Any advantage to retaining the rubber donuts

[alanr]

Ford Escorts and suchlike run the same diff with no rubber cushioning so I wouldn’t be too concerned about the diff at least.

This is only partly true!
The actual diff unit itself is of course the same as an Escort/105E and is not in itself the issue at all but it is the way it is used in a totally different drive train setup that is the problem.
The Ford diff in the Elan/+2 is encased in a fragile Lotus aluminium casing which is then merely suspended by two rubber mounts and utilises two feeble torque rods to hold it in for place back and forth movement. In other words the complete diff unit just hangs there!! Not good at all for longevity even in standard form using doughnuts without putting even more strain on on it by turning it into a more solid drive train utilising CV's/UJ's.
It is the way that the Ford diff is utilised in the Elan/+2 drivetrain that is the issue and imho definitely needs some cushioning somewhere in it.

Alan.

[2cams70]

The Ford diff in the Elan/+2 is encased in a fragile Lotus aluminium casing which is then merely suspended by two rubber mounts and utilises two feeble torque rods to hold it in for place back and forth movement. In other words the complete diff unit just hangs there!! Not good at all for longevity even in standard form using doughnuts without putting even more strain on on it by turning it into a more solid drive train utilising CV's/UJ's.

Good point!

[mbell]

The last one that burst fractured the complete brake caliper from the suspension leg casting.

I've had to repair similar damage on my car due to a failure at some point in the past. The brake caliper had been knocked off, a chunk taken out of the chassis and hole in the body work by the battery.

A previous owner had the caliper mount welded back on very badly and called it done. I had to fit a new wheel carrier, cut and weld the chassis and fix the body.

Personally I think the risk of damage to the diff from CV joints is a lot lower than likely hood of a doughnut failure. The diff failure is also less likely to cause other serious damage.

I plan to install a Lotus 5 speed I bought a few years ago in my 68 elan S3. I've heard they are fragile transmissions and have not held up well in an 83# elite or an 84# eclat.

I wouldn't say they are the world's strongest gearbox but plenty strong enough for an elan or +2.

The elite/eclat are much more powerful and heavy cars, that combined with abuse is why they don't last well.

If you do it pay careful attention to the shift mechanism on the box. They don't shift as good as the ford 4 speed rifle like action but are still very nice if in good condition.

I run redline mt90 in mine.

[Slowtus]

I am in no way an engineer; not even close but while the donuts are possibly 'gentler' to the diff the corollary to that is that the constant back and forth they have, gentler though it may be, could be actually - over time - wear the diff more than the alternatives.

Just an uneducated muse...(see 'no way an engineer' above)

Sorry, can't buy that. there's no back & forth, diff is always moving forward, it's more torque / a bit less torque, but always moving in the same direction, unless you're in reverse !
And having even one doughnut each side in the system cushions any shock load on the diff, coming on & off the throttle.

As stated 'no engineer' but the donuts DO cause a rocking/back and forth or whatever you can call it - the diff may be always moving in the forward/reverse direction but the donut wind up and the effect it has - whatever that effect may be, exists.

And I am talking about the windup effect on starting off (whether going forward or in reverse )

No engineer...

[Foxie]

IMHO as an engineer 45 years qualified, any cyclic stress over a certain minimum, whether in the same direction or reversing, will eventually lead to the fatigue failure of the component.

[elans3]

As stated 'no engineer' but the donuts DO cause a rocking/back and forth or whatever you can call it - the diff may be always moving in the forward/reverse direction but the donut wind up and the effect it has - whatever that effect may be, exists.

And I am talking about the windup effect on starting off (whether going forward or in reverse )

No engineer...[/quote]


Of course you're right , but that effect is cushioned by the rubber donut, and has less effect (shock load) on the CW & P, than the normal gearchange into second, and the on / off action of the throttle, in a more normal solidly mounted diff / axle.
As Alanr says above, the "soft " mounting of the diff in the Elan application, is more than likely one reason why donuts were used by Chapman in the Elan.

[Andy8421]

Noting the comments that the current rotoflex aren't as good as the originals, it is worth reflecting that the rotoflex weren't actually up to the job in period either.

In his book 'It was fun' Tony Rudd mentions that the rotoflex were the highest cause of warranty claims on the Elan. He had to design a modified rotoflex for the Sprint with interleaved metal plates to try and improve performance, and (again in period) 26Rs ditched the rotoflex completely and ran with hook joints and ball splines on the rear driveshafts.

I recall the Elan was originally supposed to have inboard discs, but the rotoflex couldn't cope. I have yet to see an Elan currently racing in a historic series in the UK using rotoflex.

Taken together, this doesn't seem to much of a recommendation for rotoflex. Chapman would be aware of this (hence the more robust solution on the 26R). My guess is that the choice of rotoflex was driven primarily by cost.

[Slowtus]

IMHO as an engineer 45 years qualified, any cyclic stress over a certain minimum, whether in the same direction or reversing, will eventually lead to the fatigue failure of the component.

Thanks, that's what I was trying to say

[elans3]

IMHO as an engineer 45 years qualified, any cyclic stress over a certain minimum, whether in the same direction or reversing, will eventually lead to the fatigue failure of the component.

Thanks, that's what I was trying to say

Of course it will, but if it's cushioned somewhat, by rubber in the drivetrain, it will last longer.

