The Vorsprung Secus is an aftermarket air spring upgrade that aims to get more out of your Fox or Rockshox fork’s potential performance.

Pete sat down for a chat with Vorsprung Suspension’s Steve Mathews to find out how they went about improving on their Corset air spring upgrade that eventually led to the Secus being released.

What was the impetus for designing a new air spring?

Well, air springs have mostly not felt that good, historically. You ride them back to back with coil springs and even on the same bike one feels like a DH bike and the other feels like an XC bike.

We made some considerable improvements in that regard with the Luftkappes and the (now-discontinued) Corset but it was pretty clear even before the Luftkappes were released that there was significantly more ground that could be made.

Was there one thing that made you realise you needed to offer a new spring?

Yes and no, we were never completely happy with the Luftkappes. When we developed those, we found that increasing the negative chamber size by taking volume from the positive chamber worked really well, up to a point, but beyond that point it required higher pressures and got too progressive.

Some of our prototypes were way more extreme than the production versions, and they worked super well for the first half or two thirds of the travel, then ramped up way too hard, it felt like you’d broken your wrists by the time you used full travel. We also found out that lighter riders really suffered from the lower-leg air ramp up effects.

We had one customer, a particularly lightweight woman, who couldn’t bottom out her stock Pike with no pressure in the air spring. That really opened our eyes as to how significant the non-scalable aspects of the fork’s spring curve were. Once we’d finished up development of the Smashpot we were able to really turn our attention to nailing a better air spring curve.

Once you’d decided to update the air spring, what happened next?

A lot of things happen in parallel. We looked at all the other aftermarket air spring modifications out there, and pretty well all of them were systems using 2 positive chambers that you pressurise independently. Those do work quite well at controlling the middle and end of the stroke (though they don’t do much for the initial stroke, which is the biggest source of harshness in air springs), but adding adjustments isn’t always a good thing, particularly when the variables are interdependent.

If you modify the pressure in one chamber, it doesn’t only affect one end of the stroke, and in my opinion it’s just not that reasonable to expect every rider to sit there working these things out and learning every detail about how their suspension works. It should be as simple as possible, meaning as few variables as possible.

So it became a design priority that setup was no more complex than a normal fork, and that there was virtually nothing there to confuse the rider or potentially steer them in the wrong direction. It isn’t quite as simple as a normal fork to set up though, you do have to press an extra button, but it’s a pretty binary ste, you just press the little gold button after changing the air pressure in the fork. Can’t exactly get it wrong or misjudge it.

We spent nearly 2 years going through different design concepts in CAD, in conjunction with our mathematical models of the springs. There were all kinds of crazy ideas in there to try to make the whole system self-calibrating, but many of the designs got too complex to be viable.

Along the way we realised that in order to control the spring rate in the way we wanted, we needed to also reduce the lower-leg ramp up quite a bit on most forks. That was the nail in the coffin for any way we could see to fit the system inside the fork. We got very fortunate that in the past couple of years, both Fox and Rockshox started using the volume inside the air spring shaft as part of the negative chamber volume, so we had a way to connect the Secus to it.

We looked at a ton of different configurations there, often related to manufacturability, but obviously reducing its profile and protrusion was a big concern, since anything external to the fork presents another object that could potentially be damaged in a crash. For example, we looked at using hose connections instead of a rigid machined one, but those were going to stick out too far, constrict airflow, and actually be much weaker and easier to damage, along with being heavier and more expensive, so that idea didn’t quite work out.

How many people are involved in that process and what do they do?

There were three of us primarily involved in the development. Myself and another design engineer worked on various design concepts over the space of a couple of years, and our machinist Mathieu is the guy who physically machines all the parts.

After that, during field testing it’s initially mostly myself, and once we have functional prototypes we hand a few off to other riders who I know are pretty critical. It doesn’t take long at all to validate the design concept as viable from a performance point of view, because it’s a spring, but durability, consistency, usability and longevity take a lot longer to establish.

Does it take long to get a working prototype in-hand?

Not always, but in this case, yes it does. There are quite a few parts in the Secus, many of them quite tight tolerance, and setting up the CNC machines to only make a couple of parts doesn’t take any less time than it does to set them up to make 1000 parts. There were months and months of prototyping going on at a time.

How many prototypes did you make before settling on what would be the production model?

Our CAD models had over 450 iterations. Physical prototypes, some parts of the assembly we only made one prototype of and never had to change the design, other parts went through 10+ revisions to get right.

Beyond prototypes, what form did your testing take?

The vast majority of air spring development is done mathematically. We have an extremely comprehensive air spring calculator, that’s been an ongoing development since about 2012, that we’ve validated against measured values, and that allows us to directly compare spring behaviours quickly with different setups and systems.

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It has a few important benefits in that it allows us to see the different behaviours under different conditions without measurement noise due to friction. It also allows us to cleanly see derivatives of the force vs displacement curves (the derivatives being the actual spring rate curve), which is much harder to do on data that needs to be filtered heavily to remove signal noise.

