I have a slight vibration that started at 62 mph , went away by 70 and returned at 90. I looked for 2 years for it's source, even replacing the transmission with a new Tremec, all universals were fine except I do go through a driveshaft every year for the last 3 years. I twist them so the universals no longer line up. The last one was a 3 inch with 11 gage tubing. Hopefully it will survive.
Anyway I began to wonder about the alignment of the rearend , driveshaft which is now shortened to a 23 inch tube and the transmission motor combination.
I indexed the bellhousing so I now it was perfect.
I ran a string down the center of the motor and the center of the rearend. The motor center was easy to find using a Tee square and putting masking tape on the deep sump of the oil pan. For the rear end I used the center of the bolt in the rear spring

I then made up short lengths of string and hung them over the slip yoke in the back of the transmission and on the pinion and let these bob lines hang down. I found the driveshaft was really off to one side.
Later I put the car on jack stands and started to really look at this missalignment.
First the pinion is offset to the right of the center of the rear carrier by about 1.1 inches so if it extends straight forward it would miss the back of the transmission. I know the motor is also offset but it would miss by .70 inches. I then checked the centering of the carrier in the frame and they are centered. I also checked the detent in the cover plate that holds the spring and once again it is centered.
I determed that I would have to rotate the nose or pinion output .7 inches towards the drivers side to make the pinion point to the rear of the transmssion and align the motor and the rearend. I cut and modified the front pinion mount to move this pinion .7 over.

It is not finished but needs further checking before powder coating.
I also had to slot a few of the holes in the crossmember to rotate the housing but made a precision 1/8 plate that was later welded on the top of the crossmember to assure future alignment.
After assembling the modified pieces and running a new string, along with a brass plumb bob in the hole for the spring detent , along with the plumb lines over the pinion and slip yoke everything looks like it is aligned.
I know that a .7 misalignment might not cause the problem but it might. When a u joint rotates it speeds up and slows down, it is not a constant velocity joint so it moves a irregualar speed. If the back joint and front joint are not on the same angle the shaft is subjected to this speed up and slow down at different times and the drive shaft must absorb this changing of speed. If the output angle from the transmission matches the output angle from the pinion everything speeds up and slows at the same rate . I had about 4 degrees comming off the pinion and almost 0 off the transmission. This is a miss match I hope to eliminate. Like I said I destroy a driveshaft every year do to twisting. Is it the power or the uneven movement of my u joints?
I am running this past you guys hoping someone will shed some light on this misalignment problem or get me thinking in a different direction.

 

I think the driveshaft twisting is the result of 1000 ft lbs. of torque passing through it. ;)

In a Corvette, the differential is fixed. In a truck, the diff is almost never lined up, depending on load, bumps in the street etc. I don't think that having the diff slightly out of alignment would cause the trouble you are having (vibration), or trucks would experience vibrations all the time. Of course, aligning the drivetrain perfectly would be best, especially if it seems as far out as yours (note, I have never checked mine, or had the need to). Let us know if the alignment changes or eliminates your vibration!

Other things to consider are driveshaft balance, tire balance, flywheel and clutch balance, and even maybe half shaft balance. Although I am sure that you have investigated most of these.

 

Harmonic vibrations are nasty to find. I think your on the right track for your vibration I'd be interested to see if this solves it.
I've always wondered if the basic non-movement of the driveshaft u-joints
was worse overall for wear and vibrations or better. My guess would be that they get a "memory" after awhile and would be intolerant of operating out of that range.

Scott

 

Since I am on my 3rd drive shaft I assume balance is not a problem. Each time I get a new driveshaft it is balanced. I have never balanced the half shafts. The vibration is there regardless of rpm. It is speed related. I have gone to 80 , pushing in the clutch and let it coast down to 55 and it still has that vibration going down past 70 to 62.
I thought the half shafts are not turning fast enough to cause this balance problem but I have them out and might get them balanced.
Trucks sometimes use constant velocity joints . My dodge has them. Even on big trucks the angle between output and input must be the same to prevent vibration. If you come out of the trany at say 10 degrees and your pinion angle is 10 degrees the speeding up and slowing down of the universals cancels each other out. But if one is 10 degrees and the other is 5 degrees you will vibrate. The magazine articles say that 4 degrees difference is the maximum without going to constant velocity joints.
I motor could be twisting the drive shaft but this time I went 3 inch instead of what they call 2 inch.
I know that .7 inches misalignment doesn't sound like much but over 2 feet is looks like alot when a string and plumb lines are run.
Have a nice day
:)

 

Scott even though the rear end is fixed the pinion on all our C3's are offset to the right by about 1.1 inches forcing the drive shaft universal's to work. Universals get what you call Bernelling, which is speed bumps or pot holes like roads that cause a rough feeling when flexed.

