A Guide To Fork Swapping

2000 Fisher Tassajara

 In Monday's "Randomonium" post I asked you readers if you had any questions about fork swapping or technical questions about fork geometry. One of you did ask, so that results in today's subject: "A Guide To Fork Swapping". 

So, before we get started, let's get some definitions and terms down that I'll be using today. It is very important to note that a bicycle is a 'system'. It is not wise to think of a bicycle in terms of one of its parts, or to focus in solely on a particular facet of a bicycle's geometry. So, for instance, when we discuss forks, it is not a good thing to just focus on "offset", and to disregard that "offset" is but one of a few things that ultimately determines how a bicycle will handle. So, don't fall for the simplistic idea that changing a fork from one offset to another will magically cure your handling ills. Many media writers are guilty of perpetuating this myth.

So, following are the things that are important to consider about a fork for your bicycle:

Basic fork geometry terms visualized: From http://yojimg.net/bike/web_tools/trailcalc.php

 Fork Offset/Fork "Rake": These are interchangeable terms that define how far the wheel axle is placed ahead of the steering axis. More 'offset/rake' means that the axle sits further away in front of the steering axis. 

Steering Axis: This is an imaginary line which is traced through the centerline of the head tube, fork crown, and on down towards the ground. (See the blue hash marked line in the diagram above) Steering Axis angle is equal to the "Head Angle" of a bicycle. Designers adjust this to affect overall Fork Trail.

Fork Trail: This is the distance from a point on the ground where the steering axis intersects the ground plane to the center point of the tire's contact patch behind it. More "Trail" usually means slower/ more stable handling and vice-versa- Less "Trail" means a quicker, less stable bike. This measurement is typically expressed in millimeters.

Wheel Diameter: Wheel diameter is determined by the combination of tire and wheel rim diameter you choose. So, just saying "twenty-niner" isn't good enough. Similarly, the terms "26 inch", "700c", and "27.5"er/650B" are not sufficient for determining overall handling characteristics. Not if we are concerned with detail. Here we need an actual measurement of the wheel/tire combination's diameter, which- as you probably know- is a measurement from the ground, through the axle centerline, to the top of the tire. Essentially, in visual terms for our example above, you would extend that blue hash mark line from the ground plane past where it terminates with the rear wheel axle to the top of the diagram's 'tire'. 

The axle to crown measurement is the distance between the axle to the fork crown along the fork's 'leg'.

Axle to Crown: This determines overall fork 'length'. The distance is determined from the centerline of the wheel axle to the top of the fork crown, or you can think of this as the point where the steer tube exits the fork. 

Suspension Correction: Designers account for the longer length of a suspension fork, necessary to allow for a wheel to 'travel' in a suspension design, and translate that to their rigid fork design, So, if a suspension fork has, let's say, 80mm of travel, to keep the front end of a bicycle equipped with that fork the same in terms of head angle, stack height, and more, that designer makes the rigid fork a similar length to a suspension fork with a rider on board. 

With the weight of a rider, that suspension fork will sink into its travel a bit, and this is called "sag". That is also accounted for in the rigid fork design. So, let's take our 80mm travel fork in a 29'er size and measure that. It's going to come out, nominally, with an axle to crown of around 480mm. That's a bit more than 18"'s for you non-metric folk. Now, 80mm of travel equals about 3"'s. take that off the slightly more than 18" and you are left with around 15+ inches, doubled, equals 30+ inches, or in other words, more than enough to clear a 29" wheel safely with room to spare at full travel of the fork. 

Now let's say we don't want a suspension fork, but we want a rigid one that keeps our 29"er handling similar to what we know. Accounting for some rider induced "sag", while on the bike, our suspension fork really probably is more like 470mm or so in terms of axle to crown. So, our rigid fork needs to be that measurement to preserve the handling characteristics we know, all else remaining the same. This is what we mean when we say that a fork is "suspension corrected for_____ travel". In our example's case, the rigid fork would be suspension corrected for 80mm's of travel. 

Whew!

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Choosing A Fork:

So, let's say you want a new fork for that old bike you have in the garage. Let's say it is a clapped out old suspension fork on an older MTB that you have had for years, but now you have some ideas. Maybe you want to do some bikepacking, or maybe this will end up being your 'gravel bike', or like me, maybe you want to use that bike as a townie/urban errand rig. 

Well, you know that a suspension fork is too much money, hassle, and it isn't necessary. So what do you get? Well, here is a list of things you'll need to know:

• What is your steer tube size: One inch? Inch-and-an-eighth? Is that a threaded or threadless steer tube? It is probably a straight steer tube, and if so, a tapered steer tube fork, (like the one to the left here) won't work.
  
• Wheel size, (Probably 26 inch, but there are old 650B MTB's and old 29"ers out there nowadays)
• If you have suspension, what travel length does your fork have? Older bikes had 63mm, bikes from about 1998 on had 80mm, or even 100mm in 26"er flavors. Measure the axle to crown and see if you can compress the shock to assess what travel you might have. 
• Axle type: Most probably are quick release, but there may be a few instances of thru-axle bikes in this situation. 
• What is your suspension fork offset/rake? If it was a fork made before 2007 and a 26"er fork, it most assuredly is going to have the then industry-wide standard 38mm offset. But some newer bikes had longer offsets, so check specifications if you can find them. Measuring a fork offset can be done at home, but it gets tricky to be accurate. 

Now, with that information in hand, you can start looking for a suitable replacement fork. Here's an important thing to remember though- If you veer too far from your bicycle's original specifications, you will affect the way the bike handles. And I cannot tell you if you would or would not like that. Just realize that millimeters make a big difference. It may seem trivial to you that your fork has a 440m axle to crown and the replacement you want has 420mm axle to crown, but that matters. You will feel a difference. 

And in some cases you won't be able to replace your fork with an off-the-shelf option. Let's say that you have a 1" steer tube, threaded fork with 63mm of suspension travel, (Original Rock Shox forks, as an example). Well.....you may have to get a fork custom made to suit your desires. Options off the shelf will likely be, what in the industry are called, "replacement forks', and generally are the lowest common denominator in terms of fork options. 

Many 1990's MTB's with 1 1/8th steer tubes and 63mm travel forks can be converted to rigid using a Surly Troll fork, which - while a bit shorter than the stock forks, and have a bit more offset, seem to work a trick for a weight bearing, bikepacking set up. Plus they had canti OR disc brake tabs. Now if you can actually find one......

Sometimes you have to think outside the box. Take for example the Surly Ogre Fork, which is for rigid, non suspension corrected 29"ers. It has an axle to crown of 447mm. Since it is a disc fork, it could work for a 26" wheel. If your 26" suspension fork had 4"/100mm travel, that Ogre fork might work for you. 

Besides those things, you'll need to consider brake standard, brake levers, and if the forks are different lengths, one from the other, you'll need to consider how that affects your stem, bottom bracket height, and seat angle too. 

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Hopefully that helps! But if you have anymore questions about forks, switching forks, or what have you, let me know.