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Buying a Bicycle -- A Basic Guide: Sizing Your Bike

Size matters on a bicycle. Although you can probably ride almost any size bike, you’ll be amazed at the increase in comfort, speed, and safety achieved when you get a bike that’s sized just right for you. We’ll look at three aspects of the sizing equation: fixed dimensions, adjustable ones, and modifiable ones.

Fixed dimensions are those that are given for the particular bike, like the size of the frame and the wheels. Adjustable ones are features such as the height and angle of the saddle and the handlebars—things that can be adjusted to make a given bike more fitting to its rider. Modifiable dimensions are those that can only be changed by replacing a part by one of a different size.

Some dimensions are limited by factors of design for any given kind of bicycle. Mountain bikes, for example, come with 26-inch tires, and to accommodate that wheel size, other dimensions of the frame are also bound to certain maxima and minima. Just the same, that still allows frames to be constructed that fit all but the very smallest and tallest riders. That’s why mountain bikes are available in a number of different frame sizes. You’ll be most comfortable selecting one that’s designed for riders of approximately your size.

Some bike shops have special devices for sizing a bike to the customer’s physique. Although it’s best to use those devices, when available, under the guidance of the bike shop personnel, it’s also important to understand the criteria yourself, and that’s what the rest of this section is about.

Frame Size

There are a number of different ways to define the size of the frame, as illustrated in FIG. 1. For mountain bikes and hybrids the frame size is usually given in inches, whereas road bike frames are measured in cm (centimeters—one cm equals 0.4 inches).

Manufacturers’ catalogues usually define the bike’s size by the length of the seat tube. This is measured from the center of the bottom bracket either to the center of the top tube or to the top of the seat lug. The difference between these two methods is the distance from the center of the top tube to the top of the seat lug, as measured along the center of the seat tube. On models with a lowered or missing top tube (such as those that are used, For example, on “ladies” models and folding bikes), frame size is always measured to the top of the seat lug.

Another way of defining frame size is by the straddle height (also called stand-over height). This is the height of the top of the top tube above the ground. On models with a sloping top tube, it’s measured at a point just in front of the saddle.

The relationship between frame sizes measured by the seat tube length and those measured by the straddle height is not the same for all frames, because the height of the bottom bracket above the ground may differ and the angle of the seat tube varies. On mountain bikes, the bottom bracket is usually higher than on other machines, so there a given seat tube length results in a greater straddle height than it does on a road bike.

FIG. 1. Frame size and other characteristic dimensions, also showing a definition of the rider’s inseam measurement as used for sizing the frame.

Probably the most critical dimension is the straddle height. You have to be able to straddle the top tube comfortably to be safe on the bike. Consequently, it should be at least an inch less than your inseam leg length. Test it by standing across the top tube just in front of the saddle: it’s the right size if you can raise the front wheel by about 5 to 6 inches for a mountain bike, or 3 inches for a road bike, hybrid, touring bike, or any other conventional bicycle.

Be careful if the bike has a lowered or missing top tube. In this case, you’ll have plenty of straddle height even on a bike that is otherwise too big. On machines like this, the minimum saddle height is a more critical dimension. You should be able to reach the ground with both feet when the saddle is in the lowest position. It’s too big if you can’t comfortably stand on the front of your feet, with the heels raised, too small if you have to bend your knees to have the feet flat on the ground.

What’s Your Frame Size?

To give you a rough idea of the nominal frame size range to look for, use the following method. Mark the height of your crotch above the ground, standing against a wall without shoes. Measure the distance in inches between the ground and the marker, and call that dimension X. Look for a frame size determined in the following table, depending on the method of frame size designation:

That’s the size you should look at first, but don’t take it as gospel. Once you’re in the bike shop and have checked out some frames in the appropriate size, make a note of what works best, and continue looking for bikes of that size. What also matters is the frame length, measured as the length of the top tube between the center of the seat tube and the center of the headtube. The manufacturers give the bikes with longer seat tubes longer top tubes as well. That’s fine if you have standard proportions, but if for example, your upper body is short in comparison to your legs, you may not be comfortable on a bike with the “right” seat tube length because its top tube may be too long for you.

Saddle Height

This is the dimension defined by the fixed frame size and the extent to which the saddle is raised above the top of the seat tube. To be comfortable, the saddle should be raised to the point where you can just reach the pedals with the knee almost fully stretched, with the heel of a flat-soled shoe while sitting on the saddle. Get someone to hold you while sitting like that and pedaling back. If you can do that without rocking side-to-side, the saddle is at the right height.

To be safe, the seatpost should be clamped in over at least 2½ inches, or 65 mm. Most seatpost are engraved to show the minimum insertion height. Don’t ride a bike with the saddle so high that this marker shows. If you selected the right size frame, you should never run into this problem. Since there are other criteria, such as your weight distribution over the front and the rear of the bike, it’s not a good idea to “fix” a frame that’s too short by using an extra long seatpost. Use only the seatpost that came with the bike, or one of the same length.

