Post by Mx Life286 on Sept 27, 2005 21:14:12 GMT -5
QUESTION ONE: WHY DOES A FORK HAVE OIL IN IT?
Oil is used to control how fast the fork moves when compressing and rebounding. The oil is squeezed through orifices and around washers to create resistance to movement. Oil is the reason that forks don’t dribble like a basketball.
QUESTION TWO: WHAT STOPS A FORK FROM BOTTOMING?
In the bottom of the fork there is an aluminum plug. The plug acts as a hydraulic stop. The inside of the aluminum plug is shaped like a female cone. At the very bottom of the fork slider is a male-shaped cone. As the fork nears the end of travel, oil gets trapped between the two cones and the fork stops hydraulically on a film of oil.
QUESTION THREE: WHY DOES EVERYONE OBSESS ON OIL HEIGHT?
A fork cannot be completely filled with oil or it wouldn't budge. There needs to be a relief space so that the oil has a place to go when the fork tubes collapse. This empty void is called the trapped air space (and it is measured as the space between the surface of the oil and the top of the fork tube). Because air is compressible and behaves like a spring when contained, the trapped air acts as an air spring that compliments fork damping. By adding or subtracting air from the trapped space (by raising or lowering the oil height) it's possible to tune fork action.
QUESTION FOUR: WHY ARE FORKS UPSIDE-DOWN?
Make no mistake about it, motorcycle forks prefer to be right-side-up (with the sliders on the bottom and the fork legs on the top). This layout puts the oil in a large reservoir at the bottom. It is held in place by gravity and pressurized to where it is needed by fork movement.
So, why are modern forks upside down? In order to make a right-side-up fork stiff enough to absorb the loads created by modern doubles and triples, the fork tubes would have to be enormous (at least 50mm in diameter). There came a point where making the steel fork legs bigger was counter productive. The forks got very heavy. Inverting the fork gave the triple crown a larger area to clamp, gave more room for valving, lessened flex by having the largest structure at the top of the fork, eliminated underhang and saved weight.
QUESTION FIVE: WHAT'S BAD ABOUT INVERTED FORKS?
Lots of things. Hydraulically they are all wrong. With right-side-up forks you have a vast reservoir of oil to use, but with upside-down forks, the oil has to be pumped up to the top of the fork to be used. Additionally, inverted forks are often so stiff that that transfers forces to the chassis. If you make one part stiffer, you spread the load to the next piece in line. Without fork flex to provide a forgiving ride, valving, damping, spring rates and frame design had to be refined.
Over the past 15 years, there have been many more misses with inverted forks than hits.
QUESTION SIX: WHAT'S A BUMPER FORK?
For the last three years, the battle has been between bumper forks and bladder forks--with bumper forks winning the war. But, what is a bumper fork?
When inverted forks were designed, the hydraulic bottoming system was moved from the bottom to the top. The cone system had to be made larger and it encroached on the space available for the valving cartridge. Kayaba's solution was to replace the bottoming cone system with a rubber bumper. Working in unison with the bumper was an oil-lock piston added at the end of the cartridge rod. The oil lock stopped on a layer of oil just like a bottoming cone, but took up much less room.
QUESTION SEVEN: WHAT'S A MID-SPEED VALVE?
By replacing the bottoming cone with a bumper, Kayaba got more room for valving, so they included an additional valve at the top of the cartridge assembly. This valve controlled the rate at which the oil circulated out of the valving cartridge. Called a mid-speed valve, it was supposed to keep the oil pressure from jacking up too high and smooth out the action on sudden hits. It also increased the rate of the oil flow through the cartridge system and the chance for more air and dirt to interfere with the valving.
The mid-speed valve helps hold the fork higher in its stroke, which leaves more travel for when it is really needed.
QUESTION EIGHT: WHAT'S A BLADDER FORK?
Bladder forks may soon be ancient history, but the idea was quite innovative. The bladder fork is the poor man's version of a Showa Twin-Chamber fork (which is really a rear shock mounted inside a set of fork legs). A rubber bladder (a balloon) seals the oil inside the cartridge from the oil in the main chamber.
Unfortunately for bladder forks, they didn’t work as well as bumper forks. And, on the whole, the advantages of completely separating the inner cartridge chamber from the outer tube chamber are becoming fewer and fewer.
