Any advantage to retaining the rubber donuts

[Bob45se]

Is there any vantage to retaining the rubber donuts, Is it easier on the drive train in the long run

[billwill]

Ir's softer on the diff wear ?

[steve lyle]

Originality?

[bitsobrits]

I've owned 6 Elans (still have 2). Two were running Rotoflex couplings their entire time with me, one came and went with sliding spline u-joint axles, and three I converted to CV joints. Having had two Rotoflex failures, one minor, one major, I would never go back to 'doughnuts'. The smoothness and reliability of CV joints is so much better, and there should be no concerns about diff wear as those diffs are so robust relative to the modest torque of the twin cam (ask the racers out there).

While there is a certain satisfaction to be gained from learning to drive an Elan smoothly with the original kangaroo effect couplings....given the cost, risk, and general PITA that Rotoflexes can be, it's CV joints for me from here on out.

[alanr]

I have pondered this for a while and have come to the conclusion that it is best if you can cope with the 'surge' effect to just live with doughnuts.The car was originally designed and built to have them and they suit the somewhat delicate drive train just fine.
The potential increased stress on the drivetrain of CV's/UJ's is just not worth the risk unless the car is going to be used on the track or have heavy annual mileage road use.
If I personally did change my car to doughnuts, which I have now definitely decided not to, I would want to ensure that my car had already been fitted the with uprated diff output shafts and the top diff mountings had heavy duty ones fitted.
My thoughts are that if you do a relatively low annual mileage and you can crawl underneath and check them regularly I would stick with doughnuts!

Alan.

[Foxie]

Donuts never lasted very long on my Plus 2.

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

I fitted CVJs over 20 years ago, and they're still perfect.

[elans3]

I'm in the "doughnut's kinder on the diff" camp. Having had both on various cars, both CV / CV, and U/J / U/J, I didn't like either, found them slightly noisier too.
In the end I settled on the Spyder solution, ie CV outer, and Rotoflex inner. There's virtually no surge, and the inner doughnut still cushions the diff .
No issues with these over the last 2500 miles, since fitment.

[JimE]

I switched to CV joints on my Plus 2 which changed the character of the car for the worse. Reverted back to doughnuts and got my old Plus 2 back. Jim

[StressCraxx]

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 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.

[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)

[elans3]

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.

[Bob45se]

Hi One of the reason I posted this question about rubber donuts. 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. These are heavier cars then the elan plus with the use of rubber donuts that stress and strain on the transmission might be less. Is there anyone that give me any guidance.

[rgh0]

I have used CV's in my Plus2S 130/ 5 for over 30 years and around 60k miles without any 5 speed transmission or diff issues. Also no maintenance required on the CV's in that time either compared to the need to routinely replace cracking CVs . Getting rid of the donuts improved the cars drivability especially in stop start traffic. Donuts in the lighter Elan are less of an issue with surging but still CV's are a significant improvement.

cheers
Rohan

[2cams70]

Ford Escorts and suchlike run the same diff with no rubber cushioning so I wouldn’t be too concerned about the diff at least.
As usual my concern would be more about the quality of aftermarket parts. So called “billet” driveshafts should certainly not fail.
Where access is difficult when it comes the tightening bolts to the correct torque I sometime use a reference bolt and tighten it to the correct torque whilst noting the number of turns to do so after all the clearance is eliminated. Usually if there’s a few bolts to tighten you can get good access to at least one. I then use the number of turns rather than a torque wrench to tighten the remaining inaccessible bolts.

[StressCraxx]

Hi One of the reason I posted this question about rubber donuts. 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. These are heavier cars then the elan plus with the use of rubber donuts that stress and strain on the transmission might be less. Is there anyone that give me any guidance.

Hello Bob,

Sorry if you think the thread went sideways from your question. I have often wished for a 5 speed while buzzing along CA interstates at 75 - 80+mph.

It really depends on your intended use of your car. If you enjoy your Elan as a gently driven cruiser around town or the local twisty roads in the foothills, the donuts are fine. Yes, they will provide absorption of direct torque loads more than CVs. The torque rods and rubber bushings under the diff provide damping to the transmission. The damping springs in the clutch disc also provide a cushion to the transmission. The motor mounts and transmission mounts all provide even more damping.

If you plan to autocross your car, run track days or hillclimbs, driving it really hard, the 5 speed box, or most any transmission will have shorter life. No amount of damping will save shift forks or synchros that are abused. The donuts will have a shorter life as well.

The transmissions were also used in the Elan +2 130/5 model near the end of the production run. The Lotus 5spd internal parts were from an Austin Maxi. The early Lotus 907 engines made about 130 ft/lbs of torque and as performance upgrades came along, made 160+. Lots of enthusiasts really beat on these cars. The lower torque produced by the twincam is in your favor. Gearboxes are rated by their torque capacity not HP.

An Elan twincam built to sprint specs will have 115 ft/lbs of torque. The reduced torque alone will help the gearbox longevity. Help for longevity is using the recommended gear oil from the +2 manual. Hopefully a +2 owner could advise the recommended gear oil viscosity.