Air springs are lighter, more adjustable and more progressive at the end of their stroke than coil springs, and this has made them the most common option for mountain bikes. But one of their biggest drawbacks is that the spring is usually much stiffer at the start of the travel than in the middle, which can give them a less ground-hugging feel and less mid-stroke support than their coil counterparts.
One of the most important trends in air suspension over the last few years has been towards higher spring volumes which provide a more linear "coil-like" spring curve. Vorsprung suspension has been at the forefront of this trend with their Corset and Luftkappe air spring upgrades, both of which increase the negative spring volume to offer a softer beginning stroke and a more linear spring feel. The Secus is the first air spring mod (as far as I know) which offers a truly linear beginning-stroke, so the spring stiffness doesn't drop off at all as the fork goes through its travel.
Before getting into how this $453 CAD upgrade rides, it's worth going into a bit more detail on the problem the Secus is trying to solve.
• Makes air forks perform more like coil, but with more bottom-out control and adjustability.
• Increases negative volume to give a linear-progressive spring curve.
• Also increases lower leg volume to reduce unwanted, non-adjustable ramp-up.
• Roughly 130g heavier than stock.
• Requires removing the air shaft to fit.
• Compatible with most RockShox and Fox forks plus Marzocchi Z1 (see website for details).
• Can be swapped between forks with compatible foot stud.
• MSRP: $453 CAD (including shaft clamps for at-home fitting)
The spring rate, or the stiffness, is the increase in spring force for every unit of additional travel used. In other words, it's the gradient
of the familiar spring curve with force on the vertical axis and travel on the horizontal axis. Conventional air springs have a very high initial spring rate, but a very low spring rate in the middle of the stroke. This gives the spring curve a distinctive S-shape which starts off steep, then levels off a bit before steepening again at the end. The decreasing spring rate/gradient in the beginning stroke is known as the digressive phase of the spring curve, while the increasing spring rate towards the end is the progressive phase.
Some older air springs could be ten times stiffer at the very start of the stroke than in the middle, while in modern air springs that ratio can still be around three-to-one. That's like having a coil spring suited to a 150 kg rider at the start of the travel and one suited to a 50 kg rider in the middle. In practice this can make air springs feel harsh and abrupt when the suspension extends towards the start of the travel (say after a bump or in a hole) then reconnects with the ground. The high spring force near top-out can also makes the spring keen to over-extend past sag, potentially making it feel unsettled and unable to track the ground. Meanwhile the soft mid-stroke spring rate can result in too much wallowing or brake dive. As we'll discuss later, there are upsides to this "old school" spring curve, but the gist is that the digressive phase is bad news for both traction and predictability, at least if it's too pronounced. How does the Secus work?
The Secus bolts onto the bottom of the hollow air spring shaft in Fox and RoskShox forks and connects the negative chamber to a reservoir inside the Secus. This greatly increases the spring's negative volume. As an air spring moves into its travel, the negative pressure (below the piston) drops while the positive pressure (above the piston) increases. This difference in pressure is what causes the build up in spring force through the travel. Because the volume of the negative chamber is increased with the Secus, the negative pressure drops off more gradually, so the spring force builds up more gradually especially at the start of the travel.
The Secus has another trick up its sleeve. The Midstroke Support Valve closes off some of the negative volume once the fork gets past a certain point int the travel; this causes the spring force to build more rapidly again, which increases the mid-stroke spring rate. This allows the Secus to create a spring curve that very closely approximates a linear spring curve in the first 2/3 of the travel, before it becomes progressive towards the end.
Finally, another chamber connects via the foot-stud to the spring-side lower leg cavity. This reduces the unwanted and non-adjustable progression at the end of the travel caused by trapped air in the lowers being compressed.Measuring the effect
The first thing I did after fitting the Secus to a RockShox Zeb was to put the fork on a basic spring dyno to measure the spring force throughout the travel. I've already done this on a stock Zeb, so it was easy to compare the two spring curves shown in the graph below.
This graph shows the force required to compress the forks against travel. The Fox 38 is also shown for comparison. The stock Zeb and 38 could only be measured up to 120mm due to the limitations of the equipment used. Due to the effects of friction and measurement errors, it's best to ignore the details, but the basic takeaway is that the stock Zeb is the most digressive and the Zeb with the Secus is the least digressive.
