Singlespeed & Fixed Gear - How to measure a fork?

I'm looking to replace the fork of a friend's track bike with one that has
a brake hole, but I seriously don't know where to start. Perhaps this
question shows the depth of my naivety, but I was wondering how one
measures for a new fork? What factors do you account for in buying the
correct size & rake? This kind of stuff never comes up in BMX!

The first factor is threaded or threadless headset? Then the two most important measurements are the steerer tube diameter and length. Diameter is either going to be 1" or 1 1/8" most likely if it truly is a track bike. The next factor is you need a fork that has a steerer tube long enough to accomodate the head tube of your frame. As long as it is long enough you can cut it down to the right fit. Thats a whole different topic though (use search function). Other measurements like rake etc are probably secondary at this point in the game.

Oh yeah, 1" threaded. Sorry, I can't believe I forgot
to include that.

Aside from whether the headtube is long enough, there are two major measurements that will mess with the handling of the bike: Rake, and Axle-Crown length. Rake is the distance that the center of the axle is from the center of the head tube axis (some folks call it offset). Axle-crown length is the "height" of the fork, the distance from the axle to the base of the crown, where the crown race of the headset sits.

If either of these measurements is far off from that of the original fork, it will mess with the handling.

Thanks Aeroplane I stopped before I got to rake because although I understand the concept I'm no expert at suggesting what rake is appropriate etc. Is the axle-crown length just what determines what size wheels you can run?

I stopped before I got to rake because although I understand the concept I'm no expert at suggesting what rake is appropriate etc. Is the axle-crown length just what determines what size wheels you can run?
A-C length can determine what wheels you can run (which also affects handling), but it is determined by a lot of things. Here's a couple examples:

Think of a blinged out NJS track fork. Super-low clearance, the top of the tire clears the bottom of the fork by about 2mm, and the fork crown is about 10mm thick. This makes the A-C length an estimated 325mm.

Now think of a cyclocross fork. Built for the same size of wheels, but the tires are way bigger, and there is a lot more clearance between the top of the tire and the fork. Also, the crown is thicker. You might have an A-C length of 360-380mm (completely pulling numbers out of my ass).

You put a CX fork on your NJS bike, and the bigger A-C length will pick up your front end, which will slacken the crap out of your head and seat tube angles. Depending on the rake of the fork, it will also drastically affect the trail. If you're going to replace a fork, if you can match the rake and the A-C length, you should be golden.

Good explanation. Thanks!!

Does anyone care to explain what trail is, how it works and how it's related to fork rake?

Edit durr http://www.sheldonbrown.com/gloss_tp-z.html#trail