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Gun Jam 01-05-2009 02:09 PM

pinion angle or driveline?
Okay I rebuilt my trans and it turns out the car still develops a vibration at about 83mph. It gets worse when you back off the gas. Was worse before the trans rebuild but is still too sever to ignore.

1) trans is healthy with new tail shaft bushing
2) drive shaft has all new bearings and was balanced twice by the same guys.
3) Cant really feel the vibration below starts fast and gets worse very quickly
4) is a very fast vibration like a buzz (not like a tire) You can fell it in the floor and in the shift level and gets worse when you back off the gas and slow down.
5) engine RPM is about 2500 when vibration starts. this engine runs smooth past 5000 rpm

I'm running 3:80 gears. I was told by a shop that this increases the velocity of the DS and it must be balance perfect. The shop also HIGHLY doubted it was pinion angle as that would be noticeable at 40 mph not 80...they were very sure that the DS was the problem. They suggested I get a second opnion on the DS balance

The last guy that balanced the DS was very sure it was balanced well.

What do you guys think?

if im running 3:80 gears on a 25" tire at 2500 rpm at 80mph on a .63:1 OD what is my DS RPM? IF im in 4th gear at 1:1 wouldnt DS rpm=engine rpm I dont see how it could rotate at a different speed because its a direct drive in 4th.

According to my calculation the vibration starts when DS RPM reaches 4200...Is this right?


urban_cowboy 01-05-2009 04:01 PM

Does it happen at 80 when you put it in neutral? In other words, is it speed related or engine rpm related? I had a strange vibration/noise that ended up being a worn out LS differential. After replacing that, all was good...course I replaced u joints and driveshaft before I narrowed it down to the rearend.

kalli 01-05-2009 04:21 PM

yah. just with cowboys thought. if you can run 80 in 4th gear as well try to see if you have vibration at same speed or same engine rpm.
I'm plagued by the same issue (just at 60, not at 80). and I still blaem the DS. I have just replaced my rear springs to 5leaf (3 days ago) and I haven't driven the car because of blown gaskets. I was planning to replace heads ( i guess everyone knows that by now) but I freaked at prices and shipping, duty/vat so I'm doing only gaskets.
I'm quite sure that my pinion angle changed again because of this swap. but at least now I know I won't change anything on the rear anymore, so I can get proper measurements for dennys aluminum driveshaft if needs be

I cannot tell for sure, but I'd say DS as well !AND! congrats to your 5spd rebuilt!!!!

silver91gt 01-05-2009 04:29 PM

were your tires and rims balanced? a missing weight can cause a vibration.

Gun Jam 01-05-2009 04:33 PM

Putting the clutch in causes the vibration to get worse for a short time until speed drops off. Shifting into N does not help...I did test it in 4th before the rebuild and it did the same thing.

After the rebuild the vibration was less..(had worn tailshaft bushing) now the whole thing would probably grenade at 120mph as apposed to 95mph. Its really weird the vibration increase exponentially with speed. almost none at 75 very noticeable at 80 twice as bad at 85mph and keeps getting worse. never tired 90 yet but Im guessing twice as bad as 85.

Again...DS has new U joint bearings and balanced right after they were replaced.

wheels were balanced sometime ago could have lost a weight...But this feels nothing like a tire weight (they tend to be low frequency and get worse evenly with speed) This vibration is high frequency probably 60 cycles a second and makes a horrid buzzing grinding sounds that comes from near the tailshaft of the trans. You can clearly feel each cycle of the vibration in the shifter...a lot like one of those crappy orbital sanders that make your hand hurt after a few seconds. THAT frequency of vibration.


67 evil eleanor 01-05-2009 04:36 PM

"9 out of 10 times" its tire/rim related. Heres some good reading. It seems that more often now I see a lot of just BAD tires. Either they are out of round or have a weak spot in them like a broken belt. The one way to eliminate the possibility of the tire/rims is to have them "road forced".

Gun Jam 01-05-2009 04:45 PM

Crap...Im sorry!!! I forgot the most important part of the story.....It started as soon as i did the t5 swap....never had this type of vibration before.. until I swapped the c4 for the t5. (rear end gears where swapped from 3:00 to 3:80s about 2 weeks later... not sure how much this effected the vibration as I had very little high speed time on it)

Again you dont feel tire balance issues in just the shifter and floor...they dont make a grinding sound either.


urban_cowboy 01-05-2009 04:56 PM

Can you play with the throttle so that at some point of driveline pressure the vibration goes away or it always there?

Gun Jam 01-05-2009 05:10 PM

once it starts at about 80 it never goes away accelerating make it worse and letting off the gas makes it worse especially when your well into the vibration like 85mph.
only time its not noticeable is under about 78 mph...with the exception of the shift handle which still has a little purr to it..that goes away below 70 i think.


cprstreetmachines 01-05-2009 08:01 PM

Because you said it gets worse when you slow, can all but promise it's the u-joint angles in the front or rear. When you accel, the pinion tries to shoot up. On decel, just the opposite. Your angle on the pinion in the rear is increased when you decel, and putting her past her happy point. If you test, you may notice you can downshift and engine brake in 2nd or 3rd, no going fast.....but you may feel it as low as 40 mph if you can get that pinion to really dive. T-5 conversion crossmembers aren't exactly what one calls accurate. Kinda like my spelling. But add that to Fords already 3-6 degree angle, and it's bad news.

The feeling in your shifter is called a "busy" feeling. That's what is does right before the harmonics get stupid. get yourself a good angle finder, and I think you'll find your problem.

I only say that because it's greater on the deccel. If you clutch it, and it doesn't really go away, the driveshaft speed is still high, and is probably a balance issue. Try a different driveshaft shop.

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