So I built this 20 incher up for my 44 lb seven year old. One of the primary goals for this build was to have a reasonable stack height. I feel like most kids bikes have the bars super high. Head tube length is 80mm and it’s got an eBay
Chinese carbon fork which is actually for a mini velo, but it has a nice short axle to crown height— 305mm. The only unfortunate thing about the mini velo fork is it will only fit a 1.75” wide tire. Not a huge deal, since this bike won’t see a ton of off-road use. And it kept the tires lighter.
I also wanted to have the bike ride nice so the TT is 7/8” diameter and the DT is 1 1/8”. Both are .035” wall thickness. I would have used .028 but couldn’t find it in those diameters. The chainstays are 1/2 x .028 and the seat stays are 3/8 x .028 4130. This is a pretty tiny seat stay so I was kind of forced to do disc brakes. V brakes would have spread the stays and caused chattering under hard braking. It’s much easier to brace a disc tab.
I built the wheels using Velocity A23 rims and Stan’s road bike hubs. 24h front and 28h rear with double butted spokes laced 2X. A double wrap of Muc Off rim tape and Stan’s sealant with pink Muc Off valve stems finished off the tubeless setup. I’m using Kenda 20x1.75 K1153 wire bead tires. The first bead seated quite easily but I had to use a ratchet strap around the circumference of the second tire to get to hold enough air to seat. Have not had any problems since.
Brakes are Avid Juicy’s I had in the parts bin, but they have a very large range of lever adjustment so I can get the levers very close to the bars. 140mm front rotor and 120mm rear. Rear brake mount is IS style but is not to the standard. IS stops at 140mm. Probably should have been flat mount but I didn’t want to buy new brakes.
Cranks are Sinz 140mm BMX on a 107mm Ti BB. I’m currently working on a new crank arm holding jig to aid in shortening them to 115mm. Just haven’t finished that yet. Q factor ended up around 145mm. Kinda wide but maybe I’ll build new cranks one of these days.
The chainstays are 350mm which proved to be too short for eight speed. When I hit first gear it would literally shift the chain off the front chain ring. Maybe a narrow wide chain ring would help. Currently it’s running a Specialized titanium 8 speed road cassette with the #7 cog removed to make it seven speed with shims behind the cassette. Running an X9 short cage derailleur.
I geeked out on the seat post clamp a bit just to see if I could build one, but it’s got a wedge clamp kind of like the old Tioga stems.
Total weight was a touch over 17 lbs. I could throw some more money at it and easily loose another pound or two, but I don’t know. Probably should call it finished. Anyway, my daughter loves it, so that’s what counts.