I'm having some weird problems with my '94 Cobra. If I drive over 80mph, the car starts to "vibrate." I recently had the drive shaft ballanced (due to the fact that I put a 6 speed manual transmission in the car and the driveshaft had to be cut for propper fitment) and had the u-joints replaced, but the problem persists. Someone recommended that I have the car aligned and tires ballanced, but it seems to drive rather straight on the highway. Any suggestions, anyone?

Ian

What vibrates? The steering wheel? The entire car?  Side to side, up and down?
Might want to have the tires ballanced and wheels aligned anyway.
Ice

i thought you meant CAR-15 !! [;)]

yeah, if nothing else, it could be a slight mis-alignment. make sure all tires have exactly the same preasure too!

Ian,

 That someone was right.  That is the place to start.  Although a 4 wheel alignment is costly.

 How long (miles) has this been happening?  If it has been going on for awhile (20,000 miles or so) you should be seeing some abnormal tire wear.

 However if no tire wear is showing after 10,000-20,000 miles I would have to say the drive shaft is not properly balenced and should be rechecked.

 Madmortigan

The vibration feels like it's coming through either my tranny or rear-end.

The tires show no abnormal wear, however, the rear tires do wear very fast. This may have something to do with the way that I drive [:D]

Edit- It's had this problem since I bought it back in Feb. 2001.

Thanks

Ian,
 Are you sure you've got the driveshaft
installed properly?  I don't know about the
Cobra, but the newer Lincolns we use as base
chassis's for our limousines have 4 mtg. holes
in the rear yoke, & 8 in the differential.  The
proper alignment of the two is color-coded with
a dab of yellow paint.  There are 7 wrong ways
to install it, and only 1 right one, even if it
is properly balanced.  Have you checked that?

Ian..

Did you use a different flywheel when you
changed the tranny ? No expert, but it may
have something to do with it.

[bouce]

I had the driveshaft installed at a local shop that does a TON of business. I had to wait 8 days before they could take it in. I'm assuming that they did it right, all of the other mustang guys had recommended me to them.

I'll check on it, thanks!

Ian

Edit- I'm a senior member now!

Nope, I think it's the factory fly-wheel. The car still vibrates when the car is goin 80MPH+ depsite if it is in gear or not, cluctch in or clutch out. I tend to thing it is a problem in the rear-end.

Ian

Just occurred to me...IF it did have an index
mark painted on it, they probably cut it off
when they shortened it.....

Find a tire shop that will do a balance with the wheels on, that will get all of the rotating mass balanced, not just the wheel/tire combo.
TRW

Ian,
I also have that problem with rear tires. [X]
Its coming from the rearend?  Take it to Ford and tell them to fix it.
Ice

Rotate the drive shaft 180 degrees in the rear end yoke.

Or buy a Camaro. [X]

God I hate Mustangs.... [}:)]

Ian,

Those guys in the shop are after all only human and any one can make a mistake, and that's all it takes is one mistake either in installing it or balancing it.

I liked the idea of balancing the car as a whole unit though that should show where it's coming from.

I am assuming you do not have the stock tires on it since you say you go thought them often.  Did you have the tires ROAD FORCE BALANCED?  The only machine that can do this currently is the Hunter 9700 road force balancer.  If you want you can balance a square block but it is not going to roll very well, is it?  The tire may be out of round.  It will track straight but can vibrate like hell.  I would have that checked out before anything more dramatic.  If it did it before you installed the 6spd and had a reputable shop do the drive shaft mods, I would say that is not likely to be the problem.

I thought they quit making Cobra's in the sixties.[:D]

I'm thinking here that the trans swap is real recent? Therefore, if you drove it with the old tranny and the car still did this, it isn't the tranny swap/drive shaft.

I had a car once with bad shocks that would almost bounce you off the road at 70. Changed the shocks and it became a whole different creature. In a good way. [;)]

I would start looking at the shocks and tires. As stated above, it may be an out-of-round tire.

