How do I use a freewheel as a single-speed?

There are multiple options.

1. Just use one cog and don't change gear. Cheap and easily reversible.
2. Remove and store your rear derailleur/shifter/wires, and then trim your chain to length so it only suits one cog. This may require a chain tensioner and risks dropping the chain off, if the chain line isn't good.
3. You can get single-speed kits that fit on a freehub, essentially a cassette with one cog and spacers for either side. Since the horizontal placement is adjustable, you should be able to get a better chainline. If you only have a freewheel, this is not an option.
4. The expensive solution is to get a single-speed wheel, or rebuild your existing rim onto a single-speed hub. This may give you a flip-flop setup where each side of the bike has a different gear, or the choice between a fixed-gear and a single speed.

It really depends if your frame has track ends (probably not if it has a derailleur) or some other way to set the chain tension.

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Remember you still absolutely need working brakes, front and rear. Don't remove them in your quest.

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Personally I just ride with my gears - the 1935/6 Tour de France riders petitioned long and hard to be able to use gears!

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