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Planetary Gearing


Only a few short years elapsed between the time when bicycles started to standardize as "safeties," and when planetary geared hubs, also called planetary gearsets, started to appear.

Safeties were bikes with same-size wheels and a chain drive from centrally located pedals with a large sprocket to the rear wheel with a smaller sprocket - in other words, the typical modern bicycle design. Before safeties, bikes had pedals attached directly to a wheel. The driven wheel had to be as large as possible. That way, each turn of the pedals would carry the bike a reasonable distance, so as to avoid being a slow-poke.

With a planetary gearset, the hub can turn different speeds than the sprocket mounted on the hub. Planetary gearsets are also known as "epicyclic gearing" or "gear hubs." The first ones showed up around 1880, but the first commercially successful two-speed planetary hub, called "The Hub," was in production starting in 1898. By 1902, a three-speed hub was available.

Now, more than a century later, planetary gearsets are still available, with versions containing as many as 14 distinct speeds.

The Rholoff 14-speed planetary gearing bicycle hub
The Rholoff Speedhub - 14 Speeds
via: http://commons.wikimedia.org/wiki/User:Marcela c

The planetary gearset has some advantages. The system is enclosed, so is less vulnerable to weather and dirt. There's no low-hanging chain tensioner to get bent or caught in the weeds. The chainline is simple, so easier to enclose, preventing grease marks on socks and pants. In fact, belt and fully-enclosed shaft drives work with planetary gearsets. Because a derailleur system can only be shifted when the chain is moving, the planetary gearset is easier for beginners, and commuters, who often have to shift after having come to a stop. Derailleur bikes almost always have dished rear wheels. This means that the hub flange on the right-hand side is offset toward the center of the hub in order to make room for the sprockets. A dished wheel is weaker against lateral forces. Planetary hubs can have widely spaced flanges for strong rear wheels.

The downside of planetary gearsets is that they tend to concentrate a lot of weight in the rear wheel, and may cost more.

One of the most prolific manufacturers is Sturmey-Archer, who made a hub that remained mostly unchanged for many years, and was the centerpiece of the classic European three-speed bike, which was very popular in America during the 1950s and 1960s.

Raleigh bicycle with Sturmey-Archer 3-speed hub
Classic Raleigh Three-Speed with Sturmey-Archer Hub
via Degen_Earthfast

When you come across an old three-speed, you can wipe the road grime away, and read the month and year of manufacture on the outside of a Sturmey-Archer hub.

Sturmey Archer 3-Speed Bicycle Hub

Sturmey-Archer Three-speed shifter
Sturmey-Archer three-speed shifter
via huubvanhughten


Inside a Sturmey-Archer AW three-speed hub
Photo by Keithonearth

Shimano made competing three-speed hubs, typically the FA model. It was low-cost, but unfortunately not as strong as Sturmey-Archer's most common model, the AW. A strong bike racer could wreck an FA hub within a couple of miles.


Shimano three-speed hub with coaster brake
Photo by unknown photographer, via Wikimedia Commons

The Sturmey-Archer AW, in production from 1936 until at least 2008, and possibly even today, has an alarming problem of its own: There's a neutral position between second and third gear. If the cable tension is not properly adjusted, the hub can slip into neutral. This can be a serious let-down when climbing hard up a hill.

Modern planetary gearsets are still available with three and more speeds. Shimano makes a popular 7-speed model and offers 8 and 11-speed models as well. Sachs, now part of SRAM, has been making planetary hubs since 1904, and currently offers a 7-speed model that has a wider range than Shimano's 7-speed.

Believe it or not, there can be room left over in planetary hubs, so manufacturers make models with built-in coaster brakes. For those who don't know the term, a coaster brake is a hub in which when you pedal backward, typically a quarter-turn of the pedals or so, a braking mechanism is applied in the hub to stop the wheel.

