Rotoflex question?

Post by: **pereirac** » Thu Mar 12, 2015 11:46 am

Hi,
Just wondered if anybody has tried one of these on an Elan? It seems a BMW propshaft coupling can be made to fit an Imp driveshaft so I would guess it might work for the Elan as well?

http://www.ebay.co.uk/itm/HILLMAN-IMP-ROTOFLEX-CONVERSION-COUPLING-INSTALLATION-KIT-/191533173903?pt=LH_DefaultDomain_3&hash=item2c98444c8f

Just a thought ....??

Carl

Post by: **john.p.clegg** » Thu Mar 12, 2015 1:06 pm

I don't think they are designed with the intended "angle of the dangle " of a Lotus ?

John :wink:

Post by: **pereirac** » Thu Mar 12, 2015 1:14 pm

I did wonder about that.. very few are, must stop jumping my Elan over humpback bridges .. :-)

Post by: **CBUEB1771** » Thu Mar 12, 2015 7:41 pm

pereirac wrote:It seems a BMW propshaft coupling can be made to fit an Imp driveshaft so I would guess it might work for the Elan as well?

Flexible couplings for driveshafts have come up on this site from time to time.. I would not consider this without knowing which BMW these couplings are intended for and the torque rating. Don't forget the torque multiplication going through the final drive!

Post by: **Chancer** » Thu Mar 12, 2015 9:01 pm

Unless the BMW in question has an engine torque of 4 times that of your Elan then you cannot assume that they will be strong enough, the torque is multiplied by the final drive ratio, you could argue that it is shared by two couplings but with an L.S.D...............................????

Also as has been mentioned the operating angle of propshaft couplings is much less than that of an Elan driveshaft.

Post by: **Chancer** » Thu Mar 12, 2015 9:08 pm

I have just had a look at both of the guys EBay listings, one is a complete kit, the other is just the bolts and spacers.

it would appear that the PCD's are the same as the Pimp/Elan couplings, in which case in my opinion it would be rude not to try them.

He has even given the BMW part number for you to source them yourselves, there may even be pattern parts available.

Post by: **Chancer** » Thu Mar 12, 2015 9:18 pm

Final comment from me.

I now realise that this was the coupling used on the Caterham Blackbird conversion, it was used on the front propshaft where it connected to the gearbox outputshaft and at the other end there was an U/J connecting to the reversing gearbox.

I didnt like the set up because U/J's must be used in phas opposing pairs or there will be significant vibration and a jerky driveline, the coupling seemed to cope with it but it just wasnt right to me so on the one I engineered myself, which I still own I used a pair of U/J joints.

I did own one of the 24 hour Le mans cars for a short while, the prop came apart in a catastrophic manner at this joint because they had not used bolts with a long enough shouldered portion and the drive was being taken on the threaded portion, to overcome the design fault I had to machine down and rethread longer bolts.

The coupling survived so I can tell you that its pretty robust and does damp out severe driveline harmonics.

Seemingly from a 1983 BMW 325 and several others, should be lots of pattern parts out there.

Post by: **mbell** » Thu Mar 12, 2015 10:35 pm

I am with John on this one I wouldn't go anywhere near a BMW "Guibo" to replace the elan doughnut. There main purpose on BMW (and mercs + others) is just to soften shock loads between drive train & transmission and reduce vibration. It's not designed to flex at any significant angle and I'd expect it to fail pretty quickly on an elan.

They are designed to take a reasonable amount of torque, my daily drive is rated at 324 lb/ft and wouldn't surprise me if it was the same part as used on the M5 with even higher 500lb/ft of torque. (Reminds me I must get check the condition on the guibo on my car not looked at it in a while...)

Post by: **512BB** » Fri Mar 13, 2015 7:07 am

A mate of mine is a big wig in BMW circles. I was at his workshop a while ago, and noticed a BMW donut on a bench. I said I would have that, I might have an alternative use for it.

Back in my workshop, I compared it to a Lotus donut. No good, it was thicker, and there is no way it can be made thinner.

The End.

Leslie

Post by: **Chancer** » Fri Mar 13, 2015 8:18 am

Thats a shame but thanks for explaining.

And yet the photos of the Imp installation showing where the spacers are being used would suggest that the couplings are in fact thinner.

Are there more than one type of BMW coupling perhaps? I know the Imp couplings dont have the interleaves but are they thicker than the Elan ones?

Post by: **john.p.clegg** » Fri Mar 13, 2015 8:26 am

...and are they as flexible as the Elan ones?

John :wink:

...

Post by: **rgh0** » Fri Mar 13, 2015 8:40 am

The Elan used the same couplings as the Imp until around 1970 when the interleaved couplings which are dimensional identical were introduced.

cheers
Rohan

Post by: **512BB** » Fri Mar 13, 2015 10:52 am

Chancer wrote: And yet the photos of the Imp installation showing where the spacers are being used would suggest that the couplings are in fact thinner.

Not correct. Imp donut dims. and Elan donut dims. are identical, and that is why Elan peeps sometimes go to Imp suppliers for tea :lol:

As I understand it, Lotus changed to the interleaved donut when the Sprint came out, and Hillman then started to use the same.

Cleggy wrote: ...and are they as flexible as the Elan ones?

No, they are as tough as Manny Pachio [spelling] will be on Saturday night. I very soon reaslised that because of their inflexibility, they would be no good in an Elan installation. Has anyone got pay per view telly to watch the boxing....................and can I come round? I'll bring some donuts :lol:

...... pizza..........whatever !

Chancer wrote: Are there more than one type of BMW coupling perhaps?

Now that I do not know. My mate only has M cars, 3 & 5 series, but I shall see him next week and will ask if the donuts on both are the same.

Chancer also wrote: I know the Imp couplings dont have the interleaves but are they thicker than the Elan ones?

I think I have already covered this :roll:

I believe Imps went over to the interleaved couplings, but that may have just been for competition use, but in any event, the dims. of the dotnuts stayed the same, both for interleaved and non interleaved.

Up until a while ago, I had some original Metalastic donuts, with that name embossed in the rubber. Must have been 50 years old. The shape was slightly less chunky than later versions, but not much in it. Should have taken some pics for the archives, but never did :cry: