The one thing you forget is this is a closed system. So you have to remember you end up not compressing just oil but also air. So in this case you can control how high the oil is which will control how much oil has to be displaced to start to compress the air.
Hope I'm not out of place here since this is an Ohlins only thread. The only thing displacing oil therfore creating pressure in the fork is the cartridge rod. The piston, shims, etc are all submerged in oil. Therefore they are not calculated when oil is being displaced. So, if you have a set Cartridge Rod diameter in a 20mm cartridge and in a 25mm cartridge the 20mm cartridge will always create more pressure quicker. The oil level in a fork is there for bottoming resistance. You should not adjust the oil level to try and create more or less pressure. Only to reduce or increase travel. I will be happy to answer and questions, if asked. Thanks!
The air in the fork really only affects the damping of the fork near the end of it's stroke. It does not however have anything to do with the damping characteristics of the shim stack. Yes the air compresses and provides MORE SPRING to the degree that it acts like a progressive spring. This may be another reason that linear rate or straight rate springs are mor efavorable than progressives. The progressives combined with the air in the fork can get very hard at the end of the stroke. The cavitation that I'm refering to is created immediatly upon the forks movement. This cavitation is velocity based not neccesarily stroke based. If at full extension you hit a bump very hard or do very hard braking suddendly the fluid cavitates through the shim stack, damping is eliminated and the fork now collapses without damping controll untill the velocity drops or the fluid stops cavitating. Forget about dissplacing fluid as it relates to damping and stroke. The fluid is forced through a piston that uses shims to controll it's damping curve ( reaction to velocity ). This curve does not change as long as velocity does not change. That is to say that the damping of the shims is based soley upon the velocity of the piston in the cartridge. It has nothing to do with the volume of fluid. The volume of fluid in the fork leg can be used as a way to controll travel of the fork. If you have very little oil in the fork ther is a larger volume of air. This air can be compressed more and this will allow the spring and damping parts do there job more. However the forks spring and damping parts do not stop the forks travel, only it's controll over velocity. The air inside will provide progressive resistance to the forks compression or upward travel. The less air in the fork the quicker it will build enough compression to affect the forks velocity or travel. This is why fluid level is so important. If it is not to spec the forks overall performance will not work around the way the internals were designed to work. The forks internals are designed around oil wt., oil height in the fork, and spring wt. That is why there are different shims to controll damping. These shims will help work with all the parameters. Essentially the shims don't care about spring wt. as this is factored to compensate for your weight on the machine. However the amount of fluid in the fork is important. to little and the fork will compress untill it bottoms out. This is provided there is enough force to compress it untill it does bottom out. Most people will just crank up the compression on the fork. The compression adjuster does not controll fork travel only compression velocity. If there is enough force to keep compressing the forks the internals will only controll it's damping. The air inside of the forks is what ultimatly controlls the forks overall travel. If there is enough fluid in the forks ( less air in the forks ) the compression of the air will be enough to eventually over come the force being applied to the forks. This is again based upon velocity and the length of time that the velocity is more than that of which the springs can controll. In a nutshell the air in the forks is a completly seperate damping system. it does nothing to the actuall performance of the cartridge and the spring installed but does affect the overall performance of the forks. Yes you are correct on the compression of air but you can't really call it displacement of fluid as much as displacement of air. It is the air that is trying to be displaced since the fluids volume is always the same. we are changing the volume of air when the forks compress. Or at the very least the area in which it consumes. however we are never increasing the amount of volume the fluid is consuming. The amount of air in the forks is calculated into the design of the cartridge. This is why they are so sensitive to oil height. In order to give you the damping that you want the oil level should not interfere with it at the end of the forks travel. If you are always bottoming out but have the damping that you need to get the handling you want you will have to add oil. This will change the forks performance as the forks compress.
Thanks guys. I would still love to see the comparo dyno charts even if I don't have a clue how to read them right away.
The funny thing about current trends is that sometimes they dead end. Remember when single sided swing arms were the best thing since sliced bread and all the hype? You don't really see that much of that trend now. I think the most common thing you hear from all suspension tuners is to use whatever works best for you. And the one good piece of advice that is not in any suspension DVD or article is....don't tell a guy that spent $2500 on suspension parts that what he just purchased is total shit! That's a no fun conversation. And who is this guy? trispdtrip? What, is he some kind of "expert" or something? Oh wait.......he is? I'm not going to listen to him anyway! JK!
excellent contribution. i should not have phrased it as the "best" setup, since best is relative. what works awesome for one person may not be so hot for another, etc... i meant 25 mil is the most expensive and seems to be the most saught after. also, i've read a couple threads where people have tried both and preferred the 25 mil. once again tho, it's all personal preference. you need to have the sensitivity and the pace for any of this stuff to matter anyway.
Why would the 20mm create more pressure than a 25mm? Assuming they are valved correctly. Are you saying that since there is more fluid inside the 25mm cartridge, the pressure will not be as much?
no, i think he is saying that ASSUMING ROD SIZES/DIAMETERS ARE EQUAL, a 20 mm will build pressure faster than a 25 mm simply becase the ratio will be tighter on the 20 mm, whereas with 25 mm, the rod is effectively smaller/ displaces less oil per distance of stroke. if you adjust the rod size to be in the same ratio, then they build the same pressure...
Equal size rods will displace the same amount of fluid whether its in a 20mm, 25mm, or 200mm cartridge.
Except the rod is going into a tube and that tube size is different so with a 20mm piston the rod diameter is larger in ratio compared to a 200mm piston.
And? The pressure is due to the valving resisting the fluid. Since the fluid is an incompressible liquid, the pressure is exerted equal through the fluid. So, i still do not see how the pressure will be lower? Assuming the valving is set up correctly.
Oil is compressable....just not very much. Anyhow you get pressure on the face of the piston and shim stack by pushing it through. Also get pressure by adding in another object in the oil as the oil has to be displaced.
Yes oil is compressible, not in a cartridge though. The pressures needed to compress the fluid would rupture the cartridge.
Ok, look... so, we have: 1. cartridge with a 12.5mm rod and a 20mm piston and 1 cartridge with a 12.5mm rod and a 25mm piston... let's say that we're gonna compress each of them an inch when we hit the same bump of the same size at 50mph. ok? now, the cartridge with the 20mm will build pressure faster. why? BECAUSE IT DISPLACES MORE OF THE TOTAL FLUID CONTENT OF THE CARTRIDGE "FASTER" OR WITH LESS TRAVEL THAN THE 25MM CARTRIDGE WOULD. i.e., to build an equivalent pressure in the 25mm cartridge, you may need to compress the fork another half inch, or something along those lines.
the 25mm compresses faster because it builds less pressure for a given distance of travel/time (once again, because of the ratio of the rod to the cartridge diamter). it makes perfect sense to me. i'm not sure what else needs to be said....
