RockShox Debonair C1 “upgrade”

[Nuffy]

The last update to RockShox’s Pike, Revelation, Lyrik, and Yari forks was just a change to the C1 air spring with a new foot nut and seal head. I believe this was done as a response to complaints from users that their forks were sagging into the travel and so had less travel than advertised. A 160 mm fork might have been sitting at about 155 mm at rest, for example. RockShox has touted the revision as providing a higher ride height and no wasted travel or dead space. However, suspension specialists (such as Chris Porter and Vorsprung) have pointed out that the C1 seal head creates a smaller negative air chamber, resulting in a less supple start to the travel and less small-bump sensitivity. Their recommendation seems to be to stick to the B1/B2 seal head and enjoy the better grip and sensitivity at the top of the stroke, and stop obsessing over the travel markings on the stanchions.

Based on my understanding of how the system works, it’s the longer foot nut of the C1 version that extends the fork out so it has more travel and is riding higher. It is equivalent to fitting a slightly longer air shaft. I can only hypothesise that the seal head was revised in order to reduce the negative air volume so that the fork cannot be extended deep into the negative travel to the extent that insufficient stanchion and bushing overlap become an issue. Is anyone able to confirm this, or correct me, please?

If I am right, does that mean that one could use the longer C1 foot nut to get the higher ride height, but combine it with the B1/B2 seal head for the large negative chamber, provided that the fork is not already at the maximum travel for that model? So with a 160 mm air shaft in a Lyrik and a 140 mm air shaft in a Pike this should be fine, and would give the advertised travel but also the superior performance of the larger negative air chamber?

Any clarifications, corrections, or references to further reading or explanations are appreciated!

Edited September 2, 2021 by Nuffy

[thebob]

The foot nut changes where the fork chambers equalize their pressure, nothing to do with travel. There is a lot of chat on this topic on the MTBR forum but the consensus is if you try and combine the C1 footnut with the B1 air shaft you will end up with a fork that does not work very well. If you want more travel, buy the proper air shaft for your air spring system.

From personal experience there is not a massive difference between the two springs. You can get a much bigger impact on performance by servicing the fork regularly and getting the bushings burnished

Edited September 2, 2021 by thebob

[Nuffy]

Thanks @thebob. I’ll peruse the MTBR forum for further reading.

There is a basic fundamental concept I’m struggling to get my head around: What is it that causes the B1/B2 air piston to not sit at the equalisation dimple? Surely if the positive and negative chambers have equal pressure then there is nothing to pull it in either direction away from the dimple? That is, until you have the weight of the lower legs and wheel extending the fork or bike and rider and trail inputs compressing it, but that would apply in just the same way to the C1 system.

[thebob]

When the chambers equalize, when you push the seal past the dimple, the chambers will have the same pressure. Bear in mind at the same time the negative chamber is expanding in size for a small time as this happens. When the fork goes back to it’s resting position, the size of the negative chamber has shrunk relative to when it was being equalized. Suddenly the 70PSI at the equalization volume is under pressure and is more than 70PSI, say 80PSI. This is the negative force that pushes the fork into its travel. When the negative chamber is too big or has to much pressure in it, you get fork suck down, which was happening to the B1 forks. The same can happen if you let all the pressure out of your fork too quickly or if the equalization port is plugged with grease and the negative chamber cannot release its air.

[Nuffy]

3 hours ago, thebob said:

When the fork goes back to it’s resting position

But what makes the fork go back to its resting position, if the resting position is not with the piston at the dimple?

Perhaps it’s a matter of semantics, as I agree with most of what you’ve said. Let me try to explain it a different way. According to my understanding:

B1

Starting with the assembly of the fork (let us say a 150 mm Lyrik), the air shaft is inserted into the bottom of the stanchion and the retaining ring is installed. Assuming the seal head is pushed all the way up against the topout bumper which in turn is pushed all the way up against the air piston, the air piston will not reach the equalisation dimple on installation. The positive and negative chambers both have 0 psi. Let us call this the starting position.

The lower legs are installed. A pump is attached to the positive chamber and it is inflated to 100 psi, let us say. Nothing moves, and the negative chamber is still at 0 psi and the lower legs sitting at the 150 mm stanchion marking. The fork is compressed slightly. The pressure in the positive chamber increases and the pressure in the negative chamber decreases, making the pressure differential between the two even greater. The fork is compressed further, to the point that the air piston reaches the equalisation dimple. Let us say this is a distance of 5 mm from the starting position. The positive and negative chambers equalise, both settling at 90 psi, let us say. More air is added and the equalisation procedure repeated until both chambers are at 100 psi. Now the air piston is sitting at the dimple and stays there because nothing is pushing it away in either direction. It is 5 mm higher up the stanchion than when it was installed, meaning the lower legs now sit at the 145 mm stanchion marking. Let us call this the resting position.

RockShox gets complaints because in the resting position the “150 mm” Lyrik shows only 145 mm of available travel on the stanchion.

C1

RockShox releases the C1 air shaft. It is the same as the B1, except that the foot nut is 5 mm longer and the seal head sits 5 mm higher above the retaining ring (I don’t know what the actual measurements are – I’m just continuing with my example from above). Again we start with the installation of the air shaft, except this time in the starting position the main piston does reach the dimple. Because the seal head sits 5 mm higher, the main piston is also pushed up 5 mm, and it meets the dimple. This would mean the lower legs are also 5 mm higher up the stanchions at the 145 mm mark, except we now have a 5 mm longer foot nut, which lowers them by 5 mm so they’re back at the 150 mm mark. The positive chamber is inflated, and because the piston is already at the dimple the negative chamber inflates at the same rate – no extra equalisation procedure is necessary. Both chambers reach 100 psi together, and the fork hasn’t moved from its starting position, but it’s now also at its resting position.

 

Is this an accurate interpretation of how they work? Perhaps I’ll try to create a diagram to illustrate it all.

[Nuffy]

7 hours ago, Nuffy said:

one could use the longer C1 foot nut to get the higher ride height, but combine it with the B1/B2 seal head

After further reading, it looks like the C1 foot nut can’t be used with the B1/B2 seal head because under full compression the foot nut will hit the seal head. The C1 seal head has a cavity into which the foot nut can move.