vibration at 25-40 mph

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rich006

rich006

Learning as I go
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What would cause a mild vibration at relatively low speeds? I recently replaced my stock 7 1/4 rear with an 8 1/4 sure-grip and I installed the driveshaft from the 8 1/4 donor car because it was the right length already. Since then I notice a mild vibration starting at about 25 mph. It continues up to about 40 mph and after that it's not very noticeable, either because it goes away or it's just too mild to notice over the general bumpiness in the road. In addition to the rear end and the driveshaft, I also put in brand new u-joints, brake drums, leaf springs, and rear shocks as well as new wheels and tires (the wheels aren't actually new but they were balanced at a shop I trust). The only thing I changed in the front is the ride height which had to be adjusted to match the new springs. I realize with so many new pieces there are a lot of possibilities, but I thought the fact that it happens at such low speeds might narrow down the search.
 
Balance driveshaft anytime you change u-joint, also sometimes you can flip driveshft where it goes into the rear yoke, that cost nothing.
 
You changed your axle and leaf spring. Check your drive shafts angle at your Trans and your rear axle pinion. The angles should be the same cancelling each other out. Pinion should be slightly down as it will lift under acceleration.
 
I would have the driveshaft balanced. And if you changed the front ride height you should have the front end aligned.

I had a vibration in mine and turned out the driveshaft was way out of balance. It's so much smoother now!
 
So I put the car on jackstands and left it idling in Drive while I looked under it. Not surprisingly, the driveshaft was wobbling quite a bit. I got it to wobble a lot less by repositioning the rear u-joint in the yoke. I made sure the bearing caps were pushed inboard so there's no side-to-side play in the u-joint. That didn't really affect the vibration while driving though. Also, now I have differential chatter. At least that's what I think it is. I had it right after installing the 8 1/4 rear, and it went away after I added a bottle of friction modifier. But now that I adjusted the u-joint, it's back. Any ideas?

The u-joints are brand new and they didn't seem to have any problems before I installed them.

As to the other ideas: I'll try to check the pinion angle, but I'm reluctant to get the front end aligned because I'm just about to rebuild it anyway. I just would like to make sure the rear is squared away before I tear into the front.
 
Pinion angle is difficult to check with the driveshaft installed, but as best I can tell the angles are:
- transmission rear seal angles down 4 degrees to the rear
- driveshaft angles down 2 degrees to the rear
- pinion yoke angles down 3 degrees to the rear (yoke is tilted slightly up from differential)
That's with the car on my garage floor, which slopes slightly down towards the rear of the car.

Also, here's an attempt to post a slo-mo video with the speedo reading just under 40 mph.
IMG_1307.gif
 
OK. I called a local driveshaft guy. In the course of our conversation he pointed out that there shouldn't be any sideways play in the rear u-joint cups because of the c-clips being held in place by the straps. Turns out I had failed to install the c-clips on the cups that mount to the pinion yoke. Yeesh. No wonder I had problems! This afternoon I'll take the driveshaft for balancing.

Also, after I removed the driveshaft to install the c-clips, I remeasured the angles. New measurements:
trans seal 2 degrees down to the rear
pinion yoke 3 degrees down to the rear
 
If the trans is 2 degrees down you want the pinion yoke up 2 degrees or close to it. Vice versa also works with trans up and pinion yoke down, they need to be opposite and equal. Make sure you check it with load on the suspension, not up in the air with an unloaded suspension. I've broken a driveshaft on the track, tearing up the trans in the process, because of an incorrect pinion angle.
 
I took the driveshaft in to get measured and sure enough it was bent by about 0.040" off center. The guy picked a couple of spots to heat with a torch then dump water on. After that it was a lot closer to straight. Tonight I got it back in (with the c-clips this time) and the vibration is not as bad, but still noticeable.

While I had the car running on jackstands, I noticed the wheels wobbling a little bit. Videos are linked below...are these acceptable?

Left wheel: [ame]https://youtu.be/ItKkOvtNPoc[/ame]
Right wheel: [ame]https://youtu.be/okujPya6ATo[/ame]
 
Have you checked the axles for straightness, also the hubs? Even a slight bend in the axle will cause that. Or the hub could be warped. Put a dial indicator on the hubs and check them. Shouldn't get any wobble like that.
 
With wheels removed, a dial indicator measures the following.
Driver side:
- runout (measured on flange face): .003"
- runout (measured on flange edge): .006"
- up/down axle play: .005"
- in/out axle play: .015"
Passenger side:
- runout (measured on flange face): .003"
- runout (measured on flange edge): .009"
- up/down axle play: .007"
- in/out axle play: .004"
 
Looks like a slightly bent wheel on the right side. Swap them side to side and see if the wobble follows that wheel to the left side.
 
More data: running on jackstands again, I notice that the vibration seems to come and go, one second on, one second off with speedometer reading 40 mph. Also, it might be more pronounced during coasting than acceleration, though I can't say for sure. Is this a problem in the differential gears?

nm9stheham said:
Looks like a slightly bent wheel on the right side. Swap them side to side and see if the wobble follows that wheel to the left side.
Click to expand...

I did swap on a different wheel on the right side, and the wobble is now more up/down, less side-to-side. Tire shop said the wheels were straight, but I'm not so sure.
 
If you got rid of the side to side wobble, then the wheel is a bit bent. Look at the new wheel from the side and see if you spot any obvious vertical runout. If not, then it is just the tire shape. It may or may not be any issues; radials and any nylon belted tire take a set when they sit for a while. (Rayon corded tires in the 60's were terrible for this!)

