New thoughts about the drive shaft vibration issue...please read

85_SS_302_Coupe · Aug 28, 2008

I was flipping through a Car Craft mag and ran across a reader mail piece that talked about a drive train vibration. It sounded an awful lot like the problems so many Mustangers have that's usually "solved" by getting an aluminum shaft. Well the guy answering the mail said the problem is that the guy's car is lowered (he mentioned that in the article) and that changed the transmission/driveshaft angle and that's what is causing the vibration. He said that most shafts are designed to only have around 3 degrees of angle at the universals and lowering a car causes this to be too much angle and the universals are what cause the vibration.

SO, has anyone ever considered this? I figure it's logical since most people have their Mustangs lowered. I know most people blame it on gears, which could also be an issue, but the shaft angle really makes sense to me. The guy said a quick fix is to raise the tail of the transmission up with some shims and see if that solves the problem. Is there anyone with the vibration problem that would care to give this a shot and tell us if it changes anything?

93 LX · Aug 28, 2008

My Capri had this high speed vibration. I fixed it by swaping out my Mustangs DS in place of the Capri's. At the time I have the DS's out of both cars. Neither car has any high speed vibration. Go figure! LOL

BTW my Mustang is lowered 1" in the front and 1.5 in the back. The Capri is lowered .75 in the front and 1" in the back. The Mustang sit level and the Capri sits at a slight rake.

85_SS_302_Coupe · Aug 28, 2008

Well my buddy also told me that the "dogbone" on the rear end is actually matched up to the gear ratio...and that swapping the gear throws that balance off and that's what causes the vibration. I've never heard any facts that confirm that but i also know there's not a lot known about the dogbones either and most people just pitch'em.

795.0pacecar · Aug 28, 2008

The dogbone cancels out drifeshaft harmonics. TCCoA Transmission Article - Page 17

My driveshaft had a bad vibration a couple years ago, so I put new U joints in it and I haven't had a problem since.

Mr. Rustypwnz · Aug 28, 2008

ive had 2 foxes that were lowered and they almost sat on the ground, and I never had any probs with vibes although I put new u joints in both of em, just for the heck of it...

just4bob50 · Aug 28, 2008

85_SS_302_Coupe said:
Well my buddy also told me that the "dogbone" on the rear end is actually matched up to the gear ratio...and that swapping the gear throws that balance off and that's what causes the vibration. I've never heard any facts that confirm that but i also know there's not a lot known about the dogbones either and most people just pitch'em.

can some one put up a pic of what you are reffering to as the ( dog bone ).

I had a bad vibration on the LX i bought a few months ago, turns out the DS had 7... yes 7 counter weights on it.

Now with my new motor in I get a slight vibration at 55mph or so weather Im in 4th or 5th. The rear tires have threads showing and Im assuming its rom the tires, but I would like to know about what you were reffering to.

jrichker · Aug 28, 2008

The "dogbone" is a cast iron weight bolted to the bottom of the differential housing and is shaped like an oversize dogbone. Its sole purpose is to dampen out any vibrations in the driveline.

Cougar5.O · Aug 28, 2008

Lowering the car reduces the driveshaft angles - doh! So much for what you "heard"!

My vibration got better after lowering the car due to less DS angle - so there's that.

It is possible to have the angles reduced too much which WILL lead to potential u-joint damage as the joints need to rotate to keep the bearings properly lubricated - with no angle at either end of the driveshaft, u-joint damage is likely in short order.

I have a carbon fiber driveshaft since I couldn't stand the vibration any more and needed a strong driveshaft due to my heavy KB blown car and 4" longer driveshaft compared to a Fox 'Stang.

The dogbone is tuned to the resonant frequency of your system - mostly based on the length and other characteristics of the driveshaft. Since the driveshaft spins faster with gears, the tuning will still work, but it will be effective at different speeds.

wildstang90 · Aug 29, 2008

I had a 93 LX with no dog bone and 3.55 gears never a vibration at all. I now have a 90 LX with 3.73s and its got a dog bone on it car has 180K miles on it and no vibrations on it. I have though about taking the bone off just to see what it does.

85_SS_302_Coupe · Aug 29, 2008

Cougar5.O said:
Lowering the car reduces the driveshaft angles - doh! So much for what you "heard"!

Now that you mention it, yeah that is partially true, but there's a LOT that goes into it, like the height of your rear wheels and height of the tail shaft on the trany. I think the article i read was more along the lines of a custom hotrod type car where things were built more from scratch, making it much easier to throw off shaft angles...i just thought maybe the Mustang problems could be a spin off of this too.

