Re: [CR]early, light tandems


Example: Framebuilders:Alex Singer

Date: Sun, 01 Feb 2004 15:10:55 -0500
From: HM & SS Sachs <sachs@erols.com>
To: heine93@earthlink.net, classicrendezvous@bikelist.org
Subject: Re: [CR]early, light tandems


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