MECHANICAL ADVANTAGE

The Speed Drive provides direct drive to a smaller chainring. The rear derailer The Cyclo conversion kits are no HYBRID GEARING with an overdrive of 65%, so your 50- would not require any control or cable; it longer available, but if you have an old tooth chainring offers the equivalent of an would just be set in a fixed lateral position three-speed and enjoy tinkering, you can Why choose between a derailer and internal when you can have both? 82.5. The Speed Drive is particularly use- by the use of the high- limit stop. actually do a two-speed derailer setup by Sheldon Brown ful for small-wheel bikes for this reason. It is possible to use a spring-loaded with available parts. The Schlumpf system is controlled chain tensioner intended for singlespeed Sturmey-Archer hubs have a couple by a button at each end of the bottom- use, but these have only a very limited of spacer washers next to the . If There are basically two types of gear-shifting sys- sprocket, you’d need a 5- tooth ring to get bracket spindle. You shift it by pushing take-up capacity and will only work if the you remove the spacer washers, you can the same top gear. the appropriate button inward with your chainring size difference is very small. put on two back to back. It is tems for . Derailer gears, as most modern heel. The button on one side selects high also not difficult to modify multi-speed bikes use, are the type that move the Available Systems gear; the button on the other side selects Historical Systems Cassette sprockets to fit on a Sturmey- SRAM (formerly Sachs) offered a “3 low gear. This system is compatible with Hybrid systems based on Sturmey- Archer hub—it’s just a matter of grinding chain back and forth among different sprockets, and internal x 7” model for several years, and there are all sorts of rear-drive systems. Archer hubs had a modest popularity six of the nine internal splines and round- gears, which are built into either the rear hub or bottom a lot of them on the road. These were back in the ‘50s and ‘60s. Cyclo used to ing off the corners of the remaining ones. bracket. I addressed this type of gearing in my last three-speed internal hubs with a driver Front Derailer Systems. make adaptor clusters that would allow For more information go to: that would accept a Shimano type seven- You can use a front derailer and mul- the use of two or three sprockets on such sheldonbrown.com/gears and “Mechanical Advantage” article. speed cassette. The internal gear hub gave tiple chainwheels with any internal gear a hub. I had one of these on my Elswick sheldonbrown.org/otb.html. But what about combining a high gear 136%, direct-drive middle system, but to do so you also need to Tour Anglais. The four-speed Sturmey- both of these systems on the same gear, and a 73% low gear. install a rear derailer. This is because the Archer hub with three sprockets gave me bike? This is often possible, and The current offering from SRAM is a rear derailer’s spring-loaded cage is needed a 12-speed bike in 1961, a time when 10- Sheldon Brown addresses many issues this type of setup is commonly completely redesigned 3 x 8 or 3 x 9 hub. to take up the chain slack when you shift speed bikes were the latest exotic thing. at www.sheldonbrown.com/harris called “hybrid gearing.” Although the internal gear ratios are the Hybrid gearing is not for same as those of the 3 x 7, the internal everybody, but it is beneficial for mechanism has been totally redesigned to riders of less than normal strength, facilitate underload shifting. The 3 x 8 or with heavier than normal loads and 3 x 9 systems use the same hub but to transport. It also permits limited with different cassettes and twist-grip shifting while the bike is stopped, shifters. They will work with any which is particularly useful for com- Shimano-type cassette. muting in stop-and-go traffic. Unfortunately, SRAM has not cho- Probably the most important sen to offer any choice of shifters; they G SIPLE modern application for hybrid gear- only make a combined twist-grip shift GRE ing is for bikes (or trikes) with A Shimano Nexus seven-speed manual hub. unit to fit on straight 7/8” (22.2 mm) smaller drive wheels, such as fold- handlebars. The derailer part of the sys- ing bikes and many recumbents. For example, let’s consider a tem may be shifted with any rear-derailer control with the proper bike with 44-406 (20 x 1.75”) tires. The smallest readily avail- number of speeds, as long as a matching rear derailer is used. able cassette sprocket has 11 teeth. The largest standard chain- Shifting the internal hub with anything but the SRAM dedi- ring with a triple has 52 teeth. Assuming 170 mm cated is somewhat problematic. I have seen setups using cranks, that combination gives a gain ratio* of 6.6. Shimano STI shifters with the left shifter controlling the hub This ratio is the same as 52/16 with 32-622 (700 x 32c) gear, but the cable travel is not quite right, and an overload tires. Many cyclists would spin out in such a low gear, but if you spring had to be included in the cable run. combine the derailer with an internal hub gear, for instance a Shimano is due to release a similar system this fall, but SRAM 3 x 9, that same combination gives you a gain ratio of details are not yet available. Although it is scheduled for rollout 9.0, which is a bit higher than a 53/12 with the larger wheels! at the Interbike show in October, I wouldn’t expect actual units Nobody in the world needs a higher gear than that. to be available for after-market retail sale before next spring. To get a gear that high with a straight derailer system, Switzerland’s Schlumpf Mountain Drive and Speed Drive you’d need to replace the 52-tooth chainring with a 71-tooth. are two-speed bottom-bracket systems. The Mountain Drive Chainrings this large are very scarce, and even if you find one, provides direct drive with a reduction drive of 40%. If you have a you will not find any front derailer that will be happy shifting it 50-tooth chainring with a Mountain Drive, the reduction gear over any useful range. If you managed to come up with a 9-tooth gives the equivalent of a 20-tooth “granny” chainring.

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