Length of horizontal dropout
 

I got this old Trek frame recently and want to build it up fixed, it has horizonatol drop outs but they are a little on the short side. The dropouts are ~20 mm long. Is this long enough? Or should I use an eccentric hub?

If an axle is 10mm in diameter then you have 10mms of room for adjustment. 5 in front and 5 in back theoretically.

5mm is ~.2 inch. For every tooth added to, or subtracted from the drive train, you need to move the axle 1/8th inch, or 3.175mm. So you should be able to dial it in without an eccentric hub or bottom bracket.

Great, thanks.

The worst case would be you'd have to use a half-link to get a good chain length that falls within the adjustment range of the dropout slot.

you're a horizontal drop out ;p

Your mom is a horizontal drop out and I just put my nut in her slot :roflmao2::roflmao2::roflmao2::roflmao2::roflmao2::roflmao2::roflmao2::roflmao2:

https://upload.wikimedia.org/wikiped..._%28WWE%29.jpg

JK

+1/2.

Adding/removing a tooth to the sprocket is only half the story. You're also bound by chain length increments of 1" which mean 1/2" in wheel position. So it's entirely possible that your desired gear combination will take the axle to the extreme position within the dropout.

Of course, using a 1/2 link, will allow 1/2" chain length increments, so only 1/4" of axle travel is needed.

Get a couple of half links. (So you have one on hand when you change chains.) I had an almost identical setup on a late '80s Peugeot sport bike that I set up fix gear. With the half link I could dial in the chain length so I could use 2 different cogs. I might have even been able to get a 3 on.

Ben

Half links are unnecessarily. You simply rotate the chain stay length adjustment knob. Only a little.

Chain stay length adjustment knob????

We were talking about whether an eccentric was necessary, so can't propose a solution dependent on having one.

So, for the OP, serendipity may work for you and you'll be fine. Or you can use a 1/2 link to reduce the travel needed, or you can use an eccentric.

Yes. All good bikes have one in the cockpit. You simply turn the knob to increase or decrease chain stay length. This allows you to change the wheelbase and/or weight distribution to suit you riding conditions or varying load placement. It also allows you to dial in chain tension without the need to break the axle nuts free. Your bikes don't have one? All mine do. They are quite magical. ;)