clip-in Ford throw out bearing on the back side it is cast iron and pulverized everything in there.

The really bad news I am not sure this caused the vibration because the engine doesn't vibrate in neutral.

I will have to pull both seats, the tunnel, front engine mounts, drive shaft, transmount ,
bell housing, clutch and flywheel to get the parts out. I then need to find a new Magnum force flywheel and maybe a clutch if it is damaged.

All the mounts and U joints appear to be in good order I aired the tires to 50 PSI and put them in the sun so that they could find the roundness God. I am going to up the high speed bump adjustment on my rear shocks in case the problem is in the tires other than that I don't know what to do. If I can't find the right flywheel I may not be able to run.

[ERA Chas]

Originally Posted by Cobra #3170

I aired the tires to 50 PSI and put them in the sun so that they could find the roundness God.

My method exactly.
Lykins can help you get the clutch you want.

[blykins]

If you get in a bind needing a flywheel or clutch, let me know....I can get pretty much anything and everything.

[Cobra #3170]

Originally Posted by blykins

If you get in a bind needing a flywheel or clutch, let me know....I can get pretty much anything and everything.

Brent,

Thanks, I will pull it apart tomorrow to see what is broken and what I need, I'll let you know if I can't get the parts because if I can't I will need a clutch, flywheel and throw out bearing. I am really running out of time, have to haul it to Laguna Tuesday after next.

[D-CEL]

Hey Bruce,

I sure am sorry to hear about all this. In reading all the details (good work) I would think the flywheel and clutch issues are a resulting failure but not causal. To me it sounds like the drive line is flexing at the extreme end of the load scale. You are stressing the thing far past the level mere mortals can dream. I would be looking at the trans, trans mounts, diff mounts, and rear end cradle. I wouldn’t be surprised to see a (partial) fatigue failure somewhere. Maybe at the cradle to main frame rail? Something in the driveline is moving out of alignment under (big) load resulting in a vibration.


Jason

[blykins]

Where are you located? I have access to warehouses on each side of the country and can get stuff drop shipped to you...

[Cobra #3170]

Originally Posted by blykins

Where are you located? I have access to warehouses on each side of the country and can get stuff drop shipped to you...

Brent,

I am outside of Fresno Ca, I appreciate your offer but found the a flywheel at McLeod this morning that matches what is in the car, they are in California so I should have it by Wednesday AM. I started at 4:30 AM and had the flywheel out by 7:30 worst part was getting the starter out with those #16 oil lines in the way. It seems the $#@@#$ zerk fitting unscrewed from the Ford throwout bearing and destroyed the flywheel spokes and even part of the starter mounting flange, it looks like somebody shot it with a 10 gauge using double 00 buckshot, hard to believe it could do that much damage. I am sure this is not the source of the vibration as Jason stated. I have looked at all the mounts involved and can find nothing broken or damaged in any way, I lifted the car using the front of the rear end cradle mount and it might have shifted up .060
All the U-joints also look and feel good. I am going to take my sun stretched Goodyears to have them rebalanced today and re-phase the left half shaft to match the right half shaft. I will also start at 26 and 25 PSI and increase the high speed bump in the rear, beyond that I don't know what to do other than pray and keep my foot out of it. I'll use the time I am waiting for the flywheel to do a thorough nut and bolt plus move my brake bias adjuster and radiator catch bottle (I had interference with both during the test).
Thanks to everyone for your input, this forum is a great source of knowledge.
If I find anything else I will post it but do not expect to.

[ERA Chas]

Originally Posted by Cobra #3170

I am going to take my sun stretched Goodyears to have them rebalanced today and re-phase the left half shaft to match the right half shaft.

Bruce-please try to have them shaved to true the runout-it's not helping.
Your plan and thoroughness is great. I appreciate your taking time to post results. It's invaluable coming from guys like you that are pushing the car's performance envelope and very educational for us slackers. Morris' build and testing was of very similar benefit-but you're doing it with an original. If that's a limiting factor of your car's performance, it hasn't shown it yet. Great accomplishments from both of you. It's what I come here for.
Thanks from me.

[Cobra #3170]

Originally Posted by ERA Chas

Bruce-please try to have them shaved to true the runout-it's not helping.
Your plan and thoroughness is great. I appreciate your taking time to post results. It's invaluable coming from guys like you that are pushing the car's performance envelope and very educational for us slackers. Morris' build and testing was of very similar benefit-but you're doing it with an original. If that's a limiting factor of your car's performance, it hasn't shown it yet. Great accomplishments from both of you. It's what I come here for.
Thanks from me.

Chas, I wish I could but the run out exceeds tread depth.

I took the tires in today and had them balanced on a recently calibrated Hunter.
left rear was 2.75 OZ off, Right rear was, 2.5 OZ off, RF was 1.5 OZ off and LF was 1.0 OZ off, so much for having them balanced at the track race shop.

Perhaps the imbalance coupled with the large torque load was enough to cause a stick slip in the tire and the resulting vibration, I hope so. At least something was wrong, I feel a little better about it now.

[ERA Chas]

Originally Posted by Cobra #3170

Chas, I wish I could but the run out exceeds tread depth.

Yikes-didn't realize that! Damn G'year-if the Japanese, Brits, French, Germans and Italians can get them better than that, why can't they??For sure that's a major contributor to vibration.
But I don't think it knocked the zerk off...
P.S. If you don't mind-exactly how much power and torque does the motor make?

[Richard Hudgins]

Originally Posted by Cobra #3170

I am going to take my sun stretched Goodyears to have them rebalanced today and re-phase the left half shaft to match the right half shaft. .

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......

[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 .