Tech support: Making sense of wheel sizes
Tech support: Making sense of wheel sizesBy Rick Denney
Dear Tech Support,
I have been doing triathlons for the past two seasons with a very old Schwinn road bike. The biggest question that I have is in regard to wheel size, 650 or 700? My "tank"-- as I call it -- has 27-inch wheels, which I've come to understand are about the same size as 700c wheels. Is there a great difference between the two sizes? I am planning to upgrade to a more modern bike in the near future, and can get no clear answer as to which size is better.
James
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James,
This is a good and frequently asked question. Let me start with a pedantic discussion of why we size wheels the way we do, as this will answer part of your question.
In the past, the French developed a system of standards for designating sizes of bicycle tires and rims. The standard defined the outside diameter of the inflated tire. For example, they established one standard for bicycle tires that were 700mm in diameter as ridden. They designated rim size based on the height of the tire's sidewall using a letter following the tire size designation. So, 700a rims were bigger around, and met the standard overall diameter with skinnier tires. The 700c designation referred to rims that were 622mm in diameter at the bead, on which tires that were 39mm wide (i.e., typical old-style balloon tires) would be installed.
It turned out that the 700c rim, with its 622mm bead diameter, was just about the same diameter as the rim designed for a tubular tire, and could be used on bikes designed for the latter without too much adjustment of the rim brakes found on the better bikes. So, it became a common European size for bikes with clincher tires. The tires that we put on them got skinnier and skinnier, so that the overall diameter is now much less than 700mm.
The smaller wheels used in triathlon are often called "650" wheels, mostly as a means of distinguishing them from the larger wheels. We should adopt the current standard designation agreed upon by the International Standards Organization, which defines tires using two numbers separated by a dash. The first number is the width of the tire, and the second is the bead diameter of the rim. So, a 23mm tire on a 700c rim would be called a 23-622 tire. The 27-inch rims and wider tires on your Schwinn probably say something like 32-630, because the bead diameter is 630mm. The tires on a small-wheeled triathlon bike would be designated 23-578.
Modifying a bike to use 700c rims instead of 27-inch rims is easy if the brakes will reach the new rim. Those brake pads have to be 4mm lower. If they won't reach, then it's not so easy and requires either hard-to-find long-reach brakes, or old brakes that have been modified with a drop bolt to lower them. Wheels with 27-inch rims are now obsolete, although it is possible to find lesser-quality tires from a few sources.
The size of 700c wheels presents some problems for bike designers trying to meet the needs of smaller riders, especially women, who tend to have shorter torsos and longer legs than men of the same height. This a particular problem with forward-position triathlon bikes. Also, the forward position concentrates more weight on the front wheel, which makes for less predictable handling for some riders. One solution to the weight-balance problem involves tucking the rear wheel farther under the saddle, toward the front of the bike. A small wheel can be tucked farther than a large wheel. So, for smaller frames, the smaller wheels give the frame designer more room to work.
Some argue that smaller wheels are more aerodynamic, but this has never been demonstrated to my satisfaction in wind-tunnel tests that include the whole bike and rider. Some argue that smaller wheels accelerate better because they provide a lower gear. But the legs do the accelerating, and the gears match the legs to the wheels, whatever their size. Bigger gears compensate for smaller wheels.
It is true that the smaller wheels ride a bit more roughly, because they climb any given bump more steeply than larger wheels. It's also true that the smaller wheels are a bit lighter, but, as I have written in the past, the benefits of lighter wheels are usually overstated. These effects are probably not significant enough to be worth considering. Smaller wheels can be less convenient, with a smaller selection and availability of tires in most places, especially for riders who have other bikes with large wheels.
So, the main reason to consider smaller wheels is that they give they sometimes afford a more balanced bike for smaller riders. Most experts will define the point at which the smaller wheels really help at the 54- to 58-cm frame size. If you use a smaller bike, they help a lot. If you ride a larger bike, they don't help with balance (though they also do no harm). In the middle, they may or may not present an advantage.
If I found a bike that met all my needs and cost what I could afford to pay, and if it fit and was fun to ride, then I'd buy it. Tire size isn't a big enough factor to keep me from buying the perfect bike, and it certainly should not sway me toward buying a bike that doesn't otherwise work. So, my advice is simple: Retire that old tank, and find the bike that works best for you. Take the wheel size that comes with it.
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