I hear the comments re Tony Rudd and the original Rotoflex. The original was the Hillman Imp version, and perhaps a version of poor "parts bin" selection on Lotus' part. At 105 - 118 bhp, against the Imp's 30-odd, it was never going to last.
The 26R 145 + bhp depending on engine spec, made the choice even worse, and they were bound to go to the only alternative, solid joints.
The vast majority of the market has always been for road cars, and not competition cars.
Those for the road cars re-jigged with the metal inserts did last well for a good few years, until supplier change / rubber re-mixes etc gave us a batch of poor (very poor) quality Rotoflex.
Don't know what the quality is like in the States nowadays, but in the UK, they have improved a lot over the last few years, to the point where they're lasting 7 - 10 years typically. (that's info from a circle of Elan-owning friends).
Back in the bad old days, they would start to break down just months after acquiring them, and be close to failing at 1-2 years in very bad cases.
It's a very subjective subject, but after trying CV /CV, TTR type, and of course original donuts, my choice is the Spyder solution with the inner being a donut, the outer being a CV, retaining some cushion effect.
Just my opinion of 20+ years of Elan ownership on & off, and nearly 45 years in engineering, & building competition cars.
For a competition car, then it's a no-brainer, I wouldn't fit anything else but solid shafts, but for the requirement of a nice, quiet road car, I would always want some rubber in there.

[promotor]

This is a customer's diff who asked me to post these pictures here.
After discussing what I found with the customer I was advised that the car this came from had the donuts removed and a driveshaft kit installed (I can't remember the exact design but was told of the brand that is no longer available) and that the car had have suffered damage believed to be from being airborne as the rear suspension towers were cracked and bent.

I don't know if this issue was related to loose bolts (either installed incorrectly, or loosened over time through fretting), high mileage, being airborne, solid driveshafts or maybe a combination of all four but it does leave you to wonder how this happened!

[2cams70]

This is a customer's diff who asked me to post these pictures here.
After discussing what I found with the customer I was advised that the car this came from had the donuts removed and a driveshaft kit installed (I can't remember the exact design but was told of the brand that is no longer available) and that the car had have suffered damage believed to be from being airborne as the rear suspension towers were cracked and bent.

I don't know if this issue was related to loose bolts (either installed incorrectly, or loosened over time through fretting), high mileage, being airborne, solid driveshafts or maybe a combination of all four but it does leave you to wonder how this happened!

This although uncommon is not an unknown occurrence in Escorts. I believe what happens is that the bolts can come loose over time which puts them under shear and they snap off. I always put high strength loctite on these. You’ll find many OEMs these days specify a thread locker here. I doubt it makes much difference whether or not there is rubber in the drivetrain as to whether or not the bolts snap.

By the way BMW used to at least for a very long time have rubber couplings in their tail shafts. Fair point that in that application they don’t need the same articulation but they were always quite reliable. BMW’s were renown for their smooth drivetrain and no doubt the rubber assisted this

[RichardHawkins]

2 Cams,

That’s an interesting observation. I had forgotten that when I removed a Ford MT75 gearbox from a Ford Scorpio, it had a rubber coupling on the rear of the gearbox. I also now remember being told that the MT75 was a Getrag design, and that BMW used Getrag gearboxes.

Hope this helps,

Richard Hawkins

[promotor]

This is a customer's diff who asked me to post these pictures here.
After discussing what I found with the customer I was advised that the car this came from had the donuts removed and a driveshaft kit installed (I can't remember the exact design but was told of the brand that is no longer available) and that the car had have suffered damage believed to be from being airborne as the rear suspension towers were cracked and bent.

I don't know if this issue was related to loose bolts (either installed incorrectly, or loosened over time through fretting), high mileage, being airborne, solid driveshafts or maybe a combination of all four but it does leave you to wonder how this happened!

This although uncommon is not an unknown occurrence in Escorts. I believe what happens is that the bolts can come loose over time which puts them under shear and they snap off. I always put high strength loctite on these. You’ll find many OEMs these days specify a thread locker here. I doubt it makes much difference whether or not there is rubber in the drivetrain as to whether or not the bolts snap.

By the way BMW used to at least for a very long time have rubber couplings in their tail shafts. Fair point that in that application they don’t need the same articulation but they were always quite reliable. BMW’s were renown for their smooth drivetrain and no doubt the rubber assisted this

Cheers for the info. I have to say I've not seen this problem in original spec diffs from Escorts etc, only in Elan's. I did see an English diff that had new non-original replacement crownwheel bolts and they suffered also. Not seen original spec bolts broken in Ford models from the 60's and 70's.
Some classic Ford's also used a type of cush-drive in the propshaft, I have a Corsair prop stored somewhere that has it!

[mbell]

By the way BMW used to at least for a very long time have rubber couplings in their tail shafts.

They are normally called "guibo", a few other companies use them like Mercedes. While they don't fail as often as elan doughnuts they do fail and need replacing.

Just another part on a BMW that is a service part for BMW but it's life long part on other cars. ( I might be bias with 11 years of running (fixing) a m62tu powered car...)

[elans3]

Other users of drivetrain "guibo's" are BMW 2002, BMW NK 2000, Alfa Romeo 101, 105 series, all on the propshaft.