3.90 gears...I have a vibration in the driveline..remove dog bone?
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94-302-vert
Active Member
If it didn't happen at 83 mph before then it is not the D/S. At this point the 3.55 will have the same d/s RPM as the 3.90 at 75...
So I vote install.
So I vote install.
precision just indicates how well the shaft is balanced. When I put my 3:73s in I had vibration and so I had my driveshaft re-balanced and all the vibration went away. The shaft is spinning faster and the balance of the factory is not good enough. It cost me $30 - check around and see what it would cost you.
OK Guy's ... and (did ya'll see?) ... Girl 
You can laugh if you wanna
but
Several years ago I saw the same kinda thread where this guy had the same prob.
This one fellow replied and said the prob was the ds out of balance like Tom said.
Everyone in the thread challenged this guy and just poo poo him to death with stuff like .............
if it didn't vibe before the gears ... it shouldn't vibe after.
After all the hub bub and finger pointing this guy calmly said in one reply he had a hard time believing the gear swap would cause a prob himself.
This guy went on to say he did a little test to see if the ds could be the prob.
He took a hose clamp and put it on the ds to check ... better or worse
After about three drives he found a spot where the vibes were almost gone.
He then told them he felt the test was valid enough to warrant spending money on the ds balance. He finished his post saying the balance took care of the prob.
The thread completely died at that point
Now here is the best part in that whole thread
A few days later I see a notify the thread is active again
The orginal poster did the hose clamp thing with the same results
All those poo pooers of that guy ..........
had to eat a big ole slice of humble pie
It was great ... I loved it 
Grady
You can laugh if you wanna
but
Several years ago I saw the same kinda thread where this guy had the same prob.
This one fellow replied and said the prob was the ds out of balance like Tom said.
Everyone in the thread challenged this guy and just poo poo him to death with stuff like .............
if it didn't vibe before the gears ... it shouldn't vibe after.
After all the hub bub and finger pointing this guy calmly said in one reply he had a hard time believing the gear swap would cause a prob himself.
This guy went on to say he did a little test to see if the ds could be the prob.
He took a hose clamp and put it on the ds to check ... better or worse
After about three drives he found a spot where the vibes were almost gone.
He then told them he felt the test was valid enough to warrant spending money on the ds balance. He finished his post saying the balance took care of the prob.
The thread completely died at that point
Now here is the best part in that whole thread
A few days later I see a notify the thread is active again
The orginal poster did the hose clamp thing with the same results
All those poo pooers of that guy ..........
had to eat a big ole slice of humble pie
It was great
... I loved it Grady
Blk91stang
Member
with the 3.55s when the tires did one complete revolution the driveshaft did 3.55, with 3.73s, the driveshaft does 3.73 revolutions to one revolution of the tires. And at 60, with stockish sized tires, thats around 800 revolutions per minute, with the driveshaft going 2840 revolutions per minute with 3.55s, and 2984 with 3.73s. may not sound like a big difference, but figure with stock 2.73s the driveshaft would only be goin 2184 revolutions a minute, you can see where a slight imbalance would become noticeable with almost 1000 extra revolutions a minute, at 60
Blk91stang
Member
mkk50 said:
Yes I understand gear reduction at points in a system but the driveshaft will spin no faster than before at the same rpm. It will only apply to road speed. Therefore if he did not have a vibration from 0-6,000rpms before, he still should not feel it after the gear change because it has absolutly NOTHING to do with it. Of course unless you are now spinning the motor higher than before, than you will see a problem which is not the case in this dilemma.
Change to whatever gear you want and your driveshaft will sping no faster than before assuming, Driveshaft Speed vs. RPMS.
I rest my case.
Blk91stang
Member
I am only arguing the fact that rear end gears do not change the driveshaft speed. I do agree with you that some people have imbalanced driveshafts.
why didn't the very same shaft vibrate with the stock 3:08s then? But it did with 3:73s......and balancing got rid of it. I don't argue with success.
Blk91stang
Member
tmoss said:
BECAUSE the driveshaft was pulled out and put back in when you got the rear gears changed. The driveshaft was not put back in EXACTLY on the same splines causing your vibration because the stock shaft isn't 100% balanced perfectly.
Put your stock 3.08's back in and stock balance on the driveshaft and guaranteed you will have vibation problems like u did with the 3.73's UNLESS you get real lucky and line it back up on the same splines from the factory. If this happens, take the driveshaft out and turn 180 degrees and stick back in, hello vibrations!!
Trust me, the driveshaft STILL does NOT turn faster when you change gears!! It's simple math and having the ability to visualize the assembly. I respect you for your porting work but to still think that gears have some kind of effect on the driveshaft speed which in turn causing vibration is stupid. Just think about whats happening and see how its possible.
If that wasn't enough, here is some experience with this issue. I got 3:55's put in and no vibrations, than took my shaft out and put it back in, got vibrations. Took it back out and put it back in, no more vibrations. Took it out a 3rd time, put it back in and got vibrations. How is that possible without me touching anything else? It's obvious the shaft cannot be lined back up EXACTLY as before so that is the only thing I was changing and it changed the vibrations I was getting. This WAS your problem also but you didn't get lucky enough to have it line back up and become balanced. I'm going through this right now and have done a lot of experiments to figure this out. I am assuming you got yours balanced right after you had the problem and was happy it got fixed. This leads me to believe you didnt experiment to narrow down the actual culprit of the problem and blamed the gears on account of it being a coincident.
Sorry for the long post, just trying to let people know gears don't effect the driveshaft, taking it out and putting it back in does.
you couldn't be more wrong wrong - you ass-umed I did not mark the drive shaft - but it was permanently marked on both the yoke and input shaft for alignment just for the reason stated. Never ass-ume you know what happened - ask.
94-302-vert
Active Member
The reason that we have problems with the DS balance after gears is the higher rotational speeds the ds now sees.
Now when going from 2.73 to 3.73 or 4.10's the DS is now spinning a lot faster than you probably had it before.
The difference between 3.55 and 3.90 is mild though. So while DS is impt I would not rule out the gears. This is coming from a guy that needed both.
I ruled my DS in by getting one from someone here for free that needed to be rebuilt and put it on to see if it changed the symptoms. everything was different, herego new driveshaft.
Now when going from 2.73 to 3.73 or 4.10's the DS is now spinning a lot faster than you probably had it before.
The difference between 3.55 and 3.90 is mild though. So while DS is impt I would not rule out the gears. This is coming from a guy that needed both.
I ruled my DS in by getting one from someone here for free that needed to be rebuilt and put it on to see if it changed the symptoms. everything was different, herego new driveshaft.
