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Driveline Clunk Diagnosis

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TR4A_IRS

TR4A_IRS

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We had some nice weather in the Puget Sound over the holiday weekend, which meant I got to take the TR for a few short drives. I performed a good tune-up, adjusted the idle, and decided not to mess with the carbs – I’ll leave that for another day, as the engine was running better than it had in a while, after the tune up.

While out and about, my driveline clunk, that I have been conveniently “forgetting about” for a little while, made itself known to me again. I get the clunk when I let out the clutch, from a stop, but not normally when shifting through the gears.

So I crawled under the TR to try and diagnose the clunk. I started looking for the usual suspects. The differential mounts were in good shape, and the pins were solid, although I did not drop the differential to get a full visual inspection done. I will do that when I replace the bushings, at a later date. The differential bushings were in decent shape, although I have poly bushings on the shelf to replace the stock ones that are currently installed. The U Joints are all relatively new, and they all seemed to still be in good shape, with no play noted.

I was hoping for a bad U joint, as that would have been the easiest thing to remedy. I am happy that I didn’t find problems with the differential mounts.

But, I still don’t know where the clunk is coming from. Since I was under there, with nothing to do, I started moving stuff around to see if I could identify where the clunk was coming from. I found that there was a considerable amount of lash between the driveshaft and the wheels. Isolating different components, one at a time, made me conclude that the total amount of lash was a combination of several things. It appears that there is lash in the driveshaft, the differential, and each one of the axles.

The lash in the driveshaft and axles appears to be coming from the splines that connect the two halves of each component together. I assume that some lash in the differential is expected, but mine probably is more than is normal.

Have any of you experienced the problems that I think I have with your driveshaft or IRS axles? I assume that new and/or rebuilt shafts are what I need to remedy the problem, but I would hate to go to that expense if I may be overlooking something.

Any other suggestions of what to examine?
 
Lots of stuff to check...you've got the uj's and driveshafts down...

Trailing arm bushings, hubs, half shafts, diff mounts, lots of stuff...
 
Last summer at one of our club's tech sessions, one of the guys had a clunk just as you describe. We put it up on the lift and poked around and didn't find anything obvious. The one of the guys from across the shop told us to replace the differential mount bushings without even looking at the car. The club had a set there, so we changed them and the clunk went away. Apparently, this is a common problem.

I would take a serious look at those bushings before you say they are OK.
 
I second this. Problems with the diff mounting can be prevented by really keeping an eye on those bushings. At first it's just clunky. Later on, the problem becomes when those things wear out and the amount of torquing and pulling on the pins becomes bad and the most drastic thing that can happen is them breaking free.
 
It's my opinion that you should not use poly bushings for the diff mounts as they do not absorb the shock of shifting because they are hard. That shock then gets transmitted to the mount directly. Good rubber bushes are better in this application. The poly bushes are OK on suspension attachments because you want to remove as much compliance as possible for handling reasons. Not so with the diff mounts. You want some absorbtion in the rubber at that location.

Let the flames begin. I'll stick to my opinion having several years in the rubber industry selling dynamic absorbtion products.
 
JerryVV said:
It's my opinion that you should not use poly bushings for the diff mounts as they do not absorb the shock of shifting because they are hard. That shock then gets transmitted to the mount directly. Good rubber bushes are better in this application. The poly bushes are OK on suspension attachments because you want to remove as much compliance as possible for handling reasons. Not so with the diff mounts. You want some absorbtion in the rubber at that location.

Let the flames begin. I'll stick to my opinion having several years in the rubber industry selling dynamic absorbtion products.
Click to expand...

Jerry, I'm going to back you on this one, there needs to be some amount of minor tolerance because it's such a weak part of the car.
 
I hear lots of advice to keep an eye on the differential bushings, but no real information on how to tell when they are bad. I used a long screwdriver to move the differential around, while I observed the bushings. I could detect no uncontrolled movement, and no cracking of the rubber. Based on my past experience, I would have to say that these bushings are good.

My daily driver is an older BMW, which go through suspension bushings faster than anything. I have become accustomed to detecting bushing deterioration on the BMW, and I don't imagine that my TR would hide the bushing condition from me!

I do plan to replace them anyway, as I want to get a look at the weld between the pins and the frame as a preventive measure.

Regarding the poly vs. rubber debate, I am aware that rubber has energy absorbing properties that polyurethane does not. I do wonder, however, if the amount of movement that the IRS differentials see (it gets inputs of force from the driveshaft as well as from both axles) doesn't contribute to the fatigue failures seen at the mounting pins. They don't seem to fail due to shear fracture, but rather from fatigue.

I am not intending to foster a debate about poly vs. rubber bushings, as I know there are various opinions out there.

I am still curious if anyone has experienced significant spline wear in their axles or driveshaft, as that seems to be something that is, at a minumum, contributing to my problem.
 
Ian,

I chased several clunks in my 4A when I still had it.

