DSD RUNT Review | Best Bike Upgrades – Bicycling
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Better mid-stroke support and more control of progression without altering small bump performance.
I’m going to tell you about a cool little suspension mod I run in my forks. It’s called the RUNT and it’s made right here in Durango, Colorado by a little outfit called Diaz Suspension Design (DSD).
DSD is a suspension repair and tuning shop with two silly-fast employees: founder Anthony Diaz, and Dylan Stucki. The RUNT is a $200(ish) drop-in upgrade for many air-sprung forks. It’s a handful of grams heavier than the stock cap/token stack, but it provides more tuning control and, I think, better overall feel and performance.
DSD makes RUNTs for many RockShox and Fox/Marzocchi models. There’s no version for Manitou forks because its IRT system—which is stock on its higher-end forks—is similar to the RUNT.
According to Diaz, standard air springs and plastic volume spacers are problematic, “A steady linear compression is very tough to achieve with air-sprung suspension because air naturally compresses at an exponential rate. Because of this, air forks naturally lack mid-stroke support. In order to overcome the lack of support, most manufacturers have unnecessarily stiff compression damping.
“As a result, most riders begin to run less and less air pressure to maintain efficient small bump compliance. With the air spring’s lack of mid-stroke support, most riders think that adding progression to their suspension with volume spacers will help get some support back, but it actually causes a dramatic spike toward the end of the travel. This contributes to a less-than-optimal suspension setup, leaving riders sacrificing small bump plushness for stiffer cornering performance, or vice versa.”
The RUNT adds a third air chamber to an air fork’s existing positive and negative chambers. It’s a second, independently adjustable, positive chamber that lets you fine-tune the end of stroke progression, without altering small bump sensitivity or mid-stroke support.
The RUNT’s second, smaller, positive air chamber— labeled “H” on the RUNT—is isolated from the main positive chamber by a floating piston. As the fork compresses, the RUNT’s piston is held in place by the pressure in the RUNT’s H chamber. When the pressure in the main (L) chamber builds enough to overcome the force holding the RUNT’s piston in place the RUNT’s piston begins to move which progressively increases the volume of the main (L) air spring chamber. While RUNT’s piston moves upward, the volume in the RUNT’s H chamber decreases, so the amount of pressure it takes to move the RUNT’s piston increases.
The rider can adjust how progressive the fork is, as well as some adjustment of mid-stroke support, by altering the pressure in the RUNT’s H chamber. That’s independent of the pressure in the main chamber which controls sag, as well as initial mid-stroke support.
The difficult part for Diaz was tuning the size of the RUNT so the volume of all the air springs involved work properly. That’s why there’s a different RUNT for every 10mm travel increment. However, most RUNTs can work with two travels by swapping in a different piston shape.
How is this different than a standard air spring with tokens? Whether you add or subtract tokens, the positive spring still has a fixed compression ratio. With the RUNT, the fork has a varying positive air spring volume, so the compression ratio also varies. That’s how Diaz gets the more linear spring curve while also providing more control over the end of stroke progression. Also, air pressure provides much finer control over the fork’s progression than plastic pucks.
It’s also different than MRP’s Ramp Control, which only alters end-of-stroke progression and doesn’t change the overall spring curve the same way the RUNT does.
Installing the RUNT is very simple. Write down the air pressure you’re currently running in your fork. Remove the stock air cap and token stack (be sure to let out all the air pressure), and drop in the RUNT. Attach a shock pump to the RUNT’s L (low pressure) valve and inflate the fork to your normal pressure, then move the pump to the H (high pressure) valve and inflate to double the L pressure. Equalize your positive and negative chambers (if necessary), check your sag and adjust the pressure in the L chamber as necessary.
The 2:1 pressure ratio is just a suggested starting point. Frankly, I find the 2:1 ratio too progressive for my tastes (and talent), so I wound up running closer to a 1.5:1 ratio most of the time.
A quick note on my impressions: While DSD did give me one RUNT for review, I’ve (spoiler) liked it so much that I’ve purchased two more and I’ve now tried the RUNT in a 130mm RockShox Pike, a 160mm Fox 36, and a 170mm Fox 38. Some of those forks I’ve switched onto the front of different bikes including e-bikes, so my impressions are based on riding the RUNT in many different situations.
Backing up a bit, while I think that, overall, a stock RockShox or Fox fork works very well I’ve chased my tail trying to get it to feel, for me, just right. Traction is a big challenge on my trails so I’m always trying to get more bite, but there are also a lot of high-speed trails and big hits too. So I like a lot of initial suppleness, solid mid-stroke support, and a fairly progressive finish. I’ve never been able to get a stock air-spring-and-token system just right—one part of the travel was always compromised for the others.
However, the RUNT lets me tune in my nearly ideal feel. What particularly impresses me is the RUNT makes the mid-stroke is much more stable and usable. So much that I’m able to run a bit more than my usual 20-percent sag, and I’m able to back off my compression damping a click or so too without compromising my mid-stroke support or end of travel control.
And I love how much more control I have over the fork’s progression than I get with a plastic puck/token. I can dial in more suitable (again, to my preferences) bottom-out control AND get better small bump compliance AND improve the mid-stroke support.
While I’ve been impressed with the RUNT on a regular mountain bike, I found it even more impressive and helpful in an e-bike’s fork. The extra weight and momentum acting on an e-bike’s fork makes the hole in the middle of a typical air spring curve even more pronounced, so the RUNT’s improved mid-stroke support and greater resolution of bottom-out tuning is hugely helpful.