Quote:
Originally Posted by fb39ca4
Has anyone tried using a shifting bicycle hub? The kind that bikes without derailleurs use?
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Being an amateur bike mechanic whose rebuilt a few Sturmey-Archer 3-speed hubs and knows way more about them than I probably should, I can speak on the question. (Long post ahead!)
Most hubs will not work for a drive train. This is because they are designed from the start to only be driven in one direction; the frewheeling ratchets are actually part of the internal shifting design. In essence, such hub has a single planetary stage and locks/unlocks the ratchets to make the planetary run 3 speeds (x:y, 1:1, y:x). Newer designs pushed the concept to 7 speeds (shimano nexus for example) and even 14 speeds (rohloff speedhub, well over the $400 limit @ ~$1000 per hub). However, most FRC robots need a bidirectional drive, so this isn't going to work out.
However, there are fixed gear planetary hubs... the modern Sturmey archer S3X comes to mind. What I don't know is whether such hubs maintain the selected gear ratio in reverse.
What I'd be most concerned with would be getting the input and outputs meshed into a FRC capable drivetrain... The non-fixed versions used to and still come in a format for tricycles where one of the hub flanges is replaced with a 2nd sprocket (to go to a sprocket on the rear axle); however, I'm pretty sure the S3X doesn't come in that format (you'd have to do a gut swap with another hub shell or do some other custom setup)
Shifting could also be tricky; at least the modern designs use an even pull setup (the cable distances between shifts is consistent). Older (pre-2000) sturmey archer hubs had a small gap between 1st and 2nd and a huge gap between 2nd and 3rd, with a neutral zone in between (get the cable tension too loose and you'll hit it in 2nd under load). In addition, shifting must be done with the input at rest (at least with the non-fixed hubs)
Last, these hubs are lubrication picky. They are usually lubed with oil (I use generic ATF)... and many hubs (older ones especially) could fling oil if overfilled or really old (sheet metal oil caps). Not exactly FRC legal. One could in theory run it on grease but my experience has been that these hubs don't like being run with grease.
My advice: offseason it if you really dare. It won't be an purely COTS solution, but to be honest, much more can be learnt from a non-COTS drivetrain when it comes to offseason projects.