(From left) The negative spring is formed by the space between the main air piston and a seal at the lower end of the air can. In this simplified graphic, RC illustrates the function of the Fox Float RP23 negative spring. Coil springs weigh between 400 and 600 grams (more than a dropper seatpost). Weight? Well you can figure that one out. Last time I checked, air was a bit less expensive than coil springs sold by Fox and RockShox.
An air-spring can be set to many rider weights and preferences - with a air pump. Most riders use the spring that came on the shock - some are willing to buy one with a different rate, and darn few own more than two springs. The linear nature of a coil spring, and the fact that it derives its rate from a combination of coil-spacing and wire diameter, means that coil springs are only useful for a narrow range of rider weights and/or riding intensities, so you'll have to purchase a new spring to experiment with different preload and spring rates. Unless you are rich, the big fails for coil spring are cost and weight.
Every time the shock is compressed, the air piston passes the transfer hole and the negative air spring is recharged with the static pressure of the positive spring.
Air pressure from the positive spring is fed to the negative spring through the air-volume sleeve via a tiny transfer hole near the bottom of the can. The negative spring sits just below the air-spring piston. The air canister above the shock body replaces the coil spring of a standard damper.
#Fox float rp23 stuck down series
With the tables turned, suspension designers now use an end-stroke damping booster or add an air-spring in series with the coil (like Marzocchi's 55 and 888 forks) to emulate the air-spring's beneficial rising rate.įox Racing Shox provided this cut-away illustration of its Float RP23 shock. As it turns out, however, some degree of a rising rate spring is beneficial in both forks and shocks, and suspension engineers have learned to add or subtract air volume from the air spring to tune exactly the right spring rates for mountain bike suspensions. Left to its own devices, an air spring ramps up considerably near the end of its stroke. Gasses, by nature, compress in a non-linear fashion. Compress it three inches and it stores 900 pounds. For instance: compress a 300-pound spring one inch and it stores 300 pounds. The boxing match is not over, however, because the coil spring is essentially linear. As the positive air spring is compressed (right), the minimal volume of air in the negative spring quickly loses pressure and has no effect on the positive spring rate. The addition of a second air valve (center) below the piston, like the RockShox Dual-Air forks, equalizes pressure of the positive spring and suspends the piston slightly above the end-stroke limiter. (From left) Without a negative spring, 200 psi in the air canister forces the piston to the end of its stroke, hard against the travel limiter. This illustration depicts a simple air spring - a piston in a cylinder - to explain how a negative spring functions. Video: Air vs Coil Springs and why the Negative Spring is Important The air-sprung shock's rebound damping must fight that static pressure all the way to the end of the stroke, which, until the the invention of the negative spring, increased the amount of rebound resistance required and caused the shock to ride roughly. A simple air-spring must be set at some static pressure in order to produce the proper ride height and that pressure is quite high (about 200 psi in the case of a shock). This creates the butter-soft feel that once was the sole domain of a coil-over shock. In suspension terms that means, as the coil-sprung shock or fork extends to the end of its stroke, rebound damping begins to overpower the spring as it runs out of stored energy and comes to rest at a static state. The advantage of a coil-spring is that it has a finite length at rest. The first to go is the revered belief that a coil spring delivers head-and-shoulders better performance than an air spring. Today's Tech Tuesday breaks down a couple of doors that have been locked for decades.
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