If even just looking at the bicycle in your garage makes your knee (or backside, or wrist, or neck) ache, there is good news: It's the bike, not you. Your fit is wrong—and most likely fixable. With any bike, the right fit can depend as much on your flexibility and riding style as on your physical dimensions. For example, if you're a casual rider, you may be extremely uncomfortable on the bike of a racer who is the same size as you. Here are some basic adjustments to get you started.

Set the Seat
In your biking shoes, sit on the seat with the bike fixed in a stationary position (or with someone holding it for you) and put the balls of your feet on the center of the pedals. Pedal backward so that the cranks are in line with the seat tube, right foot down and left foot up (see figure on left, above). Keep your hips parallel (i.e., don't sink your right hip to get your foot down). You want a 10- to 15-degree bend in your right knee. If you're a beginner, you might feel more comfortable with your seat lower. That's OK to start, says Gaynor Collester, chief mechanic for Performance Bicycle, a chain with several East Coast shops. But as you start putting in miles, a too-low seat can cause knee pain. As you gain confidence, raise your seat until your knee angle is in the 10- to 15-degree zone.

Knee Over Pedal
To check that your seat is in the proper fore-aft position, backpedal so that the cranks are parallel to the ground, right knee in front. Measure from the center of your knee down to the pedal with a plumb line or straight edge. The plumb line should drop through the center of the pedal (see figure on right, above).

The Right Reach
The distance from the seat to the handlebar affects your overall weight distribution. Too short a reach can hamper handling and cause you to put too much pressure on your seat; too long a reach can cause all kinds of pain from butt to shoulders to hands. Ideally, for a weekend touring rider, "you want your back at somewhere close to a 45-degree angle," says Collester. This means leaning forward, but not so much that you feel overly stretched out. Key point: Bend your arms. Casual riders should aim for a 20- to 30-degree bend when hands are on brake levers, while more aggressive riders and racers often bend much deeper. If your arms are too straight, you'll stiffen up. "The kinds of aches and pains you get in your upper body are related to that stiff arm," says Collester.

The other factor in reach is the height of the saddle vs. the height of the handlebar. For a comfort position, you want the bars a bit higher than the seat. For a neutral position, they should be close to level. Racers often have handlebars set 3 or more inches below the seat.

Once you have your setup, take a ride and do an appraisal as you go. If something still hurts, head to your local bike shop for fine-tuning. If they can't fix the problem, it's possible your bike just doesn't fit you, and it's time to invest in a new one.

Kids' Stuff
While all of these fit principles also apply to children, it can be tough to maintain a good fit during growth spurt years. As a result, parents often fall prey to the most common mistake: They buy a bike that's way too large, so the child can grow into it. Within reason, that's OK, says Collester. "At a very minimum be sure kids can safely get off the saddle and put both feet on the ground, without touching the top tube." If the bike has hand brakes (good on bikes for kids age 6 and up), check that your child can fully grab the lever with his or her fingers bent at the middle knuckle, where the hand is strongest.