BMX - 24" various frame wheel/rim diameters 507mm 520mm 540mm swap?

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riva
04-27-11, 10:24 AM
I didn't see this talked about in the archives.

What are the 24" bmx frames really designed for.. 507mm or 520mm rims?? Is it too close to matter? I'm building a junior up for my oldest and want to know if one size is preferable to the other before I order my rims and spokes. I'm not sure if he will race it or not so its not a big deal if the tires are the fatter 1.75 or skinnier 1 1/8". Or even 1" or 1.5 or 1 3/8 will work.

I see there are higher grade tires available in the 520mm size than the more common ISO 507mm wheels. I'm thinking I may go with the 507's anyway and just run the cheng shin comp III 1.75 tires though.

As far as brakes go, the 13mm difference means 6.5mm more reach is needed for 507mm. Which I don't think is any problem for most brakes.

For the 20's I know the jump from the 20x1.5 ISO 406mm rims up to the 20x1 1/8 ISO 451mm rims is big enough that it causes problems with brake reach..

Anybody have any advice?

http://sheldonbrown.com/tire-sizing.html

riva
04-27-11, 05:24 PM
Well I went with the fatter 507mm 1.75" setup even though the junior frame is probably built for the 520mm skinnies. 1.75's are cheaper and I don't see the extra weight mattering for a non-pro kid. I'll post with my thoughts on this after we try it out.
riva
05-09-11, 04:23 PM
The 1.75" 507mm wheel worked out fine in the back. Its tight but it fits. The fork looks like it will only fit a 1" tire though. I'm going to try a mixed 520/507 set of wheels.

Edit: So for anybody running across this thread confused about the various 24" tire sizes, here is the break down:

24 x 1-3/8 (ISO 540) size not meant for BMX's, for small wheel roadbikes.

24 x 1-1/8 (ISO 520) junior cruiser BMX's built for this, also small roadbikes.
24 x 1-3/8 (ISO 520) similar but slightly wider than the 1-1/8 tires/rims.

24 x 1.5 (ISO 507) full size cruiser BMX frames for these diameter tires
24 x 1.75 (ISO 507) full size cruiser BMX frames for these diameter tires
24 x 1.85 (ISO 507) full size cruiser BMX frames for these diameter tires
24 x 2.XX (ISO 507) cruiser BMX's, dirt jump bikes, some small wheel MTB's