[wvrodder]

I'm running an 8 1/4" sure-grip in my 36 Dodge street rod. I've been getting an intermitant driveline vibration or shudder on ther highway; sometimes it starts at 20 mph and sometimes it starts at 60-65 mph. It will do it under power and while coasting sometimes. The drive shaft has been balanced and has new u-joints and a new front yoke. I've played with changing the pinion angle (the car is running leaf springs) and I haven't had any luck. I'm beginning to suspect the sure-grip may be the source of my vibration. I replaced all the bearings and seals when I built the car and filled it with synthetic lube and added a tube of "positraction additive". Could the sure-grip mechanism be the problem? It always has made a "chucking" noise when backing up like other posies do. I'd appreciate any help or thoughts any of you might have.

[dklong]

first thing I would check is the emerengcy brake. if the cables are sticking on this would give you a vibration, similar to what you have here. or it may be the sure grip. when you over hauled the rear end , all the clutch plates and gears look ok , and everything is set up to mopar spec. the other year I had a similar problem with my 72 buick 12 bolt posi. when I switched to synthic gear oil, the rear end started smashing and banging, the more I drove it the worse it got, after 35 miles on it I pulled the gear oil out . and let the car sit for 2 days then put 80w 90 posi gear oil in it and never had a problem since!

[KULTULZ]

[quote]...when I switched to synthic gear oil, the rear end started smashing and banging, the more I drove it the worse it got, after 35 miles on it I pulled the gear oil out . and let the car sit for 2 days then put 80w 90 posi gear oil in it and never had a problem since!<hr></blockquote>

I think you will find the synthetic lubricant to be the problem as it does it's job too well and actually allows the clutch dics to slide within the steel plates.

FORD has had the same problem on T-5's due to owners installing synthetic lube (ATF) and causing internal shifting problems.

[wvrodder]

Thanks for the responses and suggestions. I'm going to take the cover off, drain the synthetic lube out and maybe even try to flush it out with varsol. Then I'll refill it with.......? Do I use regular 80w90 or is there special posi 80w90?
These sure grips don't have clutch discs and plates like a Ford or GM posi; they have conical shaped cast iron pieces that nest into each other (metal to metal)

[dusterbd]

just use regular 80-90. i like slick 50. its somewhere in between regular gear oil and synthetic. also, use a tube of posi additives.
ive been doing this combo in my duster for years. its always donhe me good.
mike

[bullheimer]

make sure you use "hypoid" gear oil. most of it is now but be sure

[bcwrench]

There is no way a traction differential can cause problems with vibration from the clutching system. It is inactive when going in a straight line. Nothing moves. The synthetic oil is not a contributor. Ford has been using it in the truck rears for years. It does require the additive however. One thing most owners of traction diffs do no do after installing the oil and additive is run the vehicle for about 15 miles and then find a parking lot and do 8-10 donuts hard over (both ways) to exercise the clutches and move the oil into them. Also adding a components like Slick 50 to a compound metal type diff like your than uses cones is mechanical suicide. It is OK in non metallic compound clutch type however.

Now to the problem that no one seems to be addressing and the one that is your problem. You have to know what its frequency is. This is important because you have to know what sets it off. Is it closer to wheel speed than engine speed. Since your vibe comes in at different points and loads all sorts of things can start it. Although the shaft is balanced, is it str8 when its in the car. Main tube cannot have more than .005 runout throughout the length. I have seen shops balance shafts that are not straight. Many shops cannot spin the shafts fast enough as well. If the shaft is not str8 then you have to ascertain whether it is the shaft or your yokes. Also your cancelation angles must be within .5 degrees of each other. Depending on your install, you may have a broken back system where the angles or both of a positive nature. These systems are hard to work with. As a rule alignment problems give vibration under accel and decel and balance problem show up at steady speed and most often amplify with shaft speed.

[willysjs]

you didnt say what trans you are using but i suspect a flywheel flex plate torque converter etc. i dont believe your posie is the problem at all hope this helps

[KULTULZ]

[quote]Originally posted by bcwrench:


<strong>There is no way a traction differential can cause problems with vibration from the clutching system.</strong> It is inactive when going in a straight line. Nothing moves. The synthetic oil is not a contributor. Ford has been using it in the truck rears for years. It does require the additive however.<hr></blockquote>

<a href="http://www.ring-pinion.com/posichatter.shtml" target="_blank">-Positraction Chatter-</a>

There are many reasons that will cause the problem the poster has described. But to say that clutch chatter is not possible is rather a blanket statement isn't it?

And yes, FORD does use synthetics in their new systems, but they are designed for synthetics and they have not proved trouble free.

Am not trying to start an argument, but you approach a problem in it's most simple terms. Then you progress to the more complicated reasons.

[wvrodder]

I really am amazed at all the responses and suggestions posted in response to my problem. Common sense suggests that bcwrench is right - there is no way the posi clutch system should make any noise when going in a straight line. Since the original posting I have taken the cover off, drained the synthetic out and refilled with regular 75w-90 hypoid gear lube that is supposed to work in regular and positaction diffs. I also decided to re-visit the possibility that pinion angle is the culprit. By the way, there seems to be as many theories and "rules" about pinin angles as there are carnut websites!!

I thought the theory suggested by the local driveshaft builder made sense. It goes sort of like this:

The centerline of the crankshaft/transmission output shaft should be parallel with the centerline of the pinion shaft. They don't have to be, or shouldn't be "lined up" or pointed directly at each other to form a straight line, just parallel. In most conventional rear wheel drive vehicles the crankshaft/transmission output centerline is level or angles downward slightly from front to rear. The pinion centerline will be above or below the crankshaft centerline but the angle should be the same. HOWEVER - in a car with leaf springs, under power the springs tend to "wind up" (depending on how stiff the springs are and how much power is applied). And you have to compensate for this "wind up" by angling the pinion down so that when you are under power the centerlines are parallel.

Anyhow, I now have regular 75w-90 gear oil in my differential, and I have my pinion angled down another 2 degrees for a total of 3 degrees from parallel as described above. I have driven the car a short distance and can tell a difference but haven't really tested it out yet

[KULTULZ]

[quote]Originally posted by wvrodder:


I really am amazed at all the responses and suggestions posted in response to my problem.<hr></blockquote>

It is a common problem largely misunderstood, hence the curiosity.

[quote]Common sense suggests that bcwrench is right - there is no way the posi clutch system should make any noise when going in a straight line.<hr></blockquote>

Well, never say never...it will bite you in the butt sooner or later. I admit total ignorance of the MOPAR system, but have come across many instances of clutch chatter in FORD applications, whether improper lube, wrong setup techniques or just service failure.

[quote]I also decided to re-visit the possibility that pinion angle is the culprit. By the way, there seems to be as many theories and "rules" about pinin angles as there are carnut websites!!<hr></blockquote>

Well, the best theory to follow is that of the OEM.

[quote]HOWEVER - in a car with leaf springs, under power the springs tend to "wind up" (depending on how stiff the springs are and how much power is applied). And you have to compensate for this "wind up" by angling the pinion down so that when you are under power the centerlines are parallel.<hr></blockquote>

No...the only time you need to compensate for the movement is with a competition car only, i.e. the angle must be constant only under hard acceleration. You nose it down for this purpose and not everyday driving.

You find the correct angle(s) with the use of a bubble protractor and shim it to that point. If you are concerned about spring wrap-up, you use a larger pinion snubber and/or traction/slapper bars.

On a street driven car, the correct pinion angle must be maintained or possible u-joint failure (not to mention harmonics) will be destroyed.

Fun isn't it?