MK3 rear axle (2017)

...and if the 17mm axle works strength-wise for Mk1-2 models...
I believe the previous models are riding on a 9/16" (14.3mm) rear axle.

For all practical purposes my 17mm axle is bulletproof. Something else will break, like the cast aluminum hub, before that 17mm axle breaks, so a 20mm diameter axle doesn't increase the strength of the weaker parts that will fail first under stress so a bigger axle is pointless. Since the wheel I was adapting was already from a bike using a 1 piece axle, I only had to figure out spacers to make the wheel fit centered between the arms of the norton swingarm. The internal hub spacers were already set up to work from the donor bike with the 17mm axle.

If Don says adapting a 17mm axle to MK3 has more issues than just widening the axle slot, I would believe him, since he makes the kit to adapt the early model commando to use a 1 piece axle... so he knows.

My bike had the early bolt up hub and 9/16 diameter 2 piece axle and it never failed me, unlike some other people who weren't as lucky as I. My move to a 1 piece axle came purely because it was the simplest place to adapt the chosen yamaha wheels to the norton swingarm.

Just to make Don's point about there's often "more to it", I had other issues too... I moved the speedometer gearbox to the front wheel. I made a custom brake cable to reach the new drum brake now located on the timing side of the bike hub. I had the sprocket carrier turned down .25" to align the sprockets. I welded a brake "stand off" onto the swingarm to recieve the torque arm of the brake hub on the timing side, and I welded a stud to hold the brake cable end on the timing side of the swingarm near the swingarm slots... I could list more...

So, be aware that prototype modifications can snowball from inexpensive and simple to complicated and costly. I know this from experience. I can also say that I love my bike as it is now too, but never expected to put as much into it in time and money as I eventually did.

With that being said, IF I wanted to adapt an MK3 to have a 1 piece axle, I would probably do it and bugger all the costs... but that's just how I am. Obviously, I can't leave well enough alone.... 8)

Would using the same design, but replacing the axle with a higher grade of steel, similar to a Grade 8 bolt, be a practical solution?

Deleted.

Deets55 said:

Would using the same design, but replacing the axle with a higher grade of steel, similar to a Grade 8 bolt, be a practical solution?

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Unlikely. Fatigue strength usually worsens with higher grade steels.

-Knut

"Wrong. Featherbeds and G15 series used a 2-piece 17mm axle. P11 series used a one-piece 5/8" (15.9mm) axle."

Learn something every day , cant speak of the G15 or P11 , but I better refund all those Featherbed guys I sold 9/16" One Piece Axles to.
Don

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the bearings are NOT mounted on the axle. unlike the front 17mm axle.

madass140 said:

the bearings are NOT mounted on the axle. unlike the front 17mm axle.

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Ouch, you are right, I stand corrected.

-Knut

I can't be sure about this without dismantling the assembly (the photos were taken some time ago) but to make the Mk3 rear in one-piece it would seem to need a parallel spacer to replace the shoulder of the dummy axle.




If flats were machined to match those on the dummy axle and the thread diameter also reduced to the original 9/16" size there would be no need to open the LH slot to 17mm.

Exactly L.A.B. this is what I said earlier. the only possible way is to revert to a 9/16" axle, even then the "shouldered spacer" would only have a wall thickness of approx 1.35mm which would probably still be ok, It has to have the shoulder.
And the 9/16" axle would need a 17mm shoulder on the right side for the swingarm gap.
The problem I see is owners will think going down to a 9/16" axle is defeating the whole exercise. However the one piece 9/16" axles dont have any issues.
Don

madass140 said:

Exactly L.A.B. this is what I said earlier. the only possible way is to revert to a 9/16" axle,

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The only part that needs to be 9/16" is the threaded section. Edit: plus the flats to fit the LH slot.

madass140 said:

even then the "shouldered spacer" would only have a wall thickness of approx 1.35mm which would probably still be ok, It has to have the shoulder.

