Introduction: How to Get Super-low Gears on a Bicycle
This instructable is for people who want extremely low gear range on their bicycle, for climbing extensively, hauling cargo, or pulling trailers such as kid-carriages. You can see various gear combos by using Sheldon Brown's Gear Calculator. Mine is geared to go about 2mph (requires some balance skills) to 18mph (top gear and spinning). In the 16tooth front gear, I can go about 10mph tops with a 14t rear cog. If you use a 16 tooth single BMX freewheel, and an 11-34t freewheel, you should have satisfactory gear range for heavy hauling, casual street riding, or most beginner/ recreational off-road riding (at 80 rpm, 9mph in high, and 3mph in lowest gear).
This instructable assumes you have a bicycle with a bottom bracket shell that uses Japanese style (shimano compatible) square-taper spindle bottom bracket cartridges or axles and bearing cups. If your bike currently uses a triple crankset, and has a square-taper spindle, you can do this conversion simply without buying additional parts except the crank set shown, freewheels shown, and possibly a replacement cable.
My bike (shown in this demonstration) used a 28/38/48 crankset by Shimano, with a matching front derailleur made by Suntour. The bike is widely available from Walmart. It came with an 11-26 7 speed freewheel, which would work with this, but I use a replacement 14-34 Megarange freewheel for the lowest of low gears possible. (Yes, SRAM makes a 36 tooth cassette system, but this page doesn't cover that because it's rare and expensive.)
1. Remove the crank bolts and crankset with a proper crank-removal tool. They are available at www.bikenashbar.com or www.niagaracycle.com and make removal very easy. One part threads into the crank arm's threads after the fixing bolt is removed (14mm or 15mm hex bolt), and another threads into the remover part and is turned with the wrench handle, pulling the crankarm off the spindle. It is best to remove the chain from the bike before doing this, but if you can't, leave it on the small ring, and put it on the small cog/ring when installing the new crank. For information on how to do this, if you don't know, use the tool's instructions, or visit Park Tools website for tutorials, or Sheldon Brown the bicycle Encyclopaedia (www.sheldonbrown.com) Make sure the crank spindles and crankarm holes are clean and grease free when installing cranks. When the cranks are off is a good time to do preventative maintenance, such as cleaning the bike and re-adjusting or repacking the bottom bracket (or replacing worn cartridge bb). 118mm is common spindle length for triple cranks on hybrid and mtn bikes. 115 will work. 113 might work, as these arms are widely spaced for clearance of large freewheel gears on motor "shift kits" for motorized bicycles.
2. File 1 to 2 mm of material off the right crank arm as shown in the illustration, to prevent the chain pins from brushing the crank arm. This is not necessary if you use a couple freewheel spacers (available from bike shops, or Nashbar or Niagara Cycle), or if you don't mind a little noise for a few rides while the chain wears on the soft crank arm material until it no longer grinds in the "big" 22t gear and "high" cogs in back.
3. Grease the threads on the new crank where the freewheel will be installed as a substitute for the usual chainrings. This will help prevent cross-threading, and will make removal much easier if you ever need to replace the freewheel.
4. Install the front derailleur as low as possible without hitting the frame as it swings through its range of travel. You may have to experiment a few times with the mounting angle (rotating on seatpost by a couple degrees), or even pinch the derailleur bracket with channel locks or vise-grip pliers, to get the most satisfactory shifting. The cogs are spaced more closely than a standard triple, and the derailleur must use the lower part of the derailleur bracket side plates to accomplish the shift, as the teeth are so far away compared to standard rings. A cheap road bike double front derailleur might work better. Mind the size of your seat tube, and shims as may be needed, when shopping for or selecting front derailleurs.
5. It will likely be necessary to shorten the chain. I recommend installing the Megarange freewheel (or cassette if your bike has a cassette hub), and measuring the chain for the big-big combo and small-small combo to determine chain length. (use a proper chain-breaker. They are available widely and cheaply, under $15, especially for older style multi-speed chains such as "7 speed" bikes. You can buy freewheel remover tools, freewheels such as the Megarange, shifters, economy chains, bottom bracket sets, and more tools at Niagara (my favorite shop).
6. Don't forget, the left hand pedal has reverse threads. Most pedals can be removed with a flat 15mm pedal wrench.
I recommend a combo wrench that has a 1 ft. long handle, 15mm flat wrench, and 15&14mm sockets for axle nuts, crank bolts, etc. I got one for $15. I keep it in the bike toolbag for removing wheels, and in case a crank were ever to come loose.
7. Trigger style shifters are not likely to work ideally with the front freewheel, but Grip Shift and friction shifters work fine. If you use one and it works well, please let me know what combination of shifter and front derailleur you're using, for my research. Thanks!
8. If you want to manually change front gears (while stopped!!!) you can omit the front derailleur and front shifter for simplicity. This works well. The front freewheel will easily let you push the chain onto the desired gear with 1 finger. Push on the right side of the chain behind and below the freewheel to gently derail the chain to the next lower gear, and from the left side below the freewheel to shift up, or pinch the chain and pull it onto the desired gear. USE CAUTION not to pinch your fingers, and you will need something to wipe your finger on to remove the grease.
Be sure to test all systems at your home or shop before leaving for a ride or carrying heavy cargo or pannier bags.
The front freewheel has the ability to rotate, but rarely will if the rear freewheel is operating properly and is well lubricated. The product is used more for the gear selection than freewheeling ability, and I DO NOT recommend doing anything to add resistance to the rear freewheel so that the bike operates as a "front freewheeling" bike. A misshift or a stick in the spokes could destroy the drivetrain, especially with such low gears.
The 3 speed freewheels are common and about $10 in USA. You can get one here for $10 or 11, and free shipping. http://www.sourcingmap.com/bicycle-scooter-sprocket-wheel-speed-freewheel-repair-part-p-268417.html I got one from Ebay for $13 and free shipping. Stores or online shops that sell Electric Bicycle parts might have them.