The 2021 SR Suntour Auron takes their top-of-the-line mountain bike trail fork and adds their Air EQ system to automatically balance positive and negative air chambers. Combined with their sealed, highly adjustable PCS damping cartridges, they say it gives you a supple, coil spring-like feel with plenty of tuning options.
The tricky part is how they make that work with any length travel within the fork’s 130mm to 160mm range, all without changing the spring curve. Here’s the deep dive on the tech…
How SR Suntour’s EQ air spring works
Beyond the Air EQ air spring, the Auron fork carries over from 2020, so we’ll start by explaining how the EQ works and its benefits.
Using a transfer port on the inside of the air spring, air can slip past the piston when it’s fully extended. So, as you’re pumping up the fork to the desired pressure, it’s filling both chambers simultaneously. Basically, EQ = Equalize.
Technically, that means there’s no need to cycle the fork to pressurize the negative chamber then re-check and re-adjust the positive. Buuut, they still recommend that you do push it down a couple times then re-check just to be sure.
SR Suntour’s This makes set up much quicker and simpler, and helps you get the right air pressure every time. The only reason to adjust it is if you need to add or remove sag.
The Auron can be setup with 130, 140, 150, or 160mm of travel. But, since the position of the air spring’s main piston seal (A) needs to remain constant in relation to the stanchion’s internal check valve, the only way to do that is to change the height of the rod (E).
The other parts in the photo above are the top out bumper (B), main air spring seal (C) to hold all of the air inside the stanchion, and the cap (D) that threads into the bottom of the stanchion to hold it all in.
Here’s where it gets tricky…
No matter which travel length you select, the air volume in both chambers remains exactly the same. So, if you were to only change the height of the piston rod (E), then a long 160mm travel fork would send that piston head (A) almost all the way to the top of the chamber. This would compress the air a lot and create a big ramp on the spring curve.
But, a short 130mm travel setup wouldn’t push the piston as far into the positive air chamber, so it’s not going to ramp nearly as much. Meaning, it’s going to have a much more linear spring curve. So, what’s the solution? Volume spacers!
Factory settings are for 7-9 volume spacers inside the positive chamber, with 130mm and 140mm both getting 9, then dropping one as you go up each 10mm in travel. The forks will fit a maximum of 10 spacers, and you can always pull some out if you want.
The EQ system debuted on the longer-travel Durolux enduro fork last year, and that story explains some of the testing they did to design the EQ air spring.
Auron RC2 PCS damper
PCS is SR Suntour’s “Piston Compensator System” and is their way of handling an IFP to keep pressure on the damping fluid inside the fork. Where most brands use a high pressure nitrogen-gas filled chamber, SR Suntour uses a lightweight coil spring. The result is that they can run lighter seals, so things move easier for less stiction.
They introduced this design on the 2016 Durolux and you can read a deep dive on that tech right in that story.
The other part of their damping is the RC2 cartridge, which means external adjustment knobs for Rebound (low speed only) and Compression (high and low speed) damping. They say the RC2 damper actually outperforms the R2C2’s rebound circuit, in part because it has to do less, and their pro riders preferred it. So, for North American customers anyway, RC2 is the only option available…which is lighter, simpler, and also less expensive.
Which brings us to price.
SR Suntour’s designs mean you can easily drop in a new damping cartridge if something goes wrong (which is unlikely). And air spring changes are pretty easy, too. They want you to be able to work on your fork while still offering an absolutely high-end, high-tech fork.
And they want it to be affordable. The 2021 Auron 35 EQ RC2 Boost fork retails for just $679.95, but as of this post it’s on sale for $579.95 in North America. Link at bottom is to the U.S. portal for that deal.
The Auron is available in/with:
- 27.5″ and 29″ wheel sizes
- 44mm and 51mm offsets (44 only for 27.5)
- 130-160mm travel
- eMTB rated
- Integrated fender
- 35mm A7000 stanchions
- 15QLC2TI titanium QR thru axle
- Fits modern trail rim/tire sizes
Available now for pre-order, they should be shipping soon. Available in gloss black and matte gray with 15×110 axle spacing only. We’ve got one in for review and will be testing into early summer…stay tuned.