Eccentrics in Cycling

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.

Drum Brake Overhaul – Jack Taylor Tandem

Drum brakes get a bad rap because they aren’t very effective when compared to rim brakes.  But, on vintage tandems, drum brakes are added to assist with rim braking. My 1977 Jack Taylor tandem features this front brake augmentation, which helps to keep the heavier and more powerful tandem bike’s front rim from overheating on descents.  An overheated rim can blow off a tire – not a good thing!

To accomplish this dual braking, the Mafac lever is modified to accept two cables – one for the cantilevers, and one for the drum brakes.

This bike’s drum brake hub is a cup and cone Maxi Car model featuring black dust caps.  The drum and pads were filthy dirty, and one pad had separated from the brake arms.  The pads were covered in grime and were glazed, but did have more than 2mm of depth meaning that they were still usable.

I cleaned and sanded the drum and the pads to smooth out the contact surface and remove the glazing. I also re-glued the separated pad to its arm, using an epoxy resin rated for extreme high heat (a JB weld product).  I clamped that with a C clamp and let it sit for about a week.  Then, it was time for reassembly.  Fortunately, when I disassembled the hub I took a series of photos at each stage.  I referred to this series of photos while I put the drum brake hub back together.

But, I also consulted Glenn’s Complete Bicycle Manual for an overview of the process.  This manual is the only vintage repair manual that addresses the overhaul of drum brakes in adequate detail.  Above are a few pages which illustrate the process.  In total, the manual contains 5 full pages dedicated to drum brake overhauls.  There are locknuts, spacers and nuts which need to be in the right place in order for the brake to operate as intended, which is why it’s so important to document the disassembly process.

This model of drum brake features only one spring, whereas some contain two springs.  When the arm is engaged, the spring opens and moves the pads outward toward the drum via a cam mechanism.

Once the hub was reassembled I put the wheel up into the dropouts and attached the fixed arm to the fork blade, using the original “88” rated bolt.  This designation signifies its tensile strength.

Having reinstalled the fork and headset, it was now time to install the stem so that I could set up the bars and brake levers.  I had noticed when working on the fork adjustment that there was no hanger for the cantilevers, and that’s when I remembered that this bike has a cable stop drilled on the stem.  I ended up having to gently re-tap the threads for the barrel adjuster, but the Milremo stem cleaned up beautifully after polishing it with Nevr Dull.

The next step was to set up the cable, and for that I needed to make sure I understood how the hardware should be positioned into the brake arm.

The original hardware’s end cap could not be reused because the cable would not come out, so I found a replacement and began the process of setting up cable tension.  It took several trial and error passes to get the cable tension correct so that the pads did not contact the drum when not engaged, but contacted the drum adequately when engaged without bottoming out the brake lever.

I made this short video showing the operation of the brake.  All looks good, and now it’s time to move on to the installation of the cantilevers. After that, I’ll continue on to the drive train, having previously cleaned, polished, and waxed the frame.  I’m glad to be making progress on this wonderful vintage machine.

 

1977 Jack Taylor Tandem Restoration- the Beginning

Mafac hood with dual cables

I’ve been waiting to get in the right frame of mind to begin restoration of a 1977 Jack Taylor Tandem that I had shipped from England back in 2012.  Clearly, I’ve been waiting quite a while, but during that time I have corresponded with the original owner’s son, who has provided valuable information about the bike’s history as well as a few family photos of the tandem’s exploits.

Jack Taylor frame at bottom left

I also needed more shop space to enable me to use two stands to aid with disassembly.  That finally happened last Fall, and I now have not only more shop space, but additional bike storage space as well, all in one location.

So I was finally able to remove the components to begin the process of bringing this tandem back to its original glory.  I had already removed the 650b Maxi-Car wheelset and worked on getting the wheels back in order.

I still need to re-glue one of the brake shoes in the front drum hub, but the rear wheel has been overhauled and adjusted.

Jack Taylor frames are built with Reynolds 531 tubing.  The specs for each Reynolds tubeset will vary based on customer request and on the particular application.  A serial number is stamped on the rear drop-out and the steerer tube.  You can see the matching numbers in the above photo – 7183 – indicating this is a probably a 1977 frame, based on the helpful chart provided by Joel Metz at his blackbirdsf.org site.  This chart was developed from information provided by Mark Lawrence, who happens to be the individual I purchased this tandem from.  He sold the bike on behalf of the original owners.  Mark was a long-time friend of the Taylor brothers.

Tandems differ from regular bikes in a number of ways.  There are naturally two bottom brackets, one of which is an eccentric, which aids in adjusting the timing chain so that that both sets of cranks are positioned at the same angle on the spindle, and so that chain tension is properly maintained.  The matching crank arm position is important for cornering, as you want both crank arms upright when descending at speed while leaning over.  Both bottom brackets are T.A. models, and the threaded eccentric appears to say “Rogers”.  It taps out of the frame after loosening the bolts.  Adjustment is made by loosening the bolts, twisting the eccentric to the desired position, and re-tightening.

The components on this tandem indicate that it was the Super Touring Deluxe model, spec’d with Mafac Tandem cantilever brake calipers (plus a Maxi drum brake up front), Stronglight headset, Campagnolo derailleurs and shifters, Campagnolo drop-outs, SunTour Perfect 5 speed freewheel, T. A. cranksets, plus Maxi-Car 650b wheels.

1977 Jack Taylor Tandem with kiddie crank and trail-a-bike

Four cranksets was one more than expected.  Upon corresponding with the son of the original owners, I learned that this was needed to set up the “kiddie-crank” for the young stoker.  His sister contributed to the effort on her trail-a-bike.

As originally purchased 1977 Jack Taylor Tandem

As I’ve researched the history of Jack Taylor Cycles I’ve come across a few sites I hadn’t accessed before.  One of these is the Stockton-on-Tees History site, which has several wonderful posts regarding the history of the “works” building and of the Taylor brother’s exploits:

Jack Taylor Cycles

Jack Taylor Cycle Maker

While the restoration process may take some time, I’m hopeful about the result.  This is a machine worth preserving.