Some rigid bicycle forks are classified as “suspension-corrected”. This description could be a little enigmatic because rigid forks do not have suspension after all. Hence why some cyclists may be confused as to what the term means.
A suspension-corrected fork is an elongated rigid fork designed for bicycle frames normally operating with front suspension. The purpose of the extra length is to preserve the original geometry and handling of the bike when converting from a suspension fork to a rigid one.
Why Are Suspension Forks Longer?
Regular suspension forks are long for two main reasons:
When a suspension fork compresses, it shortens, and its crown gets closer to the tire.
If suspension forks were any shorter, there wouldn’t be enough room for the suspension to operate optimally because the tire would get in the way.
Stable geometry and long travel
Frames designed for front suspension forks come with a slacker head tube angle keeping the bike stable even when the fork is compressed. Also, a “taller” head tube can accommodate forks with a longer travel.
If suspension forks were shorter, they would shift too much of the rider’s weight to the front upon compressing. Subsequently, the ride would become unstable and dangerous.
The image above illustrates how a suspension fork shortens. Upon compression, the distance between the crown and the arch decreases.
As a result, the head tube and consequently the handlebars get closer to the ground as illustrated in the next image.
The image above can illustrate two scenarios:
A. What would happen if suspension forks were shorter
If the fork in figure 1 was that short in a non-compressed state, upon compressing it would render the head tube angle dangerously steep.
As a consequence, the bike would become unstable and sketchy, especially when ridden in extreme conditions.
B. How a suspension fork compresses
Figure 1 in image 2 can also be seen as a normal fork in a compressed state.
How Long Should a Suspension-corrected Fork Be?
The objective of a suspension-corrected fork is to preserve the geometry of the bicycle when its front suspension is non-compressed.
Therefore, in theory, the length of the suspension-corrected fork has to be equal to that of a non-compressed suspension fork.
However, there’s one more factor that has to be taken into consideration when selecting a suspension-corrected rigid fork – the sag of the original suspension fork.
What Is a Suspension Fork’s Sag?
When you get on a bike, the fork will compress ever so slightly under the pressure of your bodyweight. That movement is called sagging or sag for short. It can be adjusted by playing with the settings on a fork (not all models provide that option, though)
And since the sag effectively shortens the fork, it affects the final choice of a suspension-corrected rigid fork.
Calculating the Length of a Suspension-corrected Fork
Step 1. Measure the axle to crown length of the suspension fork that will be replaced.
The axle to crown length is the distance between the middle of the axle and the top of the crown. Make sure that you measure it in a straight line.
Step 2: Determine the sag of the suspension fork
The simplest way to measure the fork’s sag would be to use a zip-tie unless of course, the fork has a pre-installed o-ring for that purpose.
The Zip Tie Method
It’s best to use the zip-tie the opposite way. The goal is to prevent the teeth from actually catching as originally intended. Hence why they’re on the outside in the image below.
This strategy allows you to remove the zip-tie without tools and use it later. It also minimizes the chances of scratching the stanchion because the teeth of the zip-tie are not in contact with it.
Loop the zip-tie around one of the stanchions and tighten it. Push it gently until it’s on top of the fork seal. Get on the bike and assume an attack position (out of the saddle, center of mass in the middle).
Then, dismount from the back to avoid leaning on the handlebars and affecting the measurement.
Finally, measure the distance between the zip-tie and the seal. This is the sag.
Step 3: Subtract the sag from the axle to crown length
If the sag is 30mm and the axle to crown length of the suspension fork is 500mm, then the axle to crown length of the new suspension-corrected fork will be around 470mm.
If you can’t measure the sag but know what percentage it normally is, subtract the percentage from the fork’s travel to find the sag’s measurement.
For example, if the sag is 25%, and the fork travel is 100mm, then the sag is 25mm.
Once you have subtracted the sag from the suspension fork’s axle to crown length, you will know the axle to crown length of the corresponding suspension-corrected fork.
Of course, you don’t have to get everything down the last millimeter, but getting as close as possible increases your chances of preserving the handling of the bike.
The Frame Matters Too
Mountain bike frames are designed for forks with different travels depending on the terrain and riding style.
For instance, cross country frames are engineered to accommodate around 100-120mm of travel whereas city bikes operate with 40-80mm. Putting a fork that exceeds that number could potentially crack or break the head tube of the frame.
Conversely, going for a fork that has a lot less travel than originally intended for the frame is also detrimental to the geometry of the bike.
Since swapping a suspension fork for a rigid one makes sense only if the frame is designed for light trails, cross country, trekking, or city use, then the needed suspension-corrected model would have to have the axle to crown length of a suspension fork with 40mm-120mm of travel minus the sag.
If you’re operating with the lower values (e.g., 40mm), the available forks would not be presented as suspension-corrected, and you would have to go by the axle to crown length alone.
If you only have a frame, you could search for its specification, find out what travel it can accommodate, and buy a suspension-corrected fork built to replace a suspension fork with that much travel.
For example, I have a cross country frame that can work with front suspension travel between 80-120mm. If I wanted to replace the fork with a rigid one, I’d have more than one option.
What if the travel of my fork is higher?
Most suspension-corrected forks are designed to compensate for forks with 100mm of travel because that has been the standard for cross country bikes for a long time.
However, some modern XC frames require 120-130mm of travel. This limits the options greatly. Thankfully, some manufacturers are making rigid carbon forks for similar frames.
Note: A fork shorter than the optimal length would mess with the geometry of the bike – the head tube angle would be steeper and the handlebars would get lower. Hence it’s recommended to find a fork that mimics the original one as closely as possible.
Will a Rigid Fork Influence the Handling?
Even if you find a suspension-corrected fork that matches the exact length of the previous suspension fork, there are no guarantees that the bike will behave exactly as before, but the chances are certainly higher. Ultimately, you will have to experiment to know for sure.
The Head Tube Diameter Matters Too
The diameter of the head tube determines the diameter of the fork’s steerer tube.
Since most suspension-corrected forks are designed for mountain bike frames, the diameter of the steerer tube is usually 1-1/8″.
However, the newer frames come with tapered head tubes that have a larger diameter at the bottom (1.5″) which decreases along the length.
If that’s the case with your head tube, and you would want to run a 1-1/8″ fork on it, you would have to buy a reducer cup for the larger, lower part.
If you have an “old-school” frame with a 1″ head tube, the modern suspension-corrected forks just won’t fit, but you may be able to find some second-hand options.
What is the preferred material for a suspension-corrected fork?
Rigid forks come in three materials – steel, aluminum, and carbon.
Steel forks are often the heaviest, but in exchange, they are really strong and have the springy feel that steel is known for. Moreover, many steel forks designed for touring are equipped with eyelets for fenders, front racks, and sometimes even water bottle cages.
Aluminum is lighter than steel and cheaper than carbon but creates a harsher ride due to its poor absorption capabilities.
Carbon is lighter than steel and provides more “road absorption” than aluminum. Its main downside is its price. Putting a USD 500 carbon fork on a USD 200 bike could be seen as an unjustified upgrade.