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.

 

Brake Pads for Steel Rims

Fibrax leather faced brake pads

I’ve been enjoying cruising around on my newly restored 1965 Sears/Puch 3 speed, but was reminded during a recent excursion on a wet and windy day how poorly steel rims perform in the rain.  I needed to stop suddenly but was unable to do so, and it took several revolutions of the brake pads against the rims to clear the water and finally take hold.  By then, it was necessary to swerve!

I’ve known about leather faced brake pads for steel rims, but haven’t tried them until now.  I ordered enough pads for two bikes from an eBay seller.  Even though these pads shipped from the Leicester region of the U.K. and were expected to arrive in 30 days, I actually received them within two weeks (supply chain problems be damned).

These Fibrax pads do not have a directional notation as did the Weinmann’s which they are replacing, but they do have an orientation requirement.  The pads are angled to match the contour of the rim.

The pads need to be positioned as shown above so that they can contact the rims evenly when the brake levers are engaged.

It’s easy to assume that these older single pivot sidepull brake calipers don’t have any kind of quick release, which is true for the calipers themselves.  However, by squeezing the pads against the rim, you can slacken the cable tension and use the quick release available on the brake levers – just pull the ferrule out of its slot and let it go.  It’s best to do this after you’ve screwed the barrel adjuster all the way down, to provide further cable slack.  A slackened cable makes it much easier to get the pads in place.

So, how do these Fibrax leather-faced brake pads perform?  I took the bike out for a spin to try them out.  Today was a warm and glorious day, and the leaf strewn streets were drying out.  So, I found a few giant rain puddles in some shaded areas and splashed through to get some water up on the rims.  Then, I sped up and braked suddenly.  And, voila, they worked beautifully and I was able to stop as expected, without multiple revolutions of the wheel to clear the moisture off the rim.  On my ride today I thought about how 3 speed cycling is something everyone should try. I ended up going on a much longer jaunt than originally planned because this kind of bike with its upright position and simple shifting encourages a relaxed pace allowing for exploration, peace, and wonder – the things I love most about cycling.