Lotus Elan

In house research: flex donut coupling improvements

PostPost by: lotustastic » Sun Apr 13, 2014 2:54 pm


Just though I'd share...When in doubt DIY...I recently completed a prototype flex donut for the elan drivetrain in tough polyurethane. Hopefully soon, will road test a set of them. Unfortunately, with my elan still in restoration, it will be some time before actual road testing. The donut prototype has improved reinforcement and superior material properties (improved elongation, peel strength, weathering, etc) as compared to the traditional rubber material used in various flex donuts (many inferior) on the market. The material chosen is very resilient, tenacious and tough which should withstand constant flexing and stresses. After looking at my original recommended oem heavy duty donuts, the design addresses common failure points and should greatly reduce the disadvantage of rubber degradation by atmosphere, chemical attack, temp cycling, etc. I've always been intrigued by improvements through the use of new materials and techniques especially to improve upon issues with material quality, longevity, driveability, etc. My goal is to see if I can maintain the dampening effects in the elan driveline, reducing vibration, harshness, wind up and prove a much longer service life than those available today. Will let you know how it progresses... :D

David D
1969 Lotus Elan S3 Coupe
Sent from my iPad
Second Gear
Second Gear
Posts: 88
Joined: 08 Jan 2014
Location: Franklin, TN

PostPost by: ken ob » Sun Apr 13, 2014 3:21 pm

That is very interesting. I hope it works out. You will sell lots.

Slightly confused by everything since the invention of the wheel.
ken ob
Third Gear
Third Gear
Posts: 230
Joined: 13 Nov 2010
Location: Wirral England

PostPost by: RichardHawkins » Sun Apr 13, 2014 7:00 pm


I wish you well with the project, I only went to CV joints because of the rotoflex problems, and feel sure I and many other users would revert to rotoflex if a good product was available

I am a retired mechanical engineer, and spent most of my working life in mining mineral processing and chemical industries. One of my colleagues had problems with a rubber covered component from a grinding/milling device. He replaced the component with one covered in polyurethane and had great success. Unfortunately that initial success was never repeated. All subsequent versions failed at the bond between steel and polyurethane. Hopefully technology has advanced since then and you will be successful.


Richard Hawkins
Fourth Gear
Fourth Gear
Posts: 778
Joined: 05 Jul 2008
Location: Lincolnshire U.K.

PostPost by: KevJ+2 » Sun Apr 13, 2014 7:05 pm

David, a very worthy challenge and one that's quite topical for me (see rotoflex quality post).
I wish you the best of luck and look forward to your results.
Fourth Gear
Fourth Gear
Posts: 819
Joined: 23 Aug 2013

PostPost by: ensign42 » Sun Apr 13, 2014 7:19 pm

Interestingly, I just did some maintenance on a marine drive system. It had these mounted. Maybe some of it will be useful to you. Sorry for the picture quality, cell phone snaps before everything was reinstalled and inaccessible again.

20140325_095042.jpg and
Marks 1
20140325_095239.jpg and
Interior sleeve
20140325_095003.jpg and
Profile 2
20140325_094948.jpg and
Flex joint end profile
First Gear
First Gear
Posts: 25
Joined: 24 Jul 2013
Location: Indiana

PostPost by: silverlink » Sun Apr 13, 2014 7:36 pm

Good luck with your project, I'm sure should you crack the rotoflex problem you'll have a real winner.
Second Gear
Second Gear
Posts: 173
Joined: 24 Aug 2012
Location: North Yorkshire UK

PostPost by: Elanman99 » Sun Apr 13, 2014 7:59 pm


If your car is not on the road how are you testing your couplings. Is the 'In House' part of your title a clue?

Like many others I hope your product is a success.

Out of interest is this a coupling that looks something like the rubber one we know and love (moulded on to inserts) or is it a totally different construction with maybe 6 separate polyurethane elements?

68 Elan S4 DHC. Built in a weekend from a kit (just like the advert said)
User avatar
Third Gear
Third Gear
Posts: 430
Joined: 11 Sep 2003
Location: Sandiway, Cheshire UK

PostPost by: StressCraxx » Mon Apr 14, 2014 5:20 am


I think many of us are interested. I was considering the rubber drive couplings used by BMW for the driveshafts. There was a discussion about them last year. I am considering buying one and seeing if it can be engineered for our cars.
There is no cure for Lotus, only treatment.
User avatar
Coveted Fifth Gear
Coveted Fifth Gear
Posts: 1066
Joined: 26 Sep 2003
Location: San Francisco Bay Area, California

PostPost by: trw99 » Mon Apr 14, 2014 9:18 am


I too wish you success with your experiments.

You said "My goal is to see if I can maintain the dampening effects in the elan driveline, reducing vibration, harshness, wind up and prove a much longer service life than those available today." I believe this will be key to your testing, as many people forget how much the Elan was praised for it's ride back in the 1960s, the doughnuts playing a not insignificant part in that.

The other aspect that I think to be important is that the finished product be black, to appear as near as possible like the original item. None of this shiny blue poly-what-not stuff beloved of Scooby drivers!

User avatar
Coveted Fifth Gear
Coveted Fifth Gear
Posts: 2848
Joined: 31 Dec 2003
Location: Essex/Herts border, UK

PostPost by: john.p.clegg » Mon Apr 14, 2014 9:57 am

If they work I'll have a set in Dayglow Yellow if that's all that's available...

John :wink:
User avatar
Coveted Fifth Gear
Coveted Fifth Gear
Posts: 5707
Joined: 21 Sep 2003
Location: Manchester

PostPost by: lotustastic » Mon Apr 14, 2014 3:22 pm

Thanks for all of the comments and support...The donuts have bonded bolt bosses and reinforcements and will be circular. After reviewing the failure points on my own hd donuts, they appeared to fail/break clean often at the bolt bosses over time. My hope is that the redesign fixes this for longevity in addition to reducing the overall material breakdown from the elements. The current donut is in natural color (white). Colorant can be added. Once I work through some more of the molding/commercial design issues, I can post pics. I work in polymers daily and am drawing on this experience to hopefully have a winner.
Second Gear
Second Gear
Posts: 88
Joined: 08 Jan 2014
Location: Franklin, TN

PostPost by: billwill » Mon Apr 14, 2014 4:24 pm

Very interesting, I shall watch out for your postings.
Bill Williams

36/6725 S3 Coupe OGU108E Yellow over Black.
Coveted Fifth Gear
Coveted Fifth Gear
Posts: 4736
Joined: 19 Apr 2008
Location: London UK

PostPost by: 65sunbeam » Mon Apr 14, 2014 5:16 pm

Another Elan owner and I were talking about original flex couplings just this weekend at a British car show and how expensive they are now and with no guarantee of how long they will last. I will buy them if you make them-thanks! Eric
User avatar
Second Gear
Second Gear
Posts: 73
Joined: 23 Dec 2009
Location: South Carolina, USA

PostPost by: Spidersprint » Sat Apr 19, 2014 6:11 pm

Miles Wilkins of Fiberglass Services has a shed load of brand new donut couplings....
Posts: 2
Joined: 09 Apr 2014
Location: Hampshire

PostPost by: richardcox_lotus » Sat Apr 19, 2014 7:32 pm

I'll be giving him a call next week :-)
'72 Sprint
Coveted Fifth Gear
Coveted Fifth Gear
Posts: 1169
Joined: 11 Jul 2004
Location: Kent

Total Online:

Users browsing this forum: digdug and 6 guests