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Diaz Suspension drops The Runt into your air fork to give it a coil-like ride

Written by Cory Benson
Diaz Suspension Design The Runt air cartridge upgrade, linear Coil suspension feel in your Air fork
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If you are looking for more enduro-ready linear suspension performance out of your Fox or RockShox air fork, Diaz Suspension may have just the ticket for you. Without needing to buy an all new fork or convert to coil internals, their simple The Runt upper air spring piston drops right into your existing fork, giving it a coil spring-like ride.

Diaz Suspension’s The Runt cartridge – Coil feel in your Air fork

The Runt is a self-contained & adjustable air spring system that threads into the top air chamber of your existing fork to reproduce the more linear performance of a coil spring. Designed and produced in Colorado, Diaz says it delivers a near linear compression curve while maintaining the infinite adjustability of air suspension.

DSD’s The Runt, linear suspension performance from an air fork

Diaz Suspension Design The Runt air cartridge upgrade, linear Coil suspension feel in your Air fork

Upgrading your fork with The Runt is simple as threading the cartridge into its left leg. It works by providing an additional floating air piston at the top of your fork leg that only is activated around mid way through your fork travel. (It is also reversible, meaning no damage to your otherwise stock fork.)

Diaz Suspension Design The Runt air cartridge upgrade, linear Coil suspension feel in your Air fork

In doing so, it effectively flattens out the middle of your fork curve (much like adding fork tokens) providing more mid stroke support, but without excessive ramp up as you continue deeper into your travel. Diaz says The Runt gives the best balance of mid stroke support without your fork diving in corners or on descents, and without harsh, stiff compression towards the end of your fork’s travel.

Diaz Suspension Design The Runt air cartridge upgrade, linear Coil suspension feel in your Air fork

They even build in an extra high pressure air chamber that can be dialed up to increase bottom out resistance if you still want some of that extra end stroke protection, while benefiting from increased mid stroke support.

Diaz Suspension’s The Runt – Tech Details, Compatibility & Pricing

Diaz Suspension Design The Runt air cartridge upgrade, linear Coil suspension feel in your Air fork

The Runt will thread directly into your existing Fox 36, or to several RockShox forks, including the Lyric, Pike, Yari, or Revelation (35mm version) – as long as they are the Solo Air versions. Diaz says it is half the price of converting to coil internals, and a 1/2 pound lighter than a coil conversion too.

Machined in Durango, CO, Diaz sells The Runt on its own for $200 and you can install it your self. Getting going is literally as simple as taking the air out of your fork, threading out the entire top left leg fork air cap, threading The Runt into place, and airing it back up.

The Runt also ships with shim kits to custom tune the damper of every fork (except the Fox FIT 4 & Grip 2 which are still in development and will ship out to buyers at no extra charge when ready), following Diaz’s simple online instructions.

Diaz Suspension Design The Runt air cartridge upgrade, linear Coil suspension feel in your Air fork

For $425 you can schedule and send your complete fork to Diaz, and they will re-valve your fork’s compression & rebound circuits for optimal performance for you & how you ride, then install The Runt, inspect your fork, and service it with new oil & seals. Maybe a good option if your fork is already in need of servicing after another hard season of riding.

DiazSuspensionDesign.com
Cory Benson
4,470 articles

Cory Benson is the EU Tech Editor of Bikerumor.com.

Cory has been writing about mountain bikes, enduro, cyclocross, all-road, gravel bikes & bikepacking for over 25 years, even before the industry created some of these names. Prior to Bikerumor, Cory was a practicing Architect specializing in environmental sustainability, has designed bike shops & bike components, and worked as a bike shop mechanic.

Based in the Czech Republic for 15+ years, he is a technical mountain biker, adventurous gravel rider, and short & medium-haul bikepacker. Cory travels extensively across Europe riding bikes, meeting with key European product developers, industry experts & tastemakers for an in-depth review of what’s new, and what’s coming next.

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Johann
Johann
6 years ago

From their graph it appears the fork will be much much less sensitive to small bumps which is kind of the opposite of a coil.
And it looks very much like https://www.chickadeehill.de/
Just saying.

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Michael McCallum
Michael McCallum
6 years ago
Reply to  Johann

Nope. You can tune the small bump compliance just like you can tune the ending stroke. That’s why there are two air chambers.

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Steven
Steven
6 years ago
Reply to  Johann

The Runt has been around in various stages of production for quite a while! Cool to see Bike Rumor picking up on it. Diaz knows his stuff.

