Rear End Vibration!?

I had a set of frpp 4.10's w/ new carrier bearings / Pinion bearings / installed on my 04 GT. On the way home, I noticed it making a rythmic roaring vibration at 70+ mph, around 85 it stops the rythem and just straight roars, no pulsing. If you are really listening for it, it actually starts around 45 mph. The car made no such noise with the stock 3.27's. I thought it might be a drive shaft imbalence, but i would have noticed it before when i had it over 100 mph. Anyone have any ideas?

I had the same thing it's like a humming/roar... and getting an aluminum driveshaft helps alot.. you have to understand everything is rotating alot faster so your going to get more noise like that. You should get the Aluminum driveshaft ASAP!!! because if not you'll have other problems like me having to replace bearings and seals on the rear of your tranny.

Did a reputable shop install the gears?

After my gear install it vibrates around 80mph or so and makes a humming noise, and REALLY vibrates over 120-125mph so bad that I let off REAL quick. With stock 3.27's I've had the car to 130-135mph and NO vibrations from the shaft what-so-ever.

Like joshheat said, the stock driveshaft is spinning much faster than it was designed to when you are flying down the road with aftermarket gears. The lower ratio you go, the more pronounced it is.

Sounds like who ever set up ur rear didn't do it right. Take it back if it vibrates.. It shouldn't do that.

The everything spining faster verdict is bs... sorry, but it should not vibrate. Now u can get some moaning from the gears if they were used before u put them in but an experiences tech who knows how to set up rear ends can do a new gear install flawlessly. You SHOULD not experience any type of vibrations from this swap.

Oh and futhermore, rear gears does not speed up the drive shaft, it still is relevant to what gear ur in (rpm), all the gear swap does is change how many times the tires rotate compared to a drive shaft rotation....

i get the drive shaft imbalance possibility, i dont think its that, just for the simple reason that

before 75mph = 2000rpm - no vibration
now 2000rpm = roughly 55mph with vibration

sooooo if the driveshaft was imbalenced before, it would have vibrated then at 75 or higher. I am wondering if the shop might have accidentally installed one of the bearing races in a little cockeyed, or if the lash is set too close. i watched him check the backlash and it was .10 which is good, but if the gears are heating up and expanding, maybe that lash is closing up and causing the roar. What do you guys think?

I think u need to take it back to them to look at it. If the car did NOT vibrate before they worked on it, then something is wrong and they need to recheck their work. Now it is possible that u have a u joint that decided to go bye bye with then new found stress from the torque multiplication happening just behind it, but I'd have the shop look at it again.


U can check u joint play by jacking the rear up, supporting it properly and look at how much play is in the drive shaft while the car is in gear, now this is not an exact science, but if it seems like there is more than an inch or so (an inch may be too much, someon help me here) of play, then u could have a bad u joint and that will cause u vibration and a real bad day if not fixed........

This is inaccurate information. Your driveshaft speed is directly related to the speed at which the tires are traveling - it can be no other way since they are mechanically connected. Lets say you are cruising down the highway in your 3.27 equipped stang at 75 MPH, your tach reads 2500 RPM, if you change up to a 4.10 your RPM at the same 75 MPH would be over 3100. Because your engine rpm translates directly to driveline rpm, your driveshaft is spinning faster with your 4.10's than with a 3.27.
I have personnally installed several hundred ring and pinions, and in any vehicle you will most certainly experience a driveline harmonic vibration when you change gear ratio if your driveline has not been properly prepared. Since Ford chose heavy steel large diameter driveshafts for our cars the vibration is quite pronounced when you change ratio. The answer is to reduce that rotating mass with a lighter shaft, without sacrificing strength, and it must have a good balance, .0010 out of balance is a nightmare to drive at freeway speeds with.

And why would you use a used ring and pinion, they're not that expensive new. You can get a good set of Motive's are like $150.

Ford racing part number M-4602-J will solve your problem it should cost around $250.00. Mine doesnt vibrate thru 130+ and you'll appreciate how much faster you can get there with those 4.10's.

