Lotus Elan

Spyder Rear Suspension Hubs.

PostPost by: vincereynard » Sat Jul 20, 2019 7:42 pm

The bearings in BOTH my twin bone rear suspension hubs have failed in about 2000 miles.

I posted a thread a while ago complaining about suddden "road noise" from the back.
This was immediately following fitting a pair of adjustable rear bones. I naturally assumed it was something to do with the bones and changed them back. With no success.

I've been building up a pair of original Issue 18 struts and having got around to fitting them I've found the culprit. Must admit I had discounted the rear hubs as possible sources as they were only a few years old and done little mileage.

Mysterious - why following a bit of work on the Arms and why both together?

The hub nuts are slim, mild steel nylocs with only 3 complete threads. That seems inadequate for a safety critical nut torqued to 110 lbs!

Also the brake pads where held in place with what looked like a sticky rubber gasket ???
Never seen that before - any ideas?

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PostPost by: c42 » Sat Jul 20, 2019 8:50 pm

I have seen that material on the back of a brake pad before and I think that it is to stop brake squeal, surely the pads must be retained by some mechanical means.

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PostPost by: 69S4 » Sun Jul 21, 2019 6:20 am

My Land Rover Discovery has pads like that with a sticky mess on the back. As far as I’m aware it’s an anti squeal thing. If it is it works as the brakes don’t squeal but then again my Elan ones don’t (mostly :D ) with the std shims.

Those better versed in engineering than me may know what factors are important when specifying a nut for a critical location like the rear hub but my survival instincts tell me that’s not suitable. It looks to be about half the depth of the standard nut. When I bought my Elan the previous owner had replaced the rear hub nuts with ones made from aluminium (he was a navy helicopter technician :? ). Within a couple of weeks the nut stripped straight off the shaft and the wheel came off at speed. Playing fast and loose in that region isn’t a good idea.
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PostPost by: Craven » Sun Jul 21, 2019 11:03 am

Most know that strength of bolts is graded, but this also applies to nuts. So a correct grade of nut is required, old imperial system use S, T, U, etc would expect a nut with an X rating.
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PostPost by: StressCraxx » Sun Jul 21, 2019 4:10 pm

Three threads is the absolute minimum of engagement.

Joined properly, the nut is not really what provides the holding force of the hub to the tapered shaft. The shaft and hub are a keyed taper that are carefully machine matched to one another. The nut pushes the hub up the taper on the shaft and the two tapered parts marry as an interference fit. A tapered fit assembly will transmit more torque than a straight keyed shaft. It takes a lot of heat and extraction force to remove a properly assembled taper joint.

In order for the tapered fit to work, the hub and shaft should have at least 80% contact between the two surfaces. Prussian blue is used to verify the contact surfaces. The torque of the nut brings the two parts together forcing the hub over the tapered shaft.

Dave Bean's Elan parts book has an excellent set of instructions for the user to make a proper and reliable joint. Of special note in his book is the inspection of the keyway. There should be no sharp edges or corners to prevent the forming of stress raisers. It is vital for one's safety to make sure everything is in good condition and the instructions in the shop manual are followed.

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PostPost by: Craven » Sun Jul 21, 2019 5:57 pm

It has been discussed by other on this forum with an expert knowledge of such things that the angle of the taper is too steep and short to be considered a correct taper engineering solution.
If a poor quality nut is used the thread may pull or stretch as the recommended torque is applied leaving an unknown and an insecure situation.
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PostPost by: vincereynard » Sun Jul 21, 2019 7:04 pm

69S4 wrote:Those better versed in engineering than me may know what factors are important when specifying a nut for a critical location like the rear hub but my survival instincts tell me that’s not suitable. It looks to be about half the depth of the standard nut. When I bought my Elan the previous owner had replaced the rear hub nuts with ones made from aluminium (he was a navy helicopter technician :? ). Playing fast and loose in that region isn’t a good idea.


