Lotus Elan

Driveshaft Dimensions - For making CVjointed shafts

PostPost by: AHM » Mon Jul 25, 2005 11:34 pm

Hi,

I'm going to get my own CV jointed driveshafts made.

I'm sure that someone must have tried this before, so maybe you can help with some dimensions.

What is the PCD of the of the bolt holes in the drive flange (that would normally attach to the rotoflex coupling) what is the hole size?

What is the plunge length? Or what is the max and min overall shaft length?

What is the maximum angle requirement at full suspension travel?

Are there any other things to consider?

Thanks,

Simon
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PostPost by: brian wilson » Tue Jul 26, 2005 12:39 am

Far easier to buy the best C/V's available from The Elan Factory

See web site for goodies

elanfactory.com.au
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PostPost by: AHM » Tue Jul 26, 2005 9:32 am

Thanks Brian,

looks like a good site. Look like well made and well designed products

I'm afraid I don't do far easier! If I did I would buy an MX5 and spend some of my time driving it, rather than all of my time (and money) repairing a Lotus.

I also worked for a company that makes CV joints = Far cheaper.

Simon
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PostPost by: rgh0 » Tue Jul 26, 2005 11:19 am

Simon

I have done this exercise twice with both my Elan and Plus 2. This was before the commercially available kits became available. These days I would buy one of the kits and the Elan Factory one is the best IMHO (the same kit is sold by Sue Miller I believe in the UK)

If you really want to go to the trouble and make you own the following may help. I certainly dont recommend it unless you are technically competent at design and specification of components and can do the quality fabrication requied or know good quality free fabricators you can use. If you have to pay for fabrication then you are beter off buying the commercial kits.

No guarrantee for accuracy of dimensions given and you need to check for your specific car and your components you buy.


CV joint

90mm dia VW late bug / type 3 CV joints. I got mine secnd hand from wreckers togehter with the shafts. Most commercial kits still use these as they are a standard joint easily available from bearing supply companies. Similar joints used on VW /Audi, Mercedes and BMW up to current times


Adapter plate details

125mm dia
12.5 mm thick
mild steel (some people have used aluminum but I would worry about thread strength for the bolts in aluminium)
centre hole 60 mm dia
countersink for mounting CV 90mm dia and 1.0 mm deep.
mounting bolts holes
spider legs - 3 holes - 96 mm PCD drill and tap 7/16 UNF
CV joint - 6 holes 77.8 mm PCD drill and tap 5/16 UNF
Bolts required
24 off 5/16 UNF 1.75 inches long socket head
6 off 7/16 UNF 1 inch long hex head
6 off 7/16 UNF 1.5 inch long hex head ( outeer blts that also go through the rear brake disks)



Shaft Details

VW shafts cut and rewelded to 294 mm overall length (original shaft length I used was 415mm). The exact length you use depends on where you want the CV's to sit in their travel in full droop and rebound and this in turn depends on how you want to restrain the droop. The 294 mm number was what I choose for the Elan. I would perhaps go a couple of mm longer if I did it again to have equal float at the full droop and rebound that I ended up with. Rewelding the cut shafts requires top quality alignment and welding. I fortunately had access to some of the best pressure piping welders in Australia when I did my shafts and they have held up for many years of racing abuse and 170 plus hp engines. I think the outcome would be more hit and miss with the average welder and why new shafts as offered in many of the kits is a good idea.

If you get new shafts made you need to detemine details of the spline and circlip on the standard VW shaft versus this overall dimension.

Seal Plate Details

The adpater plate needs to be sealed on its outer end. The end of the drive shaft protrudes through the end of the CV joint by a max of about 10mm on full compressions so the seal plate needs to be dished by a small amount to provide clearance. I made the seal plate out of 0.5mm galvanised sheet and panel beat a dish in it to suit the dish in the spider legs. Alternatively you can use a slightly thicker adpater plate and not bore the centre hole all the way though but this results in a shorter shaft and higher CV angles which is not desirable.


CV Joint Angle

The angle adopted on full droop depends on the shocks you use and how much travel they allow. The droop was orginally constrained by the donuts. In the Elan the full droop angle allowed by most shocks exceeds the angle most CV joints will allow. The limit may be the actual ball tracks in the CV starting to bind at the end of their travel or the metal cover that locates the rubber bellows cover on the CV depending on exactly what CV you are using. In general you need to limit droop to approximately the same dimensions as allowed by the donuts with either a restraint cable or strap or by shortening the shock rod.


