Introduction: Build a Small Bike
Meet the Small Bike.
This is great for vertically challenged people, women and children who wan a "real" bike to ride.
The frame is an 20" alloy race BMX repourposed as a road bike.
Step 1: Parts List.
I chose a selection of BMX parts to build into the 20" road bike.
Frame: FMF XXL frame, 7005 alloy. Any frame will do but you will find the race frames have standard threaded bottom brackets which makes crankset selection easier, 1 1/8 threadless headsets are also a bouns. BMX race Frames come in sizes for junior, expert, pro, pro XL, pro XXL and have varying top tube lenghts, go for something long..
Wheels: 20" bmx race wheels. These are available in two sizes for adults and junior riders, sizes are 406 with 1.95" tires and 451 for skinny tires. I use 406 with either Maxxis DTH in a 1.5" or (my favorite tire ever) the Maxxis Ringworm whish is only avaliable in 1.95" but it is hard and near puncture proof and takes 115 psi, read 'fast'. 406 size makes for a great fun 'town bike'.
Forks: lots of carbon candy options to choose from.. Mine are older Answer Mag forks, go for Bombshells if you can get them.
Bars: The reason for the 1 /8 threadless frame choice is that ANY stem of that size will fit your forks, you can stay with the origional BMX stem but it likely has a 22mm bar clamp. Go for a road or MTB stem that will take oversize carbon bars of your choice. I would only recomend drop road bars for children or very small riders and MTB bars in 1-2" rise for taller riders.
Crankset: You can use ANY crankset that will fit the BB shell of the frame, threaded BB frames like the FMF allow the use of Hollow Tech type Cranksets. Depending on your Leg Length, Square drive BMX cranksets come in sizes from 135mm-165 for junior riders. 165 is a standard Track length. Mine are 175mm, my prefered size.
Seatpost: The trick to this build is use a long seatpost so you can have a 'normal' seated riding position.
Gearing: Rear sprockets, for BMX wheels are often a threaded freewheel. I recomend this over the cassette system as the larger size of the two (large does 16-20 tooth small size is 13-15tooth) is the same thread as track sprockets so you cam make the bike a fix wheel bike if you choose. I like fixy..
Brakes: Bonus for BMX Race frames is they have V-Brakes on the back, they work. On the front, I use a caliper setup but this will be dependant on your fork selection, not all carbons have a hole for the caliper but a good mechanic will probably sort that for you. It is possible to use the 451 wheel size on 406 frames but the V-Brakes will not line up, there are aftermarket BMX brakes that can take care of this for you. Front brakes are optional but recomended if running a freewheel.
The FMF frame was new at $200NZ most other parts were second hand, crankset was $40NZ, wheels $50-100. Tires are $30ea for Maxxis.
Lightest build was a shade over 8kg.
Step 2: Assemble and Ride.
Im not going to tell you how to tighten a nut or adjust V-Brakes.
Although I am probably just a little large for the 20" as a road bike the handling is OK. There is the issue of weight over the front wheel and getting the bar height right, I compramise just ride it.
You can use tire sizes to adjust the handling, on a normal road bike I would use a skinny front tire and slightly wider rear, on a MTB it is the other way around. A wider front tire will give slower steering.
If you want to get fussy then tire pressure will do the same thing.
I have a pair of 20" schwalbe kojacs in storage that have not bee used yet, crazy little 406 size slicks.
Step 3: Go Further...
One of the first I made was built as a Time Trial concept, low bull horns, 451 wheels, cut down carbon bladed forks.
Oddly enough that even though I could do ~60km/hr (drafting a bus, which I overtook..) on my old track bike (fix wheel, no brakes, on the road), the 20" TT bike 'hit a wall' at 48km/hr.
I have this on Instructables to help smaller riders out, but I would love to see the kids out on these as track bikes and doing single speed criterium races.
See you at the races.
Step 4: 24" and 26" Size.
As said I am slightly too long for the 20? frames, untill I find an XXXXL frame.
There are other options though if you want to avoid the UCI 700c bikes, namely 24 and 26" wheel sizes.
-Haro 24" BMX, its a "super sport" race bike, again built with flat bars and long post. This bike is again not right for me as it is a short frame but I am learning to balance it when out of the seat sprinting. This bike is suprisingly quick, it rolls great. Yes it is also my trailer bike and daily ride..
-The Healing trail blazer is possibly the only one in existance, in 24" wheels is an oddity, old MTB frame on 24's, no V-Brakes (probably hadnt been invented then), I just removed all the gears and set it up as fix gear, very cool. The forks have too much trail but otherwise it is great. You will find the 24" wheels have a screw off cluster (freewheel not freehub) and that the thread is the same as BMX freewheels, bonus points as unlike the same trick for 700c fixy conversions, the 24" wheel does not need to be adjusted for chainline, no wheel mechanics pulling at the spokes.
-Last but not least is the Apollo, it is an MTB frame with doctored rear triangle. I cut the chainstays short, bent the seatstays in and added horizontal dropouts from a kids BMX. Running fix wheel on 26" wheels.
MTB frames are good for me as they have long toptubes and low stand over but they also have long chinstays which aree not so cool for road use. My preferance is for crit style bikes with quick steering.
Frame cutting/edit was done with an angle grindes and cutting disks, welding with flux core mig wire, finished with cutting disk and sanding pad on the angle grinder.
Interestng thing here is this bike is UCI legal, depending on tire sizes it will meet their roll out restrictions so it could be raced at the velo.
You can see from the photos the seatpost extention, all of these have a good stand over clerance, especially the Haro.
If you have a small wheel fetish like me, send your bike pic in..