*Please note this post is a work in progress (as of Dec 28/19), thanks for your patience! The information is all here, it’s just disorganized as I need to condense it all as it originates from several different posts.
In our experience, it’s better to be on a bike that is too small than one that is too big. You will find this true for your family, too, especially if you have any kids who are ‘thinkers’ or a bit tentative when trying new things.
Time to go bike shopping.
Ideally, you will have your kid try the bike in person at your rad local bike shop at which time you will check the standover height (see below) and look to see how stretched out they are (reach or cockpit).
This is your chance to choose a bike with gears for the first time. If you are going that route, check to see if a twist shifter or rapid-fire style shifter works best for them.
And, can they reach the brake levers? Check.
Fit
This is not the time to maximize your dollars and buy the next frame up because it jussssst fits. Believe me, we made that mistake once, almost twice.
Cockpit
Being too stretched out can be remedied a bit with a shorter stem (or reversed stem) or different handlebars, otherwise your little rider might get numb hands on longer rides.
Minor adjustments can be made by moving the seat fore/aft, but do you really want to be making component changes on an already spendy kid’s bike though? No. Find the bike that fits.
Standover height
If your kid is barely clearing the top tube they will be wobbly getting on and off the bike and crashes will hurt even more when they slam their crotch on the top tube before their feet touch the ground.
Measure your child’s inseam by putting a book between their legs and having them gently push the book up as high as comfortable. Measure the distance from their flat, bare feet on the floor to the top of the book, i.e. their inseam. (While you’re at it, get your bare-footed kid to stand against a wall and figure out how tall they currently are; not the most important measurement, but you can recommend it to the manufacturer’s recommended heights for their bike.)
Compare their inseam measurement to the standover height or recommended inseam from the bike’s manufacturer (see tables in the different wheel size groupings). Aim to have 3-5 cm (~1-2″) of clearance or match the recommended minimum inseam for your child for easier dis/mounting of their new bike and for less chance of them slamming the top tube if they fall forward when ripping it up.
So, to make sure you buy a bike that is not too big, aim for their inseam measurement plus 3-5 extra centimetres (or ~1-2″ more) to equal the manufacturer’s measurement for standover height.
Seat height
With your child sitting on the seat, they should at least be able to touch with their tippy-toes, preferably the full front half of their foot for extra stability and confidence. The minimum seat height should not be much more than their inseam measurement; theoretically, the inseam measurement equal to minimum seat height means they could sit on the bike with feet flat on the ground.
Notes specific to 16″ bikes:
Cockpit
Many bikes in this category come with a BMX-style handlebar that can easily be adjusted fore/aft in order to change the reach of the cockpit. We’ve always made the distance comfortably small (or short) for our young riders and then pushed it a bit forward as they grew.
Standover height
When we sized up our 4 year old to our 16″ bike, this is what she looked like on the first ride:
Note, too, that this photo was taken on a slight uphill! She still had tonnes of room for confident footing, when necessary. She took off and transitioned to this bike without a hitch. I suppose, we could have moved her to it earlier on, but this fit (as pictured) inspired conviction in her and facilitated a very smooth transition to the bigger wheels and frame. (She was previously on a 14″ Spawn Furi.) [Kid-sized pogies made by coldbike.com, FYI.]
Having the seat height at its lowest point yielding flat feet is a very good starting point, especially for more timid riders. Bare minimum, you want the front of their foot to be able to touch down. Later on you can move the seat up so that it’s just the front of their foot or even tippy toes for the more confident rider; having their seat higher gives them more power in their pedal stroke.
Notes Specific to 20″ bike
Cockpit
Being too stretched out can be remedied a bit with a shorter stem (or reversed stem) or different handlebars, otherwise your little rider might get numb hands on longer rides.
Minor adjustments can be made by moving the seat fore/aft, but do you really want to be making component changes on an already spendy kid’s bike though? No. Find the bike that fits.
Standover height
If your kid is barely clearing the top tube they will be wobbly getting on and off the bike and crashes will hurt even more when they slam their crotch on the top tube before their feet touch the ground.
Measure your child’s inseam by putting a book between their legs and having them gently push the book up as high as comfortable. Measure the distance from their flat, bare feet on the floor to the top of the book, i.e. their inseam. (While you’re at it, get your bare-footed kid to stand against a wall and figure out how tall they currently are; not the most important measurement, but you can recommend it to the manufacturer’s recommended heights for their bike.)
Compare their inseam measurement to the standover height or recommended inseam from the bike’s manufacturer (see below). Aim to have 3-5 cm (~1-2″) of clearance or match the recommended minimum inseam for your child for easier dis/mounting of their new 20 inch bike and for less chance of them slamming the top tube if they fall forward when ripping it up.
So, to make sure you buy a bike that is not too big, aim for their inseam measurement plus 3-5 extra centimetres (or ~1-2″ more) to equal the manufacturer’s measurement for standover height.
Seat height
With your child sitting on the seat, they should at least be able to touch with their tippy-toes, preferably the full front half of their foot for extra stability and confidence. The minimum seat height should not be much more than their inseam measurement; theoretically, the inseam measurement equal to minimum seat height means they could sit on the bike with feet flat on the ground.
