Diaz Suspension Design, or DSD, is a small company based in Durango, Colorado, who offer a variety of suspension tuning services and products. Their fork cartridge, the RUNT, is designed to increase tunability in a fork by replacing the stock volume spacers with an air cartridge that they claim improves the ride quality and adds a coil-like feel, with a wide range of adjustment and a greater amount of suppleness than the stock configuration.
The RUNT is available for a variety of popular forks, including Fox 34 and 36, RockShox Lyrik, Pike, Yari, and Marzocchi Z1 and Z2.
• Replaces volume spacers on air-sprung forks
• Simple installation
• Tunable progression and bottom-out resistance
• Available for Fox 36, 34, Marzocchi Z1, RockShox Lyrik, Yari, Pike, and Revelation
• MSRP: $200 USD
DSD claim the RUNT combats some issues that air springs have. One of which is achieving a steady linear compression, as air naturally compresses at an exponential rate. A lot of times, compression damping can be used to combat this but, according to them, it's not ideal and compromises small-bump performance. Running less air can help small-bump performance, but that can reduce mid-stroke support, and then things can ramp up harshly at the end of the travel.
By having an air chamber instead of tokens to reduce volume, the RUNT is designed to work around this issue and provide the correct amount of support for riders. When the air pressure in the main chamber is equal to the pressure in the smaller high chamber, there is a floating piston in the high chamber that begins to move up with the piston in the lower chamber. This allows for a more linear pressure curve and, according to DSD, a better ride.
The piston in the RUNT moves when pressure in the main chamber of the fork equalizes.Setup/Installation
The installation of the RUNT is straightforward. If you've ever changed volume spacers in your fork, it's that easy to drop in the RUNT. If you haven't, you'll need to ensure that you've let all of the air out of the air chamber on your fork (note your current air pressure ahead of time) and then you will remove the air cap and any volume spacers. Drop in the RUNT in place of the top chamber cap, air things up, and you're good to go.
Setting up the air pressure is simple. The team at DSD recommend taking the pressure you're currently running in the air chamber and using that as your starting pressure in the LOW chamber. For the HIGH chamber, you'll double that number to give a 2:1 ratio and a linear spring curve. If you want things more progressive, you can add pressure to the HIGH chamber which will make the curve more progressive in the last third of the mid-stroke. They also suggest that many riders end up using a little less pressure in the LOW chamber than they previously had been running without the RUNT.
I set the RUNT up on a 160mm travel Fox Factory 36 with the GRIP 2 damper. I started out with 63psi in the LOW chamber but ended up dropping down to 58psi there with about 110psi in the HIGH chamber. I experimented with running a little more or less in both and the differences that it makes in ride quality are noticeable. DSD recommend making small changes - 2.5psi at a time in the LOW chamber and 5psi at once in the HIGH chamber.
For damper settings, I went with DSD's recommended wide open HSC and LSC while closing the HSR. LSR was a few clicks in from fully open.
It's worth noting that DSD can include shims with the cartridge to revalve your fork to what they feel is a better tune or, you can send your fork in to them for a custom tune along with the cartridge as well to get the best performance. We tested only cartridge itself, without a fork tune or revalve for a true back-to-back in how it works in place of tokens, revalve aside.Performance
With the cartridge installed in a 2020 160mm Fox 36, the difference in the amount of support throughout the travel of the fork was noticeable. At the suggested 2:1 ratio, with the same PSI I had been running in the factory set up, it was a bit too firm for my liking. There was plenty of support, but that came at the expense of comfort. I then dropped down the air pressures in each chamber while keeping the 2:1 ratio. That made a noticable difference - the increased support was still there, but the slightly harsh feeling was no longer present. This setup felt especially good on bike park style trails, where there were high speed, hardpacked corners to push into.
I experimented with running a little more and a little less air in both chambers on various back to back runs. The cartridge allows a much wider range of adjustment than can be attained with tokens alone. It was easy to make it feel extra plush yet still supported in rough, rocky sections of trail by running a lower pressure in the large chamber and a touch more than 2:1 in the high chamber. This gave lots of active suspension and traction with plenty of ramp up to avoid a harsh slam at the end of travel. On the other end of the spectrum, it was easy to air up closer to the recommended pressures and the 2:1 ratio to ride jump lines and feel like I wasn't plowing through the travel in turns and off of lips. Even when running the fork a bit soft the transition through its travel was smooth and well supported, with a nice ramp up at the end of the stroke.
It's the ease of tunability that sets the RUNT apart more than anything else. Rather than adding or removing a volume spacer trailside, riders can easily add or remove air pressure. It allows for easier fine tuning than what's possible with volume spacers, and could be a good solution for riders who are looking to really dial in their set up.
Well made, easy to install+
Wide range of adjustability+
May not be a dramatic difference from the stock configuration-
Not a cure-all for poor suspension performance