Tandem Triangulation

The word "triangulate" is what comes to my mind when I think about my tandem. Although this blog is supposed to be about me riding a bike, and not about the actual bikes, we are stalling for time while my cold fades away and the pouring rain lets up.

"Tandem Triangulation" really means "two and threes".

Two parallel tubes meeting cross pieces at each end (forming a square) can be stiffened by a diagonal member (forming multiple triangles). A triangle is an inherently stable shape.  My bike is made up of dozens of triangles. Even the two sets of diagonals that cross the center of the bike are themselves laterally cross-braced.

Frames aren't made this way anymore, because they are said to be too complex, too laborious to build, too heavy, too stiff, etc. But this is what we have, we've ridden for 34 years, and we still like it.

This frame came from Dana Fenimore, aka California Cycles. It's special oversized Italian Columbus tubing, joined with hand-made lugs and silver-solder throughout. Dana built it for me in 1979. I cut and bent small diameter chrome-moly tubing to create the racks, which I think were brazed by Bill Holland.

If you read my post on my touring bike, the Platano, you may recognize some common features, like the overall color scheme and trim. The component group is higher quality - all Campagnolo crankset, pedals, seat posts, shift levers, headset, etc. I used a Cinelli front stem and bars, and had a custom rear stem fabricated to match the frame. This bike is brown with a pearl clearcoat.

The color of the seats match the paint - it took months to get the seats in that leather color. And the bar tape is the same too. I'm sure a similar scheme could be created today if you wanted to spent the effort tracking things down and planning "the look".

As with my touring single, I specified braze-on fittings galore - rack mounts, generator and light mounts, wire and cable guides, water bottle cage mounts, matching pump, cantilever brakes, vertical drop-outs and so on. 

The derailleurs are not Japanese this time; there is a Huret Duo-Par, a legendary titanium-bodied, dual parallelogram design with extra-long cage and almost unlimited gearing range capacity. Unlike my single whose high gear ratio is 93 inches (48 x 14), the tandem has a 110 inch high (53 x 13).

The right rear crankarm has extra drilled and tapped holes which allow a Stronglight chainring to be mounted inside the Campagnolo rings. This gives us a low gear of 24 inches (28 x 32) which you need to climb hills with the tandem.

Since we intended to tour with this bike, I wanted to fit shorter, fatter tires. They improve handling on dirt, the rubber lasts longer, the ride is smoother, the gearing gets lowered slightly, there's more clearance for fenders and mud, etc. The only drawbacks are that it was harder to get tires, and it takes more effort to pedal. And a technical difficulty was that cantilever brakes - obligatory on tandems - can't span both tire sizes. 

Think, think, think - and I came up with a solution. We used removable studs as the cantilever pivots, instead of the normal brazed-on cantilever mounts. That way I could change the wheels, screw in the stud at the proper point, mount the brakes, and ride off. 

Notice the set of Allen bolts below the brakes. I remove the bolts (plugs) move my brakes down an inch, put the bolts in the upper holes and Presto! Bob's your Uncle. Or Robert is your mother's brother. 

The rear hub has threads on both sides. On the opposite side of the cluster, there is a Phil Wood disc brake. An extinct design, it works well for me but has been superseded by other types of disc brakes. It's operated by a cable that I can pull (or set to drag) from my single handle-bar-end shifter lever.

The rear hub is 140 mm wide which allows a stable rear wheel that is evenly triangulated. There are no quick-releases long or strong enough, so this wheel is retained in the frame with Allen bolts. If I recall correctly, they are the same size as the brake boss plugs, so I need carry only one size wrench. These were very strong and very expensive wheels, and I built 2 identical sets, just differing in outer rim diameter (650B and 27 inch).

There are other nifty things on this bike. The fork crown was made by Dana to fit extra-strong fork tubes that were created by pressing one thin fork blade inside a slightly larger one. The main frame tubes are all oversized. Although they appear normal in the photos, they are larger in diameter than single bike tubing. The bottom tube is gigantic.

That's about enough. Can we finish with the old car I spotted today? A Renault Caravelle Floride.