Your 'on and off' vibration sounds like there are 2 sources of vibration; they are phasing in and out and thus reinforcing each other 1 second and then cancelling each other the next. Separate vibrations from engine or exhaust or drivetrain can do this. I can't say that this has to do with the new axle or ring and pinion; seems unlikley unless one of the bearings is worn. Have you grabbed the pinion and tried to move it around up and down and sideways, to see if there is any looseness or play?

BTW, friction modifier makes the sure grip slip more smoothly in normal turns. This is when you will actually have diff chatter. If you are getting some chatter or noise when moving straight-ahead, then that is not what friction modifier will effect.
 
The vibration is only in gear, so it's not anything in the engine. Just now I ran the car on stands with the wheels and drums removed (wood block under brake pedal) and the vibration is the same. So I know it's not the rims or tires. Also, there's no vibration with the transmission in neutral, so I know it's not the engine or exhaust.

I did replace the pinion seal before I installed the differential. I reset the bearing preload so that the turning torque was the same as before (about 6-8 in-lbs), and there was no play in the pinion.

Here's a slo-mo video of the pinion turning while the speedometer reads about 30 mph. The collar wobbles, but I don't know if that means anything.

[ame="https://youtu.be/fmb-uK1IEMc"]https://youtu.be/fmb-uK1IEMc[/ame]
 
what did U do to help the pinion nut from not loosening with such a small amount of retightening? check it again for tightness. 6-8 turning-that is pinion and? ring gear? axles? the 8 1/4 has a crush sleeve
 
Marcohotrod said:
what did U do to help the pinion nut from not loosening with such a small amount of retightening? check it again for tightness. 6-8 turning-that is pinion and? ring gear? axles? the 8 1/4 has a crush sleeve
Click to expand...

I left the old crush sleeve in there. I measured the turning torque at about 6 in-lb (to turn the pinion, differential and axles without wheels installed) before I took the nut off, and I also marked the nut position with a punch. After I replaced the seal I torqued the nut back to its previous position (with the same number of threads showing of course). That took over 150 ft-lb. Then I measured the torque required to turn the pinion again, and it was right around 6 in-lb still. Then I loosened the nut one more time, applied red Loctite, and tightened the nut down to the same position.
 
I think I might have found the problem: it seems my transmission is not bolted to the mount (see photos). It looks like the transmission mount bushing is bad. Questions:
1. Should I replace the entire mount or just the bushing? Either way they are cheap.
2. What kind of bolts do I need?
3. Should I be concerned about the fact that the base of the tailshaft housing is not sitting squarely on the mount, or will that fix itself when I install the bolts?

How I discovered this: Suspecting my bent driveshaft was causing the vibration, I had just installed a new driveshaft (actually my old one cut shorter). After checking for play in the rear yoke, I decided to check for play at the transmission output shaft. Sure enough, there was a little bit of play there. Pushing harder, I found I was able to move the entire transmission side-to-side over an inch! Seems like someone (not me this time) removed the transmission mounting bolts and forgot to put them back in. I've put 1000 miles on the car since I got it and I haven't touched the transmission. I bet those bolts have been missing the entire time (come to think of it, the speedometer cable was disconnected when I got the car). With the new driveshaft, the vibration is less noticeable, but it's still there. I now suspect the bent driveshaft was just making the transmission looseness more apparent than it was before. Plus I wasn't feeling for any vibration before, but now I'm sensitive to it.

I'm glad I didn't decide to just live with the vibration!
transMount.png


transMount2.png
 
When you loosen the big bolt to align the small holes, tighten the two tranny bolts first. This will allow the mount to rotate, and then you can secure the large bolt,locking it all in place.The little bolts have; large,thick,rectangular "washers", and bolts with integral loc-washers on them. All this to prevent them from working loose. Be careful to replace them with correct length bolts.
 
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The mount looks normal as they are made with those kinda raged looking holes.
If cleaned off you could check to make sure the rubber flexes a little if you yank it around a bit. (bet it does from the looks of it and that is normal, but of course you will have to bolt it up first.
Watch for cracks or tears in the two solid rubber sections if you can.

That's a common place for bolts to fall out of, so when you put them in use a little blue Locktite on the threads to keep that from happening.
Personally I like only big lock washers between the bolts heads and the mount metal AND blue Locktite. :D

Sorry, don't recall the size of the bolt shaft but I know the heads of them are 5/8 so they must be 7/16 course thread? X 1 inch long (the threads, not the total length with the bolt head)
 
AJ/FormS said:
When you loosen the big bolt to align the small holes, tighten the two tranny bolts first. This will allow the mount to rotate, and then you can secure the large bolt,locking it all in place.The little bolts have; large,thick,rectangular "washers", and bolts with integral loc-washers on them. All this to prevent them from working loose. Be careful to replace them with correct length bolts.
Click to expand...

Good info--are you saying there are specific bolts for this application? If so, how can I find them? I saw on another thread that the bolts are 7/16 x 14 and the length is 1"-1.5" with longer recommended if there's enough thread. But where can I find the right hardware?
 
OK, I get it now. I need some type of lock washer that is beefy enough, and I guess the mount itself can serve as the "flat washer." Loctite will make sure it stays in, and 1" bolts should be good if I don't add extra washers. And I'll check to see if the mount is actually OK.
 
rich006 said:
OK, I get it now. I need some type of lock washer that is beefy enough, and I guess the mount itself can serve as the "flat washer." Loctite will make sure it stays in, and 1" bolts should be good if I don't add extra washers. And I'll check to see if the mount is actually OK.
Click to expand...

:D Exactly, and 7/15 x 1 with a 14 thread count sounds just about perfect.
That knarly lock washer will bite into that mount plate and that bolt ain't movin.
 
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