Vibration in a 500 rpm window ?

[just4bob50]

Im having a hard time tracking down a vibration that happens around 3000 rpms in 4th and 2200 in 5th. Once Im in 5th at 2500 it is gone. In fact if Im at 2000 rpms in 5th it isnt there, just round 2200-2300. You can feel the vibration start and end in that narrow 500 rpm window. I thought it was my old worn out tires so I had some DR's mounted. It turns out one of the wheels was slightly bent on the inside but not enough that it wouldnt balance out ok. So any how I have 4:10's in an other wise stock rear with a stock suspension, no gear whine or anything. The next thing Im going to do is put my wheels from GT on the LX in the back and if the vibration is still there then I have no idea where to go next. Everything on the car is ne from the DS forward. My break in miles are almost done and I want to get it tuned but I dont want to put it on the dyno without figuring this out.

 

[Mr. Rustypwnz]

how do the u joints look? do you still have the dogbone?

one of mine would do this at around 1900-2200 somewhere in there, I swapped in a new pilot bearing (wasnt the prob) and u joints and it cleared it up alot...

 

[just4bob50]

Everything is new from the drive shaft forward. I checked both of my 5.0 and have found nothing resembling a dogbone. In another thread I asked for someone to post a pic of it.

 

[Bullitt347]

When referring to the "dogbone" I believe he is referring to the 5 lb/weight/dampener that hangs from the rear end. You say that everything is new from the driveshaft forward. Does that include the driveshaft, or just forward of the driveshaft? The fact that the vibration happens at different rpm's in 4th and 5th suggest that the vibration is mph related not rpm related. This is usually the case when a u-joint starts going bad, because the driveshaft speed changes with mph not rpm. So all rotating parts that are affected by mph and not rpm would be suspect. Wheels, tires, drums, rotors, driveshaft ect.

 

[LiquidGT]

Everything is new from the drive shaft forward. I checked both of my 5.0 and have found nothing resembling a dogbone. In another thread I asked for someone to post a pic of it.

Dog bone

Inbetween the axle and the mufflers, you cant miss it, either its there or its gone.





.

 

[just4bob50]

Thanks for the pic. I dont have it on either car. Yes the DS is new also, it is an aluminum one from ford racing. Whats pissin me off is that it is such a small window that it happens in, and then it goes away.

Also I found out the other day that if Im heavy on the throttle through 4th into 5th I dont notice the vibration, but normal driving it becomes rather obvious.

 

[just4bob50]

I changed the wheels around from my GT to the LX and the vibration was the same. If im hard on the throttle in 4th gear the vibration is hardly noticable, but normal driving it is strong enough that my rear veiw mirror will blur for the 2-300 rpms that the vibration is present. Ive had the car to 5000 rpms in 4th and almost 4000 in 5th and the vibration never returned. Im sure it has to do with the rear end, but I dont know where to go next. I would like to figure this out without spending uneccessary money, or just by throwing parts at it.

The suspension is 100% stock right now, and appears to in good shape.

 

[phutch11]

It sounds like you had a very similar problem that I had a little while back - have you lowered your car and/or installed a different engine/tranny combo than stock?

If so, I'd be willing to bet that your driveline angles are screwed up and that you need to shim up (or down) either the rear end and/or the tail of the transmission. Get yourself a magnetic angle finder and measure the angle that your rear end pinion makes relative to the driveshaft. Typically guys like to run a little bit of negative pinion - 2-4 degrees - so that the pinion can rotate upward to a near horizontal when under acceleration.

Then measure the angle that your transmission output shaft makes relative to the driveshaft - this angle MUST me almost exactly the same as the pinion angle (within 1-2 degrees) or you will have vibrations in a very narrow band like you are describing.

You must also make sure that your driveline angles are correct in the horizontal plane or the same issue can occur - however, this is much, much less likely to have gotten out of whack.

Good luck.