Mustang5L5 · Aug 29, 2008

He's talking about the pinion angle. Yes, it can play a roll in driveline harmonics. You want your pinion angle to be set at 2-3 degrees. Stiffer bushings require less angle. The reason for this is because under load, the front of the differential rises and the idea os to have the pinion and d/s now become straight. When you lower the car, you make them straight or reduce the angle to begin with. Now when you are under load, the pinion flange rises and now you have a slight offset and the potential for vibration. The larger diameter AL driveshaft just dampens it slightly due to less mass and most people think it's the cure when it's really a band-aid

Most guys lower their car without a second thought to suspension geometry anyway. Hell nobody ever worrys about their instant center when the lower their car...but they ruin it anyway. How many threads have you ever seen that say "I lowered my car..help me correct my pinion angle".

And with that said...i never adjusted my pinion angle or corrected for instant center when i lowered my 5.0

magnj · Aug 29, 2008

I had a thread a few weeks back about vibration. AFAIK my car is not lowered. Still on the fence about what to do with mine, the whole damn thing shakes. I'm going to be pulling a lot of it apart this winter I think.

Rick James · Aug 29, 2008

I just recently had a severe vibration issue starting at 4500rps and up no matter what,long story short i put in a aluminum driveshaft and bam,gone.Now on the flipside my brother in laws old notch had a almost identicle problem and we replaced the flywheel,put in a aluminum driveshaft ,harmonic balancer ,got rid of the solid motor mounts for brand new ones and it didnt fix it............he ended up selling the car.Checked everything and could not find it

just4bob50 · Aug 31, 2008

Not that I want to beat a dead horse hear, but I was under both my GT & LX and dont see anything that resembles a ( dogbone ) the GT I was the 2nd owner at 17000 miles 100% stock. The LX is all new from the DS forward. Could someone post a pic. It may help me track down a problem.

85_SS_302_Coupe · Aug 31, 2008

just4bob50 said:
Not that I want to beat a dead horse hear, but I was under both my GT & LX and dont see anything that resembles a ( dogbone ) the GT I was the 2nd owner at 17000 miles 100% stock. The LX is all new from the DS forward. Could someone post a pic. It may help me track down a problem.

It's literally bolted to the bottom of the pumpkin on the rear axle...you CAN'T miss it unless it's not there anymore.

magnj · Sep 1, 2008

Yea, you'll know it if you see it. Bolted to the bottom of the pumpkin with a bracket and 2 bolts, weighs ~8 .5 pounds.

331fastcoupe · Sep 1, 2008

Rick James said:
I just recently had a severe vibration issue starting at 4500rps and up no matter what,long story short i put in a aluminum driveshaft and bam,gone.Now on the flipside my brother in laws old notch had a almost identicle problem and we replaced the flywheel,put in a aluminum driveshaft ,harmonic balancer ,got rid of the solid motor mounts for brand new ones and it didnt fix it............he ended up selling the car.Checked everything and could not find it

Same boat can't get rid of mine etheir, and after bringing it to a mustang shop he told me it was fine after he adjusted the pinion angle and some other things tires rim's ect. Nope didn't change.
I changed the motor, trans, clutch, new rear gears,ballanced motor sfi matching balancer ,mounts ,k-member had a alum. drive shaft balanced,
nothing changes, it only does it under hard aceleration 3-4 gear right in the thick tourqe curve. It's so bad it feels like the cars drive trane will shatter in peices.
My only conclusion is a bent axle. What do you guys think ? I have 30 grand in this car and I am about to crush it.

magnj · Sep 17, 2008

331fastcoupe said:
Same boat can't get rid of mine etheir, and after bringing it to a mustang shop he told me it was fine after he adjusted the pinion angle and some other things tires rim's ect. Nope didn't change.
I changed the motor, trans, clutch, new rear gears,ballanced motor sfi matching balancer ,mounts ,k-member had a alum. drive shaft balanced,
nothing changes, it only does it under hard aceleration 3-4 gear right in the thick tourqe curve. It's so bad it feels like the cars drive trane will shatter in peices.
My only conclusion is a bent axle. What do you guys think ? I have 30 grand in this car and I am about to crush it.

could be an axle for sure, wondering the same thing myself about my issue, now I need to figure out how they hell I figure if it's straight or not?

NKau · Sep 19, 2008

magnj said:
could be an axle for sure, wondering the same thing myself about my issue, now I need to figure out how they hell I figure if it's straight or not?

Same way you check a pushrod; roll it along a level surface (like a table) with the hub hanging off the end. You'll know if it's twisted/bent right away.

magnj · Sep 20, 2008

Probably should have done that first huh. Gonna check all the tires first, pull the driveshaft, etc...opening the rear housing was not fun for me.

New thoughts about the drive shaft vibration issue...please read

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