I have balanced driveshafts for a living. The other post was correct .4th gear is 1to1.the change to alum is ok for other reasons but i go for rear and here is why.i am in the same shoe on my 03 cobra i pulled rear sent it to a pro.when 355 to 373 and complete bearing and clutches only part not done were spider gears.mine comes in at 70 but bearing noise is now getting worse.im gonna try changing pinion flange that is what centers driveshaft . But have heard storys that its normal BS

i didnt use a set of used gears. they were brand new frpp 4.10's. also, brand new timken bearings.

I just took it back to the shop. they hooked it up to a chassis ear, you chould hear something intermittantly chunking in the rear diff. so, they are going to tear it apart and check everything over on thursday. probably change the bearings again. if this doesnt fix it, i will ask them to hook the chassis ear up again, to some other things and see if we cant figure it out.

Sounds like a bearing may have taken a dump, or they forgot to properly install something. Clunking is bad to say the least, hopefully nothing got damaged.

well, after they work on it again, if it continues, i will cough up the 250 and get frpp's alluminum drive shaft

To under stand about shaft goto spicer.com a shaft longer than 72 inch centers will whip. My cobra stock came with a 10lb weight bolted to rear. The ford slip yoke havd many different harmonic balancing rings. In the 80 the trans yoke on a 2.3 turbo weighted more than 10 lbs. ford has many hidden problems. The sign for our shop says .We repair fords better idea!

Ok. Let me clarify. The drive shaft still rotates in direct coralation with the rpms. Now the GEARS do cause u to be at a higher rpm than before but ur d/s is not spining any faster than the motor. If u did NOT have a vibration before the gear install at ANY rpm then they did something wrong. Now the OP said that they heard something in the rear that was clunking so my diagnosis was right. Sorry if I was not this clear earlier. But I assume that he drives his car like we all do and it prob has seen red line before or at least some spirited driving. So therefore he would already noticed a vibration like that if it were a problem before the rear.

Now another possibility is if they didn't put the d/s on EXACTLY where/how it came off the rear yoke. Every repair manual I have ever read tells u to mark the d/s and yoke so u can put it back on exactly how it came off. That is b/c u can get vibrations like what most experience. Now an alum d/s will cause less normal vibrations b/c its less rotating mass, it will also make the everything run smoother.... again rotating mass. Quite nice pieces. So not a bad upgrade anyway. But not the fix for the op.

Yes they say to mark flange .the pininn flange has eight threaded holes .that is for other yokes .spicer has all their info that you can download also in spicer you will find a complete break down on most rears and instruction on both .

Like I said before and like other people stated At the Same MPH your driveshaft/driveline will be rotating faster and at a higher RPM. an aluminum driveshaft will solve this problem. I know because like I said before it worked for me and thousands of other people with this same problem. Other factors that contribute to more vibrations are lowering the vehicle and worn out bearings in the rear of the tranny. Lowering the vehicle changes the pinion angle and sometimes can cause vibrations. I had both lowered my car and worn out bearings. I replaced the bearings and put in a new aluminum driveshaft and .... no more vibrations.

Just an afterthought, the vibe on your car sounds like it's pretty low speed, wonder if they forgot to put friction modifier in and maybe the clutches on your limited slip are helping your vibration.......could happen. Make sure when they put oil back in they put in f/m. should only be like $5. D/S43161. Unless you use a synthetic with friction modifier included, then you should be OK.

Wouldn't that only happen when he was turning though? The diff doesn't do much untill one wheel starts spinning slower or faster than the other.

Never tried running a limited slip without it. I know it reduces friction between the clutches(therefore reducing chatter and vibration), and without it you would definitely notice around turns, at least it should make a racket. Idunno he said 40 MPH, seems pretty slow to have a vibe after a gear swap, the thing is going to vibrate, mine vibrated like a SOB at 80 after the 3.73's(from 2.73's) Driveshaft fixed it as expected, but I didnt notice anything at 40 thats pretty low speed.