Broken Axle

This is the second rear axle that has broken, clearly something is amiss. The Commando rear wheel is a brake drum joined to a hub with three stubs. The axle broke where it joins to the stub axle. Is there a flex that occurs between the brake drum and the hub? Is there a "limitation inherent in the original design" ? I am using the early rear wheel from the Atlas which does not have the rubber cush. Does it make sense to make up a new wheel, I was thinking of fabing up a conical hub to fit the swingarm. All input greatly appreciated

Not to be obtuse, but what happens when the axle breaks? I assume you are riding the bike when it breaks.

Yes the bike was underway when the failure occured, in both occasions. The axle sheared at the junction between the stub axle and the long axle at the base of the threads in the long axle. There was ten thousand miles on the axle and it was a used part when it went in. Doubt if it was prebroken. Two broken axles is too much of a coincidence

I should of been more clear. What I meant to ask, did you go for a wild ride once the axle broke?

Without revealing your weight or the combined weight of rider and passenger, could the bike overloaded? Can you check the swing arm and make sure the "ears" at the back are parallel verticaly and on the same plane horizontaly? Are all the wheel bearings OK?

Jean

Hi Norsa
I have had an axle break too. This was on my Triton featherbed but the rear wheel assembley is the same.
Mine broke whils I was two up, trundling down the road, in a straight line at about 50 mph. I felt it when I accelerated and the bike went diagonal. The failure was very gentle and didn't have us off, thank goodness!
The long axle which goes though from the left hand side was the one that broke, right at the base of the thread. On examination we concluded the failure was a fatigue failure, due to continuous flexing over the years. I am now begining to wonder if this diagnosis was correct. I did notice the axle was feeling a bit weird when I screwed it into the stub axle. It was reticent to pick up the threads in the stub axle and seemed to need a push and a tap to get the threads to engage.
I am at a bit of a loss to help, but at least you know you are not on your own.
I can only recommend you lock the stub axle up first (RHS) then the LHS after, as per the design, which allows the brake hub to be left behind.
Stu.

I tell ya these things have radioactive half lives of fissionable elements. Mr. Ed The Tomato Man had axle break today in his nice CNW Cdo but help was around to truck back home so pleased with the adventure long as it lasted.

Madass has a single piece axle for the early bikes.

is your bike an early one with the Atlas clutch or the later normal triplex chain clutch?

In defense of the 2 piece axle, Ed said he nipped his to 80 ft lb which I suspect contributed to his fracture. Just needs to be tight enough not to back off or risk another fracture too soon. If some way to retain R axle end from backing away from 750 type adjuster and also from backing out its not big a deal to ride as far as ya need on it till home awaiting next one or tow piece. Ed did have rear brake linkage come off too so the safety spring did it rare job.

Just shooting from the hip, thinking out loud, so to speak. Any hunk of metal breaks either in shear (so much weight that it gets sliced in two at the junction of the brake drum to hub), bending past its yield point, or from fatigue over time.
Did both axles break in exactly the same location? That is, did it break at the face of #23 http://www.oldbritts.com/1973_g20.html or was it down inside at the base of the threads? ('Sorry: I can't find an exploded diagram of the Atlas wheel).
How tight do you torque the axle to? If not tight enough, some repetitive flexing could be allowed at this joint, eventually causing the axle to stress fracture.
Could we get a picture of the broken pieces, especially how you found the remaining piece as it lay inside the stub axle?

Nathan

Okay, just noticed your comment about it breaking at the base of the threads. That would lead me to believe it was not tightened to the point where it would keep the joint from flexing, possibly leading to the broken axle. Still, a picture would help.

I always thought that was normal. I have at least a half dozen broken axles in my bin.

If you carry a passenger or a lot of luggage you better convert it to a one piece axle.

