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 (such as a DNP brand 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). I upgraded to cartridge 127 mm and it’s great. 118mm is common spindle length for triple cranks on hybrid and mtn bikes. 118 mm will work. 115 might work, as these arms are widely spaced for clearance of large freewheel gears on motor "shift kits" for motorized bicycles.
2. No modification of the cranks are necessary, but good dust caps, or self extracting crank bolts are nice.
3. Do not grease the aluminum threads on the crank or the steel freewheel threads. Clean both surfaces meticulously with 91% alcohol and dry thoroughly before installation. Beware and prevent cross threading. Do NOT use any washer, spacer, or bash ring/plate behind the front freewheel. It will likely strip the threads (I tried it before. One bike with no spacer went on 6 years of heavy use and counting. One with spacer stripped in a month. )
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.
Update: I found this inexpensive Shimano Altus dual pull bottom swing front mech to work well with this small front gearing. I installed the front mech with a rubber shim and gently bent it with a long wrench by leveraging inside the cage with no chain on, rotating the cage counter clockwise as seen from the drive side (photo of red bike included).
This makes the cage able to swing close as possible to the front freewheel, majorly improving shifts and lowering the cable travel needed to derail the chain over the gears.
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 a trigger shifter and it works well, please let me know what combination of shifter and front derailleur you're using, for my research. Thanks! I have had success with Gripshift SRAM twist shifters and rear mech on one bike, and Shimano Revoshift twisters and Shimano Tourney mech on a mountain bike tandem.
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. It prevents moving the chain when backpedaling to position the pedals for takeoff from a stop, averting chain suck or rear derailer jamming. If the rear freewheel freezes or clogs with mud the front may rotate on coasting to prevent jams. 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. If you want to do this, I have conversion instructions and photos on a new INSTRUCTABLE for the adventurous rider familiar with the old FFS /Positron 1980s systems. See “FRONT FREEWHEEL CONVERSION FOR MODERN SHIMANO BIKES. “
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.
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