SRAM goes beyond simply displaying its components at Interbike. Throughout each day, SRAM and RockShox tech experts demonstrate how to disassemble and service everything from bottom brackets, hubs and suspension items, to the Reverb dropper seatpost. I stopped by to discuss two RockShox Reverb questions that had been on my mind with Simon Stewart - a RockShox expert and one of the best guides I have ridden with. My first question was about how air could occasionally work its way into the oil column of early Reverbs and cause them to act like suspension seatposts. My second question was to address the possibility of RockShox being able to offer an adjustable-travel Reverb in the future, to address riders whose saddle heights prevent them from using longer-stroke posts.
The "suspension post" issue turned out to be a simple one. Reverb internals use a floating piston to separate the air-spring chamber at the top of the post from the oil column below it. The oil column is metered to change the height of the post, and also must support the rider, who is often banging on the post with his or her full weight. Because oil can't be compressed, it acts like a solid shaft, supporting the rider when the control valve is closed. The problem arises when air slips past the floating piston into the oil column. Even a small amount of air will cause the post to act like a spring. Simon explained that air could leak past the piston when side-loads or internal pressure overwhelmed the O-ring seals, or caused contact and wear between metal parts.
The first floating piston was machined from aluminum and had O-rings on its outer and inner surfaces to maintain a seal. Bushings in the form of white plastic "glide rings" on either side of the O-ring seals were used to separate the metal bits and to give the post a seamless feel. Stewart said that bending forces on the post could sometime cause the metal floating piston to contact the aluminum inner shaft and wear grooves that could allow air to pass the seals. RockShox engineers then developed version two - a plastic floating piston, which could not abrade the shaft, but still used O-rings as primary seals.
According to Simon, version two fixed 99.999-percent of the problem, but in rare cases, side loads on the longer-stroke 150 and 170 millimeter posts could still coax air past the O-ring seals. Version three - which is installed on current Reverb posts - eliminates the O-rings completely. The entire piston is molded from a seal material, and it is engineered to expand against the shafts when it is pressurized. The pressure-actuated seal is not affected by bending or lateral loading and it also compensates for wear - all of which has reportedly solved the issue completely. The new piston is also included in RockShox Reverb rebuild kits, so customers who send their posts in for service will automatically receive the upgrade.
Why the need for an adjustable stroke feature for dropper posts? Two reasons: with seat tube angles evolving steeper, riders are positioned significantly higher above the bike, and at the same time, steeper angles move the saddle nearer to where the rider must hover to control the bike on descents. In most cases, those effects require a longer-stroke dropper post to get the seat low enough to clear the rider. Unfortunately for some riders, frames with tall seat tubes create a situation where the saddle height with a 150 or 170-millimeter dropper post is just a little too tall for pedaling, even when the post is slid all the way down in the frame. In such cases, the option is to choose a post with 30 to 50 millimeters less stroke - which often places the saddle uncomfortably high in the fully dropped position. The advantage of being able to adjust your dropper posts's stroke in five or ten millimeter increments is that you would be able to maximize the amount of drop you need, rather than be forced to accept a 20 to 30-millimeter reduction in travel, downsizing to the next available post.
Simon told me that RockShox was aware that customers were asking for an adjustable Reverb, but he was not in a position to comment about whether or not the engineers were working on a solution. So, I asked if there was a simple fix, like an aftermarket spacer that customers could install under the post's top-out cushion to reduce the travel in five millimeter increments - sort of like small headset spacers. He said that, while that spacing the post would be possible, it would also be impractical. The Reverb mechanism tops out at full extension against the lower side of the poppet-valve body, which is located deep inside the guts of the post. Besides the fact that accessing the poppet valve requires a complete disassembly of the post, the cramped space between the poppet valve body and the tube surrounding it leaves little room for a spacer. If the top-out spacer failed, it might destroy the inside of the mechanism. So, the short version is that, barring a redesign of the Reverb's internals, we won't be seeing a travel-adjustable version soon.