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I've been biking on single-speed for a few months and would like to play around with a fixed gear. I've heard of a flip-flop hub, but I'm not sure exactly what it is. And without any fellow bikers - I'm completely lost.

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3 Answers 3

Paul High Flange FlipFlop Hub

Benzo and Glenn Gervais are right on, but I thought I'd include a photo for any visual learners. This is a typical fixed/free, high flange rear hub. Quite often they're available in 120mm and 130mm OLD to fit different width dropouts. These hubs generally have solid axles without quick releases to prevent the hub from slipping and slackening your chain.

The right side is for a fixed cog. (The hub in the picture has a lockring installed already). The left side is for a single speed freewheel.

Below is another example (a White Industries ENO Eccentric) with two big differences. First, instead of two opposite-threaded areas for a cog and lockring, this hub has splines for a splined cog and threads for the lockring (left side of the picture). The second difference is the eccentric axle which allows for chain tension adjustment on bicycles with vertical dropouts (single speed or track specific bikes have horizontal or rear-facing dropouts for chain tension adjustments).

White Industries Eno Eccentric

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If you have a singlespeed, you may already have a flip flop hub. A flip flop hub has threads on both sides of the wheel. Typically there is a side for a freewheel and a side for a fixed gear. Take a look at the rear hub opposite the freewheel and chain.

The fixed gear side will have two tiers of threads, one larger diameter section for a fixed cog and a smaller diameter section for a lock ring which is reverse threaded. You need to thread on the fixed cog clockwise, then thread on the lockring counterclockwise.

You do not want to attempt to put a fixed cog on the same side as the freewheel if it does not have the reverse threading for a lock ring. This is known as a suicide fixed cog because you can't put a standard fixed cog lockring on and would have to use a lockring from a bottom bracket typically to prevent the fixed cog from loosening when backpedaling or skidding. Over time this can come loose and cause you to loose the ability to brake with the back wheel as the cog will unthread as you backpedal.

You can however thread a freewheel on to the fixed side of a hub with no real issues. This would thread on clockwise like the fixed cog. No lockring would be used. This can be common when using fixed / fixed hubs which are threaded for a fixed cog and lock ring on both sides.

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As for what cog to put on, you could just go with the same size as your freewheel. However, you'll want to consider using a gear ratio that leaves you with more 'skid patches' so that you wear the tire more evenly. Some combinations of chainring / fixed cog will wear your tire in 2 to 4 spots only when skidding, causing your tires to wear very quickly. –  Benzo Sep 11 '12 at 19:02
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bikecalc.com is a good place to go to calculate skid patches, gear inches, and gear ratios. –  Benzo Sep 11 '12 at 19:02
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It's also worth noting that you may want a slightly higher gear ratio on your fixie than your single speed so that you're not so spun out on the downhills. Of course, the trade off is working harder on the uphills. –  jimirings Feb 18 '13 at 16:20

Flip-Flop hubs have freewheel thread on one side, fixed-gear threading on the other side. Many SS bicycles come with this sort of wheel, if you have a sprocket on both sides of your hub, you already have a flip-flop hub/wheel.

If you only have a single sprocket (and it is a fixed gear hub, see note below), you can purchase a fixed sprocket and replace the SS freewheel with the fixed sprocket, since both singlespeed and fixed gear hubs use the same 1.37 x 24 tpi threading for the sprocket.

Note: @heltonbiker is correct, you will need a lockring with the fixed gear hub, they have a double thread, one for the sprocket and one for the lockring. You can put a SS freewheel on a fixed gear hub, but you cannot put a Fixed sprocket onto a SS hub (it does not have the step down threading for the locking mechanism)

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Upvote for you, I had no idea that the freewheel mechanism was in the sprocket itself. You learn something new everyday. –  Kibbee Sep 11 '12 at 18:52
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I'm afraid the answer, as it is, advises a DANGEROUS adaptation, since it is not just enough to install a fixed cog on a SS hub, you need some sort of locking which is usually impossible with a singlespeed hub (I use a glued suicide setup and it works flawlessly, so I'll remove my downvote if the answer is updated). –  heltonbiker Sep 11 '12 at 23:02
    
Holy Smokes... This answer is way complex and a beginner "fixie" biker would have no earthly idea what the hell all this means, unless he or she has taken the back hub out before and seen this apparatus. Re-word to beginner's English please. –  user6166 Feb 16 '13 at 22:11

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