The nut and washer do have a secondary function of retaining the hub to the shaft (so stop the wheel falling off) but their main purpose is to hold the two halves of the taper locked together. With well mating tapered surfaces the hub and shaft are locked together effectively becoming one piece of metal. Bending and other forces from the wheel to the shaft should not impart any more tension on the thread than it had when it was assembled. The threaded portion of the shaft should not normally see any bending forces leading to fatigue.
It seems to me that there must be signs of fretting or other movement on the taper surfaces and the keyway groove and a picture or two would be useful.
I agree. Thinking about it, the wheel spinner torque should be reacted by the hub through the taper so should not play any part in the failure. Similarly the wheel torque should be through the taper, so the threads should only carry the tensile load from the nut.
That it failed on acceleration suggests movement in the hub. That should only occur if the nut torque was low. CheckIng the hub for signs of movement would show low torque, no movement would suggest a too high torque and eventual yielding.
Hope this makes sense!
Very sorry you had that experience. I had an stock inner shaft fail on my S4 shortly after buying the car, but it simply stopped going down the road. Glad you didn’t have much worse damage!
I installed TTR outer shafts on both my Elans years ago. They are excellently machined and finished, and appear to be much stronger than the original Lotus shafts. I installed CV axles at the same time. The TTR shafts do fit standard hubs (not just 26R).
no affiliation, but it does make sense to me to put in perspective the cost of some modifications for track days and sticky tires with potential damages that can result of a use a given car is no necessarily up to.
I suspect failure to lap the hub to the shaft on the failed one may well have been significant in why it failed, only the nut would have been holding the hub on, not the fit on the taper. If it had been well lapped in it would probably have failed at the inboard end of the taper. As pointed out by Rohan, this is more common and would leave the tapered part of the shaft still jammed into the hub. Good job you had a fire extinguisher to hand in the car and that it worked!
If you want to fit CVs and fit stronger output shafts on Diff then the CVs from the USA must be the best. More distance between CVs so less Angle deflextion.
Also TTR for Shafts at Chapman Strut.
Alan
Another thought, though maybe I missed it.
Torque to yield.
No mater how careful some people are, when daft garages over-torque wheel nuts…wheels fall off because bolts break at mating surfaces of wheel.
So when one whacks spinner to unknown torque, most likely it stretches hub/bolt at same location. When you hit a pot hole…when you use the same hub/bolt for 50+ years. Important to follow the engineers report (did Lotus recognize replacement of hubs/bolts as a replacement item at __,____miles?)
I suspect this is something we all need to consider.
Confused on previous comment, however tight you torque the spinner it has no direct connection to the spindle, only the hub. The nut securing the hub to the spindle would normally only be tightened once when fitting the hub to the spindle, and the torque setting is in the manual.
Agree over tightening normal wheel nuts onto the studs can be a problem with untrained wheel fitters, but that is very different to Lotus knock on wheels.
Overtime the hub tends to bell out on the shaft taper, loosing contact at each end. This is more significant with the original bolt on hubs which were not as rigid as the Knock On design. It also occurs more quickly with sticky tyres putting more side loads on the wheels.
If the hub starts rocking on the shaft then it will be putting oscillating loads on the hub nut and eventually lead to fatigue cracking of the shaft thread and the sort of failure experience. Usually though the loose shaft shears the key and spins in the hub first and you lose drive before you lose the wheel
Very sad to hear of the incident, but bravo for continuing on with your holiday…!
I’m in the process of installing hubs to brand new TTR shafts, and am currently performing the lapping step mentioned here (the parts are excellent).
Any tips for the lapping procedure itself? How do I know when I’ve done it sufficiently? I’m using valve grinding compound and have worked them several times each, but is there a finish on the surface I should be looking to achieve, or apply toolmaker’s blue to check my progress? TTR instructions also mention to heat the hub prior to installation as well.
Given the above I just want to do this properly…
(Also, sounds like you have great insurance coverage!)
You can tell by the lapping paste itself and how evenly it spreads and gets ground as to how close you are getting. I then do a final check with engineers blue to verify the fit. Heating the hub should not really be needed and I have never done it on my Elan but I guess TTR may have found it helps with some race cars. Getting the hub off in the future may be more challenging than it already is if its been heat shrunk on. I do check the hub nut a couple of times in the first few hundred Km’s and re-torque to the specified number and sometimes find the nut moves a little
I fitted the keyed TTR shafts, carefully lapped, radiused ends the keys and did not use heat when fitting them. Like Rohan, I re checked the hub nut torque over the first 500 kilometres.
Thanks for sharing the TTR list of mods. Ironically I have almost every single one of these on the car except the uprated hubshafts.
The car was returned to my house this morning so I will start looking at the damage and share more photos this evening. A quick look at the underside shows the underside of the upright heavily ground away along with the underside of the brand new adjustable bottom wishbone.
too bad… I hesitated when I did the restoration of my street S4 as it was low mileage and good mechanically, and eventually opted for the “upgraded” version in case I did use it for occasionnal track days, then I was glad I did, and nowadays track days are the best motivation to push me to fetch the car from where it is stored and use it. With sticky tires, It’s hard not to get caught into it and it does pull quite a bit (almost 1G laterally from what my phone told me) while the street car is more than 100kg heavier than the GTS… even if I try to avoid the curbs as much as possible.
Found some time to have a closer look at the damage and possible root cause of the hub shaft shearing.
Almost certainly down to a loose hub nut I think
1, The photo shows the nut to be well down the thread
2, Out of interest I checked the torque on the other side and it was below 50 lb/ft. I think the setting is 100lb/ft
The hub is heavily ground down and will need replacing.
The bottom wishbone is also scrap.
Both shafts have been off the car recently for a skim to the legs for mating up to new discs. The hub nuts and bearings are new.
as mentioned before, the rust on the shaft is a symptom of poor hub fit, eventually resulting in the nut being lose and in turn exerting a cyclical load on the threaded end of the shaft: as soon as the nut does not pull 100% axially, trouble is bound to happen, sooner rather than later.
If the Chapman strut holder is not leaking at the bottom of the shock, you should be able to keep it…
good luck for a smooth repair and back on the road !
The same happened to me back in the 80s, to everyones amazement the Lotus dealer in my intended destination (Goldblatt & Webb, Cardiff) had a bolt-on type hub for an Elan on the shelf.
Mine happened near Exeter on the A38 as it became the M5 and was probably travelling at 75-80 MPH.
The sensation of rear wheel steering was very disconcerting and caused a few moments. The wheel actually stayed more or less in place trapped by the wheel arch.
I felt very lucky there wasn’t any more damage done.
Try as I might I was unable to persuade the AA operator that my wheel had fallen off due to a broken hub, so had to go through the process of an AA man turn up having been told by the operator that I probably just had a loose wheel!
In your third picture, of the end of the shaft where the nut sheared off, it seems like there’s rust along the edge as if there was a pre-existing crack at the threads… hard to tell because of the focus.
