Lotus Elan and Lotus Elan Plus 2 Forums

Hi,

I have an issue with my gearbox on full acceleration. When I have selected the third gear and accelerate, a surge will appear for half a second or so. I have solid TTR drive shafts, so it is not rotoflex-related.
Any guess? Could there be a weakness in the gearbox? As this happens only on 3rd gear, I guess it?s not the clutch?
/Ulf

Has the surging always been there or has it come recently. Have you done anythng else to the car around the time you noticed the surging?

The other significant torsional springing in the drive train system apart from the donuts which you have removed is the ring of torsional springs in the clutch plate, if you have broken some of the springs then you will change the resonance and potentially get surging . Failures of rubber mounts which retain torsional reactions - eg engine or diff mounts could also cause a surging problem


cheers
Rohan

the gearbox is in original shape, 125 000 kilometers, no job done.

I am sure engine and diff mounts could be bettered, but as surge only happens in third gear, I am confused.

My guess is as follows - and its really just guessing without seeing and hearing the car

The surge occurs because of a low frequency resonance between the mass of the car and the torsional spring rate of the overall drive train. Normally the torsional stiffness of the drive train is high enough for any resonance of this nature to not occur especially if you have removed the donuts. The torsional stiffness of the drive train in front of the gearbox is affected by the gear you are in. Third gear appears to create the right overall torsional stiffness to excite the low frequency resonance.

I would check all the drive train rubber mounts and if they are all good then consider pulling the engine and replacing the clutch driven plate.

The other possibility is that it is a carb or electrical or some other engine problem causing surge in the engine pulling power - why you may just see this in thrid gear is not clear to me but then with a lotus anything is possible !!!

I would be confident its not a direct gear box problem as any thing wrong in the gear box that could cause signficant variable drag as it rotates to create a surge would also create a lot of noise you would notice in my opinion.

but like i say - just guessing really


cheers
Rohan

OK, many thanks Rohan.
I will start exchanging the rubber mounts to see if that helps.
/Ulf