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MK3 rear axle

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Re: MK3 rear axle

Postby mdt-son » Wed Aug 09, 2017 12:28 pm

L.A.B. wrote:
The Mk3 axle/spindle thread is 1/2" x 20 UNF (0.4387").


I stand corrected again! Thank you L.A.B.
The information is hard to come by and my source was obviously erroneous .... however, Old Britts list the thread size as well.

Sizing makes me wonder if the flexural behaviour is deliberate .... it`s frightening, in my view.
It also makes me wonder which material was selected and the fatigue life prescribed in design.

-Knut
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Re: MK3 rear axle

Postby Deets55 » Wed Aug 09, 2017 1:10 pm

I guess the two piece axle, especially in the case of the MKIII, should be considered a wear item and be replaced at some time. Has any data been compiled that shows the failure rate so that a reasonable replacement schedule can be established? According to a prior post AN seems to believe that 150,000 miles is the magic number. Would it be out of line to assume if a average rider were to change the stub and long axle at 50k miles he could sleep a little better at night? This seems to be the only solution at the moment for the MKIII riders. ( except for the inevitable " sell your MKIII and buy a real Norton" :D )
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Re: MK3 rear axle

Postby L.A.B. » Wed Aug 09, 2017 1:43 pm

mdt-son wrote:Sizing makes me wonder if the flexural behaviour is deliberate ....


I don't know, but I think they did try to prevent it, or at least, reduce it. The axle thread can't be much larger than 1/2" because the shaft of the dummy axle it screws into is also 17mm diameter.

I'm not totally sure about this without taking the rear wheel assembly apart (and I've already had the axle out once, today :wink: ), but as the axle is screwed in, I think part of the unthreaded 17mm section is also drawn into the head of the dummy axle, and the tightness of the 'fit' is probably intended to increase the rigidity of the joint, however, repeated removal and replacement of the axle eventually slackens the fit and allows movement, so periodic replacement could reduce the chance of it failing.

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Re: MK3 rear axle

Postby Mr. Rick » Wed Aug 09, 2017 4:44 pm

I think part of the unthreaded 17mm section is also drawn into the head of the dummy axle,
If this is true, then there is obviously room to make the internal threads of the dummy 17mm as well, instead of necking it down to 1/2".
Enlarging the dummy overall wd then permit 17mm flats on the dummy to go with a 17 slot in the swingarm, no?
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Re: MK3 rear axle

Postby L.A.B. » Thu Aug 10, 2017 12:13 am

Mr. Rick wrote:I think part of the unthreaded 17mm section is also drawn into the head of the dummy axle,
If this is true, then there is obviously room to make the internal threads of the dummy 17mm as well, instead of necking it down to 1/2".


The head is only a comparatively short section and couldn't be made longer as it is the 'spacer' between the LH wheel bearing and the double-row sprocket bearing. The 17mm thread would have to be the same 20 tpi as the 1/2" thread and both thread pitches would have to be aligned or the two separate threads would jam as the assembly was tightened.



Mr. Rick wrote:Enlarging the dummy overall wd then permit 17mm flats on the dummy to go with a 17 slot in the swingarm, no?


It isn't possible to enlarge the diameter of the Mk3 dummy axle, at least not without increasing the size of the sprocket bearing which would probably involve changing the complete sprocket assembly (and modification of the speedo drive gearbox) because unlike the pre-Mk3 drum brake dummy axle, the Mk3 dummy axle passes through the 17mm double-row sprocket bearing.
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Re: MK3 rear axle

Postby mdt-son » Thu Aug 10, 2017 3:52 am

Deets55 wrote:I guess the two piece axle, especially in the case of the MKIII, should be considered a wear item and be replaced at some time. Has any data been compiled that shows the failure rate so that a reasonable replacement schedule can be established? According to a prior post AN seems to believe that 150,000 miles is the magic number. Would it be out of line to assume if a average rider were to change the stub and long axle at 50k miles he could sleep a little better at night? This seems to be the only solution at the moment for the MKIII riders.


Just for the record, the mentioning of a replacement interval of 150,000 miles was my proposal for a service manual amendment. In view of L.A.B.`s recent post regarding details of the spindle / stub axle joint, I think a replacement interval of 75,000 miles is a better figure. These parts are cheap and replacing them is an easy task. As understood by the messages received from Andover Norton, no specific investigation has been undertaken to examine the failure rate of the two piece axle. I know A-N is observing this thread and I do hope they will issue a service bulletin or take a similar action.

All Mk3 owners should take notice and at least stay clear of non-approved parts.

-Knut
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Re: MK3 rear axle

Postby madass140 » Thu Aug 10, 2017 4:04 am

"All Mk3 owners should take notice and at least stay clear of non-approved parts."
approved by who?
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Re: MK3 rear axle

Postby Deets55 » Thu Aug 10, 2017 5:46 am

mdt-son wrote:
Deets55 wrote:I guess the two piece axle, especially in the case of the MKIII, should be considered a wear item and be replaced at some time. Has any data been compiled that shows the failure

Just for the record, the mentioning of a replacement interval of 150,000 miles was my proposal for a service manual amendment. In view of L.A.B.`s recent post regarding details of the spindle / stub axle joint, I think a replacement interval of 75,000 miles is a better figure. These parts are cheap and replacing them is an easy task. As understood by the messages received from Andover Norton, no specific investigation has been undertaken to examine the failure rate of the two piece axle. I know A-N is observing this thread and I do hope they will issue a service bulletin or take a similar action

-Knut


Knut,
I have personally never seen a snapped two piece axle, so what I am about to suggest is just speculation. The general consensus seems to be that the axle snaps due to metal fatigue as a result of flexing. Looking at the construction and the way the pieces mate, I wonder if overtightening the axle could be part of the problem.

When everything is fully assembled on the bike is it possible that:

a) the long axle can bottom out in the stub axle or

b) the shoulder (the area where the threaded section meets the unthreaded section) of the long axle contacts the corresponding area in the stub axle.

If overtightened, either one of these conditions would result in a high stressed area right where the axle seems to break. In my experience bolts that have snapped due to overtightening exhibit a different pattern on the broken section than those that have snapped due to flexing. As I said this is just speculation but I would be interested in your observations and opinion.

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