Chasing 100mph vibe and running out of ideas...

Well, I've been chasing this rear end vibe for the last week and I'm running out of ideas. At first, the vibration started at 80-85mph, your typical 03-04 issue. Vibe is speed dependant (also get it in 4th) but does get worse when I shift to 5th or 6th gear. It does not go away when pushing the clutch so I know it's not the transmission / clutch.

With that said, I had my tires Road Force balanced and it slightly helped. I then turned the driveshaft 180deg on the pinion flange, another improvement but the vibe was still noticeable.

Today, I measured transmission and pinion angles. Turns out the front of the engine was pointing up 3.5deg and the pinion was pointing down 1.5deg, a difference a 5deg. Started messing with shims and got the pinion angle up to 1.0deg (pointing upwards). Installed a few washers under the transmission where it bolts to the crossmember and brought transmission angle down to 3.0deg. Car is now smooth all the way up to 100mph but vibe is still there when going faster.

Thought I was on the right track and tried adding a few shims to the differential and hit another issue. The damn differential housing is now hitting the subframe which means I can't bring the pinion angle closer to the transmission angle.

For the record, I also tried ''neutralizing'' the drivetrain by backing the transmission crossmember bolts and driving the car forward / backward a few times but that didn't have any effect.

By the way, I have the urethane MM differential bushing kit and am thinking going with their aluminium kit.

I do want to get rid of that damn vibration before investing anymore in the IRS though so please help me !!!

I don't know where to tell you where to look but maybe put it on a dyno and then you might be able to see something actually going on with the car. If it doesn't do it then it's in the front end?:shrug:

a bent wheel will balance if you add enough weight.

you dont mention what tires you have - cheap tires vibrate.

put the pinion at 3.5 up and leave the engine and tranny at 3.5 down like it was designed to run at. the u-joints need to be out of phase with eachother. then loosen up the trans mount and motor mounts, drive back and forth a few times to align everything, and retighten

I would find someone else with a Cobra or Mustang for that matter and swap ALL four tires and wheels with them to make sure you eliminate the tire/wheel combo from your equation. Tires can do that. Wheels also to a certain degree! :read:

I'm running brand new 295/35/18 BFGoodrich KDW but will swap wheels with my mother's GT just to make sure...

I just spoke with a guy at Maximum Motorsports and received confirmation that their aluminium kit actually lowers the rear of the differential housing (relative to the body). This explains why getting a 3.0deg pinion angle is possible with the aluminium kit but can't be done (housing hitting subframe) using their urethane kit (which maintains crappy OEM geometry).

Had I known that when I ordered, I would have went with aluminium right away, now I need to remove that damn pumpkin once again...

Avenger said:

I'm running brand new 295/35/18 BFGoodrich KDW but will swap wheels with my mother's GT just to make sure...

I just spoke with a guy at Maximum Motorsports and received confirmation that their aluminium kit actually lowers the rear of the differential housing (relative to the body). This explains why getting a 3.0deg pinion angle is possible with the aluminium kit but can't be done (housing hitting subframe) using their urethane kit (which maintains crappy OEM geometry).

Had I known that when I ordered, I would have went with aluminium right away, now I need to remove that damn pumpkin once again...

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The pumpkin does NOT have to come out to do the front diff mounts. You can do it in the car. Just loosen the front torque brace and rear diff mount and you should be able to get everything in.

You can save a handsome chuck of change with my front diff mounts over what you'd shell out with MM. Plus my parts don't have delivery and S&H fees tacked on. :-D (see link in my sig) Just be aware of some increased NVH with either kit. (EDIT oh wait, I see you're in Canada, expect a slight adder for Canadian delivery. My kits include US delivery)

FWIW

Thanks a lot for the help Bruce, truely appreciated. You're providing the SVT community with awesome products and I will definitely be buying some stuff from you for my winter project.

However, MM states that their aluminium kit actually lowers the back of the differential housing to allow for steeper upward angle.

I experimented a little this week. I removed the differential crossmember and tried various washers combinations. Brought the pinion angle up to 1.0deg and went for a test drive, car felt better. The vibration had moved from 85+ mph to 100+ mph. I then tried removing another washer between differential ears and subframe mounting points to increase pinion angle a little further. Turns out the differential housing was hitting the subframe and I was still only at 1.5deg.

I was about to start modifying the rear bushing bracket to lower the rear of the differential housing when I decided to give MM a call. The guy told me their urethane kit did not allow pinion angle adjustment (already knew that) and that their aluminium kit actually lowers the rear of the differential to allow for more upward angle.

As far as I can tell from the pictures, their kit includes a whole new rear bushing bracket whereas yours keeps the stock geometry. Considering I can't match transmission and pinion angles, I think I need the added room that MM kit offers.

Am I doing something wrong or is everyone experiencing that issue when adjusting their pinion angle ??

Truely appreciate your input ! :beer:

You're the first one I've heard that can't get enough angle out of the front diff mounts. :shrug:

I think you might possibly be battling a double edged sword? You may have a driveshaft indexing problem along with driveshaft angles out of phase?

Good luck with it! :thumbsup:

you are deffinately on the right track...mine went away with pinon angle adjustment. i did get the alum. pinon angle kit from mm...and set everthing as listed in the install. i still had a slight vibe problem at higher speeds like you but i bought a new d\s from steve@tousley. put it in rechecked the angle and took her for a spin... all is good now

be advised if you had your tires forced balanced it is not a tire\rim prob..they would have found it then

ShelbyGuy said:

a bent wheel will balance if you add enough weight.

you dont mention what tires you have - cheap tires vibrate.

put the pinion at 3.5 up and leave the engine and tranny at 3.5 down like it was designed to run at. the u-joints need to be out of phase with eachother. then loosen up the trans mount and motor mounts, drive back and forth a few times to align everything, and retighten

Click to expand...