I had lots of stuff bad in the back. The differential bushings were bad. I dropped the diff and not only were they dry, but there was play where the diff mounting pin goes through them. Replaced those and also gussetted the front mounts for good measure.

Also had 4 bad u-joints. Replaced those.

Also had a bad half shaft. They do wear. Some of them had play from the factory. Replaced both with TR6 ones.

Made sure the shock mounts were tight. Made sure everything was greased.

I eliminated the clunks.
 
rlich8 (since I don't know your name),

I am 90% sure it's the axles, which can be expensive to fix. I had previously replaced the bushings and the U joints, the shocks are tight and the end links were replaced.

How did you determine your half shafts were bad?
 
I've owned my 1966 4A IRS since 1969. I just changed the axles and hubs because I thought I should, especially the Hubs. If you take the axles and hubs out you should be able to feel any wear in the splines or better yet while installed rotate the wheels with one hand while feeling the inner part of the axle for any movement in the splines. I'd actally be surprised if you feel any as that spline system is very robust. However if the boots were torn and allowed water into the system you may have a problem. Mine were the original boots and axles and no movement in the splines. The hubs were just begining to have some free play in them. And this car has not been a Garage Queen and is driven hard at times. I did have what's called sticktation and that's what I fixed by replacing the axles. I used Richard Goods axles and had a set of Corvair hubs built to replace the original hubs. It resulted in the next larger U-joints as well. If I were to do it again I'd just get Richards whole system and be done with it. His stuff is well engineered.
 
Don't know if you are running wire wheels but had clunking symptoms on my 3 which turned out to be a worn spline on a wire wheel adapter.
 
I was able to detect play in both axles by holding the axle at the differential flange and rotating the corresponding wheel. I am just not sure how much, if any, play is normal.
 
Ian,

If I understand your test correctly, that doesn't eliminate play in the u-joints or even the hub. I'd pull one axle and see if you can isolate that play better with it out.

Scott
 
HerronScott said:
Ian,

If I understand your test correctly, that doesn't eliminate play in the u-joints or even the hub. I'd pull one axle and see if you can isolate that play better with it out.

Scott
Click to expand...

Scott,

You are right. Before I did this, I inspected each u joint for play, to eliminate them. I suppose the hubs could be the problem. I suspected that bad hubs would equal bad bearings. I replaced a hub, with a used unit, a number of years back, as it had a noisy bearing. I kind of imagined a bad hub would make some noise.

I am hoping to make it through the summer and then I plan to drop the entire rear end for a more thorough inspection.

Thanks for all the suggestions so far.
 
Nigel_TVR said:
Don't know if you are running wire wheels but had clunking symptoms on my 3 which turned out to be a worn spline on a wire wheel adapter.
Click to expand...

This is an excellent point. Do you have wire wheels?
 
Hi Ian,

I suggest you revisit the differential as well. Where it mounts to the bridge is a common weak point, and one of the mount points could be broken. Torque from the engine will make it move, but it would be hard to move with a screwdriver. Maybe if you have it on a lift and use a pry bar you could move it. When mine broke, I couldn't tell by looking until I removed the pumpkin. Then it was obvious.

Depending on your preference, it is possible to cut an access hole in the floor to avoid removing the body. If you don't want to do that, the body will need to come off to weld the frame.

Wire wheels could make a clunking noise too, but you would hear it on braking as well.

Dan B
South Charleston, WV

66 TR4AIRS EFI
80 TR7 DHC
 
DanB said:
Hi Ian,

I suggest you revisit the differential as well. Where it mounts to the bridge is a common weak point, and one of the mount points could be broken. Torque from the engine will make it move, but it would be hard to move with a screwdriver. Maybe if you have it on a lift and use a pry bar you could move it. When mine broke, I couldn't tell by looking until I removed the pumpkin. Then it was obvious.

Depending on your preference, it is possible to cut an access hole in the floor to avoid removing the body. If you don't want to do that, the body will need to come off to weld the frame.

Wire wheels could make a clunking noise too, but you would hear it on braking as well.

Dan B
South Charleston, WV

66 TR4AIRS EFI
80 TR7 DHC
Click to expand...


Dan,

I will eventually drop the differential for a more thorough inspection. I am not yet convinced that I am experiencing the worst possible scenario. I tell ya, we got a bunch of doomsdayers on this board!

When I do take the entire rear end apart, I will report back with what I find. I know there is play in the axles, and I know I have a clunk. Everything else is speculation at this point...
 
I had a similar hard to find klunk in my TR4A. I spent many a time under the car trying to find it and in the end it was a loose lever shock. The way the shock is mounted and were the rebound bumper goes means that when the car is up in the air the shock is still under road spring pressure. I don't have the car anymore but IIRC the rebound happens in the middle (more or less) of the shock arm so when the pring pushes down so that the arm hits the bumper the presure is applied lateraly to the shock. All this to say that if you just wiggle it it will not move because you are fighting the road spring.

It is an easy thing to just see if you can tighten the shock mounting bolts. If you are lucky that will be it like it was for me.

Check for horses before zebras :smile:
 
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