Lotus Elan

Rotoflex yet again

PostPost by: TomMull » Sat Apr 27, 2019 12:19 pm

I have pretty much decided to replace the chassis on my Plus 2, which will of course require a substantial shipping expense to the US, so I'm trying to get as much in the crate as I can, and why I started looking at Rotoflex replacements. The question of choosing CV type or original rubber has been discussed at length, so my question is only about the Roroflex joints.
I have heard that the Hillman Imp used the same joints and from what little I can find on this says little to the contrary. So here s the disconnect: The Hillman Rotoflex joints go for a bit over £20 each and the Lotus ones are £75 or more. Are they not interchangeable, poorer quality or what? I will say that I have not been able to cross reference any numbers.
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PostPost by: Evante » Sat Apr 27, 2019 3:12 pm

Don't know if the Hillman Imp rotoflex is the same size but the Triumph GT 6 is the same size and is $45.00 at Rimmer Brothers in the US, about 35 pounds.

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PostPost by: 69S4 » Sat Apr 27, 2019 5:11 pm

Has anyone recently tried GT6 rotoflexes on their Elan? I would have thought Elans and GT6s would have similar requirements - power, weight etc - but if Rimmers are charging £35+VAT and our suppliers want somewhat more than that are we just paying over the odds. Or are Elan rotoflexes actually different?
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PostPost by: Fred Talmadge » Sat Apr 27, 2019 8:01 pm

Are GT6 folks switching to CV joints?
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PostPost by: AussieJohn » Sat Apr 27, 2019 9:05 pm

The original Imp ones didn't have the extra steel bits within the donut, these Imp ones were in my S2 elan in the 60's. My +2s had the ones with the extra steel.
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PostPost by: Quart Meg Miles » Sat Apr 27, 2019 9:32 pm

AussieJohn wrote:The original Imp ones didn't have the extra steel bits within the donut, these Imp ones were in my S2 elan in the 60's. My +2s had the ones with the extra steel.

I wouldn't advise anyone to use a six segment doughnut which the Imp is, they were very difficult to drive with. The real question is whether the GT6 version is 12 segment.

Surprisingly I saw an Imp being driven briskly in Bognor Regis yesterday and it was a van with side windows; I don't remember that being a variant.
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PostPost by: Classic-BSC » Sat Apr 27, 2019 10:46 pm

The Hillman Imp Van with side Windows would be a Hillman Husky.

Basically a Hillman Imp Estate.

Regarding the Imp Rotoflex Couplings, being "difficult to Drive with".

I've owned many Imps, including a 998cc Sunbeam Stiletto.
Never had any problems with the Rotoflex Couplings and
the Handling of an Imp would surprise most people.

Probably the most under estimated British Car ever built.


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PostPost by: Mick6186 » Sun Apr 28, 2019 7:55 am

I rallied Imps & Clan Crusaders in the 80s & 90s with some success, especially on tarmac stage rallies. The imp heavy duty competition donuts are the expensive ones and they are much more rigid than the originals fitted to road cars. I used a full race 1220 engine producing over 120bhp and a Jack Knight dog box with lsd. F3 slicks fitted. I never broke a donut on tarmac even after warming the tyres and doing no holds barred starts.
The only time any drive train items broke was on loose surfaces when the standard output shafts were found to be weak.
I'm not sure of the standard of donuts available today, but there are several suppliers of Imp competition parts on line. Some of these have been supplying parts for many years so may have confidence in the donuts they supply,
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PostPost by: rcfurse » Sun Apr 28, 2019 10:27 am

I am told the Ginetta version with the Imp motor was a serious hoot.
What about the oft made claims that "modern" rubber is not as good as the originals?
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PostPost by: rgh0 » Sun Apr 28, 2019 11:28 am

Its not the "modern" rubber its the cheap reproductions that are the problem. Just a pity no one is making good Elan donuts these days as there as plenty of good quality industrial couplings and drive shaft couplings being made. The Elan style and dimensions seems to have fallen out of favour with the quality manufactures unfortunately. But then again Cv's are actually a better solution given the deflection angles involved.

The imp and it variants had much smaller deflection angles which meant the donuts lasted longer and only one donut and a UJ per side which also avoided the windup / surge problems to a large degree.

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PostPost by: AussieJohn » Sun Apr 28, 2019 5:50 pm

My first Elan S2 had the early Imp donuts and the car was running 150+ bhp [ It held Marque sports car record at Warwick Farm so took a beating ] No failures at this time, maybe the quality dropped off in the 70's for these.
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PostPost by: ncm » Sun Apr 28, 2019 9:15 pm

Evante wrote:Don't know if the Hillman Imp rotoflex is the same size but the Triumph GT 6 is the same size and is $45.00 at Rimmer Brothers in the US, about 35 pounds.

Jay S.

Have you ever compared a GT6 rotoflex to a Lotus one? The Triumph component is larger, bolts to a flange with a greater pcd and uses 1/2" unf bolts. I seriously doubt that one could be persuaded to fit onto a lotus driveshaft.
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PostPost by: Quart Meg Miles » Sun Apr 28, 2019 9:31 pm

ncm wrote:
Evante wrote:Don't know if the Hillman Imp rotoflex is the same size but the Triumph GT 6 is the same size and is $45.00 at Rimmer Brothers in the US, about 35 pounds.

Jay S.

Have you ever compared a GT6 rotoflex to a Lotus one? The Triumph component is larger, bolts to a flange with a greater pcd and uses 1/2" unf bolts. I seriously doubt that one could be persuaded to fit onto a lotus driveshaft.
Brian.

The 1/2" bolt holes could be sleeved and my early S2 has a bigger PCD than later production. We need actual dimensions; has anyone got a GT6 rotoflex?

The Imp is rear engined and has just one rotoflex and a short shaft in the drive train, the Elan has the normal front-wheel drive train as well as two rotoflex and with six-segment units the transmission wind up is considerable.
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PostPost by: ncm » Sun Apr 28, 2019 10:04 pm

Dimensions available in the attached Dunlop brochure. GT6 has 113mm pcd ( source factory wsm) .
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Dunlop Rotoflex Brochure.pdf
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PostPost by: USA64 » Sun Apr 28, 2019 10:29 pm

I have read that Chapman had to chose between rotoflex and inboard brakes and chose the rotoflex. The reason was presumeably handling and some subtlety with the geometry.
I have read that the resistance to deflection the donut (4) creates a anti-roll effect on the rear. Thus if the donuts are replaced with CV joints an anti-roll bar must be added or handling will suffer.
I would guess the donuts Lotus used were off-the-shelf. Does anyone know what else used the same spec?
Does anyone know what the specs were? Not just dimensions but stiffness or whatever rubber is specced at? Or, for that matter, what was the -rubber- actually made of?
We are supposed to be having fun, are we not?
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