vibration felt in drive shaft at track

[frankym]

Ok,

Having trouble diagnosing a problem. I took my superformance to pocono last saturday at the north course. I began noticing that particularly out of tight turns on full acceleration, the car felt a little loose, like the drive shaft was vibrating or picking up vibrations from the rear end.

After a run, we jacked the rear of the car off the ground and with one man holding the alternating wheel, we rotated the opposite rear wheel front to back to determine if there was excess play in the CV joints. There appeared to be slightly more play in the passenger side rear wheel than the driver side rear wheel. We also shook the wheels at various angles while on the lift to see if there was play in the wheel bearing but it was tight. The excess travel was about 3-4 inches in outside tire circumference, and maybe 2.5-3 inches on the driver side rear, not sure if these are tolerable amounts of play.

I am not sure what could be casusing this feeling of excess play in the rear. Simultaneous to this, the car also feels a little notchy (but sporadically) when disengaging the clutch in reverse, like clutch chatter / uneven surface on flywheel.


The notchyness in reverse occured on another cobra I had with a live axle and after replacing the axle seals and putting in new differential fluid, reverse became smooth. I brought my SPF to my SPF dealer and when I took the car out for a ride with the mechanic, the car ran exceptionally well, but maybe the car needed to be driven hard for a while before the problem persists. On the track the sound and vibration out of turns was constant, and on the street, it's sporadic. Not sure what it could be but the mechanic said after doing a visual inspection of the chasis on a lift, nothing jumped out at him, not sure if he removed the axle boots or inspected the differential but I wanted to see if there are others out there that have experienced this.

I am also trying to rid the carburetor of a hesitation in performance around 2k rpm. I am going with a Barry Grant demon 750 with mechanical secondaries, my current holly has no metering block for the secondaries. so it might be harder to get the existing carb tuned right, plus, it's a stroker motor so it might benefit from a racing grade carb.

appreciate any help.

[Richard Hudgins]

Frankym,

Typical production CV joints have about 1.5 degrees radial play.

Production plunge (Tripoid) joints have about 2 degrees radial play.

Therefore, if you have CV's on both ends of your halfshafts, you would expect about 3 degrees of play. That extrapolates to about .75 inches at the outer tire diameter on a 26 inch unit.

With a CV and a plunge joint, you can expect about 1.15 at the tire.

Your problem seems to be more in the diff. Possibly excessive play in the spyder gears?

[G.R.]

I had a bad ring gear in my SPF--car always had a bit of vibration, since new.
In May it began to make noise(only around 1200mi on it)--whine and squeal, when downshifting and/or rolling out, vibration seemed to worsen--eventually it started to make noise/vibration under acceleration also--my mechanic checked everything and eventually we pulled the rearend--this was unfortunantely just a bad "from the factory" rearend--the ring gear looked like it had been run through a grinder and after a dye test was done there were 3 hairline cracks in the gear--
Had the Diff completeley rebuilt--and from the very first day I picked it back up it has been as tight and quite with no vibration or noise--even under hard acceleration/deceleration

FWIW

[frankym]

thanks guys,

to be fair, I have spent a couple hundred miles on the track now and with the stroker motor, it wouldn't surprise me that there might be some wear inside the differential. The fact that the sound/vibration was occuring more in turns led me to believe the problem was at the end of the halfshafts, but this sounds reasonable, do you recall how much it cost to rebuild the differential?

[frankym]

Ok problem has been solved.

I spoke with Dennis Olthoff and he said that the rapid vibration that I was experiencing in a full bump steer, was likely attributable to the axles making contact with the inner CV housings under full throttle out of the turns. Sure enough, when I removed the axles, there were signs that the housing was being bumped by the axles. Dennis recommended milling the inner CV joint housings by 1/4 inch. I took the axles out and brought them to a machine shop and am awaiting their return and will try to reinstall them this weekend with some help. You can see on the housings there are 3 bulbs that contain the roller bearings and there are abrasions where the parts are coming into contact under normal track conditions. For many SPF owners that don't bring their cars to the track, they may never experience the limitation, but it's very easy to do on a track with some spirited throttling out of the turns. I am not sure if SPF has recognized this as a problem or if other car owners have experienced this condition. Since the Olthoffs have extensive track and race experience, they were familiar with the condition and diagnosed itimmediately. If it's a compatability issue with the thuderbird rear and the SPF rear suspension arms and camber, hopefully the current cars have been modified for this issue.

[tommyboy]

Hello frankym,
I have SPF 1593 and at 600 total miles experienced a clunk in the drivetrian and some driveline vibration. The problem turned out to be the left rear inboard CV joint had disintegrated. The CV boot was still intact. With the car jacked up you could rotate the left rear wheel about three inches before the axle stubbed into the differential would begin to rotate. Superformance warrantied the CV joint. I ended up replacing the joint and the differential axle output seal. As it turns out, I found out that Superformance had a supply issue (shortage) and ended up installing used (rebiult) axles. I was told about 100 in number! I did not like this but they did send me new ford parts. If you have not pinpointed the problem I suggest you carefully remove the large CV boot band clamp and roll back the boot and thoroughly inspect the insides of you CV joints.