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I have a small child road bike frame that takes 20 inch wheels. It comes with no information from the manufacturer, but from what I've learned there are two 20 inch wheel/tyre standards 406 and 451.

Based on measuring the fork, 451 wheels will fit but with very little clearance. If it were an adult road bike frame with 700/23c wheels/tyres I wouldn't be concerned but from what I've seen 20 inch tyres tend to be 'fatter' or rounder than their adult sized counterparts.

So - my question - is there a standard to define how high a tyre rises from a rim? If not, is there a calculation/rule of thumb you can use? Or even a crowdsourced resource of end users real world rim/tyre combinations?

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    The amount the tire rises from the rim is going to be very close to the diameter of the tire. – paparazzo Mar 1 '16 at 17:42
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    I'm guessing that one of 406 and 451 will work and the other won't if you have rim brakes. – Batman Mar 1 '16 at 18:07
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    It's unlikely that a 20" kids bike uses 451 wheels, those are relatively rare and least unlikely on a vintage or high end "kids road bike" with drop bars, or possibly a racing BMX. If it's just a generic bike odds on it will be 406. I would see if you can scrounge up a free 406 front wheel to try in the frame - kids bikes get thrown out a lot (kids grow, cheap bikes suck!) so your local tip shop or bike co-op probably has a pile of them. – Móż Mar 1 '16 at 22:15
  • No, generally there is no standard as to how high the profile of a tire is, the reason is a 20 x 1.75 tire on a very skinny rim width will differ height wise from a much wider rim because it will be stretched out more laterally. So there for the exact same tire would have 2 different, although quite close, heights. – Nate W Mar 1 '16 at 23:35
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    Mσᶎ/Batman - as a combonation of both of your comments. I got hold of an old wheel which was actually a 451 and tried it out in the forks. There's nowhere near enough clearance for a tyre, and also the pads on the brake calipers I fitted can't be made to reach the rim, but look like they'd sit right where a 406 rim would be based on a quick estimation using a tape measure. – ilikeprogramming Mar 3 '16 at 0:38
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If you look at the ETRTO specification of a tire you are thinking of using, e.g. 47-406 (a 20 x 1.9" tire) the total diameter of the mounted tire will be pretty close to the rim diameter plus twice the tire width: 406+2*47 = 500mm ~ 19.7". (the designation 20" wheel here comes from the diameter of the common 50-406 tire). The width of the tire will vary depending on the tread, but will be within a few millimeters of the ETRTO width.

While the most common 20" (406) tires are ~2" / 50mm wide (especially on kids bikes), it is possible to find skinny 20" tires but you need to get skinny rims to go with them. ( Schwalbe has a chart of rim widths http://www.schwalbetires.com/tech_info/tire_dimensions).

As others have mentioned, if you have rim brakes, only one or the other of 406 or 451 will actually work for you and if it isn't a BMX frame, I would guess it is more likely to be the more common size of 406.

  • OP has a small child road bike frame and is measuring the fork. I read that as has no wheels or tires. – paparazzo Mar 1 '16 at 19:59
  • I got that. I made a few edits to clarify. – JP May Mar 1 '16 at 21:02
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According to Sheldon Brown, 406 will be 20 inch decimal size. You will need this size if your current tire says 20x1.75 or some other decimal number on it. 451 is a fractional size. If your current bike has 20x1-3/4 tires, or some other fractional measurement, then use this tire. As far as bicycle tires go, 1.75 DOES NOT EQUAL 1-3/4.

You should really be looking on the current tires for the actual ISO measurement. You should see either a 451 or a 406 on the tire already. This number represents the "bead seat diameter", which is the actual number that's important. If you don't choose the right bead seat diameter, the tire won't fit on the rim, never mind fitting in the frame once it's on. If you can't see the actual ISO number on the tire already, then you can either go with the fractional/decimal value to figure out which you have, or try to measure the bead seat diameter. You should be able to get close enough in this case to tell the difference between 451mm and 406mm.

  • OP has a small child road bike frame and is measuring the fork. I read that as has no wheels or tires. – paparazzo Mar 1 '16 at 19:59

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