TUBING TECHNOLOGY

Single bikes outnumber tandems by more than 1,000:1. As a result, it is not economical for bicycle tubing manufacturers to develop superior-quality tandem tubesets. Unless tandem builders pay the development costs for a custom-drawn tubeset (or buy their tubing from us), they instead use generic tubing with a lesser degree of performance.

Some builders simply use the smaller-diameter tubes designed for single bikes. Because tandems produced from single bike tubing are not stiff enough to be stable, conscientious builders realize they must provide stiffer tubing. There are but two ways to make a tube stiffer—increase its wall thickness or diameter.

While thicker tubes are propor-tionally stiffer (20% thicker equals 20% stiffer), a larger tube yields a cubed increase in stiffness (a tube that’s 20% larger is 73% stiffer). Problem is, unless a tubing mill simultaneously increases thickness, larger tubes are harder to draw. The most expensive tube to produce is one that is both larger and thinner. Because this combination can produce the highest stiffness-to-weight ratio, it is found on every model of Santana.

WHAT IS BUTTED TUBING?

Most single bikes, and a few tandems, are built with “double-butted” tubing. Buyers are often confused by this term, or think it’s just a weight-saving measure. Actually, a properly designed double-butted tubeset saves weight while increasing a frame’s stiffness, stability, strength and comfort. To understand this you will need to appreciate where and how a bicycle frame is stressed.

On a bicycle frame (or any structure), stress concentrates where the material is joined. When the joints are at corners, as with bicycle frames, stress is intensified. When joints are welded, there is further stress at the edge of the heat-affected zones. It’s no wonder bicycle frames break at the edge of the weld—this is where three types of stress (joint-, corner- and heat-) converge.

To prevent frame failure, designers specify tubing with a heavy wall thickness. However, the thickness required at the extreme ends of the tube exacts a large weight penalty on the rest of the tube (where stresses are much lower). Besides the weight penalty, thicker tubes also yield a harsher and less responsive ride.

The answer is double-butted tubing—thick at the ends and thin everywhere else.

Besides saving weight and improving comfort, double-butted tubing has another advantage, fatigue resistance. Because a thin-walled center section of a double-butted tube will absorb stress that would otherwise concentrate at the joints, the ends of a properly-designed butted tube can be thinner than a plain-gauge (unbutted) tube doing the same job. This explains how tandems built with Santana’s exclusive 7/4/7 tubes (.7mm thick at the ends and .4mm thick everywhere else) can dissipate stress better (and last longer) than tandems built from heavier tubing.

Comparisons

ADDITIONAL TUBING REFINEMENTS

Another way to improve efficiency is through the use of superior alloys. Our aluminum tandems, for example, are made from an alloy that’s 56% stronger than the aluminum used by our largest competitor. With every type of frame material—steel, aluminum and titanium—Santana has obtained custom drawn tubes in the best available alloy.

Besides larger diameters, thinner walls and stronger alloys, Santana uses many other innovative techniques to further boost performance, efficiency and comfort. An example, see function-specific tube shaping.

Setting the standard

It’s easy to become confused when every bike builder on the planet makes great sounding claims about the performance of their tubing and geometry. How is it possible to separate fact from fiction? In the case of tandems Santana alone employs measurements collected from dynamic strain gauge testing (electronically recorded during test rides). We use this data and information gleaned from the world’s only tandem test jig to refine and improve our designs.

In addition to testing our own products, we purchase and test our competitors’ offerings. And since instrumented testing can’t duplicate the human dimension, we also assemble panels of test-riders to score and evaluate the bikes we build against the best efforts of others. Finally, we benefit from the feedback of 50,000 Santana owners who have experienced 50 million miles of shared fun.

Unexpected results

While the best way to make a tandem stiffer and more stable is to use larger diameter tubes, Santana’s electronic strain-gauge testing has pinpointed two interesting exceptions. First, because measured flex across the top of a tandem frame is relatively low, testing disproves the popular notion that larger top tubes are worth their extra weight. A better answer is to enlarge the drivetrain- stressed tubes between the fork and rear axle. Second, while one overly large drivetrain tube certainly looks impressive, it can hurt a frame’s overall performance by transferring undue stress to adjacent tubes with insufficient diameters. Thus, an enormous bottom tube is actually counterproductive unless combined with proportionally-scaled downtube, seat-tubes and chainstays.

Relative Weight Index

RECENT TUBING DEVELOPMENTS

The growing popularity of tandems has allowed Reynolds, True Temper and Easton to create tandem tubesets. Unfortunately, most tubes contained in these relatively inexpensive sets were originally designed for mountain bikes. While stock tandem tube-sets are a welcome development, thicker and heavier mountain bike tubing cannot produce the highest quality tandem.

Although it is impossible to overcome the performance deficit of inadequate tubing, other builders can distract consumers with vague or inaccurate claims of having proprietary tandem tubing. In actuality, whenever we contact a tubing manufacturer for verification of the labels being used, we learn that just one or two custom tubes are included—the remaining tubes are generic.

Why don’t other builders use specially-drawn tandem-specific tubing? Price. The special tooling, inefficiency of small mill runs, and costs of testing and development means they would have to raise prices by $400 or more to offer a tandem built with optimal tubing.

Can an expensive frame from a prestigious builder who relies upon generic tubing provide a comparable level of performance to that of an entry-level Santana built with larger and thinner tandem-specific tubing? No. Even the most talented builder cannot escape the limitations imposed by inferior materials. Because they are noticeably lighter, stiffer, more stable and more comfortable, Santana tandems have won more formal comparison tests than all other brands combined.

The lightest tandems?

Since Santana uses the lightest large-diameter tubes, are Santana tandems always lighter? Actually, when other builders scrimp on tubing diameters and then install a single bike’s lighter wheels and fork, it’s not hard for them to produce a light tandem in spite of heavier generic tubing. Most of the weight Santana saves with custom-drawn tubing is reinvested in larger and more efficient tube diameters, more reliable tandem-specific wheels and components, and the largest and safest forks in the business.

Conclusion

Is it wise to spend a bit more for a tandem with superior tubing? We think so, independent testers think so, and the market overwhelmingly concurs. Only with tandems is the most expensive brand (Santana) also the most popular.