HOWTO: Repair a broken Brompton chain tensioner
After puzzling about it for a while, I think I understand what happened. I use the same chain oil on my Brompton that I use on my racing bike. The "oil" is actually a mixture of a heavy lubricant in a volatile solvent. The solvent evaporates after coating the chain, and dissolves whatever gunk has accumulated. I think the solvent damaged the plastic. I've seen this happen with some plastics when they come into contact with gasoline. The gasoline dissolves the plasticizing agents, and leaves behind an open matrix of molecules, like a very, very fine sponge. The open matrix has a huge surface area and oxidizes rapidly. Your nice flexible plastic turns into something hard and crumbly, like a stale cookie.
That's what I think happened here. The remaining bits of plastic are still relatively flexible, but the bits that broke off have turned into a powdery mess.
The guy who sold me my bike offered to let me buy an idler wheel off of one of the bikes in his stock, but I didn't want another plastic gear. Here is what I built :
I bought a standard anodized aluminum derailleur gear from a local bike shop, and attached it to the Brompton chain tensioner arm with a few pennies worth of standard hardware. The new idler wheel (gear? cog?) slides along a little stainless steel tube I picked up at the hardware store and cut to length. This gives it enough play to allow for easy shifting. The tube has just the right tolerance to allow the gear to spin very easily, but not wobble.
Here's the exploded view :
From top to bottom, the parts are :
- regular old bolt
- two washers
- stainless tube
- another washer
- lock washer
I had to saw off the plastic axle tube on the chain tensioner arm because it would have prevented the idler wheel from sliding into the right position for the outer gear. I chose a bolt with a hex-head that fit snugly into the socket on the tensioner arm (similar to the bolts on the toggles for locking the frame in place). Once the nut is tightened against the lock washer, the axle is extremely rigid. The gear slips across the tube with almost zero play.
The shifting action is actually much smoother than it was with the plastic gear, and the bike seems to make a little less noise than I remember (that could be my imagination).
Yay! I've got my bike back, and without another dorky plastic gear, too. Neat!