The rims
Two wheel sizes - 451 and 406mm BSD. 451mm BSD rims take light skinny tyres with the 20 1 1/8 and 1 3/8 designations. 406mm BSD rims take 20 x 1.5 - 20 x 1.75 size tyres that are higher volume.
451mm options
- Alienation ankle biter rims - 280g, 28h - $45 each off fleabay ($35 delivery to Oz);
406mm options
- CRUPI RHYTHM Rim - 320g, 28h $44.95 ;
- SUN Envy Lite - 330g, 25mm wide, 36h only;
- Sun Ringle ENVY - 401g, 36h;
- Velocity Aerohead 20" - 276g, 24h, 28h, 32h and 36h;
- Velocity Fusion 20" - 306g, 28h, 32h, 36h;
- Velocity Aeroheat 20" - 318g, 28h and 32h drillings;
Hubs
For kid's use, any of the lightweight road front hubs are suitable. bikehubstore have some cheap and lightweight road hubs.
BUT, there are some potential gotcha's with using road or mtb hubs:
- BMX front hubs (and consequently fork drop-outs) use a (larger than 9mm) 3/8" bolt on axle. Special care is required if you use a 9mm QR hub with the standard BMX fork to make sure the front wheel is secure;
- rear hub spacing on most BMX frames is 110mm, so a road hub at 125mm or a mtb hub at 135mm wide will be a squeeze and require cold setting the frame.