Urban Adventures & More

As I indicated in my previous post, I’ve been learning how to get my chainring straightened out. The internet, as far as I can tell, basically only has Sheldon Brown’s useful (but slightly cryptic) explanation of how to do this. However, now that I have dealt with it, I understand the issue better.

Recall that the role of the chainring is transfer energy from your feet (by way of the pedals and crank) to the chain and eventually to the back tire. Because every ounce of energy your feet put into the pedals is meant to be transferred into the chain, getting very low resistance makes a huge difference in the feel and performance of the bike. When there is too much slack in the chain (low tension) or the chain binds at any point  (high tension), that is energy that should be going from your feet to the wheels, but is instead being dissipated by inefficiency in the system.

As Sheldon Brown’s article indicates, there are two steps to attaching the chainring. The first is to get it straight so that there is no binding at any point. The second is to tighten it down so that position is held. A third step is to fix the chain tension. The way you the first step is to use the chain itself to both detect points of binding, and then to have it correct them, as well.

To detect points of binding, you do as Brown describes; put the back wheel and chain on at a good tension. When I did it, I didn’t even move the back wheel; I left it at the same tension (with no visible sag) as when I ride. It is imperative that the chainring bolts be only finger tight at this point: tight enough that it’s held securely, but loose enough that it can shift under force. To feel the tension, you rotate the cranks a little at a time, and squeeze the chain together. Note also that you should use grease (like the kind you use on the wheel hubs) on the bolts so that a tight seal is formed and the bolts hold better under force and vibration.

Go around once to get a feel for what a high and low tension feels like. Then seek out a high tension point in the chain (when the give is least). Strike the chain with a tool (I used a heavy allen wrench). This action will force the chainring to resettle slightly; this point will no longer be one of high tension. Repeat this step until the chain tension is very consistent the entire way around.

The second major step is to tighten the bolts. This is not a complicated operation. I like Sheldon Brown’s method: start on the bolt across from the crank arm, and then rotate in a single direction (I seemed to naturally want to do counter-clockwise), tightening every other bolt until you come back to where you started. Good practice probably dictates that you tighten them only a little at a time; I went around the set twice to get them to a good tightness.

The final step once the chainring is fully tightened is to increase the chain tension. Because the high tension points were the limiting factor in the wheel tension before, you should now tighten it up a little more. My technique for tightening the chain is also mostly from Sheldon Brown, but I’ll embellish his explanation a little more. The idea is that you want to shimmy the wheel back into the dropouts so the the tension on the chain doesn’t pull the whole thing back before you can tighten down the nuts on the axle. This is done by tightening one axle nut down, and then loosening the other, pulling it back, and tightening it.

I start on the non-drive side (presumably because it’s easier for the tensioned side to follow, not lead). This one is easier to pull back for me. Since I have the bike upside down, this is on the right side; I wrap my left fingers around the excess axle bolt length (there’s about 1/3 inch that extends past the frame and nut), and brace it with my thumb against the back of the dropout (this is underhand, since it’s hard to reach with a wrench when you do this overhand). I come around with the wrench and tighten the bolt.

On the other side, I switch hands. Since it takes more force on this side to get the tension to where you want it, positioning is key. I do the same as with the other side, not worrying too much about whether the wheel is straight or not (it shouldn’t be too far off, though), but just trying to pull it as much as I can. Once it is right, though I fiddle with the non-drive bolt to get the wheel perfectly straight.

Now I could appreciate just what a difference this work made. Previously, setting the crank spenning and letting it ride until friction stopped it was a pretty quick game. The pedals generally stopped in a single position, which was a point of chain binding. Now, the crank and wheel rotate long past that initial time and come to rest at a random point. Going out on the road gives the feel of the pedals pushing back much harder to the point that I scarcely feel I am working at lower RPM. I haven’t yet vanquished the sound (it seems like at this tension, the chain itself is making a lot of noise in the rear sprocket; I may have the chain too tight after all).