Sorry, this one’s a little technical, but it’s also a funny instance of me making a silly mistake (then recovering from it). I did a minor upgrade to my bike that has electric assist: I changed the front chainring from 46 teeth to 52 teeth, because I sometimes felt my top gear was not high enough when pedalling standing up, which as I’ve mentioned in the past, is something I enjoy. A very minor change, with a not so minor effect, as it turned out. Increasing the chainring size makes the chain too tight when in the largest cassette cog I should have added some links to my chain when I put the new ring on. But I was too excited and wanted to go for a test ride. So I made a mental note to stay away from the large cassette cog during the test ride.
Mental notes don’t work so well at my present age, unless I actually write them down. I was very much enjoying my test ride with the new chainring, it worked really well when standing up. Then I got to a steep hill. Halfway up it a forgot about my mental note and shifted into the large cog. Oops! The chain was now tight enough that it jammed one of the pulleys of the rear derailleur into the gear. Then the motor kicked in and applied a lot of torque. This ripped the rear derailleur right off the frame. The first picture shows how the jamming can happen with too tight of a chain. The second picture shows the results, a fractured derailleur hanger. I also bent the cage on the rear derailleur so needed to replace it.
Fortunately, I was able to track down a replacement hanger on the internet, as well as a new derailleur. They recently arrived, and I was able to fix the bike. And make the chain long enough!
This is the biggest repair I’ve ever made on a bike. My local bike shop couldn’t help because they are swamped with repairs right now, having been temporarily closed for a couple of weeks when we first got our county’s “shelter in place” order. This was harder for me than you might think, I may be a mechanical engineer, but I’m better at theory than practical matters.
So finally after a couple of weeks off the air, I was able to take the bike out again today. And it worked like a champ. So the original upgrade was a good idea. If only I’d fixed the chain right away instead of making a mental note. This turned out to be about a $75 mistake and a couple of hours of my time to do the fix. But I got the satisfaction of accomplishing the fix. And I appreciate the bike more after not being able to use it for a couple of week.s
Note to self: no more “mental notes”, unless you write them down!