I was out for a thrash yesterday on the 660 Ten. Was really going at it (about 80mph) passing vehicles, when the chain came off, not broken, just off.

Got it back on without too much aggro and headed more sedately home.

Up on the table this morning to get a better look and the damage appears minimal, scrapes on the outside of the Swing Arm but no big gouges.
Rear half of the Upper Chain guard has gone as has the lower Chain Guide to the rear sprocket .This Lower Guide was as our own Stoic Boy had recommended some while back. The only difference was that I'd split mine in two for easier fitting, as several have done on here. I'm starting to think this is not a good idea. The right angle mounting plate had been cleanly broken along the bend in the metal (3mm alum). The two mounting bolts/nuts were undamaged and still in position.

Bike is 2011, bought new 2013, 4750miles. The chain is not stretched and has still a fair bit of life left. Scottoiler from new. There is no apparent wear on the rear sprocket however there is more wear on the front sprocket than I would have expected at the mileage.

The chain may have been slightly loose but not excessively so, I'm pretty careful. I'm drawn towards the mounting plate of the Chain Guide suffering a vibration fracture but would like to hear what the collective think.

I'm fortunate I know....Could have been so much worse.

I'd put my money on there being too much slack on the chain, especially if it went back on easily without winding back the adjusters

Glad to hear you didn't come off any worse! Same happened to me a couple of years back from the ferry after a trip abroad. To be fair, my chain was near the end of it's life and I knew that on the way home. I kept having to tighten it up! Luckily it just came off without any damage to me.

My first thought was that your chain could have been slack (like mine was), but from what you have said you keep an eye on it and it's fairly new. But it could well have been that.

Like you said, the 3mm aluminium bracket could have fractured. I also have fitted a chain guide under the swingarm, but my bracket is made from 3mm thick stainless steel (fitted after my chain came off....).

Just out of interest, did you buy flat aluminium then bend it yourself or did you buy angled bracket?

The reason I ask it that I've had a 3mm thick aluminium bracket on my DRZ fail and fracture. I used it to hold a tool tube a while back before I made my own from stainless. The alloy, I suspect, was not great quality and it was bent along the grain rather than 90 degrees to it. The bracket failed at this point and I could see the fracture progression along the grain.

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XT660Z

AltecH Wheels
Wheel building and refurbishment, UK wide
AltecH Designs
XT660 Radiator guards, sprocket guards, luggage racks, DNA air filter covers

If the chain was in good fettle my thoughts lean towards the bracket due to vibration failing prior to the chain derailmeant. And thus infact causing it. My 90 degree.bracket made 3mm steel L section.

I can use it to roll the rear wheel up into the swingarm when fitting the rear wheel. It is solid.

Id be dubious about using ali. Tenere weighs a ton anyway.

Just my 2p

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To boldly go where no Tenere has gone before.

+2p

A guide in fully functioning order will guide the chain straight and true and do its job;, one that's come loose, falling apart or misaligned will just as successfully guide the chain off the sprocket.

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Thanks, guys. I'm now convinced the bracket fractured letting the whole block of rubber into the sprocket teeth.
Yes Ali, I made the bracket up, have learned a lesson!