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I've been looking around for bikes (decent frame & components), and find that hybrids do have disc brakes (hydraulic), but are never single-speed. While single-speed bikes typically have linear-pull brakes.

Why is that? Anything in the mechanics that would make the two incompatible?

(In my case, I'd only want a disc brake on the front — for commuting.)

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Not a big enough market. People with single speeds most often want "retro" parts, no brakes at all, or as light of a bike as possible. Disc brakes don't really fit any of those descriptions. Although I've thought about building a fixie with disc brakes, on the front at least. Seems like it would be a very nice bike to own. –  Kibbee Apr 28 '13 at 22:04
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Check out the On-One Pompetamine. Love mine :) –  Mere Development Apr 29 '13 at 6:42
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This might be worth looking at if you are looking to do a conversion of your current single speed. cxmagazine.com/… –  Ken Hiatt Apr 29 '13 at 18:23
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chainreactioncycles.com sells two singlespeed bikes with disc brakes under their own brand name Vitus. chainreactioncycles.com/Models.aspx?ModelID=80998 –  user1049697 Apr 29 '13 at 20:24
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chainreactioncycles.com is claiming to be the largest cycling store in the world, and they are huge and the default go-to store in Europe atleast. I think most Vitus bikes are generally seen to be decent for the price, but I have no idea about the quality of that particular model. It is cheap, but made of hi-ten steel which has a bit of an unfortunate reputation. –  user1049697 Apr 30 '13 at 11:11
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3 Answers

up vote 17 down vote accepted

Usually, singles need to have horizontal dropouts so you can take the chain slack by adjusting the rear axle position.

That means that any brake that is attached to the frame will "go out of position" when you adjust the rear axle position. That is, by the way, the reason why some horizontal dropouts are not quite horizontal, but diagonal: to be near-perpendicular to the seat stays where rim brakes are attached, so that wheel movement is "tangential" to the brakes.

Unfortunately, with regular disc brakes, this adjustment would totally misalign the brake pads relative to the disk, if the adjustment is possible at all.

Although there are some bikes with grooves for the disk caliper, where you can "slide" the caliper into, it's usually not worth the trouble, because chain slack has to be corrected very often, and caliper setup is relatively laborious.

Also, it would be possible to take the slack with an eccentric bottom bracket, but that is even more esoteric.

Hope this helps.

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There are bikes that use a sliding dropout or rocker dropout with the disc brake mounts on the sliding part of the dropout. With those, adjusting the tension via the dropout doesn't require messing with the disc brake, since it moves with the adjusting part of the dropout. I haven't yet seen this on cheap or midrange bikes, only expensive or custom so far. –  freiheit Apr 29 '13 at 4:11
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IF they really wanted to build a fixie with both, front and rear disk brakes, it could be accomplished by having a disk-ready fork and a frame with vertical dropouts and an excentric bottom bracket housing to adjust chain slack. (Agreed it doesn't fit the tipical wants and needs of the fixie rider) –  Jahaziel Apr 29 '13 at 14:26
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But this issue affects the rear. Who cares about rear brakes. If a bike was to have only one brake, you'd put it on the front wheel, not the rear, and if it was to have only one disc brake, it would go in the front. In other words, this answer doesn't explain why there aren't single-speed bikes that have a disc brake in the front. (Either just that brake, or that brake in combination with some other kind of brake in the rear, like a hub brake.) –  Kaz May 4 '13 at 0:35
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@Kaz I guess for manufacturer it's way easier to get a PAIR of OEM brakes (discs, calipers, lever), than just a single one. Maybe in the future more manufacturers take advantage of the "front-only-brake-single-speed-bike", but I'm afraid this won't happen for now. –  heltonbiker May 4 '13 at 15:51
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It might be difficult for the cyclist to get just a front brake at the retail level, but I would think not for a manufacturer who orders thousands of them from a supplier. –  Kaz May 4 '13 at 15:57
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It's a cost and market thing. Disc brakes are generally a bit more spendy and require different frames and forks. There are some single speed bikes with discs, especially in the CX world. For your application, consider getting a CX fork that is set up for disk to replace the fork on a single-speed. Note that you will need to replace the wheel too.

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Kona Unit 29er 2014 looks like the kind of thing you're looking for, the rear disk brake is mounted on the axle so it adjusts as the chain stretches.

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