The aim is to convert an old women's bike into a two-speed automatic. Also new wheels and some other bits and bobs that pop into head.
Old bikes have large chainwheels, the starting point for this project has one with 46 teeth. The low gear in the SRAM Automatix hub is a direct drive (100%) and the high gear is around 136%. For a woman's town bike we want to have the gear ratio of the chain wheel and the rear cog well below 2:1 setting the low gear light and nice. With the old 46 chainwheel the rear cog would have to be larger than 23, which is not practical.
Another factor having to be taken into account is the shifting point of the high gear in the Automatix hub. By default, It is set low - at around 12km/h or so, and we can either build the gear ratio around that - which would mean overall low gears - or we can raise the shifting point higher by tweaking the hub and tensioning the spring controling the bobweighs.
But, the first task is to find a smaller chainwheel for the Ashtabula cranks. They might be readily available but we will roll our own so that we have the flexibility to test different gear combinations. At first, we'll experiment with a 32-tooth chainwheel and a cog somewhere around 18T.
The first version looks like this. The hole diameters are 24mm for the crank spindle and 11mm for the crank pin and the distance between centre points is 41mm. The ring is machined from reasonably strong aluminium, (most probably 6061, not sure) and should be fine for a good number of miles/kms. A thing to note is that the chainguard screw behind the crank is actually omitted as there is no room for the screw head when the guard is in place. The chainguard actually looks rather plain once fitted and the bike in general is not exacly a looker - so it's just a practical bike:).
The next task is building up the rear wheel. The rim is Mavic A119 and the hub the SRAM Automatix with the kick-back brake. The spoke length calculator gives the length for both sides at around 287mm for a 3-cross pattern with 36 holes.
So how does it ride? Out of the box, the shifting point is actually a good compromise. A gear ratio around 2:1 for a 28" wheel is about right. Our's is a bit lower but is good enough for the moment.
Lovely just what I was looking for. Thanks to the author for taking his clock time on this one.
Really enjoyed this forum post. Vanderschoot