Rear brake plate location in swingarm came out

My Norton was raced in the late 80's and did a couple of meetings last year. This year I've put a Jawa engine in but the front and rear ends are the same. Testing the other day it felt odd going round a bend and a half mile later the rear locked up solid. Fortunately I was only doing about 50mph and upright. The break lever was bend in half and rotated with brake plate and hit the base of the shock. The rear spindle on the right side of the bike was under the swingarm and the wheel pushed up against the frame on the right with the spindle badly bent on the left side.
The setup had not been changed since the last race meeting and I had checked the chain and all nuts and bolts inc the swingarm nuts the day before in anticipation of riding at Cadwell this weekend.
The brake plate is a model 50 / dominator type with the 'nub' that slots into the swingarm. All I can think is that maybe with the extra power of the Jawa engine the nub had somehow twisted out of the swingarm slot. The chain adjusters were fine and not bent.
Is this something anyone has experienced? I'm concerned as the bike has run with this setup for a number of years without problems and with different riders racing it and I need to understand the issue before it goes out again.

Is this the "QD" axle, or a solid, 1 piece axle?

Originally it was a QD axle but for all its racing life it has been a 1 piece spindle with spacers.
The wheel is central and there seemed to be decent engagement of the brake plate into the slot.

Re- assemble using the Norton parts and ditch the one piece axle as, assuming it was perfectly assembled, the one piece axle clearly is not fit for purpose as it must have come loose given your description or the failure.
The Norton design only subjects the thread to high bending stress when incorrectly assembled.

Yes thats all I can think. However many rear wheels are held in with a one piece spindle so hard to understand how it came loose after so many racing miles in the past. I shudder to think what could have happened if it locked up round a high speed bend.
I've a Suzuki hub that is lighter and has a detachable sprocket to put in the back when I get round to it but seeing as it will essentially be the same setup but with a cush drive in the equation - I wonder what's to stop it doing the same thing. Certainly putting a torque bar to the swing arm would help.

I've got those cush Suzuki rear hubs on a couple bikes, and they work well. With the Featherbed swingarm, I paired it with the banjo-type tensioners, so the axle is captured, rather than relying on the forward tensioner bolts on the swingarm, found on the later Featherbeds. Having the gearing flexibility is very nice, as is the opportunity to run a narrower o-ring chain. Running the torque arm to the frame also removes some of the wonkiness in the brake lever under suspension compression. The most difficult part of the ones I've got is the brake is on the opposite side, so rigging a cable to your pedal is annoyingly complicated. Venhill makes a cable with an M6 threaded rod end that works for the brake arm adjustment. There's also a couple/few lengths of actuation arms for those hubs, so you get some choice with the leverage and pedal feel.

If the bike felt weird just before, something must have come loose (or broken) rather than everything caused by the brake locking up. Threads are good on the axle and nuts? Threads on the drum studs and nuts all good and seated tightly? No collapsed bearings or spacers? Possible that something wasn't quite seated and your last axle tightening set it up to seat itself just a teeny bit more at a most inconvenient time, leaving things just a bit loose?

The axle being under the axle slot on the right seems like it would be a clue, as the final drive chain is pulling it that way, but not your weight or suspension, but that's admittedly just pure armchair-engineer conjecture.

The QD axle does have a redundancy that a one piece axle doesn't.

Thats interesting about the Suzuki hubs. I think thats the way I'll go.
Drum studs are tight and lock wires still in place. Spindle thread is good. Spacers are as run for years and as mentioned I checked the spindle nuts the day before. I agree that something was moving as I went round the bend.
For now the bike is in the corner and won't now get touched until the end of November. I'll put the hub in with the same spacer setup and brake plate and see if I can find any variance.
Thanks

Charkmandler said:

Thats interesting about the Suzuki hubs. I think thats the way I'll go.
Drum studs are tight and lock wires still in place. Spindle thread is good. Spacers are as run for years and as mentioned I checked the spindle nuts the day before. I agree that something was moving as I went round the bend.
For now the bike is in the corner and won't now get touched until the end of November. I'll put the hub in with the same spacer setup and brake plate and see if I can find any variance.
Thanks

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Good luck. That's pretty scary and hard to trust again without an obvious cause.

Give me a holler if you have an interest in the Suzuki hub and brake set-ups I cobbled together.