Good information. Defiantly something good to try there.

What about a CF DS from www.pstds.com? It fixed my vibe, and a bunch of the other NE guys have had luck as well. If nothing else, you might give them a call and talk to Mark (one of the owners), and talk with him. From my experiences, he's a "straight shooter" and won't try and sell you something you don't need.

Well, the fact that playing with the pinion angle moved my vibration from 85+ to 100+ tells me that it is not only driveshaft related. Getting a carbon fiber DS might correct the problem but, in my opinion, it only hides what is basically a driveline geometry issue.

I know I have a N/A Cobra but I had the same problem. I getting the angles right does not fix it completely, your next step should be rephasing your driveshaft. Do not just turn it 180 degrees. You will have to move it one spot at a time. If you find a spot where it is very close, find a thin washer and add it under one of the bolts and move that around. Eventually you will find the sweet spot and it will virtually eliminate the problem. That is what I had to do to mine once I installed 4.30's.

Avenger said:

Thanks a lot for the help Bruce, truely appreciated. You're providing the SVT community with awesome products and I will definitely be buying some stuff from you for my winter project.

However, MM states that their aluminium kit actually lowers the back of the differential housing to allow for steeper upward angle.

I experimented a little this week. I removed the differential crossmember and tried various washers combinations. Brought the pinion angle up to 1.0deg and went for a test drive, car felt better. The vibration had moved from 85+ mph to 100+ mph. I then tried removing another washer between differential ears and subframe mounting points to increase pinion angle a little further. Turns out the differential housing was hitting the subframe and I was still only at 1.5deg.

I was about to start modifying the rear bushing bracket to lower the rear of the differential housing when I decided to give MM a call. The guy told me their urethane kit did not allow pinion angle adjustment (already knew that) and that their aluminium kit actually lowers the rear of the differential to allow for more upward angle.

As far as I can tell from the pictures, their kit includes a whole new rear bushing bracket whereas yours keeps the stock geometry. Considering I can't match transmission and pinion angles, I think I need the added room that MM kit offers.

Am I doing something wrong or is everyone experiencing that issue when adjusting their pinion angle ??

Truely appreciate your input ! :beer:

Click to expand...

Did you get and install the MM alum kit? I have the same issues and went throught the same steps as you. It still vibes at 100mph+, and my diff housing is also hitting the subframe around 1.5deg. 04GT with T56, alum DS and 04 Cobra IRS.

Nice to know I'm not crazy and that others are also experiencing my issue. I will be installing the aluminium bushing kit over the winter because the summer is way too short here to lose a weekend of driving fun. I want to get the pinion angle right before messing with DS indexing but I swear I will get rid of this damn vibe. Will report back next spring.

Well, I had heard from Maximum Motorsports over the phone that their aluminium bushing kit lowered the rear of the differential but since my english sucks I decided to get a written confirmation. I basically explained what's already been said in this thread and got this reply the same day.

Alexandre,

Very good observations. Most people don't notice, nor take the time to check things like pinion angle, and thus have no idea where the vibration is coming from. First a little background information about how Ford set the pinion angle from the factory.

As with their solid axle applications, Ford continued to use the same -1.0 degree pinion angle on all IRS equipped Cobras. What they didn't realize was that with an IRS car, there shouldn't be any movement of the differential housing under acceleration. Unlike a solid-axle application, where the axle winds up and the pinion rises, the half shafts do all of the articulation. Ideally, you want to set the pinion angle to compliment the transmission angle. Example: Transmission 3.5 degrees down, pinion 3.5 degrees up.

This is where you start to run into problems. Without lowering the rear of the differential as you raise the front, you will not only change the half shaft angles, but also run out of positive adjustment. Our aluminum differential bushings allow for proper adjustment by using a lowered rear mount. They also positively locate the differential for better response and power transmission to the wheels. They will completely solve your vibration problem, however they may bring some gear noise into the car due to their solid mounting style.

Once again, awesome service from MM, I'll definitely be buying the differential bushing kit to solve that damn vibration.

Good luck!

I also have a higher speed vibe - wonder if this could be it. Mine seems to come and go, very odd - sometimes it's smooth at X speed, sometimes there's a vibe. :shrug:

I have the same issue where you barely can feel it in 4th but shift to 5th of 6th it starts vibrating. I have tried everything, to get the angle closer i put shims under the tranny mount and now the angles are almost the same with the driveshaft being at a different angle then the tranny and dif. I was told by a ford service writter that one of the shafts in the tranny isn't balanced. I could break my tranny and hopefully it gets covered under warranty and i have a 50/50 chance that the tranny they replace it with might or might not have the vibe problem.

racebronco2 said:

I have the same issue where you barely can feel it in 4th but shift to 5th of 6th it starts vibrating. I have tried everything, to get the angle closer i put shims under the tranny mount and now the angles are almost the same with the driveshaft being at a different angle then the tranny and dif. I was told by a ford service writter that one of the shafts in the tranny isn't balanced. I could break my tranny and hopefully it gets covered under warranty and i have a 50/50 chance that the tranny they replace it with might or might not have the vibe problem.

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I'll be installing a Spec 3+ and 26 spline input shaft over the winter. I will definitely be taking some measurments at the same time to make sure everything is fine.

Steeda30, I can't see how an incorrect pinion angle would create a vibration like the one you describe. I can always feel mine when I hit the right speed.