Britbike forum Forums British motorcycles in general British motorcycles in general Diagnosing failed alternator rotor

Actually they're now over $120 but still a deal compared to the damage they can inflict.

I paid $81 for a Wassell one last week, brand new, on e-bay.

There is absolutely ZERO fault with your materials and workmanship. Both are superior.

But all these efforts are effectively the metallic equivalent of nailing gelatin to a board !!

All the best.

And the original design is like using a central hex nut to rotate that gelatin eh!!

Don't keep him going!!!

Just having a laugh............................

https://www.ebay.com.au/itm/Fits-Royal-Enfield-Electra-AC-DC-Rotor-Assembly-Magnet-12V-19-mm-ECs/363251330170?epid=19028371979&hash=item5493776c7a:g:uY0AAOSwMi5eOV~t&frcectupt=true

Richard---just think of Nicks modification like this.
Go from the front end of the primary case to the rear end.
On your Triumph twin clutch replace the clutch springs with solid spacers.
Then put on the clutch nuts and tighten them up.
What then happens?
you have tightened Fasteners horizontally and the friction exerted stops circumferential movement of the clutch basket with respect to the clutch center.
Nick is doing exactly the same thing with his rotor.
Just my two cents worth of course.

I think that's enough of all this.
As i said i'll just get another rotor and smile when i remember this thread.
Life's too short to be taken seriously.

But wait, I want to check the rotors on my 2 unit single dirt bikes (B44 & B50) that have been really well used (thrashed?). I seem to remember one of them has my signature spring washer repair in it. Probably ten years by now. All that blasting up sand washes, using that raw pulsating torque to romp up rocky hills and lugging it way down to ease through ruts and water cuts. This is the very definition of:

To quote Kevin Cameron. "The flywheel does not turn smoothly, but in a series of fairly violent jerks." It is this hammering effect that leads to failure.

So my poor hammered on rotors surely must have turned to Key lime pie by now or maybe those spring washers are squirting those magnets out like watermelon seeds. But both bikes still run fine, there's no low speed thunking noises, and my two sons haven't managed to kill them yet. So stay tuned. It will take me a few days to have a look and report back.

All the best,
Stuart.

OK, I've finally got to it. I pulled the rotor covers on both my unit singles. The B50 has my loose rotor fix, the B44 doesn't appear to. I tested the B50 rotor by marking it with a sharpie and then tapping on the edge of the recessed Lucas logo in both directions to find any looseness. It is tight.

You'll notice the face of the rotor has been machined as well as the recess (the hub is also machined down flush with the aluminum) for the washer to fit. I've been riding this bike for years and letting my early 20's sons thrash on it, so that has meant lots of wheelies and trying to keep up with modern bikes. This bike was a $500.00 swap meet basket case that had thrown a rod so now it has a new liner from LA Sleeve, and a Carrillo rod and JE piston from Ed V.

Now, the B44 does not appear to be modified. There is no special washer or machining visible and I don't remember if I fitted only one washer to the back or not. The fact that the crankshaft threads don't come up flush or proud of the nut makes me think I might have. But it checks tight also.

Interestingly, the B44 with the zinc pump was not wet sumped, the B50 with the iron pump was. Go figure.

So anyway, I feel reassured that a rotor can in fact be kept together using a cupped washer strategy of some sort of pinning approach. Here are shots of an original washer and a modified washer. Notice the teeth are trimmed down to avoid digging into and damaging the aluminum
and the domed side is machined to fit the back of the nut better.

So that's my take on the whole loose rotor issue. We clearly need to check for looseness because that can lead to bad damage if neglected for too long. But, if the rotor isn't very loose, one of these methods can extend the useful life quite a lot.

its all good till it isn't

Most likely a standard one.

Wow, impressive failure!!

But this brings up another issue this thread hasn't touched on yet. Rotor to stator clearance or lack thereof. The rotor shouldn't rub the stator.

If the stator isn't centered properly on the rotor and is rubbing, the resulting heat will weaken the aluminum. We used to (and I still do) use a brass .010" feeler gauge to check for clearance.

That whole mess looks like it got pretty hot before it blew and aluminum generally loses about half its strength around 600 deg.

Could some of these spectacular rotor failures be a result of improper setup? Seems to be a possibility.

Could be. You must slide the brass gauge all the way round the rotor, spin the engine and check again. Repeat as necessary.

, mind the gap


as the hex hub center loosens the hub trys to accelerate faster than the magnets and potmetal .
the magnets are hammered and cammed outward ... creating more and more space be be cammed outward
... kind of cool if its not your engine .

Another impressive failure. And your assessment and description of the cause is a good one.

That hammering and camming is exactly what pins or cupped washers prevent.

I made up a spacer to replace the original spacer so that it has a recessed inner ledge/lip. Careful measurements of the recess means that the spacer will lock both the alloy and the steel centre to the drive sprocket once the rotor nut is done up. I did this years ago. But it wasn't to fix a problem, it was to stop a problem in the first place.
IMO the the alloy and steel centre shear because of the whip generated by the rotation>>> inertia and recoil. If you can remove or abate this whip by locking the components so they are effectively a single unit then - in theory - there will be no shear.
I've never seen the magnets come loose or fall out where the steel centre and alloy are intact.
I've never seen or heard of a triple with this issue....because the rotational whip is different/smoother ...120' vs 180' = eliminating or lessening the whip is a good thing.

I've done this on a few twins [after convincing the owners of the benefits or at their request] and none have ever had an issue.

The Lucas marketing guys described the fix as a welding operation, but if you weld magnets they stop being magnets. So they did improve the stability but kept the method secret. John Healy posted the details many years ago.