If your child's bicycle doesn't fit him properly, he won't learn to ride easily and he won't be as comfortable as he could be while riding. Setting the seat to the correct height is one of the most important adjustments for ease and comfort.
Proper Bicycle Size for Children
While adult bicycles are sized by the height to the top of the seat tube, children's bikes are sized by wheel size. Two- to four-year-olds need bikes with 12-inch wheels and probably with stabilisers; five- to eight-year-olds need 16-inch wheels; seven- to 10-year-olds need 20-inch wheels. Most children nine-years-old and up can handle adult-sized bikes with 24-inch wheels.
Adjusting Seat Height
Once you've chosen the right-sized bike for your child, it's time to adjust the seat. Children should be able to straddle the bike with their feet flat on the ground, then get back on the seat and pedal away, and they probably want to be able to put a foot on the ground comfortably when they stop. To adjust the height, hold the bike upright while your child sits on the seat. His leg should be 80 to 90 per cent straight when the pedal is in its lowest position. Adjust the seat to this height, and make sure it is level.
Reasons Children's Bike Seats Are Often Too Low
Children often end up with their bicycle seats set too low. Sometimes this is because they've grown since the seat was last adjusted, but they never readjusted the seat to compensate for the growth. Sometimes they start out with it low to have a lower centre of gravity and better balance, then fail to adjust it upward once they learn balance. If a child's knees hit the handlebars, or if he's able to balance on the bike while stopped, the seat is probably too low. Try adjusting it upward in half-inch increments to give him an opportunity to get used to the proper height.