Lotus Elan

CV System identification

PostPost by: Zlinster » Tue May 23, 2023 8:51 pm

Could some kind soul please tell me what flavour of CV system I have. I've assumed it's a MM.

If anyone has the installation/maintenance instructions then that would be a real bonus.
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PostPost by: h20hamelan » Tue May 23, 2023 9:26 pm

Not sure about your 1/2, 1/2 shafts?
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PostPost by: oldelanman » Wed May 24, 2023 5:26 am

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PostPost by: Andy8421 » Wed May 24, 2023 5:29 am

I think it is the Spyder conversion. Rotoflex at one end, CV joint at the other. Have a look here:

https://www.spydercars.co.uk/lotus-elan-2-final-drive/

Frankly, I'm not a big fan. You still have all the problems of a Rotoflex, and it seems (based on a very small sample set) to be the inner Rotoflex that fails more often than the outer as it is in the hot, oily airstream from the engine.

If I was going to the trouble of changing the rear driveshafts, I would use a CV at both ends.

edit: Oldelanman beat me to it........
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PostPost by: 661 » Wed May 24, 2023 9:09 am

Agreed, Spyder. I used to have one like it
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PostPost by: Zlinster » Wed May 24, 2023 9:17 am

Thanks for the identification, guys. I guess the Spyder system at the least gives two fewer rotoflexes to fail.

Does anyone make good rotoflexes?
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PostPost by: nmauduit » Wed May 24, 2023 2:08 pm

Zlinster wrote:Thanks for the identification, guys. I guess the Spyder system at the least gives two fewer rotoflexes to fail.

Does anyone make good rotoflexes?


Some advocate that the mixed system like your is the best compromise, and the inner rotoflexes would be the less likely to fail ... do you have a beefed-up engine, or what's wrong with you rotoflexes ?
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PostPost by: 661 » Wed May 24, 2023 3:15 pm

Zlinster wrote:Thanks for the identification, guys. I guess the Spyder system at the least gives two fewer rotoflexes to fail.

Does anyone make good rotoflexes?

I ran this system for many years. The only reason I changed it to double CVs is because the rotoflexes still need changing, and it's a pain, so I'd rather not have the bother of it. I can heel and toe so I don't think there's unnecessary strain on the transmission.
The system has a fail-safe if the rotoflex disintegrates. It is well engineered.
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PostPost by: Zlinster » Wed May 24, 2023 5:31 pm

nmauduit wrote:
Zlinster wrote:Thanks for the identification, guys. I guess the Spyder system at the least gives two fewer rotoflexes to fail.

Does anyone make good rotoflexes?


Some advocate that the mixed system like your is the best compromise, and the inner rotoflexes would be the less likely to fail ... do you have a beefed-up engine, or what's wrong with you rotoflexes ?
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PostPost by: Zlinster » Wed May 24, 2023 5:33 pm

Sorry, cocked up the post. PO was surprised when we discovered a spilt rotoflex during the prebuy inspection. He replaced the recently installed pair of them. So I wondered are some better than others?
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PostPost by: nmauduit » Wed May 24, 2023 8:39 pm

It has been experienced already some years ago that recent rotoflexes may not all be as reliable as back in the days... to the point some people prefer to use older ones properly stored. I have swapped mine for a solid type (U-joint) coupling, and could not provide a definitive answer on where to get good rotoflexes today. I would inspect them regularly when starting to put them to use (and not leave the wheels hanging more than necessary).
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PostPost by: Zlinster » Wed May 24, 2023 9:17 pm

Thanks for that. It's never imagine that rotoflexes might be so difficult. Any idea why moderns aren't as good as back in the day?
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PostPost by: StressCraxx » Thu May 25, 2023 2:23 am

Zlinster wrote:Thanks for that. It's never imagine that rotoflexes might be so difficult. Any idea why moderns aren't as good as back in the day?

I wrote this post in another thread about a year ago, but still holds today.

"Sorry for the long and overly detailed response, but I felt it necessary.

The Metalastik couplings available now are not of the same quality as the original ones from Dunlop. The donuts were designed for industrial cardan shafts in the 1950's or earlier.

