Monday, April 23, 2007

Sachs Orbit Hub


If you've ever serviced a bicycle, you might be looking at this picture and wondering what these parts are. Totally uncommon and actually now obsolete, these are the parts of a Sachs Orbit 2x7 Internally geared hub. The 7 speed cassette is non-standard. it has 2 sets of freewheeling pawls: one which engages the axle, and one which engages a planetary gear reducer. The planetary gear reducer allows a 0.7 gear reduction compared to directly driving the axle. Selection between the two drive systems is done by a spring-loaded latch which causes contact either between an internal coupling or external coupling of teeth. I guess describing any more is pointless without diagrams. But basically, it's an insane idea, which actually works. Here I have taken it fully apart to lubricate everything. I took the hub to a local bike shop at first, because I had no idea what would happen if I opened it up (imagine little gears flying everywhere), but they returned it to me improperly assembled and lubed with grease. So I tackled the job myself this time, and carefully figured out the assembly, while re-lubricating with teflon oil. Grease actually made everything sticky, so the oil seems to be the trick.

Here's the little planet gears mounted to their carrier. The annular part of the gear is actually built in to the freehub body. In the future, I plan to write up a detailed tear down and reassembly of this hub to post on this website for other people out there to find, because I was unable to find anything on the net to help me with this. But for the sake of brevity and time, that will have to be at a later date.

You might ask why is this special hub necessary? The original design of the bike did not use a front derailleur, because I was a little naive about how to design a tandem using multiple chains and connecting them all together. So in the drive configuration i came up with, there was no room for a front derailleur, and hence lack of a lower gear range. In 1991, this product was introduced by Sachs (one of our coorporate sponsors) which allowed for a gear reduction without the need for a front derailleur. So I put 2+2 together and went with the Orbit. Probably not wise to rely on such a special component to drive a whole bike design, because at this point, finding spare parts for this obsolete item is near impossible.
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