Assembly

The 7/16 UNF bolts can work loose in the adapter plates which is why the Elan factory conversion uses captive bolt heads in the adapter plate and nyloc nuts rather than bolts screwed into the adapter plates. If you screw into the adpater plates make sure you use good lock washers, high stength loctite and torque properly. To be extra sure you also should consider drilling and wiring the bolt heads.


I hope the above helps. The greatest problems come from variable droop depending on shocks and variable deflection angle allowed depending on CV joint used. You need to work this issue carefully to ensure the exercise works for you. The other key issue is accurate fabrication to ensure it all fits and is straight and concentric.

regards
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PostPost by: M100 » Tue Jul 26, 2005 12:30 pm

Rohan,

glad you posted that, another design for me to consider as I'd like to homebrew rather than buy off the shelf (we have a few good VW parts places nearby) I agree with the aluminium adapter plates, strangely out of place when you are trying to improve the driveline reliability!

I got a copy of an article on an Elan CV conversion a while ago that was slightly different. It modified a VW stock part (the outboard driveshaft equivalent) so it would fit in place of the standard Elan Diff output shafts. Then all you would need is a shortened VW driveshaft as per your design and a single adapter plate for the outboard end.

I was considering doing something similar myself but doubts over the accuracy of the article came to my mind as a dimension on a drawing showed a change in the adaptor plate PCD required from (I think) 3.8" to 3.9" to fit the Elan outboard drive shafts for "later" cars. 3.8 ties in closely with your 96mm dimension (96.52mm). I wasn't aware of any change in driveshaft PCD specs - surely all rotoflex's are the same (crap!)

There is a table online somewhere of the max angles permitted for the various VW designs - not got it immediately to hand though. The price of all four CV joints (priced retail not trade) with boots worked out around 1/4 the cost of an off the shelf conversion here in the UK - its very tempting to get a quantity of some intermediate shafts custom made!

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PostPost by: rgh0 » Tue Jul 26, 2005 12:48 pm

Martin

I have seen references to the early cars having a slightly different PCD for the donuts. Never tracked them down to an authoritative source or measured up early ones to check myself. My dimensions are for my S4

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PostPost by: elansprint » Tue Jul 26, 2005 5:53 pm

I seem to remember from a coversation with mick millar a few years ago the CV joint angle was a problem due to the short length of the shaft. I believe he used joints that would handle 22.5 degrees
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PostPost by: AHM » Thu Jul 28, 2005 12:40 am

Rohan,

Thanks for the detailed answer. Certainly provides a good starting point, and as you say I will have to measure /calculate the rest.

In terms of design and spec. I am lucky enough to be having them made as a favour.

The intention is to take a current production driveshaft and make the adapters to suit.

For the closing plate, these are standard production parts, but I hadn't considered that it needs to fit inside the three flange legs - another dimension to take.

Martin,

Off the shelf joints might seem quite cheap but getting shafts custom made will be expensive. Taking an existing shaft and cutting then welding it is a good option, but as Rohan points out you need to use someone who knows what they are doing. When you weld you loose the heat treatment and much of the strength in the shaft. This also restricts you to using second hand driveshafts due to price.

In terms of getting a joint that plugs into the diff – I suggest that this is a non-starter you would be very lucky to find the correct length of stem with the correct spline fit, retention, and oil sealing (very lucky!).

Elansprint
I agree short shafts will give higher angles so it pays to keep the adapter width to a minimum. It is also important where in the suspension travel the angle occurs. 22 – 23 degrees is quite standard for a plunging joint. But in our case we need to combine high angles, probably quite a lot of plunge (to be calculated) and a flange mounting.

Will let you know what I measure.

Simon
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PostPost by: rgh0 » Thu Jul 28, 2005 10:12 am

Simon

I think you will struggle to find a curent production shaft that is short enough to fit in the Elan.

The VW's did not use the closing plate. The hubs of the VW diff and outboard shaft that connect to the CV joints was solid with a machined dish to accomodate the plunge in the CV. At least it was that way in the ones I have seen. I did not realise a dished closing plate was available as a standard component.

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