An introduction to the ever-mystifying internal geared hub
by Keith Couture
You are biking on a cool, Autumny, but bright day; a perfect day for a ride. You take the route to your favorite flat stretch of road and, feeling inspired, you decide to really push yourself this time (well, more than you usually do anyway). Just as you reach a comfortable yet brisk pace, you're passed by another bicycler on a bike with only one gear! The rider's legs push the cranks in a slow revolution. You think to yourself With a gearing that high, they'll never make it up the hill just around the corner. You grin, satisfied that you'll get to fly past the gear-less bike on the hill and watch the rider struggle to push so big a chainring with no low gear. But as you approach the hill and start spinning, you notice the other rider has started spinning, too! In fact, you don't pass the other bicycler on the hill, and for the rest of the day you are left wondering. The premise of your wonderment is What looks like a single- speed, but isn't?
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| Not Actually A Single Speed |
The answer is a bicycle with an internal gear hub. On such bikes there is only a single chain-line pulled taut with no derailleurs, just like a single speed. However, all the shifting occurs inside the hub. A common reaction to this news is an eyeroll accompanied by grumbling about dumb trends in bicycling technology. “It'll be the new BioPace Chainring” or “Everyone will forget it ever existed in a few years, just like Shimano's Positron and FFS system.” But the truth is internal gear hubs have been used for around a century, and their invention is roughly contemporaneous with any of the current derailleur-based gear shift technology. They have proven themselves as effective and durable for many decades. So why aren't they more common today?
Bicycling in the U.S., specifically race biking and race bikes, became much more popular in the 1970's. Because of the high demand for race-worthy machines and lightweight technology, derailleurs became the order of the day, and internal gear hubs were relegated to bikes that certainly weren't flying off shelves.
Internal gear hubs are heavier mechanisms than derailleurs, particularly on a bike's rear wheel, so they aren't conducive for racing. They are also more expensive and they make it more time- consuming to remove a wheel (to replace a tube for instance). As race technology trickled down to the average consumer who, decidedly, was not racing, derailleurs began to phase out internal gear hubs on the majority of bikes. However, this has already begun to change in the U.S., where a renewed interest in bicycling not for sport or speed but for practicality and commuting has increased demand for durable, low-maintenance machines.
Internal gear hubs are precisely more sensible for urban, daily-use bikes because the gears are hidden from view. That the mechanism is contained within the hub shell is the reason for their durability and longevity. Whereas derailleurs, at the very minimum, probably must be adjusted once a year, internal gear hubs are more of a “set and forget it” system. A simple barrel adjuster can change the tension on the shift cable, a task that the rider can easily be taught how to do. Furthermore, when using a bike daily (and depending heavily on it) there are so many risk factors and variables that you simply can't always be protected from: locking a bike to a bike rack, moving it in an out of a garage, house, or apartment, having children tug on it and knocking it over, etc. Better to have a bike without a fragile (comparatively) appendage dangling that can be maimed in any number of ways. Even if you are able to account for and prevent damage to a couple of derailleurs, the chain, the cassette, and even chainrings may all have to be replaced over a few years (depending on how often and how far you ride). While this is also true of an internal gear hub, it is far cheaper to buy a replacement single speed chain, a single chainring, and a single speed cog!
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| This stuff gets pretty expensive to replace. |
Perhaps the most mystifying thing about the internal gear hubs (and a question I am asked frequently) is how they work. It is nearly impossible to describe this technology without a diagram. I will provide both a diagram and a summary of the inner workings. At its most basic, an internal gear hub is a hub, inside which there is a single driver axle with a small “sun gear” attached (why it's called a “sun gear” will make sense soon). Around the sun gear there are three or four “planetary gears” (see the blue gears in the diagram below?) that interact with the teeth of the sun gear. The planetary gears are kept equidistant from each other by being affixed to a planetary cage. Outside of the planetary gears there is a gear ring (in red) with teeth on the inside that interact with the teeth of the planetary gears. Now, imagine the sun gear is spinning clockwise. It spins, and the planetary gears spin counterclockwise to tango around it (but the planetary gear cage still rotates clockwise), and the gear ring spins around the planetary gears in the same direction as the sun gear, only slightly slower. This is the important thing to understand. Confused? That's okay. Here is a great video illustrating the mechanics at work: https://www.youtube.com/watch?v=-ue_usZqHYs
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| An epicyclic (planetary) gear diagram. Courtesy: wikipedia |
Included within this hub is a driver which is affixed to the cog on the outside of the hub, and a clutch, which can engage one of three points of contact at a time. The clutch can link the driver (driven by the cog, via the chain, via the chainring, via the pedals, etc.) to the gear ring while simultaneously the hub shell (driving the wheel via the spokes) is linked to and driven by the planetary cage. When this happens the bike is in first (or low) gear. Depending on a particular hub's design and the ratios therein, this means the cog spins 4 times for every 3 times the wheel spins. When in high gear, the clutch links the driver to the planetary cage while the gear ring is linked to the hub shell. Thus the wheel spins 4 times for every 3 turns of the cog. Lastly, the clutch can link the driver to the gear ring while linking the gear ring to the hub shell. In this position there is a 1:1 ration of cog spinning to hub spinning. Of course, a video always helps, and although you will have to sift through the cheesy promotional content, here is one from Shimano that shows its 11 speed Alfine internal gear hub:
http://www.youtube.com/watch?v=2k0T2HFQTbI
And another from Sturmey-Archer:
https://www.youtube.com/watch?
Couldn't have said it any better myself! Lastly, if you have an interest in internal gear hubs (or bicycle maintenance and mechanics in general) I cannot overstate the usefulness of the eternal Sheldon Brown:
http://sheldonbrown.com/internal-gears.html
Internal gear hubs are an inventive and smart technology. I think they almost represent something more powerful though, which is a change in the way we (in the U.S.) see bicycling. The greater numbers of internal hubs out on the road today come as harbingers of a new bicycling culture in the U.S. We are developing a culture that is based on bicycling for transportation and pragmatism, not solely for exercise and environmental preservation but still maintaining the benefits of the latter in pursuit of the former.