For many years, Shimano, Sturmey-Archer, and Sachs have all incorporated coaster-brakes. You would expect trouble due to the heat generated in the hub by breaking, but in practice, this seems to be a non-problem, since there are many decades-old planetary hubs with coaster brakes that still work like new. Interestingly, the trouble-maker is Shimano's external roller brake, that can overheat on a long hill, making screeching noises, and fading in effectiveness. Some of the most recent high-end planetary hubs accommodate a disk brake around the left side.

During the 1960s, Bendix made a two-speed hub that incorporated a coaster brake and automatic shifting, sort of. If the rider backpedaled a little bit, the hub would shift into the other gear. So if it was in high, then it would be in low, and vice versa. They had three models of the "Kickback" hub, identified by colored bands around the middle of the hubs.

The 'yellow band' and 'blue band' hubs had a shoe style internal brake. When backpedaled, a course screw would push a cone against four grease-covered quarter-circle steel shoes that rode loosely along the inside of the hub shell. The cone would press them against the shell, and the bike would stop.

The yellow band had a straight-through speed, and a lower gear for hill climbing. The blue band was identical except it had a straight-through gear and a higher gear.

The 'red band,' Bendix's most common two-speed kickback model, was straight-through and low, but had a different kind of brake. This had a pack of alternating disks coated in grease. Every other disk was splined to the hub and turned. The disks in between were splined to the axle, so of course they did not turn. When the brake was applied, the pack of disks was squeezed together. Several other coaster brake manufacturers used the disk pack system, but not Bendix. Of the millions of coaster brakes made by Bendix, this was one of the few with a disk pack.

You may wonder how a planetary gearset works. I did. When I was twelve years old, I took apart my Shimano 3-speed hub to see what was inside, and couldn't quite get it back together. Fortunately, my father came to the rescue, and showed me how a couple of parts might work, and with that hint, I was able to understand it, and get it back together. 10 years later, I owned a bicycle shop, and had overhauled hundreds of 2-, 3- and 5-speed hubs. 40 years later, I took apart a Shimano 7-speed, and had a heck of a time reassembling it properly.

bicycle rear hub planetary gears
Modified, originally via PeterWiki, http://commons.wikimedia.org/wiki/File:Planetengetriebe_Prinzip.jpg

So, here's the scoop: You have a ring gear, three or more planet gears, and a sun gear all in constant contact. Through some sliding mechanisms and ratchets, you can connect the sprocket and the hub shell to various parts of the gearset. The sun gear is mounted on the axle, and does not turn, so the planets turn around it, and the ring gears is turned by the planets. The planets are kept in position by a large ring called the planet carrier. If you connect the sprocket to the planet carrier, the planets are pushed around, and because they have to rotate around the sun gear, the outer surfaces of the planets are rotating faster. The ring gear is attached to the hub, and so you have a high gear. If you connect the sprocket to the planet carrier, but also connect the hub to the planet carrier, you have direct one-to-one gearing, and the ring gear spins uselessly. Finally, if you connect the sprocket to the ring gear, and the hub to the planet carrier, you have low gear.

That accounts for a three-speed hub. For more than three speeds, you have multiple or "compound" planetary gearsets. If you have one gearset in a low gear, and another gearset also in a low gear, you have a lower "low-low" output.

A few people and manufacturers have experimented with combining derailleur systems and planetary gearsets. Your author put together a bike with a triple chainwheel, six rear sprockets, and a Sturmey-Archer 5-speed hub, for a combined total of 90 speeds. In the lowest gear, a rider could merely set the weight of a foot on a pedal, and the front wheel would jump a few millimeters off the ground.

Theoretically, with today's equipment, one could have a triple chainwheel driving ten rear sprockets, connected to a 14-speed planetary hub. This would result in a 420-speed bike, although of course some of the 'gears' would be redundant.


Sturmey-Archer three-speed hub with incorporated coaster brake.
Photo by Markus Schweiss



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