vibration after gear swap. HELP!!!!!!

[kthpt007]

I have a 99 cobra and have swapped the stock gears for a set of 4.10's After I changed these gears I noticed a vibration at 55 mph and up. I was told that this was due to changing rotation because of the lower gear. I was also told that a fms aluminum driveshaft should fix it, is this correct?? Please let me know.

[Mongoose]

Originally posted by kthpt007
I have a 99 cobra and have swapped the stock gears for a set of 4.10's After I changed these gears I noticed a vibration at 55 mph and up. I was told that this was due to changing rotation because of the lower gear. I was also told that a fms aluminum driveshaft should fix it, is this correct?? Please let me know. From everything I've read an aluminum drive shaft should cure your vib. problems

[kthpt007]

Will the FMS drive shaft bolt right up?

[khaine]

you need a aluminum drive shaft, and once you have it you can remove the dogbone as well

and yess it will bolt right up :tmb:

[kthpt007]

I was told by Gene Evans Ford that the driveshaft would not fit and that you had to modify it at a specialty shop to get it to fit. Is this true? Surely with all of the cobra owners on this site someone can tell me somthing about the installation of a fms driveshaft on my 99 cobra.

[Ken 01-Cobra]

Originally posted by kthpt007
I was told by Gene Evans Ford that the driveshaft would not fit and that you had to modify it at a specialty shop to get it to fit. Is this true? Surely with all of the cobra owners on this site someone can tell me somthing about the installation of a fms driveshaft on my 99 cobra.
I think it depends on your car. The "J" driveshaft fit my 2001 Cobra just fine, but I've heard it is a very close fit and will not fit some cars.

[99_Venom]

The IRS Cobra driveshaft is about 3/4 inch shorter. You can stuff the FMS driveshaft in there but I would not advise it. Get it trimmed at a DS Shop.

You will find all kind of information about balancing your existing shaft here:
http://www.lx.net/jlynch/

I put in a Dynotech shaft when I installed the T56 and I don't have a bit of vibration.

http://www.dynotechengineering.com/

[Ken 01-Cobra]

I think the deal with mine was that I had the rear end out and was doing the gears and rebuilding the clutch packs at the same time. I had already removed the vibration damper when I had the PHP brace installed. The "J" driveshaft fit just fine, as I said. It didn't have to be "stuffed" in...it fit. However, I've talked to a number of techs about this, and they say it is on a car by car basis. It works just fine on some cars, but is to long for others. The others are USUALLY the ones with fully stock rear suspensions.

[kthpt007]

KEN, WHAT DID YOU DO DIFFRENT ON YOUR CARS REAR SUSPENSION?

[kthpt007]

HATE TO SEEM STUPID BUT REMOVE DOG BONE?

[khaine]

Originally posted by kthpt007
HATE TO SEEM STUPID BUT REMOVE DOG BONE?

it's a 6 pound piece of metal attached to the front of your differential case that looks sorta like a bone...

it's proper name is "vibration dampener"

if you go to an aluminum drive shaft, it's no longer required

[kthpt007]

IT WOULD SEEM TO ME THAT ALL 99 COBRAS SHOULD BE SET UP THE SAME. ARE YOU SURE THAT IF I ORDER THE J SERIES DRIVESHAFT IT WILL WORK? WHEN I REMOVE THE DOGBONE DOES THIS FREE UP MORE ROOM FOR THE SHAFT?

[khaine]

Originally posted by kthpt007
IT WOULD SEEM TO ME THAT ALL 99 COBRAS SHOULD BE SET UP THE SAME. ARE YOU SURE THAT IF I ORDER THE J SERIES DRIVESHAFT IT WILL WORK? WHEN I REMOVE THE DOGBONE DOES THIS FREE UP MORE ROOM FOR THE SHAFT?

dude, caps lock key, please :)

I hear that it's on a case by case basis for the drive shaft...

and the dogbone is on the diff, it's not in the way of the drive shaft

[kthpt007]

How is this? Much better huh. Thanks for the information you have been very helpful

[Ken 01-Cobra]

Originally posted by kthpt007
KEN, WHAT DID YOU DO DIFFRENT ON YOUR CARS REAR SUSPENSION?
Other than the IRS bushings and brace, the suspension is stock. MM full lenth subs were installed, the rear gears were changed, the clutch packs were rebuilt with an extra disk, and 2003 halfshafts were put in; but the control arms, springs, etc. are all stock.

[Ken 01-Cobra]

Originally posted by kthpt007
IT WOULD SEEM TO ME THAT ALL 99 COBRAS SHOULD BE SET UP THE SAME. ARE YOU SURE THAT IF I ORDER THE J SERIES DRIVESHAFT IT WILL WORK? WHEN I REMOVE THE DOGBONE DOES THIS FREE UP MORE ROOM FOR THE SHAFT?
There is no guarantee that the driveshaft will work on your car. It really is on a car by car basis. The vibration damper is not needed if you have the IRS brace and bushings. I have not heard that you can remove it without issues simply by adding the driveshaft.

[Mike Kievit]

I have the J shaft. They measure 45 1/2". The factory 99 Cobra shaft measures 44 1/2". If I remember correctly the '01 shaft measured the same, 44 1/2".

As mentioned in the other reply's. Your install may vary.

GENERAL RULE. Make it the same length as the one you are taking out.

You would measure the centerline from each ball joint to get the length you need.

I ordered mine from a friend who had it shortened at a driveshaft shop.

As luck would have it, mine balanced out with out weights. Not a single vibration.

Now here is the kicker. Last week I installed 3.73's.(spare pumpkin) I didn't think I had a vibration, but now with the 3.73's the car is smoother.

Mike