Children's bicycles
 
Bicycles for children should be strong enough to withstand abuse, be reliable and simple to maintain. Strength equals weight; reliability and low maintenance rules out sensitive derailleur mechanisms, and a coaster brake is preferable to a hand brake. Accessories (like levers on the top tube of a child's bicycle)are to be avoided because they can injure. When children are given a bicycle, they should also be given a helmet and expected to wear it – just as their parents do. Children are at risk of trapping fingers in the chain, and it is wise to consider buying a bicycle that has a shield over the chain.
 

Adult frames
 
 A well designed bicycle and rider fit together in a very harmonious way, but it is a fragile relationship. There is not much to vary from within the range of accepted traditional frame geometry and rider set-up without compromising the pedalling effectiveness in one or more of the important positions.

For the cyclist interested in performance, good bicycle fit is paramount. A properly fitted rider will feel efficient and comfortable. An efficient position is one that enables the rider to produce more power for a given effort, and to produce that power without working muscle groups excessively or needlessly.

Comfort and efficiency may be at loggerheads in certain riding conditions, however, riding position may have to be modified to favour one quality over the other. In a time trial, for instance, efficiency matters most, so a position allowing the best aerodynamics is desirable. In a long road race an uncomfortable bicycle may ultimately be more fatiguing than the alternative loss in aero efficiency.

Framesets are generally measured from the centre of the bottom bracket, to the top of the seat tube. This sounds as if it’s easy to specify what size frame or bicycle you will need to buy but its not quite that simple, as almost all bicycle and frameset are measured differently!

Like cars, bicycles are made in different models for different purposes. A touring bicycle differs from a road racing bicycle much as a touring car differs from a sports car; the latter is more responsive and less comfortable. Don't worry though, all you really need to do is actually get on a bicycle and test it. It's the best way of buying a bicycle with a frame size that fits properly since frames are obviously not designed to fit all body types.

As a guide you need to get the basic frame size correct and then you can go about adjusting reach, saddle height and position by using different sized bicycle components, building a bicycle that is the right size for you.

Good bicycle fit has three general parameters:

• correct saddle height,
• correct saddle-to-bar distance, and
• correct fore and aft saddle position.

The established methods for finding the correct saddle height (seat tube length) and saddle-to-bar distance (top tube and stem length) seem fairly straightforward. For maximum power, the cyclist's leg should be nearly extended at the bottom of the pedal stroke. This advice is good news for your knees, as the straighter your legs are during the peak power portions of the pedal stroke - usually between 100 to 130 degrees from top dead centre - the better it is for your knee joints. The reach to the bars should allow the cyclist to assume a comfortable position whether sitting or not.

The primary difficulty in finding the correct fit for a road cyclist – however - is in determining the correct horizontal saddle position.

Saddle position has a direct bearing on frame geometry. It affects seat tube angle as well as the top tube/stem length dimension. The correct seat tube angle allows correct weight distribution for both in - and out-of-the-saddle riding. The cyclist will find the handlebars comfortable in all riding positions. The overall weight distribution of the bicycle affects the dynamics of the vehicle and may need to be considered, though that is not generally a problem with road frames.

• KOPS. The traditional method for achieving optimal saddle position is to position the rider's saddle so that the bump below the knee is over the pedal spindle with the crank horizontal to the ground; the "KOPS" (Knee Over Pedal Spindle) method (see figure).
• The common way to check this position is with a plumb bob. Typically, a frame builder will choose a seat tube angle that places the saddle at the centre of the seat post’s clamp, with the rider's knee in this position.  
• The KOPS rule seems sensible, but interestingly enough would seem to be a coincidental relationship that puts the rider somewhere near his or her “correct position”. With a recumbent bicycle the location of the rider's knees are completely different, yet the position is just as effective in pedalling, which illustrates that the KOPS method has no real basis in physics, or physiology. But, at the same time, it works well enough and we can use it.

Finding a good-fit frame size.

Measure your inside leg length. To do this, stand barefoot against a wall with your feet +/- 15 cm apart. Place a book between your legs and press it firmly up against your pelvic bone. Make a mark against the wall and measure the distance from the floor to the mark.  Multiply this measurement by 0.65, and the result would be your road bicycle frame size (in cm).

Note:  Women generally have longer legs but a shorter torso than men, with the result that a frame sized as above would often have a top tube that is too long, rather go for a frame 2-3 cm smaller than suggested hereunder.

