There are countless forums on the internet with complaints such as my Rockshox fork is harsh or my Fox fork is harsh, or my fork has poor small bump sensitivity. The discussion ends up attributing the harsh and insensitive feeling of the fork to the damper, or the seals, or the air spring, and goes around in circles with no good solution being arrived at. Invariably, one thing that gets overlooked is the chassis of the fork and the bushing system which is the main contributor of friction and harsh feeling to the fork, far more significant than friction from main seals or air spring seals. There are solutions for a harsh feeling fork from friction in the chassis which usually make an incredible difference. Reducing chassis friction first allows much better results for tuning dampers and spring systems afterwards.
Almost all modern mountain bike forks are telescopic forks which means they rely on sliding motion of one tube inside another. Most forks use a "conventional" configuration where the upper stanchion tubes slide into cast one piece lower legs shown in figure 1 below.
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Figure 1: Schematic showing stanchion tube, bushings, and lower leg tube.
Stanchion tubes are typically hard coat anodized which is thick and durable meant for mechanical applications where the tube will see sliding motion and abrasion. If the stanchion tubes slid directly on the metal of the lower casting, the stanchion would immediately be ruined destroying the fork. To prevent damage from occurring, the stanchions slide on a bushing. Bushings in modern mountain bike forks consist of a thin layer of PTFE on an aluminum backing which is rolled into a round shape and then pressed into the lower casting as seen in figure 2 below. The PTFE on the surface of the bushing is extremely slippery and soft which effectively prevents wear of the stanchion tubes, allowing smooth motion and low friction. However, PTFE alone is not sufficiently low friction, oil lubrication is also required to further reduce friction. To have proper lubrication, there needs to be a thin film of oil between the stanchion and the bushing itself, which requires a small amount of clearance between the bushing and stanchion to form an oil film. This is where problems start to occur with mountain bike fork bushings.
Figure 2: Typical bushing used in mountain bike fork lower legs.
Fork bushings are installed into the lower casting with a press fit. This means that the outer diameter of the bushing is slightly larger than the hole in which it's installed, and is held in place solely by a large amount of friction resulting from the interference. Bushings are installed into what is known as a "seat" (Figure 3) which is used as cast in the lowers with no post machining to save on manufacturing costs. These seats are not subject to 100% quality control and can cause issues for entire batches of forks.
Figure 3: As cast bushing seat in a lower leg casting.
The most common source of friction in forks is bushings or seats being out of round, or oval shaped. There are a couple contributing factors for out of round bushings. During the manufacturing of fork bushings, they start as a flat strip of aluminum with a thin layer of PTFE attached to the surface. These strips are then cut into a very specific length and rolled into a cylindrical shape, which is why there is a split in the bushing and not one continuous round piece. The rolling process is generally good at producing a cylindrical shape and is an economic process to produce large quantities of bushings quickly and inexpensively. To roll the bushing round, the aluminum backing material is in a very soft state with little memory, or spring back tendency so that once the cylindrical shape is created, it doesn't open back up. This soft aluminum is easily prone to permanent deformation. The other factor contributing to the out of round condition is bushing seats becoming out of round after casting which appears quite frequently and with certain lower leg designs. If a bushing is installed into a fork with an out of round seat, the bushing may go in and still be round from it's production... great, so what is the problem? It's an issue because of the production method of the bushing with the very soft aluminum backing that has little memory to it. As the fork gets ridden and the bushing is loaded in different directions by the stanchion from bumps and jumps, the bushing is eventually deformed to the shape of the seat its in. If the seat is out of round, the bushing will be too. If you've have had a fork where it felt great when brand new but performance degraded after some time of riding and a service did not bring it back to the same performance level as brand new, or made it feel good only for half a ride, the bushing has taken the shape of the out of round seat. The reason that an out of round bushing is a huge source of friction is because once the bushing is oval shaped, it will contact the stanchion at only a few points, not a larger surface area as shown in figure 4 below. Having contact at only a couple small points instead of over a larger surface area greatly increases the surface pressure of the bushing against the stanchion, which can be much greater than the oil film can support. The high surface pressure scrapes the oil film away from the interface between stanchion and bushing, causing excess friction from no oil film supporting the interface which results in a harsh feeling fork. Out of round bushing seats in lower legs occur more commonly on forks where the entire outer diameter of the bushing is not supported by the seat such as ones with grooves behind the bushing intended for oil flow.
Figure 4: Point contact of stanchion on an out of round bushing leading to extremely high surface pressures that the oil film can not resist which ultimately creates excess friction and a harsh feeling fork.
Luckily there is a solution for out of round and bushings that are too tight which is bushing sizing / burnishing and it solves these issues permanently. The burnishing tool works by inserting a steel mandrel of an exact size through the bushing to deform the PTFE layer to be round because the burnishing head it self is round. This also leaves the optimal clearance for a perfect oil film. It fixes both out of round and tight bushes because once the bushing has conformed to the out of round seat, it can't deform anymore. The burnishing heads are designed so that they are larger than the final desired bushing size as the PTFE has a small amount of spring back and will shrink a bit after the tool has passed through.
It is important to remember that bushings conform to out of round seats while riding. If sizing / burnishing a brand new fork that is unridden or has been ridden only a short amount of time, the fork will need to be revisited with the burnishing tool at a later time. The bushing sizing / burnishing process will not make a bushing conform to an out of round seat. The the fork may initially feel incredible, and then might feel awful a short time later, the bushings have taken the shape of the seat and need a revisit from the sizing / burnishing tool.
Shouldn't the suspension fork Manufactures do this at the factory before shipping out forks? The cost to size / burnish bushings at the factory would be extremely high and isn't economical for the manufactures to do. It is less expensive to warranty problematic forks instead of performing this procedure at the factory. Furthermore, because the bushing needs riding time to conform to an out of round seat there isn't much point in the manufacture doing this process at the factory because it'll have to happen later anyways.
Doesn't the width of the lower legs at the hub affect the friction of the fork by pressing the bushing one way or the other into the stanchions? Shouldn't this be aligned to minimize friction? Yes, it does affect the friction. Many forks come with a floating axle design to help align the lower legs with the stanchions but there are 2 problems with trying to "align" the fork lower legs with the stanchions. The first is only the lower bushings will really move sideways. The upper bushings are close to the arch which is the bending fulcrum, so if the stanchions are not concentric with the upper bushings / lowers there is no real way to rectify that short of replacing parts. The second problem is that it assumes the stanchions or the lowers are splayed either towards or away from each other in the direction parallel to the axle when this is almost never the case. Forks with floating axles shoud be taken full advantage of to minimize friction as much as possible but it isn't as simple as just making sure the lowers at the axle are at the right width. Sizing / burnishing a bushing to make it round and very slightly opening it up helps alleviate the friction from these issues as well because it provides a tiny bit more clearance in all directions.