Drive Shaft Failures

[rgh0]

Recent topics around replacing donuts with CV joints have raised the issue of drive shaft failures with this conversion so I thought I would share my experiences with this topic.

I have seen drive shafts fail in 3 places and all of them on cars running donuts, 2 of the failure locations have occured on my Elan but I have also seen all three on other cars.

Failure point 1 - diff output shaft splines. I have seen the splines twisted in virtually all pre 1970 car diffs I have seen dismantled. Failure is virtually guarranteed in any pre 70's car used enthusiastically. The third photo below shows a twisted shaft, look at the splines carefully about 2/3 of the length from the end.

Failure point 2 - diff output shaft just inboard of bearing at diameter change, not a common failure but occured to me while my car still had donuts probably a combination of torsion and bending loads when using the car hard in competition. This is shown in the second photo

Failure point 3 - wheel hub shaft between outboard bearing and wheel hub. never seen occur in a road car with standard tyres and wheels as the wheels break first. Common in competition cars with sticky tyres due to bending loads when cornering. This is shown in the first photo

If I had a pre 70 road car I would pull the diff shafts and look carefully for twist in the splines. 70 and later cars had stronger diff output shafts and dont seem to suffer from this. The date is my best estimate based on observation as the actual change by Lotus is not documented, it probably occured at the same time the sprint diff brace was introduced. However I dont believe that Cv's make a failure at this point any more likely they will fail in either case.

If I had a car used in competition or enthusiastically on the road with sticky tyres I would replace both the diff output and wheel hub shafts regardless of using CV's or donuts as both will fail with the extreme use.

regards
Rohan

[bcmc33]

Rohan,

Thanks for your analysis on the subject - it's put my mind at rest to a large degree. Until yesterday's input from Jason1 I don't remember any of the historical threads on the solid driveshaft issue mentioning failures.
I am still on track for getting a SM CVJ set at Stoneleigh for my Sprint, but maybe you're in the best position for saying what the difference is between the Elan Factory kit and that from Susan Miller.

Brian Clarke

[types26/36]

......and this is what happens when a C.V. adapter lets go!

[bcmc33]

Brian,

How and why did the adaptor let go?



Brian Clarke

[rgh0]

Brian

From the photos I have seen the Miller CV conversion looks very similar to the Elan factory conversion. I know the Elan factory exports kits to oversees suppliers and Sue Miller may be one of them.

The key element to look for are the use of captured bolts in the adapter plate connecting to the Lotus drive shaft spider with nylock nuts. Bolts that screw into the adapter are definitely not as secure and really should be loctited and lock wired into place.

The Eloan factory conversion also has grease nipples fitted to the adpater plates to allow in service lubrication without dismantling.

The CV joint bolts into the adapter plate should also be secured with locking tab washers between pairs of bolts as is done with the original equipment use of these components or alternatively drilled and lock wired.


Any CV adapter kit from any supplier will suffer 2 potential fit problems

1. The tolerance that Lotus drilled the holes in their shafts for the donuts is not very precise as the rubber donut could accomodate any variation. Some reaming will probably be needed to get the more rigid CV adapter plate to fit the Lotus holes

2. The CV's are at the limit of angular movement when used on an Elan and may exceed that limit depending on the specific shocks used and some droop restaint or modification of the shocks to limit droop may be required. This generally not a problem on a plus 2 with its longer suspension arms and thus less angular defelection at full droop.

regards
Rohan

[types26/36]

First of all no manufacture's can be blamed for the failure, I made up the C.V. conversion myself with VW stuff.
How it happened............Well I was going along at about 100kph and BANG! no drive and a loud clang,clang,clang! adapter bolts had pulled out of the adapter.
Why it happened..........the original doughnut bolts are 7/16 UNF and so to keep the shank of the bolt the same diameter I tapped the adapters to 7/16 UNF.......BIG MISTAKE! as we all know aluminium and cast iron is normally threaded with a UNC thread.
Although I specified a dural 6082 or 6062 for the adapters (cant remember which one) I am not certain that is what I got, I suspect a lesser grade was supplied.
The bolts were not locktited but were fitted with shake proof washers and I can only surmise this was not enough.
After the "incident" I made up steel adapters using both shake proof washers and locktite and at one end lock wired the bolts as well, I could not use lock wire at both ends due to clearance issues.
I feel a lot more confident with the steel adapters as when the bolts are tightened I can tighten them without the fear of stripping the threads (always worried me with the dural adapters)

[Jason1]

Hi guys

Very interesting pictures Rohan; can I ask when these shafts twisted/broke was the car on the track or on the road?

I can understand the need for tougher shafts for a track car as it is exposed to much more punishment than a road car. It seems from other people that are running CV shafts on the road that these failures are very rare?

I am still concerned by these stories of doom but am reasonably reassured by the other road car users

Cheers

Jason

[nebogipfel]

Brian

From the photos I have seen the Miller CV conversion looks very similar to the Elan factory conversion. I know the Elan factory exports kits to oversees suppliers and Sue Miller may be one of them.

Mick Miller (with GKN I think) designed the shafts and they are manufactured in the UK.

I guess double CV shafts are going to look pretty much the same

[rgh0]

Jason

The diff output shaft broke at the bearing soon after I started club level track events in the car in 1981. It failed during the start of a hill climb. The twisted diff output shaft was what I pulled out of the other side when I replaced the shafts with new high strength billet shafts. I have seen many failures of the diff output shaft at the spline and this is the normal common failure point. It happens in any pre 70 car that is used "enthusiatically" with hard starts on the road or track regardless of whether donuts or solid rear end are fitted.

The wheel hub failure occured on the track in the mid 90's after a number years of competition and road use on sticky tyres. The RH rear wheel came off while going through a 100 km/h LH corner. I was amazed at how stable the car was on 3 wheels as I could just drive to side of the track and park it with no damage beyond grinding the metal cap off the base of the strut and the failed shaft itself. I have subsequently also had to change to high strength billet machined steel rear wheel hubs as I found that the hub itself was deflecting and fretting on the shaft taper under corner loads. I have seen reports of similar failures but not observed them directly myself, all the reports seem to be related to competition use but the same cornering loads can conceivably occur with hard road use on modern sticky tyres.

regards
Rohan