July 26, 2015 Masters Nats 55+ Team Sprint
Rock Hill SC
Report from Lionel Space, Phoenix AZ

My equipment was carefully selected and prepared prior to this event. I chose the barely used 49 Sugino chainring off my aluminum bike, rather than the one I normally train on. For sprint work, I usually use a 47 link chain with my training wheels, but the smaller tires allowed me to move the wheel farther forward using 46.5 links (utilizing a half link). I only use this chain a few times a year, mostly just for Nationals when I'm using my 19mm tires, so it's only had a dozen or so rides with it. It was freshly lubed prior to leaving home. I had a brand new 14 tooth EAI Gold Medal Pro track cog on the rear. My bike had not been changed since the 500m TT the day before and I had also done a complete check of every nut and bolt to make sure everything was tight. Ultimately, the half link would be involved in a bizarre situation that doomed our race.

At the start of my Team Sprint on the last day of the 2015 Masters Nationals, I was third rider, up above my two teammates. At the countdown to zero, I gave my usual strong start and heard and felt a very loud clunk in the drive train area. My immediate reaction was to halt putting more energy into the bike that might result in my crashing (and possibly taking out my two teammates below me as well). At first I thought that it was the typical right axle nut that had slipped, but upon looking down (instead of accelerating), I saw clearance to the chainstay and the bike seemed to still be rolling just fine. Not being able to define a mechanical, I began my start again, but now with a 4 second delay and 10 lengths behind my lead rider, necessitating riding all 3 laps solo and of course finishing well out of the medals at dead last.

After the ride, close scrutiny still did not expose anything drastically wrong. Everything was tight and in place.

The next day I closely examined the chainring, which has seen a fair amount of accelerations and standing starts. I usually do my standing starts with my left foot forward and I noticed excessive deformation in the area where the chain and chainring contact while the pedal is in this position.


Compared to the teeth where the chainring would not be subject to maximal forces:


And the right side power stroke connection position showed some excessive deformation as well:

Obviously it would be wise to rotate the chainring on the spider periodically, rather than use the same position constantly.

However, there is that fresh shiny spot in the first picture at the bottom of the tooth profile. It happens that I was using a half-link in the chain. While the rest of the chain has chamfered plates on the inside links, the half link has a square edge. It seems that the square of the half link caught on the edge of the bottom of the tooth profile and popped down (the shiny area on the top picture being the resultant damage).


The normal position of the chain tracking the hollow would look like this


But I was able to simulate flexing of the frame, crank spider, chainring and chain to make it ride the shoulder of the hollow, almost a millimeter higher.

Thus when it slips off the shoulder of the bottom of the tooth profile on a standing start, it will make a loud pop and take a part of the chainring hollow off, but then continue to operate normally. It sounded and felt like the chain had broken, except that the cranks weren't free.

So if using a half-link for a standing start, one should make sure the half-link is not at the top, close to the chainring when putting the bike on the start line. I'll try to file the edges of the inside of the half-link plates to a rounded edge to help avoid this situation again.

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