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The axle could still be 17mm, so the spacer would be the same OD and length as the 'head' of the dummy axle.

https://andover-norton.co.uk/en/shop-de ... dummy-axle

madass140 said:

And the 9/16" axle would need a 17mm shoulder on the right side for the swingarm gap.
The problem I see is owners will think going down to a 9/16" axle is defeating the whole exercise. However the one piece 9/16" axles dont have any issues.

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The axle as I see it can remain at 17mm. Reducing it to 9/16" isn't necessary so no additional spacer.

Mr. Rick said:

...and if the 17mm axle works strength-wise for Mk1-2 models...
I believe the previous models are riding on a 9/16" (14.3mm) rear axle.

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True, but those models had a bolt-up hub /drum-sprocket, which distributes loads closer to the S/A supports. Load distribution is not comparable to the cushioned hub design.

-Knut

Knut, the pre MK3 hub is a "cushion" hub and uses the same 9/16" axle set up as the earlier bolt up hubs

madass140 said:

some may not agree but many have had axle breakages because of a combination of things, such as , not the best design having a relatively
small diameter axle (9/16") and threading it in to the other half of the axle assembly (the stub axle) sometimes these threads will strip or
the long axle will break at the end of its thread, remember that the root diameter of the thread is only about 1/2" so it doesnt take much
ill fitment to put stress on the axle.

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If I remember correctly, the Mk3 axle thread is 1/2" UNF so even less root diameter!



According to Joe Seifert, 'ZFD', in a previous 'broken axle' thread, there were five alterations made to the original Mk3 rear axle drawing.

I asked in that thread (and again in another thread) if he could give us the details of those alterations so those Mk3 axles more prone to breakage could be identified, however, nothing was ever heard.

what a can of worms, If I owned a MK3 I would be fitting a 9/16" one piece axle and appropriate spacer , but for now I'll keep away from MK3 axles.
I got to much other stuff on my plate.

madass140 said:

If I owned a MK3 I would be fitting a 9/16" one piece axle and appropriate spacer

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But then you'd have to fit a 9/16" ID x 17mm OD spacer (or up to three spacers, as a short wider section would be required to take the place of the dummy axle head) the total distance between one axle plate and the other, as sprocket section, hub section and caliper plate are all 17mm.

Edit: Make that up to four spacers as the disc-side spacer 06-5545 could be replaced with one of 9/16"

Makes you wonder: Why was it okay to slot the RH swingarm plate to 17, but not the left? Coulda run a 17 axle (or 2-pc with dummy) all the way across...

Mr. Rick said:

Makes you wonder: Why was it okay to slot the RH swingarm plate to 17, but not the left? Coulda run a 17 axle (or 2-pc with dummy) all the way across...

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The flats and narrower LH slot prevent the dummy axle rotating with the nut or the main axle.

L.A.B. said:

The only part that needs to be 9/16" is the threaded section. Edit: plus the flats to fit the LH slot.
< >
The axle could still be 17mm, so the spacer would be the same OD and length as the 'head' of the dummy axle.
< >
The axle as I see it can remain at 17mm. Reducing it to 9/16" isn't necessary so no additional spacer.

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I agree, and machining the flats to a one-piece axle is obviously the best solution, although there is no need for an axle torque restraint when using a one-piece axle.

It seems we need to source a supplier in the UK for this.
L.A.B., do you know of someone who could undertake making a small batch?

I would like to propose using AISI 4340 (EN10250, DIN 1.6511) in the conditions Normalized + Quenched @ 800 ´C + Tempered 2 hrs @ 540 ´C. This will yield a high cycle fatigue sustainable stress of 500 MPa / 70 ksi (R=-1) which exhibts an elongation at break Ae = 16% approx. Stainless nut and washers, as suggested by Don.
The spacer replacing the dummy bolt head may be made of AISI 4340 in the normalised condition.

-Knut