Children's MTB buying guide: From 16 to 29 inches - the right size for a children's bike

The wheel size

The wheel size is the first and therefore most important size that parents look at when buying a children's bike. The range of Balance bike and children's MTB ranges from 12 to 29 inches. The balance bike from the age of 1 has even smaller wheels, but is not really built for off-road use. For sporty use, you should always aim for the largest possible wheels. The 29er principle has made children's MTBs much better. It puts the rider as far as possible between the wheels and thus makes the handling safer. Especially when going downhill or over obstacles.

On everyday bikes, which usually have one to two classes smaller wheels than sports bikes, children ride much less safely than on smaller 29er bikes. The limit is the saddle position. The rear wheel must still fit under the seat. This results in a minimum saddle height, which can also be expressed as stride length. The table at the bottom right gives an indication of the earliest possible wheel size. For the sake of simplicity, a body height is often also given as a guide. If in doubt, try it out to be sure.

If you run through all wheel sizes, the dwell time per wheel is quite short. This becomes expensive unless you have a finely graded family team or other buyers. However, if you put the children on large wheels as early as possible, you can skip a step. Then the bike career of 3-11 year olds can, for example, run over the steps 16, 20, 26 and 29 inches - two sizes are thus left out. Which size can be omitted depends on when and where you start. In our experience, early 26-inchers, from around the age of six, have a relatively long half-life of up to four years and a high utility value. During this time, kids make rapid progress but don't grow as much. 27.5 inches can easily be skipped in favour of 29 inches. The smallest 29ers are rideable from 1.45 metres tall.

In detail, however, the frame geometries differ in length and height with the same wheel size. Not all manufacturers go to the limit and shrink the frames to the minimum, resulting in different minimum body sizes for bikes with the same wheel sizes. Some manufacturers also offer two frame heights or more per wheel size.

The frame size

More important than the frame size, which is not clearly defined, is the minimum seat height that a bike requires. This is the distance from the centre of the bottom bracket to the top of the saddle. With the Step length - Press the book into your crotch and measure the distance to the floor (see photo below) - this gives a good guide to the seat height: crotch length times 0.885 is an approximation of the seat height.

If not specified, the seat tube length will help. Seat tube length plus eight centimetres for the seat post head and saddle height are approximately the minimum seat height - provided the rear tyre or the top tube is not in the way. You can also look at the crotch clearance when standing over the frame, but this is not so important in practice.

The weight

Children are light. Children's bikes are often not. Unfortunately. This can mean that children can barely lift their bikes up a flight of stairs or pick up a fallen bike on their own. If a 20-kilo child rides a ten-kilo bike, the equivalent for an 80-kilo adult would be a bike that weighs 40 kilos. Children, who are already really good bikers at around 30 kilos, are still at least 2.5 times lighter than adults.

In short, even the lightest children's bikes are relatively heavy. This is because hardly any high-quality components are specially designed for children's weights. Even lightweight components are safe for 80-kilo riders and correspondingly oversized for kids. On the other hand, they are disproportionately expensive and therefore rarely found on children's bikes. But there is some room for manoeuvre: Impellers for example, can be built for children with few and very thin spokes. The Tyres may be very delicate at first. Tubeless tyres save additional rotating mass. Saddles, seatposts and handlebars can be very light.

The sensitive subject of suspension forks: Up to 20 inch wheel size, suspension forks are nonsense - even if they may be the most important part of the MTB for many children. At 24 and 26 inches, they are debatable depending on the route, but should be of high quality, be adjustable to the child's low body weight and have good damping. Poor rebound damping that is too fast will cause the front wheel to bounce uncontrollably off-road. Dispensing with the suspension fork saves at least one kilo - an effect comparable to 3 kilos on an adult bike.

The crank length

Children have short legs. And short legs go well with short cranks. Logical, right? Unfortunately, the word hasn't got around to Shimano & Co., which is why cranks under 165 mm - shorter cranks are not available in mass production - are a niche product that is the preserve of lesser-known manufacturers, importers and specialists. Rule of thumb: Crank length = 10 per cent of body height. This makes it clear that cranks with a length of 165 millimetres are only available from

Ideally, the cranks should grow with the rider. A step in 10 mm increments is completely sufficient. In practice, the problem lies in actually procuring the cranks. The parts are difficult to find or not available.

Vpace has a good, reasonably priced range (picture above). Federleicht produces high-quality lightweight cranks in short lengths (picture below). Other suppliers are T&A, Hope and some frame builders with their own crank line. However, a common feature of the very classy custom cranks is the unfortunately high price level, which makes their use on children's bikes unrealistic, apart from idealistic exceptions.

Children's MTB - Editor's verdict:

Buy your kids proper children's MTBs! As light as possible, with large wheels and short cranks. Then the chances are good that children and parents will have fun together! - Robert Kühnen, BIKE tester and father of three