Makes one wonder where he got an aluminiun UNF nut from? Taking added lightness a bit far. Chunky would approve! (Assuming the ali. nut was cheaper.) Plus be careful accepting a lift in a Navy 'copter.

What surprised me is, it is so pointless to use a half nut. There is plenty of axle thread, why compromise on such a vital assembly.

Craven wrote:Most know that strength of bolts is graded, but this also applies to nuts. So a correct grade of nut is required, old imperial system use S, T, U, etc would expect a nut with an X rating.


Could I expect the nut to be marked in any way? These are not.

StressCraxx wrote: It takes a lot of heat and extraction force to remove a properly assembled taper joint.
Dan Wise


You got that right! Even with the rear hubs I gave up and took them to a man who has proper kit.

Craven wrote:It has been discussed by other on this forum with an expert knowledge of such things that the angle of the taper is too steep and short to be considered a correct taper engineering solution.
If a poor quality nut is used the thread may pull or stretch as the recommended torque is applied leaving an unknown and an insecure situation.


Funny you should mention that. The Engineer mentioned the thread stretch as a reason by a good new nut may only thread down a short way with no obvious sign of damage.

Apparently the same (even more so) for the damper rod. The thread can stretch during use so the castellated nut will only spin down flush and no further.
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PostPost by: StressCraxx » Mon Jul 22, 2019 2:45 am

Craven wrote:It has been discussed by other on this forum with an expert knowledge of such things that the angle of the taper is too steep and short to be considered a correct taper engineering solution.
If a poor quality nut is used the thread may pull or stretch as the recommended torque is applied leaving an unknown and an insecure situation.


Craven, I agree with you and the others on the forum, the taper is not the most desirable, but it is still the taper fit that retains the hub, not the nut. If the hub is loose or a poor fit on the taper, the hub will exit the shaft regardless of the nut attached.
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PostPost by: Craven » Mon Jul 22, 2019 10:52 am

Posts can often be interpreted in what the reader has in mind and not what was intended by the author, my point was if the required torque is to be applied, then the nut must be of suitable strength to withstand the force. Nothing to do with how the drive is transferred to the hub.
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PostPost by: rgh0 » Mon Jul 22, 2019 11:49 am

Craven wrote:Posts can often be interpreted in what the reader has in mind and not what was intended by the author, my point was if the required torque is to be applied, then the nut must be of suitable strength to withstand the force. Nothing to do with how the drive is transferred to the hub.


Theoretically 3 threads is typically what is required for a bolt and nut to handle the full design torque of a bolted connection ( it varies in theory a little based on thread form) . But in practice there are many more variables such as nut and bolt materials, thread friction, flatness of the mating faces, lubrication of the threads etc. i would aim in practice for at least 5 engaged threads on any critical bolted joint that is heavily loaded such as the Elan rear hub to the outboard shaft.

The taper itself is fine as long as it is fitted right and properly lapped in. As observed once the taper is locked properly it will not move, the hub nut just makes sure of ongoing compression for the taper lock and that lock is not lost with flexing of the hub and shaft as you drive. The circular key on the taper achieves nothing if the taper lock is lost and will shear off ( been there done that). The key does however weaken the shaft which is why TTR sell a version without it. I prefer to set up the hub and shaft using the key per the excellent guidelines in the Dave Bean Manual.

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Rohan

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PostPost by: Craven » Mon Jul 22, 2019 12:25 pm

One has to wonder just how many man hours were taken to lap the hubs on the production line.
And me thinking the thickness of a full nut was the required thread engagement.
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PostPost by: rgh0 » Mon Jul 22, 2019 12:31 pm

Craven wrote:One has to wonder just how many man hours were taken to lap the hubs on the production line.
And me thinking the thickness of a full nut was the required thread engagement.


I doubt they bothered lapping the hubs on the production line. Their machine setup for shafts and hubs would have been accurate enough for the initial build. The current replacement parts vary in accuracy and often do not match well. Used parts also wear and distort and often do not match well on reassembly.

Bolted joint design is not simple :roll:

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