Fabrication

 

Constructing a 135 to 110 BCD Adapter

7 Jan 2007

One of the problems of having a 20" drive wheel on a recumbent bike is that reduces your gear inches at the top end. To compensate for this you can use a combination of shorter cranks and larger chainrings. Sheldon Brown <www.sheldonbrown.com> has a gear inch calculator that you can use to compare different wheel sizes, crank lengths, chainrings, cassettes, and even internal hub gears. Using his calculator I decided that for my 20" drive wheel I needed a 60 tooth chainring and 150mm cranks for the 6 speed cluster I have on my Sachs hub which has 3 internal gears as well. So I waited patiently and eventually found both of these items on eBay -- the only problem was the chainring was for Campy cranks with a 135 BCD spacing while the cranks were 110 BCD. So I needed to make an adapter. Then while I was browsing through the list of other chainrings up for auction on eBay, I got the idea of using a solid BMX chainring to create a double and act as the adapter. This actually turned out to be cheaper than the 6061 aluminum plate I would have needed to make a straight adapter.

Here is the 44 tooth Tioga CD solid BMX chainring. Its CNC machined from 6061-T6 aluminum. Although you can't make them out in this photo, there are pencil markings on the chairing that I am about to center punch.

I started by drawing five lines between each pair of holes (BLACK), and marking the point at the center of each (BLUE). Then using a ruler, I drew a five radial lines straight out from the center of the chainring (MAGENTA). I did this by centering the ruler on the center of the hole on the opposite side. With these radial line, I overlaid the larger crank on the smaller one centering it on each line and clamping it lightly. Next I traced the holes on the larger chainring (RED), and then drew center lines across each perpendicular to the radial lines (GRAY). Finally I lightly center punched the center of each of the larger chainrings holes which lay at the point of intersection between these lines (MAGENTA & GRAY).

Now starting with a " bit at high RPM I drilled a pilot hole at each center.

Now using a step drill (panel punch) I successively stepped down until the desired diameter hole was reached.

Now using a set of Shimano Dura-Ace chainring bolts with spacers I attach the larger chainring.

Finally I used the remaining chainring bolts to attach the smaller chainring to the cranks. After mounting these on my frame, I noticed that the chain line was out, but this was easily fixed by mounting the small chainring on the reverse side of the crank spider.