Cool tech: SRAM Automatix 2-speed rear hub

#bikes #singlespeed

I had the pleasure of taking apart and servicing this cool piece of cycling history for one of my builds recently, so I decided to take a couple of pictures and write a few words about it.

The SRAM Automatix was1 a truly automatic internally-geared hub. Unlike a regular kickback hub, where you have to pedal backwards to engage the gears, the Automatix shifts automatically based on your speed. Here’s the gist of how it works: a couple of pawls inside the hub are being pushed inwards (towards the axle) by a specially-calibrated spring. At the same time, as the wheel spins, they are being pushed outwards (towards the rim) by the centrifugal force. When the wheel reaches a certain speed, the centrifugal force pushes the pawls far out enough that they engage with splines on the outside wall of the hub, thereby shifting into second gear. When you slow down and the rotational speed drops below the pre-set level, the spring pulls back the pawls until they disengage, thus shifting back into the first gear.

I went into this project fully expecting this hub to be a clunky mess. I thought for sure that the parts would grind together when the pawls came into contact with the splines, and I expected shifting to be totally random in regards to the speed it happens at. After all, how precisely calibrated can a tiny spring be? And how well would it be able to maintain its calibration over time?

Well, I am pleased to report that my expectations were blown away. The hub shifts up and down smoothly and consistently, in no way worse than your average derailleur. This was by no means a new bike–the pictures below are taken after I had thoroughly cleaned the parts–but you wouldn’t be able to tell that from how it feels to ride. My only gripe would be that it seems to shift a bit earlier than I’d like it to–or maybe I just have weak legs!–but that’s okay, because the shift point can be adjusted by carefully bending the tension spring slightly. Making the spring stronger or weaker respectively increases or decreases the speed at which the hub shifts, so that even someone with noodle legs like mine can enjoy a nice ride out on a pretty bike.

  1. I say “was” because, unfortunately, since 2017 SRAM has stopped production and sale of all internally-geared hubs. I don’t know of any other hub that works just like the Automatix, and that’s a real shame. ↩︎

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