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Daily Archives: May 4, 2022

The Key to Synchronicity on a Tandem; The Eccentric Bottom Bracket (And What To Do When You Can’t Get An Allen Key To Fit Between the Chain Ring and the Left Side Set-Screws!).

I finished up the maintenance on my tandem Monday night. I could have gone for a ride, maybe should have, but there have been a few deaths close to us (one big one) that meant my wife and I were going to be out running around much of the night once she got home from work so I concentrated on the tandem and stayed home so I could be ready to go as soon as I was needed.

The chains had rusted over the winter and I decided they’d been causing a bit of a noise, so it was time to broom them. While the synchronizing chain had plenty of stretch left, the ten-speed drivetrain chain was close to the end of its useful life, anyway. So I took to changing them last evening before my wife got home from work. Now, this has an interesting twist. If you’re not familiar with tandems, the synchronizing chain is obviously going to stretch over time which would lead to sag in the chain… which would lead to the chain falling off if you hit a decent bump. To fix this, they’ve come up with an ingenious little workaround; the eccentric bottom bracket. For mine, you loosen four set screws and rotate the bottom bracket until the slop is gone from the chain. The bottom bracket on our Co-Motion tandem is top shelf, too. It’s excellent. The only problem is, a normal Allen key won’t fit between the synchro chain ring and the bottom bracket unless you’re at exactly the right angle with the crank in exactly the right position, and even then, the key would only go in crooked. This meant the left side set screws were exceedingly difficult to tighten.

That is, until I had the bright idea to take one of my Allen keys to a grinding wheel in the garage. And that’s exactly what I did after I got the chains off and to the proper length.

Et voila! All of a sudden, my special new Allen key fits perfectly and I can tighten up the eccentric bottom bracket easily.

Fun fact: A synchronizing chain on a tandem is longer than a typical chain by a third. You have to use two chains to get one synchro chain. What you do is you break the second chain at exactly the right link but you only push the pin out far enough that the pin is out of the way enough you can separate the useful end from the leftover, but not so much the pin falls out of the back plate. This way, you take the first chain and join the second part to make one full chain and use one quick link to join the two ends.

With the chains together and my new Allen key cut to the desired length, I went to work on loosening the eccentric bottom bracket. With the tighter chain, the old position would be too stretched. Surprisingly, I found the left side had worked itself loose – likely because it was too difficult to tighten with a standard Allen key. That was the cause of my click. Not the chain. Not dirt in the bottom crank assembly. Not an issue with seat posts… I put the shiny, new, not rusted chains on, tightened up the bottom bracket (and thus, the synchro chain), and with that, was done. The tandem is like new, now.

It will be silent when we roll next weekend… and all because I cut down that Allen key with a grinder. Actually, we just might take it for a spin Thursday… we’ll see.

Next up will be new brakes for the tandem. Jess and I will have to talk about that – we’ve got brake calipers meant for a flat bar brake lever. Flat bar brakes have a lot more pull on the lever/handle than that of a road bike integrated shifter/brake lever. This means the brakes aren’t quite what they should be and we’re putting in enough miles on the bike that having the proper brakes is a good idea.