The current Reverb AXS.
The patent makes plain why SRAM might want to make this change: “Some known dropper posts locate the control module on the seat clamp. However, this location can interface with the rear tire clearance. Therefore, locating the control module (222 ) at the upper end of the lower tube (202 ), adjacent the overlap region, can be advantageous to improve rear wheel clearance. This location also helps maintain a minimal ratio of drop height to overall length.”
I haven’t experienced the rear tire hitting the control unit on Reverbs in the past (it’s something that would be more likely on smaller frames), but clearly, that would be far from ideal if it did happen. My main gripe with the current post is that the travel maxes out at 170 mm, while many riders on modern bikes prefer at least 200 mm of drop. Perhaps this design will allow SRAM to increase the travel without the overall length becoming too long for most frames. It also allows for a return to the twin-bolt clamp head, which is more foolproof than the design currently used, which can twist if not torqued up correctly.
The motor and gearbox assembly (far right) is used to open the valve on the current Reverb AXS.
It’s what’s inside that counts
But there’s a lot more new stuff happening on the inside.
The patent discusses using a solenoid (a spring-loaded electromagnet that moves forward and back when an electric current is applied) instead of a rotating motor to operate the valve that allows the post to move or locks it in place. Again, SRAM spell out why they think this could be an improvement: “motor systems require complex gear systems to create the linear motion to actuate the valve…[and] motor position encoders and monitoring systems to ensure proper valve actuation… [they are] complex to manufacture and can be relatively expensive… The solenoid-operated valves disclosed herein are advantageous because they do not require position monitoring … [this] reduces manufacturing complexity and costs. Further, the example solenoids only make small, linear movements. This reduces the space needed to incorporate the actuation components in the seat post.”
In addition to being more compact, simpler and cheaper to produce, the patent later says that they may use less electrical energy. Perhaps this could mean a longer battery life.
The solenoid which opens the valve is located inside the upper tube of the seatpost. Meanwhile, the controller is located on the outside of the lower tube. In order to transmit electrical signals and power from the external controller to the internal solenoid, a pair of wires run down the lower tube (532 & 534 ) and connect to another pair of wires (538, 540 ) that run up inside the piston shaft (508 ) which holds the piston assembly (510 ) in place.
A suspension(ish) seatpost?
But that’s not all. From what I can tell, the spring and locking mechanism seems to be adapted from their Reverb AXS XPLR, which is part gravel dropper post and part suspension seatpost. That is to say, it could use an air-only mechanism to act as a spring and locking mechanism, with no hydraulics at all. Here’s how it works.
When the post is set somewhere in the middle of its travel (as in Figure 6 above) it works a bit like an air spring with a positive air chamber (516 ) above the main piston and a negative chamber (518 ) below. Both can be pressurised and adjusted via a valve on the head of the post (200 ). This will allow some squish, like the gravel version already in existence. According to the patent, “in an intermediate position, the seat may sag a small amount (e.g., 40 mm or less) as a result of the weight of the rider.”
When the rider presses the remote button, the solenoid opens the valve (512 ) which allows air to move freely between the positive and negative chambers. This, in turn, allows the seatpost to compress under rider weight. When it reaches full compression, there will be a “hard stop”, giving a solid feel.
The reason the seatpost can extend when no weight is applied and with the valve open is that the piston shaft (508 ) effectively reduces the area of the bottom of the piston relative to the top, so when the pressure is equal in both air chambers the force on top of the piston is greater, which causes the dropper to extend. Another way to think of it is that the shaft (508 ) is being forced out of the pressurised air chamber (518 & 516 ) into the atmospheric pressure in the chamber below (526 ). This may be enough to extend the post on its own, or the chamber below (526 ) may also be pressurised independently to provide more return force.
When the post reaches full extension (as in Figure 5 ), the volume of the negative air chamber (518 ) is almost zero. As a result, after the valve is closed, the seatpost will feel much firmer when force is applied to the saddle than it does in the middle of its stroke. The effect is similar to pressurising the air spring in a fork to a high pressure without equalising the air chambers – the spring is topped out and requires considerable force to make it compress even a millimetre. According to the patent: “the seat sags less than 10 mm as a result of the weight of the rider on the seat when the seat post is in the fully extended position.”
So to summarise, this seatpost may have a few millimetres of sag from full extension, feel firm when fully compressed, and could function almost like a suspension seatpost when set to half-mast.
Having said that, the patent stipulates that the collar-mounted controller and solenoid-operated valve could be used with a more conventional hydraulic mechanism: “While the example valves and solenoids disclosed herein are described in connection with seat posts having a pneumatic platform, the examples disclosed herein can also be used in connection with hydraulic platforms.”
What we’d like to see
The Reverb AXS is over five years old and it’s starting to show its age. The 170 mm maximum drop is not enough for a lot of today’s bikes and riders. To realise its full potential, we’d like to see versions with well over 200 mm of movement. I think it would be nice to see a return to a two-bolt seat clamp and if battery life and tire clearance could be improved then so much the better. The idea of a seatpost with up to 40 mm of sag, when set to the middle of its stroke, might be a tough sell for mountain bikers who can remember the Cane Creek Thudbuster, but I could see it being a benefit in some situations – so long as the sag at full extension is minimal and doesn’t increase over time.
Will we see something like this on bikes any time soon? Given how old the original Reverb AXS now is, and how competitive the wireless dropper post market has become, I’d say it’s likely. Needless to say, it was a “no comment” from SRAM on this one.
No products in the cart.