What wheel size is best for tandems?

With new wheel sizes seemingly coming out every year,

choosing the right size for your tandem isn’t getting any easier. As a maker of
primarily 700C-wheeled tandems, we’ve won a lot of converts to the traditional
road-sized wheels for those who tandem on pavement. 26”, 650b (27.5” and
29”-wheeled tandems are a great choice if your goals are to ride both on
and off-road, or you have specific fitting needs that can only be answered by
going to a different wheel size, such as our PeriScope Scout with 26”
wheels, or our Mocha with 650b wheels, which makes it a great alternative for

riders to the larger wheeled Java. We have found that many
new tandem riders buy inexpensive first tandems with 26” wheels, hoping to
enjoy some of the versatility that they offer. Often they can become
disenchanted when they find it difficult to keep up with others riding
700C tandems on group tandem road rides. We have noticed the same effect
on tandems that are otherwise

virtually identical. It just seems more difficult to maintain high speeds
on a smaller-wheeled tandem. If you’re interested in fast road riding, and
you don’t need the flexible sizing of the PeriScope or off-road ability of
our Java or Mocha, we’d recommend 700C wheels.

There are several interesting reasons why there is a
distinct difference in the ride qualities of 700C vs. 26”, which has long
been debated. 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”).

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 a few examples:

1. 700x28 tire (680 actual dia.)/ 54T chain-ring/ 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 chain-ring/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
chain-ring/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 we might not be much help in
settling that argument. 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
roll-out from the smaller wheels?

The answer relates 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 chain-ring to a 60T ring. There
are a lot tandem riders who use big chain-rings, and probably even more who
have tried extra-large chain-rings once and then returned to using more common
sizes. It doesn’t take long to discover that front shifting is not helped by
big chain-rings. The arc of a front derailleur cage is usually matched to the
arc of a 42, 48 or 53T chain-ring. A mismatched arc forces derailleur placement
higher than optimal shifting requires, since part of the cage will drag on the
chain-ring otherwise. A further complication is that in order to cover a
reasonable range, each chain-ring 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
chain-ring 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, most of our tandem models are
fitted with 700c wheels. For those seeking other experiences, we very happily
offer other choices.

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