en.m.wikipedia.org/wiki/Bicycle_gearing Internal (hub) Main article: Hub gear Internal gearing is so called because all the gears involved are hidden within a wheel hub. Hub gears work using internal planetary, or epicyclic, gearing which alters the speed of the hub casing and wheel relative to the speed of the drive sprocket. They have just a single chainring and a single rear sprocket, almost always with a straight chain path between the two. Hub gears are available with between 2 and 14 speeds; weight and price tend to increase with the number of gears. All the advertised speeds are available as distinct gear ratios controlled by a single shifter (except for some early 5-speed models which used two shifters). Hub gearing is often used for bicycles intended for city-riding and commuting. Internal (bottom bracket) Current systems have gears incorporated in the crankset or bottom bracket. Patents for such systems appeared as early as 1890.[11] The Schlumpf Mountain Drive and Speed Drive have been available since 2001 [12] Some systems offer direct drive plus one of three variants (reduction 1:2.5, increase 1.65:1, and increase 2.5:1). Changing gears is accomplished by using your foot to tap a button protruding on each side of the bottom bracket spindle. The effect is that of having a bicycle with twin chainrings with a massive difference in sizes. Pinion GmbH introduced in 2010 an 18 speed gearbox model, offering an evenly spaced 636% range. This gearbox is actuated by traditional twist shifter and uses two cables for gear change. Pinion system is well suited for mountain bicycles due to its wide range and low gravity center suitable for full suspension bikes, but it is still somewhat heavier than derailleur based drivetrain.[13] SRAM Dual Drive combination derailleur gears and hub gear Internal and external combined It is sometimes possible to combine a hub gear with deraileur gears. There are several commercially available possibilities: One standard option for the Brompton folding bicycle is to use a 3-speed hub gear (roughly a 30% difference between gear ratios) in combination with a 2-speed deraileur gear (roughly a 15% difference) to give 6 distinct gear ratios; this is an example of half-step gearing. Some Brompton suppliers offer a 2-speed chainring Mountain Drive as well, which results in 12 distinct gear ratios with a range exceeding 5:1; in this case, the change from 6th to 7th gear involves changing all three sets of gears simultaneously. The SRAM DualDrive system uses a standard 8 or 9-speed cogset mounted on a three-speed internally geared hub, offering a similar gear range to a bicycle with a cogset and triple chainrings. Less common is the use of a double or triple chainring in conjunction with an internally geared hub, extending the gear range without having to fit multiple sprockets to the hub. However, this does require a chain tensioner of some sort, negating some of the advantages of hub gears. At an extreme opposite from a single speed bicycle, hub gears can be combined with both front and rear derailleurs, giving a very wide-ranging drivetrain at the expense of weight and complexity of operation- there are a total of three sets of gears. This approach may be suitable for recumbent trikes, where very low gears can be used without balance issues, and the aerodynamic position allows higher gears than normal. Others There have been, and still are, some quite different methods of selecting a different gear ratio: Retro-direct drivetrains used on some early 20th century bicycles have been resurrected by bicycle hobbyists. These have two possible gear ratios but no gear lever; the operator simply pedals forward for one gear and backward for the other. The chain path is quite complicated, since it effectively has to do a figure of eight as well as follow the normal chain path. Flip-flop hubs have a double-sided rear wheel with a (different sized) sprocket on each side. To change gear: stop, remove the rear wheel, flip it over, replace the wheel, adjust chain tension, resume cycling. Current double sided wheels typically have a fixed sprocket on one side and a freewheel sprocket on the other. Prior to 1937 this was the only permitted form of gear changing on the Tour de France.[14] Competitors could have 2 sprockets on each side of the rear wheel, but still had to stop to manually move the chain from one sprocket to the other and adjust the position of the rear wheel so as to maintain the correct chain tension. Continuously variable transmissions are a relatively new development in bicycles (though not a new idea). Mechanisms like the NuVinci gearing system use balls connected to two disks by static friction - changing the point of contact changes the gear ratio. Automatic transmissions have been demonstrated and marketed for both derailleur and hub gear mechanisms, often accompanied by a warning to disengage auto-shifting if standing on the pedals. These have met with limited market success.
Posted on: Sat, 24 Jan 2015 02:10:35 +0000
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