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Maciej
Maciej
6 years ago

Chickadeehill AWK System. These guys are making this for years now. I have it. Works like a charm. fahrrad-fahrwerk installs it. If you wan’t a system like that do it with a change in your damper. Linear characteristics will not play well with a damper that is made for an air spring. In fahrrad-fahrwerk they are using MST. Everything together works really great. As far as I remember cheaper than 425$ and it’s proven to work. But this is Europe specific so not for the Yankees.

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Tom
Tom
6 years ago

Looks intriguing.

Test needed!

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Wade
Wade
6 years ago

Been riding and testing The Runt for 3 months in a Fox 36 equipped Yeti Sb5.5.
More supple, even noticeable on the climbs, and way better support in the corners and over the rough stuff it keeps the wheel glued to the ground, but still poppy if you need or want to jump something. It has changed the way I ride

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Anthony
Anthony
6 years ago

Maciej, Yes I completely agree with you! Any good suspension tuner or company would not sell a product that changes the spring rate without changing the damper to work correctly with the new rate!!
That is why every runt will ship with a shim kit to re-valve the damper. We have easy to follow instructions on our website. Anybody capable of working on suspension should not have problem custom tuning their damper. We have tables that make it easy to get the right valving based on body weight installed!

We are aware of other similar products in the industry. We actually started working on this back in 2012 ! and filed in 2013! I know its been a long time! We have had a lot of set backs but its still pending.

Thanks for pointing out the custom damper tuning ! as you know its actually a big part of the package! I believe bike rumor left the included shim kits out of the article but it looks like they have updated now.

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Anthony
Anthony
6 years ago

Johann, It actually mimics a coil spring very closely. The increased mid stroke support lets you use less air pressure and compression damping, resulting in a much more supple ride with out compromising support. Here is a link on spring rate testing by Peter Verdon. I picked it because it has a few different coil springs graphed to compare. Even the lightest of the springs does not produce a slope anywhere near stock air spring rates!
https://www.peterverdone.com/springs-cause-its-spring/
If you have any questions, please contact us.

Tom, Testing has been done! We have many happy customers in the area and are working on our feedback/review section. We have actually had test riders compare our system with a coil conversion in a Fox 36 and they could not tell the difference!!!!

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switzi
switzi
6 years ago

https://chickadeehill.de/ –> is what I also wanted to post 🙂

I have it in my PIKE, best fork upgrade ever made for ~120€

The first graph of the RUNT does look excessively optimistic…

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Anthony Diaz
Anthony Diaz
6 years ago

Switzi, Actually the first graph is the most accurate! It was made using data collected from a fork put on a dyno at Vorsprung! The other graphs were made from taking pressure readings from the forks air spring. Exactly the same way Shockwiz works. Upon compression pressure builds in the lowers and a vacuum forms behind the air piston in the negative air chamber creating spring force. This can vary fork to fork based on how much volume the damper takes up where the air spring seal head sits within the stanction tube and if it floats or not and of course the amount of oil in the lowers. This is why you see a difference in the graphs. How did we get a “five token ” mid stroke line in a 4 token max Rock Shox? Its a combination of the above and it turns out that the 8 mm hex through 4 tokens is a lot of volume.

I believe our graphs are very accurate and have been tested over and over again.

What I would call excessively optimistic, and even a little dishonest, is other cartridge manufactures claiming that one cartridge size will cover a range of travel lengths and still produce the same spring rate!!!!

To keep the spring rates the same when using air volume for tuning you have to adjust it every time you change the travel of the fork. This is why forks come stock with a different number of volume spacers based on the forks travel. Claiming the same rate over a 20 mm spread is like claiming that there is not a deference between 3 or 4 tokens! (Rock Shox tokens are 20mm in length) Some of them are claiming the same rate over a 40-50 mm spread!!! This isn’t even possible!!

We have a specific set up that changes with every 10 mm of travel to insure that our customers are getting the rates that we publish !

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Samuel V
Samuel V
6 years ago
Reply to  Anthony Diaz

Regarding the first graph…

I assume all “tokens only” -lines on the graph are with same air chamber pressure?

If this is the case, take the 0 tokens -line and increase the air pressure so that you have same bottom out force than with 4 tokens, and the graph should look much like the Runt -line…?

Just thinkin’ out loud…

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Anthony
Anthony
6 years ago

Samuel,
Yes I can see how the 1 token line looks linear and it is. The problem with it is the slope of the line. When you add air pressure you don’t change the slope of the line, you only raise or lower it as a whole. Yes the negative air chamber equalizing theoretically should bring the graph all the way back to zero and still be supple, but it doesn’t completely. A 30 psi difference can be very noticeable in the initial stroke. If I remember correctly from running numbers it takes about 25-30 psi to get the same bottoming resistance and that is more than enough to change your initial suppleness. Check out our web site, there is another graph that helps explain this.
https://diazsuspensiondesign.com/runt/

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