bicycles

Bicycles for children under 12 should be strong enough to withstand abuse, reliable, simple to maintain, and safe–not necessarily the priorities one would set for an adult's bike. Strength means weight; reliability and low maintenance rules out finicky mechanisms like sophisticated derailleurs. Children's hands are not as strong as adults', so a coaster brake is preferable to hand brakes. Do not put accessories like levers on the top tube of a child's bicycle, because in an accident they can injure a boy's testicles. When children, no matter how young, are given a bicycle, they should also be given a helmet and expected to wear it–just as their parents do.

Road Bicycles

Like cars, bicycles are made in different models for different purposes. A touring bike differs from a road racing bike much as a touring car differs from a sports car; the latter is more responsive and less comfortable. The information below is meant for adults buying their first serious bicycle, to help them recognize what sort of bicycle they are being sold so that they can avoid models that don't fit them or suit the intended use.

Frame Size

The frame size is the distance between the top of the seat tube and the axis around which the cranks rotate, measured along the seat tube. Frame sizes generally run from 19" to 25" in increments of 1".

To find a good frame size for a rider, have the person stand in bare feet on an uncarpeted floor. Measure the vertical distance from the floor to the head of the femur (see drawing). Subtract 13.75".

Adjusting saddle height is the second step in fitting a bike to a rider. With the rider standing in bare feet on an uncarpeted floor, measure the vertical distance from floor to crotch. Rotate the pedals so that the cranks are parallel to the seat tube. Adjust the height of the saddle until the distance from the top of the bottom pedal to the top of the saddle is equal to the rider's crotch height times 1.09 (a factor developed by Vaughn Thomas at Loughborough University, England).

Vaughn Thomas.
Scientific setting of saddle position.
American Cycling, June 1967, page 12.

If adjusting the saddle height results in less than 2.5" of seat post inside the seat tube, you need to purchase a longer seat post. Seat posts are manufactured in diameters from 26.2 mm to 27.2 mm, in steps of 0.2 mm.

Sometimes the problem is that the seat cannot be lowered far enough. If a smaller frame size is not available, it may be possible to lower the seat by inverting the clamp, or, if the saddle is leather, by switching to a plastic saddle (they're not as deep).

Opinions about saddle tilt differ. Some say nose down, at a 10° angle; some prefer horizontal.

Gears

Cycling enthusiasts have a peculiar way of stating gear ratios: a gear ratio is given as the diameter of an imaginary driven wheel directly connected to the pedals that would move the bicycle the same distance as one rotation of the pedals using a particular gear ratio does. This system comes from the days of the high-wheeler bicycle when the pedals actually were directly connected to the front wheel, and a higher “gear ratio” could only be had by using a bigger wheel.

To find the “gear” for any combination of sprockets, divide the number of teeth on the chainwheel by number of teeth on the rear sprocket (which gives you the real gear ratio), then multiply by the diameter of the rear wheel. For example, suppose the chain passes over a chainwheel with 40 teeth and a rear sprocket with 20, on a bicycle with a 27" wheel. The gear would be (40 ÷ 20), × 27 = 54.

Handlebars

Pedals

Pedals are sold in pairs; the one under the rider's right foot has an ordinary, right-hand thread and the one on the left a left-hand thread (tightens as it is turned counterclockwise). In a pair, the one with a “D” or an “R” stamped on the end of the axle is the right pedal; the other one is a left.

Inexpensive bicycles often have one-piece cranks; these are tapped ½" - × 20. English, Japanese and Italian pedals are ⁹⁄₁₆" × 20F, though the Italian pedals have a slightly different thread. French cranks and pedals are 14 mm × 1.25.

Saddles

Saddles are attached to the seat post by a clamp that grasps two parallel rails under the saddle. On most saddles the distance between the rails is 36 mm and the flat stretch on which the clamp can be placed is 60 mm long. A much less common type is made for riders who need a greater range of back and forth adjustment: it has a flat stretch 120 mm long. Its rails, however are 20 mm apart, so the two types are not interchangeable.

Typical characteristics that distinguish a road bike intended for touring from one designed for a racer:

		Touring	Racing
hubs		low-flange	high-flange
wheelbase		long	short
number of spokes	front wheel	36	often 28 or 32
number of spokes	rear wheel	36 (40 for a heavy rider or lots of luggage)	36 or fewer
frame angle		71°–73°	72°–74°
saddle		relatively wide	relatively narrow

Mountain Bicycles

Mountain, or off-road, bicycles have heavier tires and more robust frames than road bicycles, and the frames are a different shape. They often have suspension systems, on the front fork, the rear fork, or both. An individual requires a frame size two to four inches smaller than his or her road bicycle frame size.

Resources

David Gordon Wilson.
Bicycling Science. Third Edition.
Cambridge (MA): MIT Press, 2004.