The original couplings used a one piece solid steel insert molded with the donut to hold each bolt. The bolt was inserted into the drive flange, into the coupling using hardened steel washers. The assembly was torqued to spec to properly tension the bolt so it would survive the loads in shear and tension. As long as the bolts remained properly tensioned, the couplings worked well. I used to check/torque mine at every oil change. I also marked the nuts with a dab of paint to visually indicate if the bolts started to loosen. Once a bolt loosened, the cyclic loads caused cyclic fatigue and bolt failure. If the bolt failed, the coupling would often fail with it unless it was caught in time.

Unfortunately for all of us, the Dunlop/Metalistik accountants thought they could build a better (read: Cheaper!) coupling. They saw the expense of the bespoke steel bolt inserts as a cost savings/profit opportunity and the original inserts were substituted with some folded mild steel pieces that were spot welded together and formed a square hole for the bolt in the coupling. The new leaflets would barely hold the tension created by the bolted joint. If the bolt did not have enough tension, the joint would start to work back and forth and the bolt would fail from cyclic fatigue. If the bolt was torqued a little too much, the steel inserts would collapse immediately and the bolts would fail from cyclic fatigue. Then the coupling would fail.

It's very difficult to get a torque wrench into the area to perfectly torque the bolts under an Elan, especially at the diff. The last time I replaced my donuts, I torqued one of the bolts too tight and immediately felt the insert collapse. I removed that new coupling and threw it away, ordered another and installed it.

The point of all the above? The donuts are not as originally engineered and supplied. The design has been compromised. The angle at full droop on an Elan is well beyond the 5 degrees specified in the cut sheet from Dunlop. So, no matter if one uses donuts or CV axles or Hooke's joints, one has to decide what compromises one is willing to make for the sake of originality, reliability and safety.

https://www.robush.com/wp-content/uploa ... oflex-.pdf

Lastly, I race Formula Ford. The pre 1973 cars all used donuts or donuts and u-joints. They are raced that way today. I was witness to a donut failure at Sonoma three years ago. The flailing shaft tore the left rear corner of the car off and the resulting spin put the car into the tire wall at 100mph. A beautiful Titan Mk 6 was written off."
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PostPost by: Andy8421 » Thu May 25, 2023 6:31 am

nmauduit wrote:Some advocate that the mixed system like your is the best compromise, and the inner rotoflexes would be the less likely to fail

That has not been my experience. The inner and outer Rotoflex adopt the same angle, so the only difference in longevity would be the environment they operate it. The inner Rotoflex is in the hot airstream blown down the transmission tunnel, and as far as I can tell usually fails before the outer Rotoflex.

This is important, as in the event of a failure, the outcome is worse if the inner joint fails and the driveshaft keeps whipping around driven by the road wheel until the car stops.

I take on board StressCraxx helpful note above, but I don't think Rotoflex were that great in period either. Tony Rudd in his book 'It was fun' mentions that Rotoflex failures were the biggest cause of warranty claims during the production run of the Elan.

When I was still racing, I spent quite a bit of time looking at my competition in the pits and how they had set their cars up ('know your enemy...') - all the Elans I raced against used TTR hooke jointed driveshafts.

So, my conclusion has been that the Rotoflex belong on a shelf in the garage for historical interest, along with a few other original bits I have changed out to make the car move drivable (radiator, dynamo...), and that a decent CV conversion is the way to go. I am pretty sure Chapman would have used plunging CVs instead of Rotoflex if they had been cheap and available at the time the Elan was built.

A couple of caveats - some people (not me) like the 'bounce' that Rotoflex exhibit, and there is an argument that the resistance of the Rotorflex to displacement of the driveshaft adds to the spring rate of the rear suspension. I am not sure about the second point, but could easily be compensated for by slightly higher rate rear springs.
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PostPost by: l10tus » Mon May 29, 2023 7:46 pm

In addition to the above, it was suggested to me by a Girling brake engineer, that the carbon black rubber compound has been found to contain carsonagenic material and has been banned from World wide use.

The result is a inferior product has replaced it with less strength and longevity.

Apparently Girling are really concerned about this, as all their brake seals used to contain the same carbon black which has been replaced with the only poor alternative, this being built in obsolescence with a possible early failure result.
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