Children's Bikes - Coaster 20"
Shopping Advice (scroll down past the advice to the products if you like)
Size - These bikes vary a bit in their frame sizing. The difference can be in the distance from the seat to the handlebars, as well as the height of the seat and handlebars. Different brands will tend to use different geometry. Then there are "mini" versions also. Overall, the 20" coaster range can cater to kids from about 5 years old up to 10 years plus. Typically though, around 6 or 7 years is the time to step up to this size.
Brakes - By definition, coaster bikes have a coaster, or back pedal brake. Simple, intuitive and reliable, it's the best choice for young and inexperienced riders. Choosing a different type of bike (a mountainbike with gears, or a freewheeling BMX) will probably mean a lot more parental input is needed to get the child riding well, plus the bike will have to be checked from time to time by a competent adult to make sure things, especially the brakes, are working properly. Please note there are kids' mountain bikes available now that have a simple 3-speed gear system in combination with a back-pedal brake (they're on the MTB 20" page).
Strength - A reckless rider will require a strong bike. By the time a child is big enough to ride a 20" bike, that child is getting big and heavy enough to damage a weak or poor quality bike. If your child is interested in doing jumps and tricks on their bike, then they might be better off on a proper BMX bike with hand brakes.
Frame and Fork - A larger diameter fork, and larger diameter tubing on the frame mean, all else being equal, a stronger bike.
Pedals - Pedals without ball bearings are common among 20" coaster bikes. They are surprisingly durable, but not as durable as pedals with ball bearings, nor will they run as well.
Wheels - A 28 spoke wheel will not be as strong as a 36 spoke wheel.
Handlebar Stems - Most 20" coaster bikes come with a 4-bolt stem, which clamps the handlebar over a bigger area and is stronger than a single bolt stem.
Saddles - Apart from comfort issues, you'll find that some saddles have a hard nylon edge (with padding in the middle), making them difficult to damage when the bike is dropped.
Weight - All else being equal, a lighter bike will be easier to ride than a heavier bike.
Crank Length - Hopefully chosen to suit the frame size and shape, cranks for 20" BMX bikes are typically either 145mm, 160mm or 165mm long. A longer crank is for a bigger rider.
Gearing - Chainrings are commonly 40 teeth, rear sprockets 18 teeth. If you come across a bike with a 44 tooth chainring and a 16 tooth rear sprocket, expect your child to dislike hills!