Cobra Vibration Problem

[Cobra #3170]

Hi Everybody,

I encountered a serious problem today while track testing CSX3170 prior to the Monterey Reunion. I am hoping that with all the gear heads on this site someone will have an idea or a similar experience. The following describes the problem and what I plan on investigating, any other thoughts would be welcome.

Problem: Excessive Vibration in drive train under moderate to heavy acceleration:

The car has a vibration that is so violent that it blurred my vision in 2nd, 3rd and 4th gears.
The vibration is directly related to applied torque in that if I eased off on the throttle but maintained speed the vibration goes away. It was so violent that I was afraid to use anything over half throttle in my initial test session. The car is equipped with vintage Goodyear rubber that was dynamically balanced. After checking the car over and finding nothing out of the ordinary I tried raising the tire pressure to 30 PSI from 25, this
reduced the vibration by about 50%. I am not sure if the reduction is do to less tire distortion or just less torque through the drive line because of slippage at higher pressures.

What I plan on checking:

Flywheel spokes for cracking in the spokes, happened before but may have been a symptom not a cause.

Half shaft phasing I checked at the time I installed new u joints but maybe they are off

Drive shaft phasing I had it built at a local shop and they may have it miss indexed

Tire run out

Center section mount distortion, the front of my cradle mount (used with a Jag center section) shows signs of rubber damage and the cradle mounting bolts seem to loosen
every session. I replaced the original 3/8” bolts with 7/16 studs and welded reinforcements in to the cradle structure because of previous problems with this area.
The cradle is from Brian Angless and was built in the early 1990's.

Differential damage on the ring and pinion

I will check engine and transmission mounts too.

The U joints were replaced on both half shafts and the drive shaft last month with heavy duty Spicer units.


I don't think it is a tire balance or engine balance issue because it only shows up under extreme torque loading.

The kicker, I had this problem with Hoosier A-6's at Fernley race way but it was only in high gear. When I took the car apart I found cracks in 3 of the six spokes on my Mcleod Magnum Force flywheel the cracks emanated from a stress riser caused by rivet holes in the flywheel. I have a new flywheel but the vibration is back in spades

[mln385]

How much use on the new flywheel? Do you do allot of compression down shifting . This can heat up the clutch and mating surfaces resulting in stress fractures. I had this years ago in my mustang and the pressure plate actually grenaded in 3 peices while on track.

[ERA Chas]

What I'd check in addition to your current checklist:
Tire runout-always a G'year achillies heel.
Respect what you say about the power loading but I'd carefully inspect the balancer and its bolt and pulley bolts.
The new driveshaft-did they balance it? Did it shed a weight?
Best luck and please post your finds.

[A-Snake]

Only because it was one part of the drive train not mentioned in your post, would an issue with motor mounts cause this?

[Cobra #3170]

Originally Posted by mln385

How much use on the new flywheel? Do you do allot of compression down shifting . This can heat up the clutch and mating surfaces resulting in stress fractures. I had this years ago in my mustang and the pressure plate actually grenaded in 3 peices while on track.

One autox and no down shifting I can go 83 in 1st so don't shift in autox, It happened as soon as the oil was warm enough for some throttle so I think it is something else.

[Cobra #3170]

Originally Posted by ERA Chas

What I'd check in addition to your current checklist:
Tire runout-always a G'year achillies heel.
Respect what you say about the power loading but I'd carefully inspect the balancer and its bolt and pulley bolts.
The new driveshaft-did they balance it? Did it shed a weight?
Best luck and please post your finds.

I think it would shake even with no load, I can ignore a lot of things but this is a very violent event, feels like it is going to break something. I misphased my half shafts back in the 60's and it felt like this, shook only under power.

It was very smooth at well over 100 with light throttle but look out when the go pedal is down. I only ran about 8 laps because I was afraid it would break something serious and I wouldn't be able to attend the event. I asked Roger Kraus if the tires might be flexing in the sidewall and doing a slip stick under power, he did not think that was the problem. I did notice that one time it buzzed the tires and the vibration went away, but the torque loading would go away too. It feels like the drive line is being pulsed by some huge force.
Thanks Chas

[ERA Chas]

For as hard as I know you run the car, I'd nut-and-bolt the whole driveline. The caps on the u's and yokes, etc.
Finding nothing there, I'd uncover the R & P to inspect inside. I once found (without vibration) a broke-clean-off tooth on the ring gear.
Hoping it's minor but that much tremors suggests it's not...

[brandx]

This may be a stretch but do you have another set of tires to try?

[Rick Parker]

Any multi disc clutch chatter/slippage or tires slipping on the wheels?

[Cobra #3170]

Originally Posted by brandx

This may be a stretch but do you have another set of tires to try?

Yes, I have a matching set I was saving for the race and you are right it is worth a try if I don't find anything when I inspect parts.

Rick,
I should have marked the tires and rims but did not, it could have slipped them or maybe set up a harmonic in those big side walls. It was breaking traction whenever I had the guts to get it all the way down. I am going to look at the pinion bearing and the slip yoke bushing too. I set ride height for handling/ rake and am wondering if I am getting an angle or no angle on the half shaft joints that might cause a problem.

[ERA Chas]

Thanks for the feedback-watching intently to learn.

[fordracing65]

I hope you get it figured out, your car is too awesome not to have it win the race, good luck.

[Cobra #3170]

I won't be winning, I have to keep it in check because it would not be fair to other cars that are closer to stock configuration. I am just part of the 50 year anniversary and that is an honor in itself. It should be a really great event with all the Cobra's both inside and outside the paddock, I just don't want it to break and spoil the fun.

[Rick Parker]

Just a thought since in auto X you ARE in 1st all the time and the acceleration is able to keep up with the engine (poor explanation) and then on the Road Coarse you use the upper gears (loading)it is possible the tires have spun on the wheels and become unbalanced or or your clutch is chattering. Does it do this in all gears now? Does it do it in 1st?

[Rick Parker]

Maybe you reached the point of too much HP vs Tire without a wing and the tire is actually slipping on the asphalt?

[bobcowan]

Generally speaking, when tires or drive shafts are out of balance the vibration is speed related. When engines, clutches, and transmissions are out of balance it's RPM related. Generally speaking.

When they're load related, look for things that are broken or loose - U-joints, transmission mounts, motor mounts, etc. Dodge trucks are famous for this kind of vibration with bad U-joints.

Also look for things that change relationship under a load. Like a rear suspension that gets compressed, or a front suspension that isn't. That also includes motor, transmission, and differential mounts. Try using a crowbar or jack to move some of those components around.

[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.

[ERA Chas]

From post #3:

Originally Posted by ERA Chas

Tire runout-always a G'year achillies heel.
.

I think what you found so far improves your problem(s) by 98%.
Does Hoosier make a compound for your usage? The only problem I had with TD's was circumference differences, giving stagger. Adjusted by over-inflating the small side and heat application. The runout was always less than G'years.
Possibly yours can be trued by that's a lot to shave.
Japanese tires are always very true but I don't know what you can use.

[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