SuDiBe Tandemseiten 


Open Frame Designs

by Bill McCready

(Haben Rahmen ohne Lateralrohr Vorteile? - Hintergrundinfos und ein wenig Werbung. Aufbereitet aus der englischsprachigen Mailingliste, 7/2006)

Rudy wants to know if Santana is ready to make the jump. An answer to Rudy from Bill at Santana.

Since Santana's founding in 1976, we have built, ridden and/or scientifically tested all conceivable tandem designs. My own experience started years earlier, when I commissioned various custom builders (such as Jack Taylor) to build nearly-identical tandem frames that could then be tested against each other.

While Rudy has lumped at least four distinct designs together under the banner of "bruni" or "no lateral", this inexact terminology is a bit confusing.

First, a definition. For over thirty years English-language magazines, catalogs and books have used the word "lateral" to mean any diagonally-sloping tube that bisects a tandem's main-frame. By this long-accepted definition the Schwinn Duo-Sport tandem that Rudy fondly remembers was NOT a no-lateral frame---in actuality, the Duo- Sport with its rear lateral was a "mixte-back" design. Mens/mens tandem frame designs with rear lateral tubes include the century-old "double diamond" and the briefly-popular "uptube" (a reverse double diamond). Because all three of these designs had laterals, calling them "no lateral" is misleading.

Because no-lateral designs predated Tom Bruni's frames by a half- century, giving this older design the name of a modern builder is also confusing.

When it comes to frames sans laterals, for at least 30 years English- language magazines, catalogs and books have called this an "open" design. Originally popularized in France sixty years ago, tandems built with this simple and inexpensive design include those by Rene Herse and Alex Singer. Dormant for decades, the "open" frame was revived in the U.S. twenty years ago by John Slawta, under the trade- name Landshark. While Herse, Singer and Slawta (were / are) highly respected artisans, the performance of their beautiful "open" tandems was dreadful. (I have not only personally ridden examples of all three frames, I've also personally sold new tandems to owners of these brands who desired frames with greater performance.)

Did Bruni copy these older French and American designs? Tom's contribution to the previously-discarded "open" design was the use of larger-diameter top, bottom and down tubes in the belief that these larger and stiffer tubes would make laterals redundant. Unfortunately, Tom (a brilliant designer and craftsman that I shared a number of long discussions with) never had the means to verify this theory through scientific or direct-comparison testing.

Other tandems lacking laterals---designs totally different than those of Herse, Singer, Slawta or Bruni---are frames of exceedingly small stature. In this group I include Bike Friday's "Two'sday" and Co- Motion's Periscope. What these smaller "open" frames have in common is that there isn't much room between their top and bottom tubes.

Finally, all these former designs are quite different from a new fashion in superlight tandem frames that has captured Rudy's attention. In comparison to these newest "open" frames, Bruni's "open" frame was, according to Tom, an attempt to balance framebuilding costs with performance. Tom simply thought it more effective to re-invest the weight of the laterals in larger diameter top-, bottom- and down-tubes. While Bruni frames were lovingly-built and beautiful to behold, Tom never claimed that they were particularly light.

The newest versions of the open design---all appearing in the last year---are born of the desire to build a tandem frame as light as Santana's 3-year-old Beyond with IsoGrid weldable carbon tubing. When introduced at 5.5 pounds (in a medium frame size) the IsoGrid Beyond was a pound lighter than a same-sized Calfee or 'Zona.

A fourth contender in the sub-6 category is the magnesium-framed Paketa. The only way these others have been able to join Santana in the sub-6 category, was to shed a pair of lateral tubes.

If Santana were to build an IsoGrid Beyond without laterals, the weight (in a medium size) could be 4.5 pounds --- lighter than the lightest Calfee, 'Zona or Paketa.

When will Santana do this? Don't hold your breath.

Unlike those other builders, Santana commissioned the electronic strain-gauge testing that used real frames and strong riders to scientifically confirm the contribution of lateral tubes to the pedaling efficiency of a tandem frame. We have also shared design information with three companies that computer-modeled the pedaling efficiency of various tandem frame designs. All three independent analyses verified our strain-gauge testing. In summary: while you can remove a tandem's lateral tubes to make it lighter, because the resulting bike will have a lower level of pedaling efficiency, it will also be slower. While lighter or weaker teams might not be able to detect the loss of performance---repeated scientific testing shows that no competitive tandem team is so light or weak that they should prefer a frame without laterals.

The great news for 99% of all hobbsians is that you can put away your checkbooks and/or hacksaws. Your existing tandem frame (with its pound of lateral tubes) has NOT been rendered obsolete.

Bill McCready

PS: If there are dealers or customers who want to verify this truth for themselves, you can duplicate the type of testing I did prior to 1976. Simply commission 'Zona or Calfee to build same-geometry tandem frames with and without lateral tubes. After you build them up identically and then ride them both critically, you'll be able to confirm that the lighter frame is not faster.

PPS: But if think real performance is somehow less important than bragging rights, your most effective and exclusive answer would be a custom Beyond without laterals. At 4.5 pounds (in a medium size with full-length top tubes), you won't be able to buy a lighter rideable tandem frame.

PPPS: Remember those trendy mountain bike frames with one oversize main tube connecting the head-tube and seat-tube? Did these lighter frames with their missing tubes win many races? Where are they today?

D. Bettge; letzte Änderung: 3.10.2006