If it is a lightweight then you can probably get by with a new steel [not stainless] axle with about 80ft lbs on the left and 50 ft lbs on the right. Jim

They ALL Always break at the last thread valley about even with the dumb axle face. Easiest is just replace with each change of new owner or every 25 yrs. Next batch of cryo tempering I'll send Trixie's 1 yr old axle with it. My 2 had good reasons, Peel form over load on cement pad hyw seams at 90 and trying to hi side Trixie at 10 mph in parking lot and would of too if old axle held.

hobot said:

They ALL Always break at the last thread valley about even with the dumb axle face. Easiest is just replace with each change of new owner or every 25 yrs.

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This is indicative of an inherent design flaw. There must be "micro-flexing" occurring in the long axle, and the spot Hobot describes is exactly where I would expect the flexing to work harden the axle, and eventually break. Sort of like bending a metal tab until it breaks off.

I am wondering if insufficient torque on the three wheel mounting studs might be a factor.

Slick

If the axle was one made size bigger likely would stop the fission reaction at thread valley stress riser. Same with the AMC shafts too, which bend more than break though. I've heard of dozens of fractured axles but not one crash from it though its a bucking bronco to start rolling the 1st 5 mph then steady unless ya try to lean and turn with any spirit. I've only heard about 4-5 rear brake cable or links coming apart to drop the foot lever over a few decades or reports but brake safety spring is very common, so I'd think there is a market for LH axle safety retainer.

Probably exposing myself here but never had an axle break on a drum brake Commando. I know the symptom from Mk3 disc-brake rear wheels in the 1970s which is why the drawing for the Mk3 rear axle was altered later, probably by Richard Negus.

In 30-odd years, including racing two Commandos regularly for now two decades under various riders and riding Commandos on all sorts of roads at all sorts of speeds I never broke a rear axle on the drum brake models. I hear from one dealer particularly he finds bent rear axles in many old Commandos. Again, haven't had a bent one yet so it makes me wonder how people manage to do it. Are the bent ones the ones that eventually break? And how does one bend an axle? The mind boggles, not to mention the axle.....

Was the axle made from some nondescript stainless, as most "improved on original specification" pattern parts are, I should not be surprised, however.

Joe/Andover Norton

Looking at all the comments here I would come to the conclusion that the later cush hub on the commando's must be a better design that the earlier Atlas. For that reason I would recommend you convert to the later design. It is quicker to change wheels, and I have never experienced a broken axel on any of my Commando's.
regards, Dereck

Dear ZDF your confusion is part of the reason I refer to these split personality axles as having a radioactive half life as its almost completely random when one suddenly splits but like like nuclear fission it helps speed up decay if bombarded by neutrons which in our case is rough shocks from surfaces and the loads applied to the axle. Keri, The Only Cdo axles I ever heard of breaking were all the cush hub quick change kind. I have only heard of decades old axles breaking so wishfully assume a new one is good for at least another decade. More than any other failure prone item on Cdo other than Amal slides and various leaks, broken axles may top the list. A simple loop type adjuster to hold long axle forward, with drilled nut to safety wire from working out would go a long way to stifle the rodeo onset but may work so well you'd have to check some way to know it'd broken and wearing on other stuff. Besides cryo temper maybe removing the last few threads to polish out stress risers plus some shot peen might protect for a few decades or more. At least its only a rodeo ride taking off a few mph and I'd aslo assume pitching into a freeway exit after sailing along fine @ 70+ to put some significant side loads on >>> WEEHOOdoggieDOODOO>

ZFD said:

I know the symptom from Mk3 disc-brake rear wheels in the 1970s which is why the drawing for the Mk3 rear axle was altered later, probably by Richard Negus.

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What was that alteration exactly? If there are two variations of MkIII axle, is it possible to identify which one is which?

There are five alterations to the original drawing. I thought I remembered one was in the region where the thread ends and the plain portion begins. Originally these had like a sharp cut going straight down to the thread diameter and later got like a gradual decline. They always broke at the end of the thread. Haven't had that for a long time.
I remember selling these rear axles all the time in the late 1970s. The drawing looks different now to the axles I remember breaking, including the one on my own Mk3 which broke in the first year of ownership.