The model gets validated on the dyno from time to time just to make sure we aren’t going off on some tangent that turns out to be wrong, but dyno testing of the air spring is significantly affected by shock pump precision, preload distance inconsistencies and friction to the extent that it actually ends up less consistent than the calculations without a benefit in accuracy.

With the Secus we also found it very helpful to plug in pressure sensors to the various parts of it and make sure that what we were calculating and expecting was an accurate representation of what was actually happening. That part went pretty smoothly fortunately.

Beyond that, we took them into the field, rode them for a long time (we had prototypes running well over a year ago), tested them to structural failure just to see how hard it was to damage, redesigned them for better damage resistance, re-tested, and repeated that whole process a few times until we were happy with it.

How important are athletes to testing new product?

If by athletes you mean professional riders, that depends on the product in question. If you want to test structural strength of a frame, fork, wheels or cranks, it’s absolutely imperative that you give them to the guys who are going to ride them the hardest. Nobody complains about not being able to use the full structural strength of their frame, but if it breaks at a critical moment it can have severe consequences, so that really does have to be biased towards the most extreme use cases. In the case of spring systems like this, it’s not necessarily all that useful, because many professional athletes have setups that are kind of extreme.

At a minimum they’re usually running their suspension stiffer than the average rider, and often more progressive, so if for example we want to ensure that people can use full travel, then it’s more important to give it to a lighter and less aggressive rider to check that for practicality than it is to give to someone like Remy Metailler who’s going to go send huge stepdowns on it, and who needs a much more progressive curve than most riders.

What is important is making sure it works for a range of riders. We have a lot of riders in and around Whistler who ride hard, on steep terrain with lots of big compressions. If we build something that only works for them, it can easily end up more progressive than it needs to be for a lot of riders elsewhere in the world, so testing things for lower intensity riding actually became somewhat of a priority. It’s easy to make air springs more progressive if need be.

Is modularity with older models of fork important when designing a new product?

To a certain degree yes, if we can maximise the number of fork fitments then obviously we will, but at the same time the people who want to spend the time, money and effort to really get the most out of their suspension are often also the riders whose priorities mean they’re typically riding fairly new bikes/suspension all the time. The guys who are still loving life on a 10 year old bike aren’t usually the ones buying fork upgrades, so there’s a market reality to face there.

Did you have a Eureka moment when you knew you’d got it right?

When we first assembled the first working Secus prototype and got it on a bike, and it felt as good as I had hoped and expected based on the numbers, I was pretty stoked. I went and rode it round to all the local bike shops with a big dumb grin on my face and made people push on it because I was so excited about the way it felt.

My favourite thing about the Secus (like the Smashpot) is that you don’t have to be a pro level rider to notice the difference, in fact it’s so significant that you’ll feel a distinct difference just literally standing there and pushing on the handlebars.

What are the challenges of offering an aftermarket suspension product?

The big one is sheer volume, the industry moves very fast and every year we’re having to adapt to the huge number of variations that the big players create in their products. For example, there’s two different air top cap thread specifications for Pikes. The new one came about when they changed the axle spacing to Boost.

Go figure why a new top cap thread was necessary for that. Meanwhile, Giant are selling certain e-bikes with “e-bike specific” Fox 36s on them, that are actually standard 36s with a different sticker on them, meaning that we have to tell people wanting a Luftkappe that the Luftkappe they need for their 36 is a 36 Evol Luftkappe. Unless it’s an e-bike, in which case they need a 34 Evol Luftkappe, unless the e-bike is a Giant, in which case you need the standard 36 Evol Luftkappe again. So just constantly having to cross check compatibility is very difficult, it changes all the time.

Favourite moments?

In the case of the Secus, that first bounce on it. That was when we knew this was a concept worth putting into production. We got a million questions about the prototypes whenever anyone saw us out testing them, so I kind of enjoyed trolling people a bit with bullshit explanations about it being a counterweight (since you have a derailleur at the back on the right hand side, you need a counterweight on the front left otherwise the bike will obviously fall over). Got a lot of confused looks from that one.

Any disasters?

The Secus was originally going to be called the Ventilys (Ventil is the German word for valve, and the system was designed to let the fork “breathe” a bit better). We had a ton of boxes printed with that name on it months and months ago… then a couple of weeks after we got the boxes, Covid-19 hit and everyone was talking about ventilators all the time, so we had to change the name at the last minute because we just didn’t want any association with that.

Anyone to thank?

The rest of the crew at Vorsprung for their contributions and hard work making this a reality. Also special thanks to Clint Gray and Tom Lamb for facilitating some winter testing in NZ when we couldn’t ride here in BC.

Finally, thanks to Fox and Rockshox for making solid fork platforms that give us such a good starting point to work with. Whilst nothing is beyond criticism, modern forks certainly aren’t shit out of the box, and it’s a pleasure to direct our work towards turning them into something special rather than just trying to fix poorly designed/built or broadly unreliable units.

You can check out the Vorsprung Secus and the rest of Vorsprung’s suspension tuning products on their website here.

Read all of our product development interviews on our features page here.