1958 Eccentric bottom bracket on Rene Herse Tandem – Daniel Rebour drawing.

Eccentric bottom bracket on 1977 Jack Taylor tandem

As part of restoring my 1977 Jack Taylor tandem, I needed to rebuild the eccentric bottom bracket and set it up correctly.  There’s been a bit of a learning curve, since I haven’t previously restored a vintage tandem.

The use of an eccentric appears to first date to the steam engine era, to optimize the efficiency of a rotating shaft.  And eccentric bicycle components have been around for over 100 years.  Yet, it can feel foreign, and daunting to work on a bike that features this technology.

What is an eccentric?  The official definition from Wikipedia is:  “in mechanical engineering, an eccentric is a circular disk (eccentric sheave) solidly fixed to a rotating axle with its centre offset from that of the axle (hence the word “eccentric”, out of the centre).”  The above photo depicts an eccentric pedal design introduced in 1903. 

Here’s a dual eccentric bottom bracket that can alter the gearing ratio at the crankset, also introduced in 1903.

Fred DeLong’s experimental fork dropouts

Fred Delong even spec’d a bike that had offset fork dropouts, to enable him to quickly change his bike’s rake and trail characteristics.  Although not technically an eccentric, this dropout design allowed him to choose when to optimize for trail and wheel flop, as shown in the table above.  A similar result could be achieved with an eccentric front hub.  Currently, eccentric hubs are limited in application to the rear where they are used to adjust chain tension on fixed gear bikes.

My Jack Taylor tandem features an eccentric bottom bracket that is similar in design to the 1958 model depicted at the top of this post. The threaded portion features a T.A. spindle and cups. There is a single hole drilled into the offset section.  Newer eccentric bottom brackets have multiple holes drilled, so that a pin spanner can be used to ease the adjustment process.  I can’t quite make out the lettering on this model – does it say “Rogers”?

I had wanted to rebuild the bottom bracket while it was off the bike – one of the benefits of this design.  However, when it came time to install the rebuilt BB back into the shell, the fit was very tight and needed to be accomplished with a mallet.  Not wanting to damage the cups or bearings, I disassembled it and then rebuilt it again once installed in the shell.  But, when I did so I ended up positioning the spindle toward the front rather than the rear of the bike.  That’s not what you want, since the whole idea of this design is to allow one to move the spindle toward the front as the timing chain stretches with wear.

I made this video showing how to adjust the eccentric bottom bracket on a model where only one hole is drilled. After loosening the bolts in the shell, and using an Allen wrench inserted into the hole and then leveraging that with a heavy duty screwdriver against the spindle, you can get the eccentric positioned where you want it to be.  It’s clear to me now that overhauling a tandem is much more involved than a typical restoration.  Still, it’s been fun to learn new techniques.  I’m looking forward to the rest of the process.