Lotus Elan and Lotus Elan Plus 2 Forums

Hi all,

As the title says I have a vibration that goes through the whole car (S3 SE DHC) it starts at about 60kph and progressively gets worse the faster I go, it stops on coast and overun immediatly.The whole car vibrates like the car is running on three cylinders not four but much more severly. It is running on all cylinders.

It feels like a worn uni joint but I know that both are in excellent condition. I have read in the archives and there are some comment about the alignment betrween the gearbox and the differential which apparently can cause the problem I am trying to describe, the comments mainly relate to ensuring the the spacers between the gear box mounting and the gear box crossmember are fitted. Mine are. I have new engine mounts, new gear box mount, new diff muntings as well.

I am thinking that perhaps the tail shaft may be out of balence but then why would it only occur on acceleration and not coast and overun?

At the moment the car is pretty much undriveable in this condition so I am looking for some suggestions as to how I can measure the gear box to diff alignment or any other suggestions that might explain this problem.

All and any assistance will be gratefully recieved.

Tony W

Sounds like classic prop shaft.....loose bolts on diff flange? Loose or broken gearbox mount? That'd be the first two places I'd look

Could it be the exhaust manifold catching on the chassis? with the torque reaction on acceleration is the engine "twisting" and the exhaust just rubbing? then on coast and overrun the engine does not move so much so the exhaust does not catch......it happened to me once.

Hi Tony,
For correct propshaft alignment I believe the diff pinion axis should be parallel with the gearbox tailshaft/engine crankshaft axis. Somewhat difficult to check but maybe you can measure the angle of the engine to the horizontal (on the rocker cover) and the angle of the diff flange to the vertical (probably have to disconnect the propshaft) - the two should be at right angles.

You mentioned the gearbox mount spacers - you don't have a Spyder chassis do you ? I don't think spacers are required with a Spyder mount, only with the Lotus one.

Also if you have knock-on wheels check the nuts are tight and the drive peg holes are not elongated.

A correctly balanced prop shaft will certainly make things
run a lot smoother, cost to balance will be about $160.00.

Sources of rotational vibration.
>> Check side to side play of front prop shaft yoke
>> Check for excessive play in rear extension housing (Just in front of front uni joint)
>> Check rear uni joint yoke for side play.
>> Check that rear donuts are all sound.
If engine mounts and gear box mount ar OK they should not
be the cause.
Here's an interesting promo video on tail shaft balancing (No commercial connection)
http://www.youtube.com/watch?v=tm7DhP3M4AA
Watch it to learn how it's done.

Good luck with sorting the problem.
Ceejay.

Thanks for the replies,

My chassis is an LR chassis brand new, I will check the exhaust manifold later but I think there is clearance. I will double check the gearbox mounting as well.

The universal joints are installed correctly and are new.

Ceejay, I have no problem getting the tail shaft balenced but I would like to understand why it only vibrates on acceleration and not coast or overun, what I do not want to do is simply replace everything until the problem gets fixed....I can see the bottom of my bottomless pit if you get my drift. The Utube video was interesting thanks for the link.

I forgot to mention that I have the universal joint style axles which are in good condition but I cannot see how they could cause the problem but any suggestions welcome.

Tonyw

Tonyw wrote:I forgot to mention that I have the universal joint style axles which are in good condition but I cannot see how they could cause the problem

How are the UJ's aligned? Input and output the same or opposing?

edit
Sorry to be clearer - they need to be in phase ie both ends the same when you look at them. If you haven't lined them up on the sliding spline correctly the sine waves will not cancell.... and you will take-off!

Hi everyone,

I just found this and it makes sense of my problem and also suggest a way to measure and correct it.

http://www.hotrodhotline.com/md/html/dr ... monics.php

See what you think, I will try and mewasure it next weekend and report back my measurements, I may still have an out of balence tail shaft and other issues but at least I should now be able to check this part.

Tony w

AHM,

I assume you are talking about my half shafts? if so I will double check thanks for the reminder.

Tonyw

Tonyw,

Yes halfshafts - the propshaft is welded.

For UJ shafts, as oldelanman says the input and output axis should ideally be parallel - This is because if they are, the input and output angles will be equal and the sine waves will be of the same amplitude and therefore cancel. However the diff is offset and the engine and gearbox mountings mean that the gearbox output is canted to one side. So the angles aren't equal and there is not a lot you can do about it. Don't worry about it that is how it is designed. Provided the mountings are all good it will be as every other elan.

Similarly with the half shafts, the suspension geometry dictates that the flange axes are not parallel.
The difference is that the angles in the half shafts are much greater than the angles in the propshaft so the amplitude of the sine waves are much greater, and where they don?t cancel is consequently bigger - you will feel it even if they set-up right.

When you apply the power there is no lash in the drive train and the entire car experiences an acceleration/deceleration force. On the overrun the lash in the drive train will consume some of the irregularity and only the mass of the drive train and engine will experience the acceleration/deceleration.

Sloppy tailshaft bushing ?

You've checked those propshaft to diff flange bolts, yes?

Hi everyone and thanks for the suggestions.

I know my tailshaft bolts are tight as I recently checked them.

Thinking about what Elanman said about the diff and gearbox being in a parralell plane makes sense but I can see how it would be very easy to get this out of alignment if the engine mounts have sagged abit or oil had got onto the gearbox mount and made ir droop a little so how close should the alignment be do we think? and is it assumed that the rocker box would be in the exact same plane as the gearbox output shaft? because if that is the case I could easily check this on the weekend.

On the issue of half shafts given that mine are the UJ type I did check to make sure that they were in the correct plane/maching end to end do we think that they are problematic? has anyone had anybad experiance with these causing my vibration on acceleration?

I also wonder if the LR chassis had been configuired to do away with the need for the spacers between the gearbox mount and crossmember, in fact I wonder if it is worth removing the spacer to see if this makes my problem better or worse.

Regards,

Tonyw

From distant memory 5 degrees operating angle before it is perceptible by the average person - which is why you will notice the halfshafts, and not the misalignment of the input/output on the propshaft.

If your engine/gearbox mounts are shot replace them. Measuring the angles will tell you little.

The spacers - not sure why your car should be different.

It will cost you nothing to check the bolts again.... Or the bushes, mounts, bearings

In the article you linked the last part explains how to identify which part of the drivetrain may be the cause.

Not mentioned above I don't think, but have a look at at the torque rods and bushings at the diff. If the bushings aren't in good shape and tight, accelerating causes the diff to twist upward changing the angle of the input. When they get to the point that they are actually loose you get a thump when taking off. I seem to remember mine showing as a vibration before escalating to a thump upon accelerating.