Handlebar Height

To ride comfortably in the long run, the highest point of the handlebars should be no higher than the top of the saddle. Racers put their handlebars even lower, while on utility bikes, used for short distances in an urban environment, many riders prefer the handlebars a little higher. (Of course, recumbent bikes answer to a different piper altogether).

Never raise the handlebar stem so far that it’s clamped in by less than 65 mm (2 inches) below the top of the headset. Failure of this correction is likely to lead to complete and sudden loss of control over the bike, and often causes a fall resulting in serious injuries.

Although traditionally handlebar height has been an easily variable dimension, it’s no longer so, especially with modern mountain bikes. This is due to the use of a headset referred to as “Aheadset,” which puts the handlebars in a fixed position. Here the only way you can change the effective handlebar height is by replacing the handlebar stem, or by choosing a different handlebar design. Even if the bike has a conventional headset, the preferred method of handlebar height adjustment is to get a stem that has the requisite rise rather than raising it.

Front Length

This is the horizontal distance between the saddle and the handlebars. For reasons beyond my comprehension, this is often referred to as “cockpit size” by bicycle magazine editors. The fixed point on the saddle is considered the point where a line through the center of the seatpost intersects with the top of the saddle. The fixed point on the handlebars is the handlebar’s center. Clearly, this allows for some variation. The saddle can be moved back and forth by about 2 inches (“adjustable” dimension). The handlebars can be moved closer or farther out by selecting a longer or shorter stem, or you can choose a different handlebar design (“modifiable” dimensions).

The bike should be comfortable, but you’ll probably find that the way it’s set up at first may not be comfortable if you’re a beginner. It takes some practice to get used to the rather stretched-out posture for which most modern “real” mountain and road bikes are designed (recreational bikes and hybrids tend to be significantly shorter). Besides, if you have a long or a short combination of upper body and arms, the standard configuration the way the bike came from the factory may not be suitable. Specifically, many women have this problem because their arms and upper body tend to be shorter than what is the average for most men of the same size—and that’s what most manufacturers use as the basis for their frame designs.

The most elegant, but expensive, way out of this is by getting a custom-built bike frame, literally made to measure. If you can’t find a bike with a short enough top tube, look into special bikes intended for women, such as those offered by a company called Terry Precision Cycling for Women. Without resorting to special equipment, this kind of problem can often be alleviated by making adjustments and modifications to the position of both the handlebars and the saddle. To maintain your weight distribution the way the bike was designed, I recommend you don’t just use a longer or shorter handlebar stem, but to “split the difference” by also moving the saddle back or forward by about the same distance. To get 2 inches more effective front length, get a stem that’s about 1 inch longer, and move the saddle about 1 inch back on the seatpost.

Other Dimensions

There are three other variables that matter for your comfort, and all three can only be affected by replacing certain components (“modifiable” variables). These are the size and shape of the saddle, the size and shape of the handlebars, and the length of the cranks. It will be best to get these changes made at the time you buy the bike. That’s when it’s cheap and easy to exchange, and you’ll be comfortable right from the start.

The saddle should be comfortable. That means it should have the right shape and be firmly flexible, not soft and broad.

If you have a relatively wide pelvis (most women do), look for a saddle that is 1 or 2 inches wider in the back portion and with a shorter nose with a soft section in the middle near the front.

You may find such specific women’s saddles most comfortable.

FIG. 2. The optimal crank length is about half the length of your thigh. as measured per this drawing.

The handlebars need to be consistent with your shoulder width. If you sit on the bike, your arms should be parallel to each other when you hold the handlebars in their normal position. The proper size for mountain bike handlebars and other flat designs is about 2 to 4 inches wider than your shoulders. Drop handlebars should be about 2 to 3 inches narrower than your shoulders for road bikes, about 1 to 2 inches narrower than your shoulders for touring bikes.

The optimal crank length is one half the length of your thigh bone, as measured per FIG. 2. Most manufacturers take care of that by installing slightly longer cranks on their biggest bikes, shorter ones on their smallest models. The range is quite limited. Most crank manufacturers make them in sizes 165, 170, 175, and 180 mm, a range of only 1 inch.

Although there is little risk involved in mounting shorter cranks on any given bike, there is a potential risk associated with installing longer cranks. In the first place, it brings the cranks and pedals closer to the ground when they are in their lowest position. This can be dangerous going around a curve. To prevent this risk, don’t pedal through the corners (on fixed gear bicycles that is impossible because the pedals turn with the wheels, so never put longer cranks on those machines). In the second place, it brings them closer to the front wheel when they’re in their forward position. That is dangerous because they may interfere with the front wheel, especially at low speeds, when the steering deviations needed to balance on the bike are much bigger than at high speeds. This, too, is critically important on fixed-wheel bikes, on which you have to continue pedaling when cornering.

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