single speed lesson
Since finishing school I haven’t been commuting daily on my Marin Hamilton 29er single speed bike. I finally decided that the time was right to slap on my knobbies and hit the trail! Unfortunately, the stock gearing on the Hamilton is set up for the street so it was a bit more complicated than swapping out the tires.While performing maintenance on the Hamilton in the past I discovered that the freewheel is not the typical single speed BMX style set-up. Instead, the Hamilton features a mini Shimano-style freehub on one side and a standard fixed gear + lock ring set-up on the other:
This created a challenge because the only single speed cassette style sprockets I have been able to find come with single speed conversion kits generally designed for street bikes. So, I stopped by the local bike shop and dug through a few boxes of old cassettes to find a 19T Shimano style sprocket. I then dug out an old chain from my road bike and my knobbies and fitted everything up.
The new set-up seemed to work fine during the test ride around the apartment complex. The gearing felt a little high given the mass of the 2.3″ wide 29er tires (a 20T sprocket would likely be a better companion to the 33T chainring) but I figured it was good enough for me to give it a trail test.
It has been at least 16 years since I last rode the Bratram Trail. Back when I first started college I used to ride the Bartram 2-3 times a week. This morning I woke up at dawn, loaded up the Hamilton and headed out for the trail. The first section of the trail is a technical (sand, roots, ruts, tight turns) downhill followed by a short climb up to a long rolling section. I floated downhill the first 300 yards on my enormous knobbies and was feeling pretty good. Then, I hit the first climb and with the first application of significant power POP! The next thing I know I’m dislodging my stem from my gut:
It seems my thigh took the brunt of the impact:
I assumed my old chain had broken so I was rather surprised to find the chain had simply come off the rear sprocket. I was even more surprised when I was able to easily spin it back onto the rear gear. I was further surprised that my chain was perfectly tensioned once I got it back in place (my second assumption was that the rear wheel had shifted in the drop outs, loosening the chain).
At first I thought maybe it was just a fluke but after 3 more derailments in the next 100′ I gave up and walked the 1/4 mile back to the parking lot. The lesson learned here is that the sprocket and chain combination I was using was designed for a multi-gear set-up and not single speed use. They are designed to release the chain during shifting instead of holding onto it at all costs. The chain line is pretty much perfect so I guess the torque of trail riding combined with a little sand was enough to “shift” the chain off the sprocket. Single speed bikes need single speed parts. Lesson learned.
Back to the drawing board.
UPDATE: I found a 20T single speed sprocket for $4 at JensonUSA so I went a head and ordered it and a new $10 single speed chain. The parts just arrived so I may get to try this all again tomorrow!
BTW, that bruise on my thigh has developed nicely over the week, showing that I took a harder hit than I originally thought…
UPDATE II: All mounted up and ready to ride! (Note leaf bits on chain from vigorous test ride):