In house research: flex donut coupling improvements

[512BB]

Good morning David,

I have come across a few of the metal bolt bosses recently. Do you still need these, let me know if you do and I will send them

Leslie

[lotustastic]

Hello Leslie,

Thank you for the offer. Through purchases of older couplings and the kindness of forum members, I've acquired enough for producing my initial sample units, prototypes. I expect to receive my first samples of improved metal bosses/plates in from the fabricator this week.

Thank you,

David D.

[Famous Frank]

Keep up the project Dave. That's how CV joints came to be. i had my first CV joints in 1976. Used VW axles with the CV's that we cut, shortened, then welded up. After the first burnout, we found the weak spot in the cleave and changed the design somewhat.

Frank

[trw99]

Hi David

I've been wondering how you are getting on with your improvements. Have you had a chance to make any progress?

Tim

[lotustastic]

Hi Tim,

Thanks for the message. Yes, things are progressing, albeit a little slow as other projects have needed attention. A few more couplings have been made along with a second mold. I did have some metal parts fabricated for feasibility, they look good, but I'm still in a bit of shock over the cost to have them machined even for a start of several hundred pieces (enough for about 20 elans). I guess it's considered a small project for a fabrication shop. Am getting some other quotes. As I'm just nearing completion and testing of my twincam for my elan in progress, I'm hoping to find someone local with a running elan who can have a go with a full set of couplings in the Spring and give them some road time.


Thanks,

David D.

[trw99]

Thanks David and well done; please keep going!

Anyone near David who can help?

Tim

[trw99]

Hi David

Its been over a year since we last had an update from you. Have you managed any further research in that time? I know life has a habit of getting in the way!

Tim

[lotustastic]

Hi Tim,

Thanks for the followup. I did obtain some more palatable machine shop quotes (although not inexpensive) from suppliers for fabrication of plates, etc which may allow this project to continue to be feasible assuming there is sufficient interest by those who continue to use the rotoflex couplings. Not sure how many are still out there.
I did do some manual cycling by flexing the axle incorporating my donuts and they do show somewhat stiffer properties as compared to OEM in terms of force to deflect given same same amount of deflection. I can modify this stiffness via the material formulation. I think the final tweaking will be best determined by real world sound/feel of the driveline and takeoffs when driving an elan with the donuts vs the traditional ones. Its difficult to predict how noticeable differences will affect the real world driving experience, but I suspect right now they would perform somewhere (in terms of shock absorbing) between the OEM donuts and the rigidity of CV joints. I still need to do or have someone happily volunteer to perform this testing on an elan. I plan to make another full set for this purpose in the future as my own elan is nearing completion but will probably be late summer at earliest before this can be done. I will keep in touch.

Thanks,

David D.

[trw99]

Thanks for your interesting update David and well done on the progress you are making.

The alternatives receive an undue amount of coverage! I suspect more Elans are still on doughnuts though. Furthermore, with the higher importance placed on originality in the market place currently, I believe there would be sound demand for your new and improved replacement doughnuts. At the appropriate time I'd suggest you chat to the likes of Susan Miller, Paul Matty and Dave Bean to get a better gauge on likely demand.

Good luck and please do keep us abreast of your testing.

Tim

[Quart Meg Miles]

David,

It is a huge task you've taken on and many people will be interested. I'm a serial user of Doughnuts, which have variable lives, and I'd be happy to install a set of your prototypes for trial any time they are available.

Looking forward to your progress in any case. PM sent.

[ditechspain]

Don't forget that the Imps out there are increasingly running on Lotus spec donuts, since there may be more tuned 998s than standard 875s.

I just bought some for an Elan from Miles Wilkins and he seems to be getting though a few hundred a year at least.

If my Ginetta G15 gets back on the road this year after a 15 year hiatus, I might be in the market for 2. Haven't got round to seeing how they've lasted in the garage.

[Chancer]

i dont want to pour cold water on the OP's enthusiasm but all this talk of new and improved doughnuts may turn out to be no more than talk albeit very enthusiastic and with the best of intentions.

There are reasons why modern replacements of engine mounts drive shaft couplings etc in many cases are just things that look the part but will often fail within days in not minutes, those reasons are the complex manufacturing conditions necessary to produce a strong rubber to metal bond which involve high temperature, high pressure, surface treatment and precise control of all of them. As good as modern adh?sives may be they will never be able to replicate the rubber to metal moulding process the Equipment for which is the domain of the global mass producers and not a small scale replacement parts manufacturer and with the best will in the world not a home garage operation.

My parents both worked for the UK company Silentbloc in the 70's and early 80's they made suspension bushes, engine mountings and flexible drive couplings for all the major motor manufacturers although I believe that the doughnuts were made by a competitor Metalistik.

My father ran the rubber mould shop and as it was the nightshift I would often work there myself at nights on my own projects, I can remember a lot of what he taught me (the age where you suck up info) a correct bond is so strong that the metal will often fail Under destructive test before the bond does but the control of manufacture has to be absolutely precise to achieve it, any variation in material composition, surface treatment, mould pressure, temperature and time will result in the sort of stuff that is produced today

Critically some of the processes essential to creating said bond are no longer practiced in industry these days because of health and safety except for in the huge global manufacturers who have the resources to control and implement the safety of the operation, it was dirty dangerous job and despite being well paid his workforce was 100% immigrants even back then.

The 2 essential processes (and there probably are more) which I believe you wont find these days especially in a home operation are cadmium plating and trichloroethylene vapour degreasing let alone the dedicated moulding presses and the tooling.

I sincerely hope that I will be proved wrong on this. I bet Miles gets his from Metalistik who in turn almost certainly have them manufactured in another country, at least they will know what is required of them and how to test and inspect them but that also is something which is conspicuous by its absence in modern UK manufacturing.

[trw99]

Not quite sure about what your point is here, Chancer.

I can understand that current rubber to metal bonding is very different to what it was back in the 60s & 70s and that rubber composition has changed muchly. But David is purposefully using an alternative compound, which in itself may prove to be better in a number of respects to rubber. Part of his testing of course, is to get the compound to bond successfully with the metal inserts, but I suspect that his efforts to do so will involve a totally different set of challenges to those you have experience of.

Chemistry and physics were not my best subjects at school by any standard and I may well be speaking out of my ask no questions! But I do applaud David in his efforts and hope he achieves something that we all are able to benefit from, as I appreciate you have said you do too.

Tim

[Quart Meg Miles]

... a correct bond is so strong that the metal will often fail Under destructive test before the bond does ...

No doughnut has broken metal in my usage and I'm not sure I'd want it to. You can inspect a bond failure.

You may be right, Chancer, but let's see what David produces and take it from there.

[Chancer]

I was talking about a destructive test done in the QA department of the factory, I saw the results, one was a Macpherson strut top mounting the other a flat plate engine mounting, they were shear tests.

The service failures that I have seen of doughnut couplings have either been the rubber degrading and fracturing or shear failure on the bond line with the interleave plates and/or shear/tensile failure around the bolt sleeves.

I reckon the reinforced Guido couplings are the way to go.