Drivetrain Troubleshooting

[mct340]

While driving my Elan I can hear a faint knocking sound coming from the drivetrain. It sounds like the driveshaft is contacting the frame once every revolution. That isn't the case. I've jacked up the car and rotated the entire assembly and can't find any contact marks or loose u-joints. The diff. has no play. The rotors are running true. Nothing is loose. If you run the engine and put it in gear while on jacks there is no noise.

The sound disappears on the road when the brakes are applied. When the gas pedal is applied it comes back, especially when making a left turn. If you coast down a hill in neutral you can hear it .....thump , thump, thump (muffled).

I have to think it might be the tranny. Is there a layshaft or something in there that could be doing this? The car has less than 1000km on it since total rebuild.

I'm running Spyder sliding splines and single doughnuts. Maybe the splines are binding

Very annoying. Any guesses?

Thanks, Mike

[elans3]

Hi Mike, Have you checked how much clearance there is between your diff and the chassis ? I've seen this many times where the diff can almost be resting on the chassis due to worn diff housing mounts, and the noise manifests iteself more when you put the power on and take it off. Not so noticeable without the weight of the car on the wheels, as you note,
Regards
Elans3

[1964 S1]

If it's really not doing it while on jackstands running, but it is under load, I'll go for a slightly binding spline. I had them on my plus two and removed them because of a binding vibration. Not sure about the downhill in neutral tho' , does it sound more pronounced when weaving back and forth, "the bad wheel bearing test"? (Performed ONLY while stone sober)

[ElliottN]

This is an outside chance but.... check the diff torque rods and make sure they're tightened to spec and all the bushes are sound.

[nigelfletcher]

I've had something similar to this when a rear wheel spinner wasn't quite tight enough, causing a wheel to wobble.

Under acceleration the noise stopped, because the torque seemed to take up the slack, but whilst coasting force is coming from the road into the drive train causing the nut to undo slightly - hence the reason for hand of thread for the wheel spinners.

Nigel

[elj221c]

Mike,
I've had something similar but can't remember if it was right or left handed cormers that was the problem. The cause of my knock was a worn prop shaft UJ. Depending on the corner the diff keeps it tight one way and not the other.
Roy
'65 S2

[Emma-Knight]

:huh: Thanks for the topic,
my diff was "sleeping" on the chassis and caused vibrations
like a boom-box

[berni29]

Hi

My money is on the donut bolt(s) being slightly loose. On my plus 2 the twisting of the donut would load the apparently tight, but not quite tight enough bolt up and it would click as the donut rotated. Loose wheels that seem tight but aren't will make a different but equally perplexing noise.

Good luck!

Berni

[types26/36]

Dont think its been mentioned but have you checked for a cracked wheel around the drive pegs?
Brian.

[types26/36]

..........and I forgot to say check drive peg holes are not elongated.
Brian

[mct340]

Thanks for all the input...... Brian was close.
I found that one of the adapters for the Revolution knock-on mags hadn't been screwed all the way onto the original hub. This left a small gap between the hub and the mag even when the wheel nut was tightened and allowed the wheel to wobble; hence the noise disappearing when the brakes were applied or the car was turning.
I guess one should always give the tire a shake after tightening the knock-on nut!
Glad to have figured it out before the wheel failed.
Once again this forum proves it's worth.....
Regards,
Mike