Bottom Bracket Eccentricity

The Raleigh XXIX that I use as my commuter has an eccentric bottom bracket, so that you can adjust the chain tension while enjoying the benefits of vertical dropouts (unchanged brake alignment after tension adjustment, no axle slippage, etc.). I am not really impressed with the set-up, since I am unable to run the exact gear ratio I want due to limited adjustment range.

I also don't care for the mechanism for fixing it place. The Raleigh designers chose to go with a bb shell which uses two set-screws, rather than a split shell with pinch bolts. These two bolts are recessed into two extensions which project out of the shell, angled slightly toward the front wheel. These act as intakes for road crud in the rain and snow.

Of course, when I went to adjust the chain tension, last night, the screws were frozen in place, thanks to the crud intake design. I inverted the bike, used compressed air to clean out the bolt wells, and then filled them with Liquid Wrench penetrating oil. (As an aside, I detest the smell of Liquid Wrench, and I always seem to get it all over my hands, which then reek for hours.)

A couple of hours later, I took a 4mm Allen wrench and attempted to turn the bolts. Still no go. I refilled the bolt wells with Liquid Wrench, and let it sit for another hour. The bolts would still not budge. That, of course, signaled a need for more torque.

So, I went and grabbed a monkey wrench, and used it to extend the torque arm of the 4mm wrench.

The high-tech tools used in this task.

I hooked the monkey wrench onto the long end of the Allen, and leaned on it. Nothing. Again. Still nothing. Once more and, with a harsh buzzard-like squawk, the bolt let go.

On to the second bolt: More of the same. On the fourth try, I felt something give. I couldn't tell, right away, if the bolt had come loose or if the wrench had broken.

The Allen wrench didn't quite break, but it thought about it.

I cleaned the bolts, greased them and the threads inside the extensions. Then, I threaded them back in.

In this shot, you can see how the bolts are recessed.

After I got the chain tension correct, I locked the bottom bracket in place and filled the bolt heads and the extensions with grease, in an effort to keep the water and road goo out of them.

Just another reminder of how tough winter road conditions can be on a bike.

x