GTR broken crank

[lookmc]

Has anyone ever broken a crank or seen this before, my bike started to run on 1 cylinder only on my last ride, and at a quick inspection lost compression on the left cylinder. No wonder why.

[moonpup]

I had a broken crank that came in a box with my parts bike. There was some discussion about the causes here.....

http://bridgestonemotorcycleparts.com/index.php?topic=2305.0

Anyway..... that sucks!

[OldSwartout]

From the visual results, that's a rotating bending failure, not a torsional failure, meaning the crank was dynamically doing a rotating sine wave while running, the same failure mode the 175's have. Apparently, being out of balance a little or some other factor causes a resonance.  On the 175, street bikes infrequently fail the crank center pin, but a race bike is guaranteed to fail rather quickly. This may be due to both higher RPM and/or higher compression/advanced timing causing more force to induce the resonance. I'm assuming from that, that the resonance is RPM related, and spinning a 175 beyond about 9500 RPM will break one pretty quickly. I used to break SR175 cranks flat-tracking somewhat regularly, but not motocrossing or cross country. My road racer ran several years without a broken crank when I had chambers that worked to about 9500 RPM. When I got new chambers that let me run it to 10,500 or 11,000 RPM, they broke with regularity. That's why Vince Gunning had to go to a double center bearing support on his 200 racers. That stiffens the assembly by providing additional support, raising the resonance above the operating range. I suppose there may be a threshold above which a 350 shouldn't see sustained RPM. Maybe the previous owners of those bikes with the broken crankshafts liked to keep them in the upper RPM ranges.

[slawsonb]

Sounds like some finite element modeling is in order. Unfortunately,I no longer have access to those tools.
With the relatively robust bearing set-up in a 350 it seems that very little amplitude (of sine wave) would be possible, so a cyclic failure could take a long time. A defect in the part or induced during the assembly process could have been the trigger as well as bad balance per Karl's suggestion.
lookmc, how high did you rev the motor before failure?
...bert

[OldSwartout]

With the relatively robust bearing set-up in a 350 it seems that very little amplitude (of sine wave) would be possible, so a cyclic failure could take a long time.

That bearing setup is probably why there are so few 350 failures. It's similar to Vince's 200 modifications for racing.

[slawsonb]

Vince's motor is a work of art, awesome! Lots of very trick stuff in a very tight and fast (and winning) little package.
...bert

[lookmc]

I'm not sure exactly how high it had revved too but it wasn't just a leisurely Sunday ride. It was going throughout it's paces. The spare engine is now in whilst the crank is being re-built.

[slawsonb]

I would say this failure falls in the category of - Vely Intelestink! and annoying.
 

Good that you have a spare. Who is doing your crank rebuild?

...bert

[lookmc]

I am in Australia, I have a local race bike engineer building it up, he just finished one last week for my race bike.

[slawsonb]

Thanks for the info lookmc, but a little far to ship from Arizona.
...bert

[BRT-GTR]

Yes, I had a crank that looked exactly like that, way back in 1969.  Was just tootling along at low rpm when she went onto one cylinder. No bad noises or other indications, switched to reserve on the fuel tank but no response from the second cylinder. Checked plugs,  points, fuel supply etc but couldn't find problem so pushed it 3 miles home to be on the safe side.
  Only when I whipped the heads off did I find that one piston was not going up and down with the crank.  Oh sugar, I didn't earn much in those days and didn't relish the thought of the cost of a new crank.
   Pushed it another 3 miles to our local BS dealer who just happened to be the UK importer Bill Smith in Chester. Was told there had been a recall on my bike (GTR) for replacement of a crank part that had been machined with the wrong root radius but the previous original owner had not bothered to take the bike in. Anyway he honoured the recall and replaced  the crank for me at no cost, phew. Great service. Maybe you got one of these faulty parts on your machine. Do any of you BS mechanics from the late 60's remember this recall, I've never seen any reference to it on this site. Sounds like you've got a good guy on the crank rebuild. Good luck.
Brian the Brit.

[Malcolm Moore]

Another of the 'lucky ones'!!! My 350GTO suffered the same problem in exactly the same place. The points had moved advancing the timing on one cylinder. I assumed this broke the crank. Have now fitted the Pazon unit intended for an Ariel Leader.