A properly fitting bike allows a comfortable and safe sitting position for children and teenagers. The size of a child’s bike is usually based on the wheel diameter.
This is based on the frame size or seat tube length in adults.
Proper bike fit allows the muscles can deliver optimal strength. This is less tiring and avoids overloading the knees.
How to size a kids/youth bike
Because children have less experience and balance, they are more susceptible to falls.
However, the habit of cycling is taught at a young age, and a seat tailored to a child’s height ensures optimum comfort and encourages them to ride on their bicycle.
All the while, we are teaching them how to ride more intuitively. So how does one choose the right size of a child’s bicycle? Here is a table for a general rule.
|Age||Body Length||Bike Size|
|2-4 year||87-103 cm||12 inch|
|3-6 year||96-117 cm||14 inch|
|5-7 year||110-123 cm||16 inch|
|6-9 year||117-135 cm||18 inch|
|7-10 year||123-140 cm||20 inch|
|8-11 year||129-146 cm||22 inch|
|10-14 year||140-164 cm||24 inch|
|> 13 year||160-176 cm||26 inch|
Here is a graphic that describes the size of kids better. (I do not own the rights to this graphic, credit goes to http://www.wiggle.co.uk/)
Case 2: Size Based on Inseam and Height (More Advanced)
The size chart for children is based on age and height. Since not every child is equally tall at a certain age, choosing the bike size based on body length is better.
A study  in which young children between 7 and 14 years show that choosing a bike based on the bike size is not the proper way.
The most influential parameter for a sitting position is the inseam, along with the bike size.
Kids Bike Size Chart
This is significantly different for children, even those of the same height. Based on the body size of 1,400 youth, one arrives at a conclusion summarized in the following table, which takes into account the growth:
|Body Length||Inseam||Bike Size|
|120-140 cm||< 52 cm||16 inch|
|52-74 cm||20 inch|
|> 74 cm||24 inch|
|141-160 cm||< 62 cm||20 inch|
|62-80 cm||24 inch|
|> 80 cm||26 inch|
|> 160 cm||< 74 cm||24 inch|
|>74 cm||26 inch|
The study was conducted with over 1400 young folks. Nine body measurements were taken, which determined the size of the bike.
With these data, one could determine which size fits the children for anybody’s length. The only thing you need to adjust is the seat and handlebars.
A criterion of good seat height is the distance between the pedal and the saddle, corresponding to 88.3% percent of the inseam. Children between 120 cm and 140 cm tall fit almost on a 20-inch bike.
For children between 140 cm and 160 cm, the average bike size is 24 inches. For teens more significant than 160 cm, the best fitting bike would be a bike with a wheel size of 26 inches.
Although most would prefer to go for the standard and easy-to-use first case, some like to go beyond paying close attention to detail and choosing the bike that would ideally suit them.
This would require more hard work, but it would make your purchase all the more enjoyable in the end. You can always check out the comprehensive study referenced below for more information.
Kids Bike Sizes: How To Choose The Right Children’s Bicycle
 Laios L, Giannatsis I. 2010 Ergonomic evaluation and redesign of children bicycles based on anthropometric data. Appl Ergon 41: 428 – 435.
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