 

Wonder if that is what I am getting in my drive train. It just all of sudden developed a nasty chatter bump vibration. Have not diagnosed it fully but found the tail shaft of the tranny has a good bit of play at the yoke .

 

1.1 inches, interesting I would of thought on centerline. I know what your saying about u-joints, I work on alot of industrial and agricultural equipment.

Scott

 

Norval I know that you like everything perfect!:) Any defection to a U-joint is going to cause your twisted shaft. I wonder if your offset is caused by time and frame torque

Years ago when I went to my 700R4 and shortened drive shaft i took the utmost care in aligning the crankshaft to the rear end. I only had Vertical missalignment. That's when I first found out that the motor was offset to the passenger side. My U-joints last about 2 years so that must mean that I only have half of the power you do :smash:

 

Very :cool: thread, thanks mr. norval
:chevy :chevy :chevy

 

Norval;
I like the string Idea. That looks like a great way to measure centerlines...
It's also interesting to see that you found basically the same thing I did, mismatched u-joint angles.

As far as destroying drive shafts due to this vibration it is very possible in my mind. I mean as the shaft rotates it twists 4 times per rev. The force required to twist it is added to the engine load and could really add up!

Personally I'd call inland empire and tell them your problem of destroying shafts, if anyone can help they can.

Inland empire has a great discussion about this on their web site
Click on the "DRIVE SHAFT ANGLES RE-VISITED" link

http://www.iedls.com/guide.html

 

Scott78. If you look at a rear end sitting on the bench you will definitely see the pinion comming off to one side. I measured it's offset using the yoke seals flat area with a straight edge. It is very noticable with the eye.

George My offset is not caused by frame twisting or age. My frame is perfect without rain since 1980. I started checking for anything bent but everything measures out equal. If anyone has a spare rearend sitting on the bench look at it and you will see the pinion really offset to the passengers side. I know the motor is offset to the right, I found this out when cutting the hole in the hood for the blower but it still missmatches the transmission by .7 inches and once you run a string a plumb lines hanging off the yoke and pinion it really looks off.
With the radical modification of the front rear end bracket everything looks in line.
The last 3 shafts that I have taken in for testing have all been twisted. The universals are no longer in phase. They have been the 2 inch shafts and I have now upgraded to a 3 inch. I believe it is 3rd gear that is starting to hook where I am twisting them.
George back to the power thing I certainly don't have twice your power. Wish I did but only Merlin or Monty might aproach that much power.

427V8 Keith thank you for the link. I will certainly look it up and try to get some answers. I have been reading alot of 4 wheel articles and they can't stress enough about the matching of angles between the output and input to prevent drive shaft vibration. My dodge truck has a short front driveshaft and it uses a constant velocity joint. Alot of big trucks also use constant velocity joint. I was in the drive shaft shop and they were building a huge drive shaft, at least 4 inches and very long and it had a constant velocity joint.

 

U-Joints need to rotate. That is why the trans and pinion are offset. I don't think you are doing the right thing by changing the pinion mount. Look at the pinion yoke. Mine is weighted.
If it is a torque issue, now that you went to a biger driveshaft, the next part that will twist is your halfshafts.

I work at GM Racing and we did a study a few years ago to find the optimum drive shaft angle for Winston Cup cars. I know that zero was not good. I am looking for the report, but I beleave that the magic number was 4 deg. When I find the report, I will pass on the number.

