Lotus Elan

Soft thumping

PostPost by: david.g.chapman » Sat Mar 16, 2024 12:01 pm

I noticed a soft thumping noise on the road the other day, at wheel speed on my +2. You could hardly hear it. It seemed the same on drive or coast, so I suspected the rear drive train.
I then jacked up the offside (drivers side uk) A frame to load the suspension, and spun the rear wheel. Sure, enough, there was a soft thump once per wheel revolution.

I now suspect the cv drive shaft, which has a few thou backlash and 60000 miles wear. Is there anything else that might be the cause?

Dave Chapman.
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PostPost by: alanr » Sat Mar 16, 2024 3:30 pm

A wheel bearing maybe? Jacked up with suspension fully loaded grab the wheel at the top and bottom and check for play in the bearing.

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PostPost by: khamai » Sat Mar 16, 2024 3:50 pm

Check your tires. Could be tread separation.

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PostPost by: Baggy2 » Sat Mar 16, 2024 7:06 pm

FWIW I had a strange - wheel speed related noise on my +2 recently . I was on a short journey of only about a mile. During that journey my mind was going through all the terrible things it could be and how I was going to deal with them. When I arrived at my destination I found I had not released the handbrake properly !
The moral of the story is check the simple things first and "Don't Panic"
Good Luck
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PostPost by: david.g.chapman » Sat Mar 16, 2024 10:23 pm

The wheel bearings on that side do have a lot of play - which you can see and feel when the wheel is rocked. I'll be changing them soon.

I'm not sure how you get a knock from bearings though. It's at the same point every revolution. I'm happy to be enlightened though!

The tyres are 7 years old and nearly worn out. There is no tread separation evident.

I'll disconnect the drive shaft and check the bearings again. Then if the bearings aren't knocking I'll clean out the drive shaft and examine it. Watch this space if I find anything interesting!

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PostPost by: david.g.chapman » Sun Mar 17, 2024 2:21 pm

Good news! I removed the drive shaft and spun the bearing - to reveal the same knock once per revolution. I can only imagine that a pothole or something has shocked the bearing and marked one of the races. Lummie!
As I mentioned, this bearing had a lot of play, and was only changed a couple of thousand miles ago. The bearings on the other side are fine.

I'll see if Sue M has any tomorrow.

Cheers,

Dave Chapman.
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PostPost by: types26/36 » Sun Mar 17, 2024 2:25 pm

Well if it “thumps” while jacked up then you can rule out tyres/internal band separation as the tyre is not in contact with anything …….
Play in rear bearing sounds ominous as there is no adjustment on the roller bearing (unlike the front taper bearings) …….the rears do have a habit of wearing the strut housing but unless the bearing is “tilting or rocking” in the housing I don’t really see how bearings would make a “thumping noise” but the play needs investigating.
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PostPost by: david.g.chapman » Sun Mar 17, 2024 2:47 pm

Yes, I've never had a knocking bearing before - I thought they whined or rumbled when worn.

When I've got the bearings out I'll post a picture of anything interesting.....

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PostPost by: 512BB » Sun Mar 17, 2024 6:57 pm

'the rears [bearings] do have a habit of wearing the strut housing'

Only when removed by people who do not get the housing good and hot before bearing extraction, a la U Tube.

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PostPost by: MACCA.GLM » Mon Mar 18, 2024 10:50 am

Hi

Could be a worn output drive shaft bearing journal

Regards

G
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PostPost by: david.g.chapman » Mon Mar 18, 2024 7:07 pm

I isolated the "thumping" to the wheel bearings and hub shaft assembly, as mentioned earlier.

I have now removed the shaft leaving the inner bearing on the shaft, and I've driven out the outer bearing. Both bearings spin without knocking now the bits are out of the car, which deepens the mystery. It sounded for all the world like something was hitting something else once per revolution, but I could not see anything with the shaft, and everything else assembled on the car.

I now have the cv drive shaft out and just to be sure, have spun both ends of the drive shaft in circles with the other end clamped in a vice. No noises.

I have also rotated the diff output shaft. Again, just a normal whirr from the diff. The output bearing there is locked in place and not spinning.

I'm not happy with the condition of the shaft (some evidence of spinning in the past), so I've ordered a new one from TTR, long with a bearing set. I'll post again when I've finished.

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PostPost by: StressCraxx » Mon Mar 18, 2024 9:51 pm

Some suggestions, based on both my industrial and personal Elan and formula car experience.

Good on you to take this seriously. I suggest the following to take this a little deeper:

Mic the diameters of the bearings and the diameters of the bearing bores in the housings. Measure in 8 places in the bores.

Cut the bearings open with a cutoff wheel. Cut the ball cage. Look at the bearing balls, the inner race and outer race. I have found deep chunks out of the inner and outer race surfaces. Other times some weird noises due to ball skid. Once with multiple circumferential lines on the balls called "ball skid".

Remove your CVs and inspect the bearing surfaces. On my FF I found metal loss in the races of the outer shell.

Lastly, to those of you who detect this soft thumping noise on a car that has donuts, carefully inspect your donuts for fractures in the donut or loose donut bolts. My first experience with the Lotus West Tech Manual after I bought my Elan in 1979, was finding the brief article called "Donut Rumble". That article saved me from a huge failure. Either the donut has already failed or a loose bolt is about to suffer a fatigue failure.
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PostPost by: RichardHawkins » Tue Mar 19, 2024 9:58 am

Dave,

If you don’t feel confident to follow the advice from StressCraxx about doing you own bearing failure analysis all the reputable bearing companies offer a failure analysis service (I should say they did before I retired). However this service is expensive.

Hope this helps,

Richard Hawkins
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PostPost by: rgh0 » Tue Mar 19, 2024 10:26 am

Most bearing companies provide on line information on bearing failures.. I like others here have done bearing failure analysis for 50 years in my mechanical engineering career and it is not that hard. At times I have used various metallurgical and bearing companies to assist in specialist testing but that is rare and unusual cases.

Post potos of the bearing internal balls and races if you supect them of the thumping and I am sure you can get a good analysis from the group here. Thumping typicall relates more to the mounting of components rather than the bearings internals.

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PostPost by: alan.barker » Tue Mar 19, 2024 3:38 pm

hi Dave,
it seems strange the bearings only lasted 2,000 miles.
when you assemble with new bearings be care of what assembly procedure that is used.
Alan
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