Lotus Elan

"Knock" sound when setting of

PostPost by: hansaxl » Thu Jul 20, 2017 7:55 am

Dear all,

recently I have noticed a faint, single "knock" sound in the rear of the car (Elan S4/Sprint), when setting off. The knock does not always appear, sometimes it is in 1st, sometimes in rear. It "sounds" like it is coming from the left rear side of the car.

Now, my friend who has a +2 tells me that he had the same phenomenon, and 2 k km later his transmission was gone.

I consider this a worst case, and, frankly, I hope some of you might be able to come up with another recommendation/experience/suggestion what to look for. Wheel bearing..? CV Joints...?

Some info n the car: Engine is on QED 420s, approx. 150 hp, I overhauled the transmission 15 k km ago with no issues since, diff was out and found to be good last year, uprated diff output shafts, Spyder drive shafts. Normal, regular use on the street with occasional track days. And I want to go to Spain in September and wonder now...

Thanks for looking and for any reply.

Cheers, Hans
'70 S4/Sprint
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PostPost by: nmauduit » Thu Jul 20, 2017 8:18 am

hansaxl wrote:Dear all,

recently I have noticed a faint, single "knock" sound in the rear of the car (Elan S4/Sprint), when setting off. The knock does not always appear, sometimes it is in 1st, sometimes in rear. It "sounds" like it is coming from the left rear side of the car.

Now, my friend who has a +2 tells me that he had the same phenomenon, and 2 k km later his transmission was gone.

I consider this a worst case, and, frankly, I hope some of you might be able to come up with another recommendation/experience/suggestion what to look for. Wheel bearing..? CV Joints...?

Some info n the car: Engine is on QED 420s, approx. 150 hp, I overhauled the transmission 15 k km ago with no issues since, diff was out and found to be good last year, uprated diff output shafts, Spyder drive shafts. Normal, regular use on the street with occasional track days. And I want to go to Spain in September and wonder now...

Thanks for looking and for any reply.

Cheers, Hans


Hallo Hans,

first thing I would check is the condition of the diff bars bushes (stock are rubber - you may want to check the top diff attachments if convenient): that and the amount of space between the diff lower front angle and the chassis can produce an interference.
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PostPost by: john.p.clegg » Thu Jul 20, 2017 8:38 am

Engine mountings , exhaust/chassis clearance , propshaft u/j's , diff lower tiebar to chassis , doughnuts etc etc

Good hunting.
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PostPost by: miked » Thu Jul 20, 2017 2:24 pm

Embarassingly I found the bolts that mount the top diff mounting rubbers had worked loose. This was aided and abetted by mounts (HD) that had thicker oval flange thickness. I was astounded and fitted longer bolts and even put nuts and washers on the underside.
I also had another experience were it would knock coming out of islands under power. This was a soft top mount allowing it to stretch under torque so it knocked the underside of the diff on the chassis, even with good static clearance. Only really found by levering that mount in a vice with a bar. Not scientific but did it.
One other was a lotocone poor fit for the spigot type top damper nut. This was moving inside and had nice witness marks from to and fro movement.
All sent to drive you bonkers. :roll:
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PostPost by: miked » Thu Jul 20, 2017 2:32 pm

Just one more. I chased a click on pull off on a Plus 2. It was the needle roller inside the gearbox between the first motion and main shaft. Engine out second time to find this. Had been through all the mentioned stuff and even a new clutch. Sounded for the life of me that it was from further behind. Perfect sound box to confuse!
This realigned at speed and only happened on pull off. Was worse when warm when gear oil had thinned.
Good luck.
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PostPost by: hansaxl » Sat Jul 22, 2017 10:13 am

Thanks for all the suggestions. I will go hunting, but do have a gut feeling that it is going to be in the transmission... Looks like the sound is transmitted rearward via the prop shaft. At least that was the case with my friend`s car.

We wll see.

Cheers, H.
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PostPost by: billwill » Sat Jul 22, 2017 3:59 pm

My bet would be the top bolts of the diff, they only have to be a quarter turn loose to produce mysterious knocks & clunks, as I found out some years ago.
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PostPost by: david.g.chapman » Sun Jul 23, 2017 10:29 am

It might be also worth checking the tightness of the rear hub nuts...

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PostPost by: hansaxl » Mon Sep 04, 2017 11:51 am

Dear all,

finally I found the time to havea look into the cause of the "knock"... Before, a wedding party and extensive house renovation works kept me from doing it...

A bit embarassingly, it was only the four little bolts connecting the prop shaft and the diff that had come a bit loose and caused the sound.. Another typical case where you spend considerably more time thinking about a problem than actually fixing it... But I am quite relieved and will embark on a 5-6 k km trip to Andalucia with a friend next week. Hpefully all will stay tuned.

Cheers, Hans
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