Lotus Elan

Vibration under load.

PostPost by: Keith Scarfe » Wed Jun 21, 2006 10:49 am

Hi.
I was wondering if anyone can offer some advice.
I have just started running my car (1967 S3 DHC) after approx 1 1/2 years of it being sat laid up. Before laying it up, I had done much work on it including body / interior and some running gear (brakes mainly) restoration, but it hadn?t run much directly after.
Now I have started to use it again, I have noticed a bad vibration in what seems to be the transmission (i.e. I feel it through the car rather than the steering wheel). It is worse, much worse, under power (i.e. accelerating and cruising at high speed), but when the clutch is dipped or it is knocked out of gear the vibration goes away completely. At low speed cruising, vibration is much less.
My thought is that it is something like propshaft imbalance or bearings, but I don?t think this would go completely when clutch dipped. Wouldn?t there still be the vibration at speed if it was the prop? I don?t think it is engine as when revving (out of gear there is no vibration) and power is good so I don?t think it is a misfire (but could be under load I guess).
Does anyone have any suggestions of where to start? I have not looked at all yet as it was only yesterday I noticed it. If I can?t see anything obvious I guess I might try a rolling road session to see if that shows anything up.
Thanks for any advice.
Keith.
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PostPost by: types26/36 » Wed Jun 21, 2006 10:57 am

Keith,
Start at the easy things first, check the engine mountings as they have a habit of collapsing and it becomes metal to metal contact, next check the diff is not touching (see recent threads) and check the exhaust is not touching the body/chassis anywhere.
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PostPost by: steveww » Wed Jun 21, 2006 2:41 pm

Most important - check the drive shaft donuts :shock:
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PostPost by: Keith Scarfe » Thu Jun 22, 2006 7:25 am

Thanks for the advice. Still not had chance to look. Maybe this weekend. Donuts were replaced recently so hopefully not these but will check. I dont see how they can cause this vibration but will see. Will check for anything touching. Yes I read the diff thread.
Thanks again.
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PostPost by: Bullitt3 » Fri Jun 23, 2006 3:38 am

Is the vibration worse in 1st gear than the others? Less in 2nd, 3rd, 4th? If so, then you may have a problem in the gearbox bearings. Many years ago, I had the metal cage (spacer) around the balls of the output shaft bearing break up on my Lotus Cortina. Main symptom was vibration under load, worse in the lower gears. Turns out the rpm rating of that bearing is way less than the 7500 rpm my motor liked. Nevertheless, the problem is relatively rare.

I hope that is not your problem as pieces of the ball cage can run through gear teeth, usually with $$$ results.

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PostPost by: Keith Scarfe » Wed Aug 23, 2006 3:17 pm

After eventually getting round to having a proper look under the car, I think I have found my problem. Not fixed yet so not sure, but is looking likely:

I think it is the diff touching. When looking at it there was clearance but only a tiny amount. I'll try nipping up the bolts a bit. That'll pull it away from the chassis. Applied spanner the nut goes tight(ish) then loose. Uh ow. Thread gone on bolt. I got quite a few turns on one side before it let go and the diff defo raised up a bit. Took a bit of head scratching to get the bolt out. Angle grinder on the nut (in the boot). Then it bolt wouldn't go past the rotoflex, so cut with hacksaw. It is off now and new pair ordered. I am pretty sure this is the problem. Will see next week when assembled and road tested.

Cheers.
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PostPost by: Keith Scarfe » Thu Aug 31, 2006 10:12 am

Hmmm. No defo not the dif touching. All assembled and checked but it is still the same.
It is more like a shuddering. Like the clutch slipping then gripping very quickly. Does not feel like something touching/rubbing. Maybe a misfire, but it is strange it only does it in 3rd/4th gear. Maybe also a gearbox problem as mentioned. I am puzzled. Anyone know anything else I can do to try and diagnose it?
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PostPost by: types26/36 » Thu Aug 31, 2006 10:59 am

Keith,
Does the vibration come in at a given speed/revs and is it always at the same speed/revs? a propshaft balance often goes through a "vibration period" when it comes in then goes out if above/below the revs/speed, do you have an engine driven fan or electric? could it be a fanblade problem or even generator/alternator bearing problem, I have seen the generator mounting holes elongated or the bracket broken.......(just grasping at straws :roll: )
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PostPost by: david.g.chapman » Thu Aug 31, 2006 11:11 am

You have probably looked at this, but what about the gearbox mounting? is everything secure there?

Dave Chapman.
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PostPost by: Keith Scarfe » Thu Aug 31, 2006 1:13 pm

Thanks for the replies.
The vibration seems to be only 3rd gear (bad) and a little in 4th. It doesn't seem to do it in 1st or 2nd. It is really bad on very light throttle, accelerating gently. If booted it is not noticeable and the engine pulls well suggesting no misfire. I guess it must be road speed related as it seems to come in throughout 3rd gear and a little in 4th then is a bit less at higher speeds. This does suggest prop imbalance, but if I dip the clutch when it is really bad it completely goes. The prop is still rotating so shouldn't it still vibrate. It is only when drive is going through it. Could it be prop UJ bearings I wonder. I guess I am only going to find out by pulling it all apart.
I cant believe it is generator bearings as the whole car shudders. Mountings all tight - checked that. Fan is engine driven but I took it off for a test and it is not that. Will have to crawl under again and check a few more things like gearbox mount. Doughnuts all checked ok.
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PostPost by: types26/36 » Thu Aug 31, 2006 1:38 pm

I re read your original post and see the car has not been used for about 18 months, if it was O.K. prevously the prop cant really go out of balance by standing but you may have a point with the unversal joints. Not being used rust could have got to the joints if not well lubricated, I know its difficult but try to see if a slight about of (red) rust is around the cups, you may be able to see the front one after removing the rubber gromit in the tunnel.....maybe with a mirror?
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