Lotus Elan

Quite a moment!

PostPost by: el-saturn » Sat Oct 31, 2015 1:54 pm

i'd say neither one was properly lapped first: both surfaces shouldn't show the corrosion they show. it takes some engineers ink (?) and lapping paste (course and fine) PLUS enough MEAT left to get them right and perfect! sandy 4982 (issue 18s at the rear)
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PostPost by: Chancer » Sat Oct 31, 2015 3:43 pm

A fatigue failure with the crack propogating from the root of the thread, interesting in that it has propogated circumferentially rather than transverse but understandable as the load travels around the circumference as the Wheel rot?tes, it has finally sheared in tension at the core of the shaft after the surface area remaining to resist the load is reduced and the stress going up to yield point then swiftly UTS.

I think even with rolled threads if they about a flange the last thread may not be properly formed.

I've had a few wheels overtake me in my time! Once I was towing a Customer in his Mini with my Mini, I had the sunroof open and felt a slight snatch, lokking up to the rear view Mirror I saw the Wheel going over my car through the sunroof, loose Wheel nuts, not my fault that time!!
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PostPost by: john.p.clegg » Sat Oct 31, 2015 4:01 pm

Did someone point out that there was no radius on the start of the threaded portion?

John :wink:
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PostPost by: alan.barker » Sun Nov 01, 2015 5:50 am

what tyres do you have fitted on your car. I see you have wheels like Minilites so i imagine you have fitted big sticky modern tyres. This puts a big load on the driveshafts
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PostPost by: rgh0 » Sun Nov 01, 2015 7:49 am

Generally the stub axles break at the bearing or the inner end of the keyway when loaded heavily and frequently, with sticky tyres. I had the right rear tyre pass me over the hill at the Phillip Island track at about 125 km/h many years ago fortunately the car is pretty stable on three wheels !

For the shaft thread to fail in fatigue suggests it had lost tension and was seen cyclic stresses. The fact that the hub was fretting says that the hub was moving on the taper which also says it was not fully clamped on by the end nut.

Overtime the taper on the hub opens up into a hour glass shape and it moves inwards on the shaft if it i not properly fitted and locked on the shaft. This what I suspect caused the failure. If you are reusing your hubs check the taper fits along its whole length when you lap them it onto the new shafts. Also re-torque the axle nuts a couple of times after each 100 km until they stop moving.

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PostPost by: JJDraper » Sun Nov 01, 2015 11:22 am

Thanks Rohan, the hub has been scrapped as a precaution. No doubt the hub was moving, but what caused the loss of clamping force? I suspect the crack was opened by the last tighten of the hub nut (four years ago, by a respected garage), and as the crack opened further, clamping force was reduced until the hub could move. Things moved pretty rapidly once this happened. In the preceding weeks I did hear a new creaking noise, but that was one amongst many.

As they say, keep your nuts tightened and check them regularly.

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PostPost by: Chancer » Sun Nov 01, 2015 12:41 pm

They may not have tightened the nut correctly and even if they did there is a strong probability that the hub was not lapped to the shaft, were they lapped in the factory? And more importantly were they always lapped or was it something that was found to be needed after failures in the field.

If your nuts need frequent tightening investigate the reason, had things worked out differently in the past this shaft could have been retorqued perhaps after noticing play but the crack was still creeping.

I had a similar thing on a Caterham 7 I rebuilt after a crash, the front hub is like the Elan but aluminium (maybe the Elan one is?) it seemed OK, no sign of cracking or run out but the fact that I had cause to take play out of the bearing more than once should have been a warning to me, I eventually wised up when the crack around the bearing location became visible.
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PostPost by: gus » Tue Nov 03, 2015 1:02 pm

I am betting that the nut was never tight, ever, there should be no rust no nothing on the taper or pin. The taper could be wrong[the modded rear axles on my former seven were like this]

This goes back to the repair, and I would tell the insurance company so.
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PostPost by: Robbie693 » Wed Nov 04, 2015 12:31 am

Looks like a narrow escape Jeremy, glad everything is (relatively) ok.

Surely if the hub had been loose for a while this would have been picked up on the MOT during the wheel bearing 'wiggle test'?

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PostPost by: rgh0 » Wed Nov 04, 2015 11:21 am

If the hub had never been tight on the taper i would have expected the round key to have sheared and the hub just spin on the shaft before the nut failed and hub came off. The round key can handle very little torque and a normal traffic light start can shear it if the taper is not locked (been there done that)

I suspect there was a very short time between the nut coming off with the thread section breaking and then the hub coming loose and departing the shaft.

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