• Road bicycles. 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”), and most riders are suitably positioned with a seat angle that falls between 72 and 74 .5 degrees.
• Mountainbikes. Mountainbikes have heavier tyres and more robust frames than road bicycles, and the frames are a different shape and may incorporate suspension systems on the front, the rear, or both. As a general principale a MTB frame size should be two to four inches smaller than a road bicycle frame size. To size a mountain bike, subtract 10 from the above result and divide by 2.54 to get a frame size (in inches).

Crank length: In general, if your inseam is:

• less than 29 inches, use 165-mm cranks;
• 29-32 inches, 170 mm cranks;
• 33-34 inches, 172.5 mm cranks, and,
• more than 34 inches, rather use 175 mm cranks.

A crank’s length is measured from the centre of its fixing bolt to the centre of the pedal-mounting hole, usually stamped on the back of the arm. If you use longer cranks than recommended, you’ll gain leverage for pushing big gears but lose pedalling speed / cadence.

Pedals. Pedals are sold in pairs; the one under the rider's right foot has an ordinary, “normal” right hand thread and the one on the left an “abnormal” left-hand thread (tightens as it is turned counterclockwise). In this way drag in their bearings tightens rather than loosens them. 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 the left.

• Road : LOOK pedals are the standard in this category, Visit your LBS and see what options are avaialble!
• MTB : Shimano’s SPD pedals within a flatpedal are the standard in this category. Find a pair that fits your pocket! 

Saddle Height: Adjusting saddle height is the next, crucial step in fitting a bicycle to a rider. A seat should never be so high that your hips must rock to help you reach the pedals!

• Multiply your inside leg length by 0.883. The result is your proposed saddle height, measured from the middle of the crank axle, along the seat tube, to the top of the saddle.  This is commonly accepted as Greg LeMond’s advice on seat positioning, but he assumed the cranks as a 'standard' size (suited to your length.)
• Another manner of determining the proposed height of your saddle (incorporating crank fluctuation) is to measure your inside leg length / crothch height (against a wall as set out above), and adjust the saddleheight as equal to the distance from the top-of-the-bottom pedal to the top of the saddle being equal to the rider's crotch height multiplied by 1.09 (a factor developed by Vaughn Thomas at Loughborough University, England) (Note: rotate the pedals so that the cranks are parallel to the seat tube!)

Add 2 to 5mm to the seat’s height if you have ‘long’ feet (in proportion to your height).

In any event, if it feels wrong it probably is, and the saddle should always be re-adjusted to your 'unique physique'.  Note: If adjusting the saddle height results in less than 2.5" of seatpost inside the seat tube, you need a longer seatpost.

Saddle Tilt: The saddle should be level, which can be checked by comparing it to a tabletop or windowsill. A slight (read ' very slight!’) downward tilt may be a bit more comfortable, but is not recommended. More than a degree or two could cause you to continually slide forward, putting pressure on your arms and hands.

Fore/Aft Saddle Position: Loosen the seat post’s clamp, mount up and sit comfortably in the centre of the saddle.  Click into the pedals, set the cranks horizontal, and slide the saddle to get your knee over the pedal spindle (KOPS). Seated climbers, time trialists, and some road racers may like the line to fall a centimetre or two behind the end of the cranks to increase pedalling leverage. Remember, if your reach to the handlebar is wrong, use stem length to correct it, not fore/aft saddle position!

Top-Tube and Stem Height: Combined, these two dimensions determine “reach.” Depending on your anatomy and flexibility, your reach could be longer for better aerodynamics, or it may need to be shorter for back or neck comfort. Start with the top of the stem about one inch below the top of the saddle. This should give you comfortable access to every hand position. As time goes by, think about lowering the stem as much as another inch (not all at once) to improve your aerodynamics. If your lower back or neck starts complaining, or if you notice you’ve stopped using the drops, go back up. Never put the stem so high that its maximum extension line shows, or it could be snapped off by your weight on the bar. For most riders, when they’re comfortably seated with their elbows slightly bent and their hands on the lever hoods, by looking down the front hub will be obscured by the handlebar.

Handlebar: Bar width should equal shoulder width to open your chest for better breathing. A bit too wide is better than too narrow. Make sure the hooks are large enough for your hands. Modified “anatomic” curves may feel more comfortable to your palms. Position the bottom, flat portion of the bar horizontal.

Brake Levers: Move them around the curve of the bar to give you the best compromise between holding and braking when your hands are in the hooks. The levers don’t have to be positioned symmetrically - Andy Pruitt’s rule - if your reach is more comfortable with one lever closer to you than the other, put ‘em that way!  

Voila! Your ironhorse awaits your input...