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I'm looking to replace the suspension fork on my bike with a rigid one. After much soul searching I've tentatively settled on the Surly 1x1. What's got me wrapped around the axle (pun intended!) is the axle-to-crown measurement.

The fork I have now is the SR Suntour SF13-XCT, which came stock on my Specialized mountain bike. This fork is listed as having 80mm of travel and a 459mm total length. When measuring the distance between this fork's axle & crown I've gotten very close to the 459mm listed on Suntour's site, which leads me to believe that the 459mm length listed by Suntour is correct.

The Surly 1x1 I'm looking at has two versions: one "corrected" for suspension travel of 80mm (axle-to-crown length of 413mm), the other "corrected" for suspension travel of 100mm (axle-to-crown length of 453mm).

Both of the Surly A-to-C measurements are below the 459mm that I supposedly have to put in if my geometry & handling are to stay the same.

In addition, the zip tie method allowed me to measure my sag as 18mm. (I normally keep my fork on its highest preload setting.)

Now, for the question itself: Which of the two Surly 1x1's should I get? Should I, or should I not, take my sag into account, considering that Surly's forks appear to have been "corrected" for suspension travel already?

Thanks in advance for any guidance.

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    The XCT is a long fork for being 80 mm and the Surly's are short for what they say, it seems. I'd probably go with the 100 mm fork in this case, or more likely, go with a non-Surly fork (e.g. Kona Project 2 or some On One fork). You might not get an exact geometry match, but likely, you can tweak a fork closer to the specs of the XCT fork to work.
    – Batman
    Jun 2, 2014 at 5:47
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    The difference you have between the 80mm and 100mm rigid forks is 40mm, rather than 20mm, which suggests to me that the 100mm fork is a 29er 100mm fork, not a "26er" 100mm fork.
    – Emyr
    Jun 2, 2014 at 9:00

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Definitely you have to account for sag when measuring axle to crown distance, because that is the real geometry of the bike when it is actually being ridden. If you mention in this bike sag is 18 mm when you ride it, it should mean that your actual axle to crown distance is 459-18 = 441, thus suggesting you should use the fork corrected for 80mm travel.

Ideally you want to keep the geometry of the bike, particularly head tube angle and bottom bracket height (or pedal to ground clearance). Both are important. Head tube angle affects the steering response of the bike. Bottom bracket height defines how tall an object can be for you to roll over it without hitting it, and how much you can lean towards the side before the pedal hits the ground while pedaling (you should not put the "inner" pedal down while turning)

A bike with suspension fork actually changes it's geometry while riding due to the travel of the fork, and this is no trouble at all (for most riders), that means that a small variation in axle to crown distance would not terribly affect the bike handling characteristics.

In this case, if you chose the taller fork, you'd give your bike a slacker head tube angle, making it feel a bit less responsive on the steering, which in turn means (a little) more stability at high speed, and you would gain a few millimeters on pedal clearance. If you had the option for a third fork, shorter than the ones you already described, you'd get a bike with a steeper head tube angle, which gives a more responsive steering, the bike would feel very agile, but less stable, and you would loose a little pedal clearance.

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