I finally got my 3.73s in, and I got them installed along with my zexel torsen unit yesterday. Under deceleration and over speedbumps, I feel a vibration now. At first I thought that maybe I had a control arm or the torque arm loose, since it started when I got my gears installed, but I spoke to the shop today and they never touched any of that. The guy did tell me though that he found my driveshaft bolted down only on one side. The other side was only hand tightened. I guess in my hurry to put the car back together two weeks ago after replacing the throwout bearing I forgot to tighten ALL the driveshaft bolts(Yes, I feel stupid now ).



Would this throw my driveshaft out of balance? If it is, why doesn't it vibrate under acceleration, or at a steady speed?

 

if you suspect you have a drive shaft problem have it checked for run out. i've shortened and made a boat load of drive shafts and never balanced anything. all the parts are "balanced" to start with and doing the assembly doesn't change anything. i'd suspect the problem lies inside the rear end cover. is the shop experienced and have the correct tools for doing a ring and pinion install?

 

The more I think about it, the less sense it makes to me that it is a driveshaft problem though. Supposing it were damaged, or out of balance, it should be vibrating all of the time. But it only vibrates under deceleration, speedbumps, and when driving on an uneven road. I had to drive through a little field of grass today, it would vibrate alot regardless of whether or not I was in gear or in neutral. I'm gonna try to take a look under the car tomorrow. Would a loose torque arm mount do this?

The shop was recommended to me by a local performance shop. I spoke to the guy who did the install, and he was pretty knowledgeable. I know they work on Ford rear ends pretty frequently, so they should know what they are doing.

 

I'm sure you had them replace all the bearings and races in the diff. I think the slp posi comes with the carrier bearings, but no races--using those with the your used races could cause problem. I've also seen some shops reuse the axle bearings and/or damage them when installing the axles. Could also be the pinion bearings, if they didn't get the correct preload. Take it back and see what they have to say. If you didn't get bearings, that's where I would start, fresh bearings and races for everything. Oh, and make sure it has enough fluid.

 

So you went from some grocery-cart setup like 2.73s to 3.73s, right?

So your driveshaft RPM at 60 MPH went from whatever it used to be at 60 MPH (probably somewhere around 2200 RPM), to to where now at 60 MPH it's about what it used to be at around 85 MPH (somewhere around 3000 RPM). It's natural that there will be more vibration.

But since it changes with the direction of power flow, it sounds to me like the pinion preload isn't right.... sounds like it's too loose.

This is why I always use a "crush sleeve eliminator".... it allows you to set the preload and then tighten the pinion nut until it screams for mercy, instead of having to be dainty and lady-like when tightening it to avoid putting too much preload on it. Correct bearing prelaod is usually attained at around 125-150 ft-lbs, which is inadequate for keeping it from backing off. Plus, if the crush sleeve crushes a bit too much, then the torque on the nut will just "go away" all on its own, even if it hasn't moved; but the pinion will be free to wander around in its bearings instead of being firmly held in the perfect place.

 

Yeah, I got new axle bearings installed when the gears were put in. I decided it would be a good idea to replace them while the rear was apart.

I understand that any vibration that would have been really subtle would be more pronounced with steeper gears if it was a driveshaft related problem, but I don't feel a vibration at high speeds. The vibration is noticeable under deceleration, but mostly over bumps. Especially at low speeds.

Oh by the way, I used a solid crush sleeve when the gears were installed. I had forgotten to mention that.

What exactly is a crush sleeve eliminator? I've never heard of that. Is that by any chance another name for a solid crush sleeve?

 

92blue i just want to know how your car feels now.

that 305 with 3.73's and a T-56 ought to move!

 

KingTal0n, the car certainly runs better than before (not surprisingly), but its still not very quick. I have a set of sfcs that need to get welded in, and then I'll start looking around for a 350 block. Little by little, I'll build it as I get money. Hoping for 375 -400 hp @ the flywheel. Stay in touch man, I'd love to hear how your car is coming along.

Anyways, I found the "vibration" today. I crawled up under the car, and started checking things. After a while of looking around, I noticed my left front shock was wet. I took a closer look at it, and its leaking. I took the car down from the ramps, and tried to push down on that end of the car, and sure enough, the car vibrates on its way down. I guess it was jsut a coincidence that it started to vibrate the day after I got the gears installed.