 

Norval.
All C3 (and I presume C2) Corvettes have the engine offset to the right by one inch. Just check the length of the engine mount brackets welded to the frame- the left one is about 7 inches long, the right one about 5 inches. I believe this was done so as to give the driver 's side more foot room, especially if the car was fitted with three pedals. Imagine the problems we Aussies have when we convert to right hand drive! Especially with a manual! As for offset, it is my understanding that some lateral (or vertical) misalignment doesn't matter, as long as the output shaft of the transmission and the input shaft of the differential are parallel to each other. Just think of how much a driveshaft drops in a car with a live rear end and leaf springs. They'll do hundreds of thousands of miles without trouble. I strongly agree with the post to speak to Inland Empire or any reputable driveline shop.

Regards from Down Under

aussiejohn.

 

Dana has documents that show 3 degrees for the max U-joint angle. Anymore and you start to loose u-joint life. Less is better

Live axle cars do have changine U-Joint angles but as the axle moves up and down the angles STILL cancel.

If I have learned one thing, it's that race cars and street cars are different. Just because a Winston cup cars needs 4 degrees of pinion angle for best performance doesn't mean a street car does. After all a winston cup car is running full throttle most of the time, my car rarely sees full throttle.
I'm pretty convinved that you want the u--joint angles to be as small as possible. They will never be zero no matter how hard we try.

Also the pinion is offset because it has to be. Thats the only way to keep the half shafts the same length and use a ring and pinion gear.

I think Norval is doing the right thing. I moved the transmision 3/4" towards the passenger side to make the angles better. Believe me it made a world of difference. His solution may align everything even better with smaller u-joint angles.

Norval, have you looked into constant velocity joints for the Vette???
It could solve all the problems, it they can handle the power...

 

Thank you 69schemers. The four degrees is the critical angle that you can not exceed in difference between the two ends or vibration will occur. I can see that a little movement of the joint is required for lubing. If no movement the same needles would continually take the load and universal life would be short.

 

aussiejohn I know about the offset and when I stringed the motor I went down the centerline of the motor and while the front of the driveshaft lines up pretty well the pinion was way off. My 2 angles are not nearly the same so the driveshaft must absorb the speeding up and slowing down of the universals.

427V8 I could easily install a constant velocity joint but it is big and has and extra univeral so my shaft would have 3 in total plus with the room in the tunnel it could cause interference problems. I am not modifying anything that can't be put right back with just another front pinion mount. I will go with the realigned pinion for now and if it doesn't work I will go back to the old setup. I also feel that the better I can align things the better. I now have the 3 inch diveshaft and hopefully it will last.

 

OK,
First, A Winston Cup car goes from off throtte to wide open twice every lap. Coming off the corner the throttle is feathered do to lack of traction. A u-joint can not be zero or the bearings will not rotate. This rotation is important for u-joint life.

Second, GM has used this trans / pinion angle for 14 years with no changes to this alienment. Don't you thing that if there was a problem GM would have changed it. I don't know, maybe you know more then the Corvette engineers knew. It just does not make sense.

 

I'm not an expert, but I thought that the biggest concern was that the output shaft of the tranny and the input shaft of the diff are parallel. They don't need to be on the same centerline. When people lift their trucks some tilt the diff upwards to cut down on the angle, and taking things out of parallel causes more vibrations


[Modified by stickboy, 10:16 AM 10/17/2002]

 

[QUOTE]... I do go through a driveshaft every year for the last 3 years. The last one was a 3 inch with 11 gage tubing. [QUOTE]

Hey Norval - are you using steel or aluminum. I switched to 3" aluminum. The aluminum driveshaft can flex a lot more than steel without breaking. It can absorb the shock of high torque starts better, thus putting less stress on the drivetrain.

 

I certainly don't know more then the chevy engineers but then again my C3 never came with an overdrive transmission or my extra short driveshaft. On the build sheet for the driveshaft they stated at 23 inch tube. This is alot shorter then the stock 350-400 setup. I also would like to find this source of vibration so I am exploring anything and everything. Something is not right. I have gone thorugh 2 different transmissions, 3 driveshafts and 2 rearends , both new rear axles and still the vibration is still there. It is speed related and doesn't matter if you are speeding up or slowing down , clutch in. It is in the drive train. All that is left is the half shafts and I will have them checked and balanced.