By Dwan Shepard, co-owner Co-Motion Cycles, Inc.

As a maker of almost exclusively 700C tandems, we've won a lot of converts to the traditional road-sized wheels. Much been said about the durability and versatility of 26" wheels, but they do have their shortcomings. We have found that many new tandem riders buy inexpensive first tandems with 26" wheels, hoping to enjoy some of versatility that they offer. Very often however, they become disenchanted. They find it difficult to keep up with others riding 700C tandems on group tandem rides. I have noticed the same effect on tandems that are otherwise virtually identical. It just seems more difficult to maintain high speeds on a 26"-wheeled tandem.

It is very difficult to prove this phenomenon, but there are several interesting reasons why there is a distinct difference in the ride qualities of 700C vs. 26". First, it helps to understand the history of tire-sizing nomenclature. Both designations, 26" and 700C indicate tire diameters that are only nominally related to the actual diameters commonly used. Most of the 700C tires in use today on road bikes are about 675 to 680mm in actual outside diameter, (not 700). The rim size is the same as that used on zillions of working-class bikes around the world, although their tires measure around 710mm. The 26" size that most of us are familiar with is descended from American balloon-tired bikes, which had tires about 1.75" wide and actually measured at about 26". When narrower tires are used, the overall diameter invariably changes too, as the tire assumes a relatively round cross-section no matter what kind of casing is used. Amazingly, on a 26" wheel, the small 1" or 1.25" tires can result in a diameter of little more than 610mm, (24").

The reformers in our midst want to change this vernacular to a more sensible system. They've got an uphill battle, though (hiya, Sheldon). People like to stick with what they know, and nobody seems to know what their rim diameters are.

Some of you may see what all of this is leading to: Development and gear ratios. Development (or roll-out) refers to a combination of the gear ratio multiplied by the actual tire diameter times Pi (3.14- the circumference conversion figure). The result is the actual distance your bike travels for each crank revolution. Let's look at some examples:

1. 700x28 tire (680 actual dia.)/ 54T chainring/ 11T cog (54 divided by 11 times 680 times 3.14) Development =10,481.8mm or 412.67"

2. 26x2.0" tire (667mm actual dia.-- not likely to be used for road rides)/54T chainring/11T cog ( Development=10,281.5mm or 404.78", or not much different from 700x28.

3. 26x1.25" tire (610mm actual dia., a common road size)/54T chainring/11T cog Development =9,402.8mm or 370.19", or 42.48" fewer inches traveled per 360 degree pedal stroke. That's a big difference.

The point is, a 26" wheel with a small tire is actually little more than 24" in diameter. With gearing identical on all three examples, you can see the difference in the amount of ground covered by each wheel per crank revolution. Another way of looking at is that the smaller wheels must rotate more times to cover the same distance as the larger wheels. People have been arguing whether wheel diameter is a factor in bicycle efficiency for generations, and I might not be much help in settling that argument. I am not offering any scientific data here, after all. But there is another issue that affects bicycle efficiency that has more to do with economics and applied technology than science. Which does help to settle the next obvious question: Why not just gear up to get the same rollout from the smaller wheels?

The answer related to a problem that tandems already suffer from--component compatibility. To make example #3 above travel the same distance per crank revolution as example number 1 requires a ratio increase of 11%. That difference can be made up by exchanging the large chainring to a 60T ring. There are a lot of people on this list who use big chainrings, and probably even more who have tried extra-large chainrings once and then returned to using more common sizes. It doesn't take long to discover that front shifting is not helped by big chainrings. The arc of a front derailleur cage is usually matched to the arc of a 42, 48 or 53T chainring. A mismatched arc forces derailleur placement higher than optimal shifting requires, since part of the cage will drag on the chainring otherwise. A further complication is that in order to cover a reasonable range, each chainring must be compatible with, and not too disparate in size from the next one.

Traveling a mile with example #1 requires the 700x28 wheel to rotate 753.8 times, or 153 pedal revolutions in the 54x11 gear. Example #3 takes 171 pedal revolutions per mile with the same gear, while the wheel rotates 840.36 times. The equation changes when #3 is equipped with a 60T chainring and the number of pedal strokes required roughly matches example #1, but the #3 wheel still has to rotate 86.56 more times than the larger wheel does each mile.

The combination of losses in efficiency due to component compatibility, wheel revolutions, and gearing compromises add up to a significant enough difference to make 700c wheels appealing to people who spend most of their tandem miles on the road. Since our experience suggests that the road is where most people ride their tandems, that's where we concentrate our efforts.

Dwan Shepard