What's the deal with the Rockshox C1 airshaft?

Posted by: D_C_

^^^

I think his comment is referring to the speed the fork moves though its travel, not the speed the rider is moving forward.

That doesn't make a difference.

The energy that the system needs to absorb as part of a deceleration (either vertically in the case of an impact, or horizontally in the case of braking) would be related to the speed and mass of the rider.

The absorbed energy is the area under the spring force curve, a lighter rider has less kinetic energy and therefore needs less area under the curve to absorb it.

The actual spring force curve shape (how progressive or linear it is) is a separate thing. In general, a lighter rider is smaller and has smaller muscles which means the maximum force they can withstand is lower so adding a significant force ramp up at the end of the stroke probably won't work very well for them. On top of that, the lower leg ramp up becomes a big factor when dealing with smaller riders, as they can't adjust it and it becomes a big percentage of the total spring force when running lower pressures.

A light aggressive rider needs proportional support(force) at the end of the curve, they don't need equivalent support to a heavier rider, they would crumple

Lighter riders can benefit from more linear fork curves (with a tune-able end ramp) just as much as heavier riders

Posted by: wingelabs

Posted by: D_C_

^^^

I think his comment is referring to the speed the fork moves though its travel, not the speed the rider is moving forward.

That doesn't make a difference.

The energy that the system needs to absorb as part of a deceleration (either vertically in the case of an impact, or horizontally in the case of braking) would be related to the speed and mass of the rider.

The absorbed energy is the area under the spring force curve, a lighter rider has less kinetic energy and therefore needs less area under the curve to absorb it.

The actual spring force curve shape (how progressive or linear it is) is a separate thing. In general, a lighter rider is smaller and has smaller muscles which means the maximum force they can withstand is lower so adding a significant force ramp up at the end of the stroke probably won't work very well for them. On top of that, the lower leg ramp up becomes a big factor when dealing with smaller riders, as they can't adjust it and it becomes a big percentage of the total spring force when running lower pressures.

A light aggressive rider needs proportional support(force) at the end of the curve, they don't need equivalent support to a heavier rider, they would crumple

Lighter riders can benefit from more linear fork curves (with a tune-able end ramp) just as much as heavier riders

I think you’re right. Looks like I need to bone up on my armchair physics.

Posted by: D_C_

I think you’re right. Looks like I need to bone up on my armchair physics.

Thanks, that feels like internet achievement lol.

Not sure if you've seen this before. Someone on the MTBR forums made it. Its not a true model of the airsprings, but it does allow for some cool comparisons.

Play around with the table at the top left. 

Scenario 1 C1 (although i'd change the X_port value to 0)

Scenario 2 is SA

Scenario 3 is B1

https://www.desmos.com/calculator/w9tcc3upuh

Posted by: wingelabs

*This is honestly not an advantage but its probably a big issue for RockShox and is apparently a sufficiently large sticking point that even Steve from Vorsprung mentioned on a different forum that the Secus preloads the top out ever so slightly (less than the C1) because people were unhappy with the Luftkappe robbing them of travel

I've been kind of surprised by this, mtb is such a dynamic activity by it's very nature yet people are so concerned with what the part is doing while just sitting there.

Maybe the issue is we've been thinking about setting sag from the backwards point of view. Perhaps we need to view sag as a measure of how much travel is available for use, as opposed to how much of available travel is used in a static position. The 2 are really the same thing, but the difference is if the fork sinks 10-15mm into its travel, and then you add an extra 20% of sag into the equation, you are left with less travel (and a fork that's probably set up too soft). But if you include whatever your fork sinks into under its own weight as part of the overall sag measurement, then life is good. 

But maybe I'm just oversimplifying the 'problem'

Posted by: UFO

I've been kind of surprised by this, mtb is such a dynamic activity by it's very nature yet people are so concerned with what the part is doing while just sitting there.

Maybe the issue is we've been thinking about setting sag from the backwards point of view. Perhaps we need to view sag as a measure of how much travel is available for use, as opposed to how much of available travel is used in a static position. The 2 are really the same thing, but the difference is if the fork sinks 10-15mm into its travel, and then you add an extra 20% of sag into the equation, you are left with less travel (and a fork that's probably set up too soft). But if you include whatever your fork sinks into under its own weight as part of the overall sag measurement, then life is good. 

But maybe I'm just oversimplifying the 'problem'

I think that one issue is that we talk about sag as something you can measure with an o-ring balancing against a wall on flat ground. Dynamic sag isn't something you can measure with an o-ring, you need to determine it by feel or with instrumentation. Really we need to get our dynamic sag in the right range and then adjust other factors, rather than trying to have a supple initially stroke while also never bottoming out (packing your fork full of spacers).

I think the idea of "using the travel I paid for" is also part of the problem. Bottoming out a fork hard is an easy way to lose control. I want travel in reserve to cover me when I mess up.

If a B1 air shaft causes you to loose about 10mm of travel should you bump up to the next travel? IE if you have a 160mm Lyrik with a C1 air shaft should you but a 170mm B1 air shaft?

Posted by: heathen

If a B1 air shaft causes you to loose about 10mm of travel should you bump up to the next travel? IE if you have a 160mm Lyrik with a C1 air shaft should you but a 170mm B1 air shaft?

I wouldn't think so. Even if the fork sits a bit into it's travel you still set it up for the same sag.

Posted by: heathen

If a B1 air shaft causes you to loose about 10mm of travel should you bump up to the next travel? IE if you have a 160mm Lyrik with a C1 air shaft should you but a 170mm B1 air shaft?

I have a Norco Sight and with the "Ride Aligned" recommendations I'm running 91 psi in my fork with 2 tokens. My sag is at 25%, but after riding for a couple minutes if I get off my bike it's at the 10% sag marker with no weight. After a ride I always have about 20mm of travel left. 

I'm wondering the same thing as you. 

What better, 170mm B1 airshaft or 160mm C1 airshaft performance wise?

You don't 'lose' that travel though.  When you're riding that 10mm will come into play as your fork extends into bumps and whatever other side of roots or whatever. 

If you consistently have about 20mm of travel left and you're happy with where your sag is set, that probably means you could drop down a volume spacer or half?

I'm aware of that, I should've asked if you wanted to increase your riding height what would be the best option? 170mm B1 airshaft or 160mm C1 airshaft performance wise?

Any other feedback on the newest spring?

I gotta think those norco guys' settings are for the old spring? 

Compensating for the soft top end - no spacers, lots of pressure, no compression damping.

My first ride on my 2021 lyrik yesterday with factory settings was a little underwhelming. 

At recommended pressure it felt way firm off the top. Granted it was raining and slimy so conditions would have favored a softer setup. 

Just harsh and no traction. I let 5psi out and it was better but on the verge of mushy in the midstroke. 

Only one ride in, but I get the sense SRAM compensated for the previous spring's "suck down" so much that this one is going to be happier with less pressure and more tokens, to regain some sensitivity.

^^^

Pretty sure you’re right about the Norco setting being for the old spring. The C1 feels better closer to RS’ recommended pressure.

The 'Ride Aligned' from Norco's website is with the B1 debonair.

Just upgraded my lyric to the c1 shaft. Its hard to compare as I went from 160 to 170 and put it on a different bike but I will say I think I like it better than my 36 factory.  It does ride high and was giving me good support. I think I put in 5-10% more pressure than I needed. Will try a lower psi next ride.

Gonna upgrade my pike next.