3.90 gears...I have a vibration in the driveline..remove dog bone?

Pokageek · Sep 5, 2006

Should I remove this? Could it cause a vibration since going from 3.55's to 3.90's? OR could it be my almost new FRPP aluminum ds that doesn't like higher speeds? It happens in all gears and neutral at like 75mph and up. Thanks.

final5-0 · Sep 5, 2006

I'd dump it

See what happens

Simple to get off and back on if you wish

Grady

TorqueO · Sep 5, 2006

Sounds like its the drive shaft or the gears were not installed properly. The dog bone is there to prevent vibration with the stock gears. I have removed mine some months after I had my gears installed.

tmoss · Sep 6, 2006

get a precision balance on your drive shaft and the problem will go away........

Pokageek · Sep 6, 2006

tmoss said:
get a precision balance on your drive shaft and the problem will go away........

Thanks. What is a precision balance? Does it cost as much as the driveshaft? I have heard balancing can cost a lot.

94-302-vert · Sep 6, 2006

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.

tmoss · Sep 6, 2006

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.

Pokageek · Sep 6, 2006

tmoss said:
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. Thanks very much. Any recommendations around PA or MD?

EDIT - I found a machine shop that will do it but also having the rear checked. TMoss thanks man!

final5-0 · Sep 6, 2006

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

Pokageek · Sep 6, 2006

Wow! A clamp! That's amazing.

Blk91stang · Sep 6, 2006

tmoss said:
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.

Why would your driveshaft spin faster???

mkk50 · Sep 6, 2006

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 · Sep 10, 2006

mkk50 said:
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

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.

tmoss · Sep 10, 2006

Got rid of my vibration as well as others - I guess we'd rest our case.

Blk91stang · Sep 10, 2006

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.

tmoss · Sep 11, 2006

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 · Sep 11, 2006

tmoss said:
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.

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.

tmoss · Sep 12, 2006

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 · Sep 12, 2006

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.

Pokageek · Sep 12, 2006

What spins the ds, the gears or the engine..

3.90 gears...I have a vibration in the driveline..remove dog bone?

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