1 Test ride of basic operation at low ground speed.
Video 2, rear shifting with bike in a stand
2 Test ride showing system failure caused by letting the front derailer overshift and throw the chain. The chain rotated, scratching the crank arm very slightly, and caused the foam clutch to slip as intended in case of emergency. No parts were damaged except the light scratching.
3. RJ video demonstration antique Shimano Ffs in good operation
4. RJ video demonstrating chain jam (clothing) on antique FFS system.
This covers my experiments in converting a relatively modern bicycle to a newer , simpler version of Shimano’s obsolete 1970s bicycle component system known variously as FFS, front freewheel system, or Positron. If you are not completely familiar with the 1970s-1980s FFS operation, they used an Ashtabula type crank which featured a double chainwheel mounted to a freewheel mechanism, which threaded onto the one piece crank similarly to conventional cranks.
The rear freewheel was a special unit which had five individually ratcheting cogs and was very stiff. This allowed the rear freewheel to be basically locked unless a jam occurred somehow. It pulled the chain over the rear freewheel, which allowed the rider to shift at will, pedaling or not. It was handy for novice riders learning to shift, and an entertaining luxury for experienced riders.
The point is to have a bicycle with a fixed or normally non ratcheting multispeed rear hub, and the ability to shift front and rear derailers simultaneously, at will, while coasting (feet still), or even while walking and pushing the bike (helps on hills and while loaded with cargo).
My design uses the trials-bike or motorized-bike style threaded freewheel crank demonstrated in my previous Instructable (link), How to get super-low gears on a bicycle.
This design adds to that one by using soft foam spacers (“donuts”) and a steel “pie plate” (spoke protector/dork disc ) to pin the rear freewheel to the hub wheel so it drives the chain any time the wheel turns.
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Step 1: Front Freewheel Retro Bicycle Modification
Step 1 is to follow the previous instructable, “How to get super low gears on a bicycle.”
If you do not want extra low gears, you can use a front freewheel crank, matching freewheel spider, and one or two matching chainwheels of your choice, such as 20/32, 24/40, 36/48, or 42/52. Sick Bike Parts and other electric bike and motorized bike companies sell these parts and have install info.
Some front spiders use conventional 5 arm bicycle chainrings and chainring bolts in an industry standard (110mm bcd)mounting pattern. Others use chainwheels made for e bikes and require small machine bolts and hardware sold from those venders for a few dollars.
Once you have installed your front freewheel crank and chainrings, size your chain properly for your application and bench test and ride test the bike.
Assure your chain does not overshift in any gear and limit screws are properly set in case of shifter maladjustment.
Step 2: Bike, Parts and Tools
You need a common bicycle with 26”/559 or 27”/700c wheels, a JIS Japanese/Shimano style square taper bottom bracket axle, and a threaded rear freewheel hub. You need a multispeed rear freewheel (5,6,7,8 speed) and matching shifters and derailers. A freewheel with a complete lowest cog works best, versus the kind which uses a carrier or spider. You need the correct freewheel tool for your selected and existing freewheels.
You need a 1/4” thick or 1/2” thick soft urethane foam pad, such as a foam knee pad for gardening, and an exact-o type hobby knife or model car razor.
You need a threaded crankset removal tool and possibly bottom bracket tools such as a splined cartridge bb installer bit.
You need Allen keys and maybe a small Phillips screwdriver for derailer adjustments , and blue loctite thread lock, to set your shifting adjustments carefully.
If any of these operations are new or unfamiliar to you , hire help or learn these methods. RJtheBikeGuy on youtube is a great help.
Step 3: Danger Warnings.
All bicycles are inherently dangerous. You are on your own judgement and risk to use or emulate any of this technology.
A front freewheeling bicycle requires critical adjustments of derailers and shifters to prevent chain jams or drops, which could damage the bike or cause a wreck.
This design uses friction of a foam spacer to hold the freewheel. It is expected to give way in the event of a chain jam or other mechanical failure.
Keep pants, socks, shoes, laces, hair, fingers, and any items and debris OUT of the drivetrain and wheels.
Step 4: Fixing the Rear Cog Set.
With the rear wheel off, remove the freewheel and save it. If you are re-using it, make sure it’s not dirty or gritty. Oil it’s bearings very lightly through the lock ring gap and wipe any excess.
Use the metal pie plate to trace onto the foam pad where you need to cut. The center hole is most important. It should slip over the hub’s threaded portion easily. The piece should be at least the outer size of the freewheel’s largest cog.
Install the pie plate on the bike wheel. Install the foam part.
Press down the freewheel over the foam part to compress the foam and thread on the freewheel. DO NOT CROSS THREAD it (permanent damage). If your foam cushion is too hard to compress, you may need a thinner one. I cut my 1/2” thick piece in half like a sliced bagel and it worked. Some types of bikes may need a thicker or thinner foam piece. You might have to try a couple times.
It should hold the freewheel firmly against turning backward (ratcheting), but should not balloon through the spokes or prevent complete installation of the freewheel.
After installation is complete, trim the excess foam around the teeth of the largest cog. The chain, or a chain whip, can help form the last bit out of the way of normal chain operation.