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So, my friend's kid's bicycle has an 18" tire that is completely worn down. I'm having a really tough time finding 18" tires on local buy/sell websites, but there are a bunch of 16" and 20". Does anybody know if putting a 20" (or 16") wheel on my friend's kid's bike will work? No gears, just back pedal braking, and a hand brake for the front wheel.

Thanks a lot.

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    What sites are you looking at? While it's not the most common size, it seems that there are plenty of options on Amazon and eBay, as well as brick and mortar stores like walmart. Putting a 20 inch wheel is likely to not have enough clearance, and dropping down to a 16 inch would put the bike lower to the ground making pedal strikes more likely. It's hard to say whether it would work without mounting it to the bike, but my first thought is it's not going to be optimal. – Kibbee May 20 '16 at 13:56
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    With a front hand brake, the brake pads will not line up with the rim if you change sizes. – BPugh May 20 '16 at 16:45
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    If it's a bike with only a coaster brake and there is clearance for the larger tire then it's probably not a problem. However, unless you can find the wheels used somewhere the cost of new wheels & tires would be more than the cost of a new bike. And it's very iffy whether you can get rim brakes to work after such a conversion. – Daniel R Hicks May 20 '16 at 18:37
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    How tall is the kid? The gosh-durn things just keep getting bigger, so going down is a bad idea. Can the kid ride a 20" bike or even a 24" ? Mine went from a 16" wheel bike to a 20" to a 26" wheel bike in under 12 months. – Criggie♦ May 21 '16 at 2:43
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There are several different 18" sizes, and several different 16" sizes. See Sheldon Brown's tire sizing page

If the 18" is the 355mm used by Birdy, and the 16" is the 349mm used by Brompton, that's close enough that it might be possible.

People have converted Birdys from 18" to narrower 20" (406mm) by moving the brake posts (see "brakeboss relocator").

I changed the obsolete 500A (440mm) tyres (and chromed steel rims) on an old Dawes Kingpin folder to 20" (406mm) ones on alloy rims, the widest ones that would fit in the fork to reduce the bottom bracket drop, and was able to find suitable brakes to fit.

But for a typical child's bike, I very much doubt it's worth it.

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Unless the bike was specifically designed for a wheel size conversion, like the 18/20 Prodigy from Hoffman in 2003 or so, changing the wheel size is generally not a safe and practical option.

Brake mount positions and tire clearance to the frame prevent it.

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