Lotus Elan

CV Conversion

PostPost by: surveyor » Mon Jan 01, 2007 4:01 am

Having experienced a rotoflex breakage on my '68 +2, which though at low speed took with it a halfshaft and the handbrake linkage and tree, I'm going to go down the CV route. As I have the necessary machining facilities and access to VW (IRS beetle) parts I'm toying with a diy approach rather than buy a complete kit.

I've found a lot of really useful information on the forum but maybe someone can help me out with a couple of questions:

1. Bolting the diff/hub shafts to a threaded plate seems to me a no-no due to risk of the bolts possibly working loose - I don't want to replace one worry with another. Several suppliers advertise the use of "captive bolts" through the plates which appear to be countersunk head allen bolts. Is this the case and, if so, are the plates mounted up first then the cvs and shafts fitted after? With the dimensions I have for cvs and the pcd for the allen bolts it doesn't look possible to insert a key to tighten these bolts with the cv in place.

2. Has anyone shortened VW shafts for use on a +2. If so what length shafts did you end up with? I know I should be able to work this out once everything else is in place, but why reinvent the wheel.

cheers
dennis
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PostPost by: rgh0 » Mon Jan 01, 2007 8:17 am

Hi Dennis
If you do this conversion yourself you need to be good at machining and very good at welding the shortened shaft to get a safe and vibration free end result. Having given the warning and taking no liability for the outcome my answers to your specific questions as follows

1. Shaft length should be shortened to approx 365mm for a plus 2. This is with 12.5mm adapter plates, a 1.0mm countersink to centre the CV in the adapter and a 0.5mm end plate on the other side of the adapter from the CV to seal the outer end of the CV. Note that the shaft ends stick out through the ends of the CV at maximum travel inwards of the CV's by 10mm plus so the end seal plates need a slight dish in them with the setup I described above

If your using different component dimensions you need to adjust accordingly. The shaft dimension should be checked on individual cars as the tolerances on the overal diff and suspension manufacture, setup and subsequent possible machining of inner and outer drive shafts can affect this by about +/- 5.0mm. I have seen lengths quoted or used between 369mm and 359mm on a number of Plus 2s. The dimension is also affected by how far the suspension droops which is different depending on which shocks you use, you may need to lengthen the drive shaft a little or fit a droop restraining strap if you have shockers that result in excessive droop once the donuts are removed. I would be interested in what length you end up with to add to my understanding of the tolerance range. The suspension movement requires about 20mm of plunge on the CV's and 2 CV's together can accomodate a maximum of 34mm of plunge so in the end you have some choice in how long you make the shaft also.

Having said all the above 365mm should work on most cars I believe if all other components normal and the adapters as I describe.

2. I dont think you cannot get at the heads of counter sunk socket headed bolts once the CV is fitted to the adapter. The bolts are at a PCD of 96mm and the CV outer diameter is 90mm, the hex socket on the 7/16 inch bolts is around 9mm across the points, you may be able to get a suitably ground hex key into the exposed part of the socket to hold it while tightening or find socket headed bolts with a smaller internal hex size .

An alternative I have seen used is a Tee head bolt sitting in a milled slot so it cant turn. If you cant locate a suitable Tee head bolt you could modify a socket headed bolt or standard bolt by drilling the head and fitting a pin through it that would sit in a similar milled slot in the adapter.

Another possible way to capture the bolt would be to partially counter sink the bolt in the adapter and mill off the needed section of bolt head so it is locked in place underneath and by the outer edge of the CV when it is bolted in place.

hope this helps
Regards
Rohan
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PostPost by: surveyor » Mon Jan 01, 2007 8:40 pm

Hi Rohan,

Happy New Year and thanks for the quick and very comprehensive reply.

I'm reasonably content on the machining front, but would be going to a specialist for the welding as my skills there are a bit too hit or miss for something like this.

Your assessment of the countersunk bolt access dimension corresponds with mine. But sitting back and looking at the VW installation I reckoned that it could be easier to mount up the plate to the stubs with countersunk allen bolts and locknuts, torque up and then install the shaft/CVs assembly. Once the plates are fixed in place I'd essentially be reproducing the VW fitting procedure.

This should solve the problem of trying to capture bolts in the plate though subsequent service checking of tightness would not be possible without dismounting the shaft/CV assy. Your idea of milling the bolt heads to provide a capture against the CV joint could help obviate that problem as well as providing some additional insurance.

I'll keep you posted on progress, though at the moment a ground-up rebuild of my '61 Frogeye is taking up all my time, and garage space, so the +2 shafts may be a late spring endeavour.

cheers
dennis
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PostPost by: ecamiel » Tue Jan 02, 2007 10:40 pm

Hi
My S1 has a VW based CV system that uses VW differential stub axels, VW outer stub axels and a shortened and sleved axel. The outer axels and hubs have the bearing diameter enlarged. There is an adapter for a flat brake disk. It was made by someone in NY state in the late 70s.
It has been raced and held up well after a redesign of the axel shafts.
If anyone knows what VW parts may have been used in this system I'd love to know.
Eric
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PostPost by: surveyor » Wed Jan 03, 2007 1:03 am

Hi Eric,
It sounds as if someone with access to, and the ability to use, some very high-tech machining facilities has created the setup on your car. It sounds an ideal solution, but for us lesser mortals the adaptor plate route is probably the only reasonably practical one.

I seem to remember seeing, when searching the archive, a comment from Rohan about the difficulties in getting everything just right with such an approach. However, it is obviously possible given the right facilities.

Have you ever photographed this setup, I'm sure it would be of interest to many of us.

cheers
dennis
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