Good luck.

You might want to consider finding a shop that specializes in locating vibrations and noises.  Call around to independent shops and they should know of someone with a vibration rack to test your car.

You might want to have your axel shafts checked as well.  If the wheel flange gets bent so that the axis of rotation isn't normal to the flange face, your car will have a wicked side to side vibration at high speed.  I actually have this problem on my bronco, but I'm too cheap to have it fixed, I just drive slower.

If the problem was present before the tranny change, then its problem wheel/tire problem, get a good balance on them also check you tires make sure the are rated for the speed. If the problem start after the tranny install I would look at the drive line indexing.

Clinth

Stay below 80?  

Seriously, if it just started when you got the new tranny, it sure sounds like the driveshaft was balanced or mounted improperly.

Stock driveshafts are often balanced with external weights. If the shop shortened the driveshaft, instead of just replacing it with one from a manual transmission car, they could have easily screwed up the balance. Make sure that the yoke isn't damaged in some fashion, too.


Switch from an A4 to an M6 (and vice versa) is a pretty common mod in the f-body world. Usually people just get the appropriate driveshaft rather than having the current one modified.

I agree whole-heartedly with Comp1911.

F-BODY!!!!!!!!!!!

either that, or buy a corvette.  I've got a 95, and that thing wont vibrate 80, 90, 100...


Seriously, its gotta be the driveshaft.  

What kind of engine is under the hood of that thing?  A friend of mine has a cobra, not a bad car.

I would say that most of the obvious points have been covered. A couple of minor points. When you check for abnormal tire wear, how do you do it? A good way is to take your hand and run it back and forth over the tread. You feel a sawtooth or cup wear pattern that you can't see this way. When you apply the brakes at speed, do you feel the vibration in the pedal?
Try looking around. Jack up the rear of the car, block it up safely, and go sightseeing. Did the mechanic tighten all the bolts properly? Put the tranny in neutral and spin the driveshaft. Feel anything unusual? Give it a good shake-after doublechecking your blocks. Did you get a bad u-joint? How about the angles of driveshaft to tranny and pumkin? Since it was shortened, these angles were changed slightly. It shouldn't matter but if you also modified the suspension-raising or lowering, it could have compounded to a problematic degree. The shorter the driveshaft was to begin with, the more this angle change-and its effects- would be compounded after shortening.
Try going out with a friend driving beside you. He may be able to observe tire vibration or hear a sound that you can't.
When the vibration is occuring, put your hand on the shift lever and see if you can feel the vibration through the linkage. If the driveshaft is shaking the tranny, you might. Was the correct mount used for the new tranny?

Also, a tire balance problem probably won't show up in the tracking.

Ian-
Where did you have the new transmission replaced? Blumenthal's?

What you drop in there?

A Tremec or Borg Warner ??

Ian,

As someone who has been in the automotive business since the '70s, maybe I can help you analyze the situation.

1)Did you have the problem prior to the trans swap ?? If so, then tires,tire balance, wheel bearings,axle bearings or carrier bearings. It could even be an out of balance rear brake drum(if not 4 wheel disks).

2) If problem is since swap, then we have more possibilities. Was the trans new ?? If used, it could be the output shaft bearing in the tailpiece.However, this would usually cause a leak at that seal. It could be improper indexing of the driveshaft during installation. Did they install new u-joints ? I hope so, and I hope they used x-tra heavy duty ones. It could be a change in driveshaft angle caused by installing the new trans. If it isn't mounted at the same angle as the old tranny, this can cause problems. Either the trans mount will have to be changed/modified or you will need to change the angle that the differential "looks" at the driveshaft with shims. Another possibility is that they didn't weld the shaft back together concentricly. Was the shaft dynamically balanced after they fabricated it?If not, I suggest you take it to a high performance shop that specializes in driveshafts.

If you answer the above questions, I can reply with more suggestions.

bowhuntr