How do I measure a fork for suitability?

Question

I have an old bike I bought from ebay that looks to be '80s vintage. I'm converting it to a single speed. I have some new(ish) wheels that I plan to use. I tried to just slot in the existing wheels and struck the following problems:

1. The axle would not fit into the dropouts as it's too big.
2. The axle also wouldn't fit between the dropouts as the space between the dropouts is too narrow
3. Even if I could mount the wheel the brakes I have don't reach the rims, as the distance from the dropouts to the brake mounting point is too great.

So I think I will upgrade to a new fork. My question is how do I ensure that these measurements will be correct for a new fork? I've measured a fork from another bike that I know fits with my wheels.

1. The axle is 10mm diameter (on the old fork it's 7.5mm)
2. The inside measurement between the dropouts is 100mm. Outside measurement is 110mm. (On the old fork, it's 95mm and 100mm)
3. The distance from the dropouts to the brake mounting point is 355mm (on the old fork it's 375mm)

Will this fork fit? It looks like it will but I'm not sure if the measurements supplied are inside or outside. I'm aware that it's for a threaded steerer and I know it's a 1" steerer tube (see this question for the information about the existing fork.

Also, since the replacement fork is 20mm shorter from the dropouts to the brake point is that going to throw out the angles of the bike?

Update: I bought the fork and it's a great fit for the wheel. The offset against the original fork is less than I expected, somewhere between 10-20mm so the handling is not too bad.

Answer 1

The overwhelming majority of modern road forks are designed for 10mm axles and 100mm-width hubs. (Since the hub width is the important measurement, it's the inside measurement between the dropouts which should concern you).

After you've ensured that the wheel sizes are the same (700c), the largest concerns are the axle-to-crown measurement; or very nearly what you're measuring as "the dropouts to the brake point", the steerer length, and the fork's rake.

1. Axle-to-crown. As you've surmised, changing this will modify the angles of the bike. Increasing it will rotate the bike back, slackening the head and seat tube angles. Reducing it will steepen them. Unless you've got a particular desire to change the handling of your bike and are willing to work out the geometry (see Damon Rinard's article for starters), it's advisable to keep it as close to "as-designed" as possible. 3-4mm is a minor change. 20mm is significant. It will also reduce the range of tire sizes you can run and most likely require new brake calipers, as they typically have only 20mm of total adjustment. I don't actually see the axle-to-crown measurement on the fork you have linked.

2. Steerer length. This is the amount of steerer tube from the top of the fork crown. On the fork you linked, the steerer length is listed as 250mm, with the top 130mm threaded (leaving 120mm unthreaded). You'll want to remove your current fork and measure the steerer tube. If it is longer than about 220mm (rather long) it probably won't have enough room for you to install the headset and get everything threaded on - you'll need a longer fork. If it's less than 150mm (rather short) you may not have threads far enough down the tube to tighten everything, so you'll need to get more threads cut at a local shop.

3. Fork rake. This is the distance that the axle is offset from the centerline of the fork. Changing this dramatically will also change the handling of the bike, so try to stay close to what you have now unless you want to work out the maths. You can measure it by having a friend hold the old fork vertically and then sighting down the steerer:

There are lots of resources to learn more about how fork rake affects handling, Dave Moulton has a good introduction. The fork you have linked doesn't show a rake measurement.

Answer 2

It'll probably fit. You might have to cut the steering tube to the correct length. (Use a hacksaw with a miter box, then a file to smooth the edges. Make sure to clean up all the steel filings before you install it - you don't want that stuff getting into the bearings.)

It'll change the geometry of the bike, which will affect handling, but hopefully it won't be too drastic. And that fork is cheap enough that you can afford to experiment.

I built a bike a lot like this - new fork on an old frame, and fixed gear. A potential problem you should be aware of is toe overlap. My new fork has a much lower rake than the old one, so the wheel's further back, far enough that sometimes my toes can hit the tire when turning. This is actually not as bad as it sounds, since it only happens at extremely low speeds. When turning at moderate-to-high speed, you don't turn the handlebars very far at all. A lot of track bikes have major toe overlap.