20mm vs 25mm Fork Cartridge Kit FAQ
Max: First and most important is the differences between 20 and 25mm cartridges. The 25mm cartridge is all hype in my opinion. I don’t think they are better than 20mm, or else I would be building 25mm cartridges, not 20mm. We do have 25mm Axxion Valve Kits (not cartridges) for Ohlins forks, KYB forks, and Showa forks now. The first myth about 25mm cartridges is that they “move more fluid”, and as a result, you have “more fluid to control”. This is not true. Fork cartridges work by creating pressure differentials in the fork, and then by fluid displacement. To simplify: If you have a bathtub full of water, and you get into it, water spills out. That is fluid displacement. If you had the tub covered with a lid, and a hole big enough for you to get into, you still couldn’t get in, because you couldn’t displace any water. If you opened the drain, you would create a route of escape for the water, and you could then get into the tub. If you crammed yourself into the capped tub, you would increase the pressure in the tub, and then force water out of the drain hole at a much faster rate. So that is pressure differential and fluid displacement in crude terms. In a fork cartridge, the only thing displacing fluid is the rod that is entering it. In aftermarket 25mm kits, they have a 12.5mm rod. GSXR1000s have a 12.5mm rod with a 20mm cartridge. If both forks move 100mm which displaces more fluid? Neither. They are the SAME. Back to the bathtub, if a 175 pound man gets into a full bathtub of water (i.e. 20mm cartridge), he will displace “One Man’s Worth” of water. If the same man gets into a 40,000 gallon swimming pool (i.e. 2000mm cartridge) he will STILL ONLY DISPLACE “One Man’s Worth” of water. The reason properly built 25mm cartridges don’t work as well as properly built 20mm cartridges is that a “pressure differential” has to take place inside of, and outside of, the cartridge before any damping can occur. Due to the increased volume of oil (and air) inside the 25mm cartridge, it takes a longer period of time (and distance) for damping to occur. This results in more drop and dive, and a longer time before damping occurs each time the fork changes directions. So, what is more critical than the diameter of the cartridge tube is the relationship of the diameter of the rod to the diameter of the tube it rides in. Note that Yoshimura and Honda have had some noteworthy success using 20mm pistons, which includes most of the Superbike and Supersport Championships in the US in the last ??? years. By choice, the Axxion Cartridges will be 12.5mm rod with a 20mm tube. A second negative byproduct with 25mm cartridges is a massive increase in unsprung weight. The body of the cartridge is bolted to the leg of the fork and that is weight that has to move up and down. 20mm is simply and inarguably better in this respect. This is why your axle is hollow, and your brake caliper bolts are gun-drilled. In fact, why Suzuki went to 10mm head pinch bolts instead of 12mm head pinch bolts. They are trying to shave FRACTIONS OF GRAMS off of the forks to improve response. So it doesn’t make sense to add a POUND back on!
Q: How much does increasing the diameter of a thin wall aluminum tube by 5mm really make? And what about the increased size on the damper rod?
Max: There is a significant increase in the weight of the cartridge because you have actually added about 20% more material to the tube itself. As bad or worse than that, the slugs of aluminum at either end that hold the compression valve and the bearing assembly are WAY heavier since they are solid parts, and have increased in size almost DOUBLE (cross sectional area increase is much greater than wall thickness of a tube increase). The increase in the weight of the rod is NOT critical because the rod is sprung mass, not unsprung mass.
Q: Don't most bikes come from the factory with 10mm damping rods, with the exception of the GSXR1000?
Max: As far as rods go, it is a hodgepodge of rod sizes from 10, 12 and 12.5. GSXR750s have been 12mm since 1996. The 04 600s now have the same 12mm rods. There are lots of 10s out there.
Q: Wouldn't the rebound valves be larger on a 25mm cartridge and doesn't the rebound valve assembly also displace fluid?
Max: An interesting thing to note is that the relationship of the size of the rod and bore in the 25mm aftermarket cartridges. The rod is 12.5 and the bore is 25, which is 2:1. This is similar to a 20mm cartridge with a 10mm rod. I believe it is better to have a relationship that is lower than 2:1. I have seen some kit Showa parts from Japan that used a 25mm piston, but the rod was bigger than 12.5mm (I can’t say how much bigger…haha!). I think Ohlins is aware of this problem with their cartridges, as they are going to a bigger rod. I don’t know if they are available to the public yet. As far as the 25mm rebound piston displacing more fluid, it does not. It is already submerged in the oil, and moves in it. It does not displace any more or less fluid as a result. Again, only the rod entering and leaving the cartridge has any effect.
Max: As far as there being more options to tune with a larger valve, that isn’t true either. The combinations are basically mathematically infinite regardless of piston diameter. Any number of hundreds of shim diameters and then shim thicknesses could be combined in any sequence on either configuration.
Max: The Axxion Cartridges will have all damping surfaces hold a tolerance of .0005” (one half of one thousandth of an inch…). They will be highly polished aircraft aluminum, and then hard coat anodized. This prevents contamination of the fork oil by oxidation. The bearing design is a never before seen setup that will help seal the cartridge to improve low speed damping and response time, and will virtually eliminating stiction. To go to extremes to create the least unsprung weight possible, the top out spring will be attached to the rebound mechanism, instead of to the cartridge tube like other designs. What is included in the Axxion Cartridge Kit? Each kit will include AK20 Cartridges, our innovative Omni Springs to suit your weight and intended use, with pre-cut spring spacers, new fork caps (if required), and our AK20 cartridge tool for removal and installation.
Q: What are Omni Springs and what are the advantages of using them over other aftermarket spring kits?
Max: Omni Springs are different in that we have one spring that fits most bikes. All Omni Springs are manufactured from lightweight chrome silicon spring steel and are guaranteed for life to be within 2% of the claimed rate. Instead of fitting springs by OD to the tube they ride in, we slide a “buffer” down the center of a lightweight spring to hold it square while it compresses. All other brands of springs drag the steel wall of the tube they ride in. This creates a black oxidation in your oil. Worse than that, the spring “flakes” tiny chips into the oil, which invariably get caught in your compression valves. This results in poor damping and performance.
Max: One additional benefit of the Axxion Cartridge Kit is you will not have to ship your forks out if you don’t want to. This will save time, and risk of potential damage and loss from shippers (although it is very rare). Since the kits use our world famous Axxion Valves, we already know baseline setups and adjustments, so you won’t be “in the dark” at the track, even if I am not personally there to help you. One other great byproduct is that you can remove your Axxion Cartridges, and use them in your next bike by having us retrofit (if needed) them to suit another model. Being that the Axxion Cartridge can be easily transferred to a different bike with little to no modification the savings over a couple years could be very substantial. You can also put your forks back to stock easily when it comes time to sell your bike.