GS450 Catastrophic failure

[free99]

Hey all. Pretty sad, my GS450 that I've put a lot of work into has failed in a pretty serious way that's made me lose all trust in her.

The other day, I was zipping along the highway at about 75 to 80, no apparent issues. I took the exit, stopped at the light. Light turns green, I accelerate and the bike kind of doesn't want to roll. I'd been having an issue with a sticky rear brake, but a roll back by about an inch or two would always unstick it. So, rolled it back, tried accelerating again.

A heavy *crunk* from the engine, I stop the engine.

After determining the bike won't roll (even in neutral!), my buddy tows it to his place, we think it through and hope it's just the shift forks. After removing the oil pan, there are teeth in the bottom.

I've got some theories on why this happened, but I don't have the privilege of experience you friends likely have. What are your thoughts on why this happened? Get technical, it helps us all.

1. Changes to the bike include an 18tooth primary sprocket paired with a 40t rear. Stock is 16t/42t per JTSprockets. Was I applying too much torque for the gearbox to handle? It'd been in this config going on two years now to keep RPMs down a bit since I was mostly cruising around, albeit at 80mph or so.
2. I've on occasion mishifted and spun the bike up to redline (say once in 9 months), but it's been quite some time since that's last happened. Besides, expected damage would be to the cylinder pins right?
3. Oil viscosity? Using honda 10w-40, she was about due for an oil change.
4. General unknowns? I found the bike for $200 with 14k miles on the speedo. I've added another 18-20k myself on long trips over the last 3 or so years. Maybe something happened before I'd found the bike in those 14k miles?

Full size: https://ibb.co/album/fDL7xy

[Grimly]

I'm eyeing that broken circlip, as it could have broken, come adrift and jammed itself between two gears.

No particular reason to distrust the bike - find another gear set or complete engine and ride. Shyt happens, get on with it.

[free99]

Grimly, distrusted because if this had happened 5 minutes earlier, that'd be skating along the highway on my ass. Most I'm hoping for is a track bike out of this, replacement set of gears on ebay is $40. But you gotta agree, a failure leading to locked rear wheels is quite a bad thing.

What would cause the circlip to jump though? Is that a problem you've seen before?

[rphillips]

I've been around a bunch of GS's for a lot of very abused miles, and long Drag strip days. Only transmission problems I remember were worn dogs on gears or a bent shift fork, nothing like what you've got... I can see your concern, but my opinion put it back together and ride it forever, for that to happen is rare as winning the lottery, to happen 2 times to the same bike, I'd guess it has never happened.

[Rob S.]

For $200 you certainly got your money's worth. And if a wheel locks, better the rear, right?

I always thought it would be valve train most at risk past red line. Remember the CB350? Tach went to 12, 'red' was from 9 - 11. Manual said short bursts into red were okay, just don't go past.

[Grimly]

Grimly, distrusted because if this had happened 5 minutes earlier, that'd be skating along the highway on my ass.

That applies to any motorcycle, and yes, sometimes you get a gearbox or shaft lockup out of the blue, even on a GS.
Just ask some BMW owners.
Not worth worrying over, as the chance of it happening is low - you got unlucky, that's all. Well, lucky, as you say.
Makes me wonder if the PO was in there, as quite often catastrophic failures can be traced back to ham-fisted previous owners or their mechanics. It's the reason why workshop manuals often recommend the replacement of such clips, and not the re-use.

[Cipher]

wont they replace the teeth on the NHS? Just put a suit and tie on the tranny and take a shot.

[free99]

Welp.. replacement set of gears on the way via ebay.

I wish there was a way to know with better certainty that it was the clip, but it kind of makes sense. The clip may have finally worked loose right at that stoplight, that difficulty in rolling was the first time it was interacting with teeth. 20ft later, that's all she wrote.

[rphillips]

Way easier to see snap ring breaking teeth than teeth breaking snap ring... I've been waiting for someone else to chime in with catastrophic failure of a GS trans.

[Rob S.]

I've been waiting for someone else to chime in with catastrophic failure of a GS trans.

Don't hold your breath (you were being sarcastic, right?) I've been waiting to hear about the gigantic marketing mistakes Honda made in the U.S. in the '60's.

Or De Niro to Secret Service in Taxi Driver: "You're waiting for the candidate? I'm waiting for the sun to shine."

If the sun don't come
You get your tan
From standing in
The English rain

Hope I don't have egg on my face!

[Nessism]

In all my years here, this is the first transmission failure I can remember. I agree with the assessment that something may have got stuck in the gears. Loose parts inside an engine are rare, but not unheard of. And for what it's worth, your gearing change, via final drive sprockets, is pretty extreme. I've never heard of anyone jacking the gearing so tall before. I don't know if that could have somehow overloaded the trans gears, but...

[Rob S.]

...your gearing change, via final drive sprockets, is pretty extreme. I've never heard of anyone jacking the hearing so tall before. I don't know if that could have somehow overloaded the trans gears, but...

Up in the front, down in the back. That's like upshifting 6 gears on my 10 speed English racer.

[free99]

Culprit's found.

I'll be installing the replacement gears later this weekend, but I'm also going to try to do a post-mortem on which circlip actually came off (looks like 4 total in the fiche), and see if one of my hunches on what actually happened is a winner. I'll keep you all posted, have a nice weekend.

eta: Anyone know why there's a notch on all the teeth of the gear closest to the roller bearing on the left? It's present on the replacement set too. What's that for?

[GregT]

Notches are probably identifiers for semi-skilled labour on the assembly line.
Before assembling the box, check the endfloat on the shift drum. If that is excessive it can put load on the circlips between gears. Should be only just discernable movement - about 003- .005 in.

[free99]

Thanks for the advice GregT, but the engine's been buttoned up already..

Gentle riding, oil change after 100 miles, and everything seems ok so far. I reverted the final-drive gearing back to stock, the limited top speed is annoying but it'll have to do till I get the GS1000 online.

Happy 4th everyone!