Re: [CR]early, light tandems


Example: Component Manufacturers

Date: Sun, 1 Feb 2004 15:39:34 -0800
To: classicrendezvous@bikelist.org
From: Jan Heine <heine93@earthlink.net>
Subject: Re: [CR]early, light tandems


Harvey,

Thanks for your input on tandem design and Santana. The French did have some rather sophisticated tools - their government labs actually dealt with bicycles in the 1940s and 50s. There was an article by Barra in Le Cycliste showing that oversized aluminum frames could be stiffer around the BB than standard steel ones of varying tubing gauges. If I remember correctly, they made a bunch of identical frames (except material and tubing gauge). Of course, Nicola Barra was making welded aluminum bikes... But at least his article had verifiable measurements to back up his statements, not just claims like most of today's mags.

I wonder whether anybody really has a good handle on what the loads are on a tandem while both rides are pedaling hard. If you don't know what matters, it's hard to measure it. The design Santana "invented" was popular in the 1920s, before people went to the triangulated frames. So people did have the opportunity to compare back then and preferred the twin laterals.

I know the old twin laterals work well, and the big tube connecting head tube and rear BB (as used today) does, too. The all-open frames and double diamond (I don't see any advantage to that second down tube) can work, too, but probably only for relatively short frames. I suspect tubing diameters are more critical there. (Singer and Herse adopted the open frame when they switched from oversize Vitus tandem tubing to more oversize Reynolds tandem tubing.) Numerous people felt the open frames were a step back at the time. -- Jan Heine, Seattle Editor/Publisher Vintage Bicycle Quarterly http://www.mindspring.com/~heine/bikesite/bikesite/

A few of Jan Heine's comments wish some response, but I've <snipped> everything else. <snip> JH: By the 1930s, the configurations popular today had evolved. Back then, they preferred the triangulation with twin laterals, and it's hard to argue with the resulting machines. In fact, a few years back, Co-Motion offered an "extra-high performance" tandem in exactly that configuration, if I remember correctly.

HS: Three factors have largely driven the twin laterals off the market. First, it is simply more expensive to build. When Santana converted from twin laterals to an "internal lateral" (round tube from head to rear BB), they eliminated at least 9 joints that had been brazed and finished, and made painting simpler, too. This is real money saved.

Second, in Santana's case, they made the transition after buying the pre-cut tubing stock of a bankrupt competitor. They played with the tubes, building prototypes, and found that the new design worked better, too. I think that the issue is that the craft builders of the past did not have access to very sophisticated tools, such as a strain table, on which they could study more-or-less realistic loads. If you assume that the loads on a tandem are primarily "bending" in the horizontal and vertical planes, twin laterals look good. However, if you think about "twisting" deformations (such as the head tube twisting out of the plane it shares with the rear seat tube, the value of a big round diagonal tube (and hefty down and "boob" tubes) becomes apparent.

Third, styles have evolved that have added about 5 - 10" (12 - 25 cm) to the wheelbase of tandems, primarily to give stokers riding positions more similar to those on single bikes. Susan and I have put many thousands of miles on a very short (60") on-topic Schwinn Town & Country, and on a 69"WB off-topic Cannondale. Both are great fun. The longer wheelbases do want some pretty hefty main-frame tube diameters for torsional stiffness.

Just my 2c.

Harvey Sachs
McLean VA