Oil is used to control how fast the fork moves when compressing and rebounding. The oil is squeezed through orifices and around washers to create resistance to movement. Oil is the reason that forks don’t dribble like a basketball.
QUESTION TWO: WHAT STOPS A FORK FROM BOTTOMING?
In the bottom of the fork there is an aluminum plug. The plug acts as a hydraulic stop. The inside of the aluminum plug is shaped like a female cone. At the very bottom of the fork slider is a male-shaped cone. As the fork nears the end of travel, oil gets trapped between the two cones and the fork stops hydraulically on a film of oil.
QUESTION THREE: WHY DOES EVERYONE OBSESS ON OIL HEIGHT?
A fork cannot be completely filled with oil or it wouldn't budge. There needs to be a relief space so that the oil has a place to go when the fork tubes collapse. This empty void is called the trapped air space (and it is measured as the space between the surface of the oil and the top of the fork tube). Because air is compressible and behaves like a spring when contained, the trapped air acts as an air spring that compliments fork damping. By adding or subtracting air from the trapped space (by raising or lowering the oil height) it's possible to tune fork action.
QUESTION FOUR: WHY ARE FORKS UPSIDE-DOWN?
Make no mistake about it, motorcycle forks prefer to be right-side-up (with the sliders on the bottom and the fork legs on the top). This layout puts the oil in a large reservoir at the bottom. It is held in place by gravity and pressurized to where it is needed by fork movement.
So, why are modern forks upside down? In order to make a right-side-up fork stiff enough to absorb the loads created by modern doubles and triples, the fork tubes would have to be enormous (at least 50mm in diameter). There came a point where making the steel fork legs bigger was counter productive. The forks got very heavy. Inverting the fork gave the triple crown a larger area to clamp, gave more room for valving, lessened flex by having the largest structure at the top of the fork, eliminated underhang and saved weight.
QUESTION FIVE: WHAT'S BAD ABOUT INVERTED FORKS?
Lots of things. Hydraulically they are all wrong. With right-side-up forks you have a vast reservoir of oil to use, but with upside-down forks, the oil has to be pumped up to the top of the fork to be used. Additionally, inverted forks are often so stiff that that transfers forces to the chassis. If you make one part stiffer, you spread the load to the next piece in line. Without fork flex to provide a forgiving ride, valving, damping, spring rates and frame design had to be refined.
Over the past 15 years, there have been many more misses with inverted forks than hits.
QUESTION SIX: WHAT'S A BUMPER FORK?
For the last three years, the battle has been between bumper forks and bladder forks--with bumper forks winning the war. But, what is a bumper fork?
When inverted forks were designed, the hydraulic bottoming system was moved from the bottom to the top. The cone system had to be made larger and it encroached on the space available for the valving cartridge. Kayaba's solution was to replace the bottoming cone system with a rubber bumper. Working in unison with the bumper was an oil-lock piston added at the end of the cartridge rod. The oil lock stopped on a layer of oil just like a bottoming cone, but took up much less room.
QUESTION SEVEN: WHAT'S A MID-SPEED VALVE?
By replacing the bottoming cone with a bumper, Kayaba got more room for valving, so they included an additional valve at the top of the cartridge assembly. This valve controlled the rate at which the oil circulated out of the valving cartridge. Called a mid-speed valve, it was supposed to keep the oil pressure from jacking up too high and smooth out the action on sudden hits. It also increased the rate of the oil flow through the cartridge system and the chance for more air and dirt to interfere with the valving.
The mid-speed valve helps hold the fork higher in its stroke, which leaves more travel for when it is really needed.
QUESTION EIGHT: WHAT'S A BLADDER FORK?
Bladder forks may soon be ancient history, but the idea was quite innovative. The bladder fork is the poor man's version of a Showa Twin-Chamber fork (which is really a rear shock mounted inside a set of fork legs). A rubber bladder (a balloon) seals the oil inside the cartridge from the oil in the main chamber.
Unfortunately for bladder forks, they didn’t work as well as bumper forks. And, on the whole, the advantages of completely separating the inner cartridge chamber from the outer tube chamber are becoming fewer and fewer.