Hanging By A Thread

Pics: David James

VJMC MEMBER AND REGULAR CONTRIBUTOR ROBERT CARMICHAEL RECOUNTS HIS MOST MEMORABLE RIDING MOMENT, WHICH INVOLVED A YAMAHA SR500 SWINGING-ARM HANGING LITERALLY BY A THREAD.

For a number of years, on weekends I worked for the Road Traffic Authority as a casually employed motorcycle instructor for their prelicence Learner Rider Course. Instructors had to do an advanced rider training course run by Driver Education Centre of Australia (DECA) located up at Shepparton in central Victoria. Because the SR500 was my training demonstrator bike, I rode it up to Shepparton and successfully completed the course on it, including doing some fairly rapid cornering on the closed to traffic track where we practiced counter-steering, trail-braking and nailing late apex corners. The course was great and we learned a lot about riding skills and road-craft, which we’ll go into some time.

After the training session I was riding back to Melbourne at legal speed when I felt that there was something wrong with the way the bike was handling. Looking down between the petrol tank and my left leg I saw that the swinging-arm pivot shaft had worked itself about half way out of the frame and the whole swinging-arm assembly was wobbling about. See photo for the location of pivot shaft on the frame gusset just behind the engine.

Note the locknut on the end of the shaft. Loss of this nut meant the pivot shaft could work its way out on the bike’s other side. I thought to myself, “Now if I hit the brakes, that shaft will come right out and the whole swingingarm assembly will crash to the ground and I’ll go down the road at about 100kph”. So very gently I gradually rolled the throttle off without touching the brakes, but by this time the shaft had nearly worked its way completely out and the whole rear end of the bike was slewing from side to side. Eventually the bike came to a stop and with a huge sigh of relief I put the bike on its side-stand and inspected the situation.

On the left hand side there was only about four or five centimetres of the shaft still in contact with the frame of the then actually ‘swinging’ arm assembly. The shaft had fractured near the right hand end and both the locknut and the brokenoff threaded end of the shaft were missing. What

caused the shaft to fracture there is open to speculation, but I suspect it was a combination of metal fatigue caused by the passing of time (1978 model bike), and increased stress on the shaft from the high-speed cornering training session at DECA.

I was just starting to wonder how I would make my way home, with or without the bike, when I heard the sound of another motorcycle going down through the gears and stop. It was one of the other guys on the training course. I explained to him that without the locknut there was no way to secure the shaft and so I was going to walk back towards Shepparton in the vain hope of finding the locknut lying on the ground and if I didn’t find it, I’d just keep walking to stay the night in Shepparton. My fellow RTA instructor suggested that I should first put the bike on its centre stand, which as I did, we both heard an audible ‘clunk’- it was the sound of the locknut with the end bit of the broken shaft still in it falling to the ground. It had been trapped on top of the centre stand. An absolute fluke that it hadn’t fallen out, but that fluke meant that rescue was now possible.

We had a close look at the shaft and removed the threaded bit of broken shaft from the locknut. There were only a few lands of thread left on the shaft, but we worked out that if we reversed the locknut, the rounded end had a bit better purchase on the shaft and there would be just enough thread to engage the shaft with the locknut. The whole swinging arm assembly was therefore literally hanging by a thread! We did the locknut up as tight as we could with a spanner and In case there were any further problems, my fellow RTA instructor kept me company until we reached Melbourne.

Then, in what was a fairly typical display of camaraderie between motorcyclists, with a wave to each other we went our separate ways, him on his Honda 750 K1 and me on my SR.

I made it home safely and that night with beer in hand I counted my blessings that I had looked down when I did and that the locknut didn’t fall out until it was within hearing range.