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

[94-302-vert]

Both.

The engine supplies the power that spins everything, but the relationship between driveshaft and tire RPM (speed) are the rear gears.

To make the math easy:

When your tires are roating at say 500 RPM (call this 100 MPH) with 2:1 gears your driveshaft is rotating at 1000 rpm.

Change the gears to 4:1 and your driveshaft is now turning 2000 RPM at the same tire RPM (speed) as before (500 tire RPM).

Now to look at this another way if you keep your driveshaft RPM constant at say 3500 RPM. With stock gears this may have been at 130 MPH, now with say 3:73 gears this occurs at 95 MPH (130 * original ratio / new ratio).

Did I lose ya?

 

[Blk91stang]

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.

You are missing my point. Look at the output shaft on the transmission, than look at the yoke for the driveshaft. Tell me exactly how you managed to slide it back on the EXACT SAME splines. How did you mark the splines on the output shaft while being able to line it all up upon installation??? I'd like to know this because I cannot think of any way. Your thinking of the rear end side of the driveshaft which is correct.

 

[Blk91stang]

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.

NO NO NO!!!! The driveshaft does NOT SPIN FASTER!!!! The ONLY WAY a driveshaft will spin faster is if you rev your motor up higher than before. THAT IS THE ONLY WAY MY FRIENDS!!!!! Think about what is going on before posting.

 

[94-302-vert]

And after gears at the same speed you run at higher engine RPM...

Basic physics man.... Trust me they don't give mechanical engineering degrees to just anybody...

 

[Pokageek]

So all of that = the drive-shaft is spinning faster BUT only because I am revving the engine higher to get the same speed out of the car. I think I got it now.. Thanks guys.

But just to be sure..the driveshaft max spin is going to be subject to how high I can rev the engine, so in that sense it will never spin higher than the revolutions of the engine.. Say 6,000 RPM right? At 75mph, and 3.08's say I was spinning (just for comparison) 1850rpm. Add 3.90's and I am spinning like 2400RPM same speed. Yes, I am spinning faster BUT I would have felt the vibration at 2400 in first, second, third and fourth...but in my case, I don't. I only feel it at certain speeds like 75mph and up.

I still think my bearing is going bad but will report back..Thanks for all the input.

 

[Blk91stang]

So all of that = the drive-shaft is spinning faster BUT only because I am revving the engine higher to get the same speed out of the car. I think I got it now.. Thanks guys.

But just to be sure..the driveshaft max spin is going to be subject to how high I can rev the engine, so in that sense it will never spin higher than the revolutions of the engine.. Say 6,000 RPM right? At 75mph, and 3.08's say I was spinning (just for comparison) 1850rpm. Add 3.90's and I am spinning like 2400RPM same speed. Yes, I am spinning faster BUT I would have felt the vibration at 2400 in first, second, third and fourth...but in my case, I don't. I only feel it at certain speeds like 75mph and up.

I still think my bearing is going bad but will report back..Thanks for all the input.

You are 90% correct. You have the right idea but you still wouldnt feel it in 1st 2nd and 3rd gears because you have a gearbox (transmission) which is giving you the change in ratio as you shift gears. The driveshaft spins faster, the faster you go which is why you are feeling it at 75mph now. Lets say you had stock 3.08's again and ran it to about 95mph bearing the same rpm at 75mph with 3.90's, you should feel the same vibration. It's just that you didnt reach that rpm in that gear ratio before the gear swap, hence the vibrations you have now.

 

[Blk91stang]

And after gears at the same speed you run at higher engine RPM...

Basic physics man.... Trust me they don't give mechanical engineering degrees to just anybody...

Wow, thanks!!

Pokageek pretty much hit the nail on the head with his last post and am thankful that someone understands what is going on.

Assuming the same 5th gear ratio, RPM vs. Driveshaft speed will NEVER change no matter what gear you use. It's only the fact you have never achieved those rpms with the stock gears that now you are having a problem.

I think this pretty much sums up everything that I wrote in the last 10 replies.

Funny, I'm in my 4th year becoming a Mechanical Engineer. I DO know exactly what I'm talking about, it may just be difficult for me to type it out hence the confusion. Hope this post helps tho.

 

[@smodeus]

Wow this is great have a 94 mustang and put 3.73 about 2 years ago and had this wicked vibration above 85 mph, thought was my tires so on and so on never that it could be the drive shaft and needed to be balance. Been lazy about since the only time I really go over 85 is the inerstate. Going to go get that done.

 

[Pokageek]

As usual I like closure on the questions so the problem was.......

LOOSE DRIVESHAFT BOLTS!!! They were getting more loose every day! Whenever a mechanic puts a new rear in make sure they tighten the damn bolts.