This shows the Secus does what it says on the tin. There's no digressive phase at all, but rather it's roughly linear in the first part of the travel then progressive at the end. If anything, the curve I measured is slightly progressive throughout, but this could be caused by the build up of pressure in the lowers, changes in friction, or just a measurement error. Installation
Installing the Secus is a little intimidating but isn't too hard if you're used to servicing forks. You'll need everything you'd normally use for an air spring service including replacement bath oil (you may be able to retain the old oil if the fork is fresh, but if you're like me you may end up with an unknown quantity of oil on your shoes, and you don't want to guess how much to put back in) and snap ring pliers to remove the air spring. You'll also want a heat gun to remove the old foot stud and a torque wrench with crow's foot adapter to install the new one, plus shaft clamps, which you can buy with the Secus if you don't have some lying around. Clear instructions can be found on Vorsprung's website though, and if you don't fancy the task it shouldn't cost too much extra to get a suspension tuner to do it for you if you're having a service anyway.
Speaking of which, it's worth pointing out that if you're swapping the oil while fitting any fork modification, the fork is likely to perform better anyway thanks to the lower friction. In this case I used a Zeb which was freshly serviced before I fitted the Secus.
It's important to remember to press the gold Midstroke Support Valve button on the base of the Secus after pressurizing or changing pressure in the fork. If you forget, the spring curve won't be as intended. Setup
Vorsprung recommends 20% more pressure in the spring with the Secus fitted. I'd run the stock Zeb at around 64psi, so that would put me at 77psi. However, with the stock 170mm travel I preferred it a little firmer (around 80-84psi) to hold it up, but after I'd fitted a longer air shaft 77psi was about right. I found I could run the rebound a little faster (around 14 clicks from closed) as the softer spring force near top-out made it more settled into its travel even with the rebound fast. I set the high-speed compression fully open to compensate for the stiffer spring in the mid-end stroke and allow more use of the travel. I stuck with zero tokens throughout testing.Performance
The effect of the Secus is not subtle. It's immediately noticeable that the fork sags into its early travel more easily than the stock Zeb or even the Fox 38. It even engages its travel when pushing the unloaded bike over rocky ground. When riding over rapid-fire roots rocks, the front wheel feels more stuck to the floor, especially with the rebound on the faster side. Yet when braking hard or landing, the support builds up smoothly but strongly, resulting in very little unwanted movement or dive, something which I found to be an issue with the stock Zeb.
However, on some trails with consistent braking I was finding the fork was sitting a bit too low. Raising the bar height helped, but in certain situations I felt the steeper dynamic head angle caused the bike to feel more "upright" - less willing to tip into turns. Raising the pressure above 80psi made it a little harsh in big hit situations.
At first I was running the Secus in a 170mm Zeb on a Privateer 161, so I considered upping the travel to 180mm, but as the Zeb is available with up to 190mm of travel I decided to go all in. That way I could have a similar (or slightly higher) dynamic ride height as a stock 170mm Zeb while running lower air pressures than I had been using with the Secus. I also switched to a MegNeg air can on the rear shock to give a more similar spring feel at the rear of the bike.
Of course the 190mm fork makes the bike taller and slacker when unloaded, but it doesn't matter how a bike handles when nobody's riding it. Admittedly, the extra travel definitely creates more wheel flop on the steep climbs, where the front wheel is so lightly loaded that any fork is going to be very close to the start of its travel. But the Privateer's 80-degree seat angle meant this was not an issue and the softer beginning stroke is appreciated when tackling bumpy climbs. Once descending, the fork settles further into its travel giving a similar dynamic position to before, but with more negative travel to extend into holes and keep the wheel in touch with the ground. I found I was able to look a bit further down the trail and trust my bike to deal with the small stuff without losing grip.
But there's a downside to everything. Because the Secus makes the fork significantly stiffer after the sag point and towards the end of the travel, you do get more feedback on big, simple impacts like a big root or rock. Just as a thought experiment, if you're riding along with 300 Newtons of force on the fork, then hit a bump that creates a force of 1000 newtons (ignoring damping forces), judging by the force-travel graph above, the stock Zeb would go from about 20 to 120mm of travel (using 100mm of travel), but with the Secus it would go from about 40mm to 110mm (using just 70mm of travel). This is why in this simple situation the stock fork can be more forgiving, though the flip side is obviously more movement when braking or pushing into a turn etc. In theory the added harshness on bumps can be offset by reducing compression damping (especially high-speed), which will have less of an effect on brake dive.
And although I was running a 190mm spring, this was purely for geometry reasons. At 77psi and with zero volume spacers I I was rarely using more than 160mm of travel, and the highest I've seen the O-ring was about 170mm from the bottom. My local trails are not the stuff of Josh Bender
, though, and it's nice to know that if I ever accidentally find myself on a Rampage line I've got something in reserve. But for me I would like to be able to run the fork a little less progressive if using more than 170mm of travel.
Overall, I'd say the Secus offers a significant improvement over the stock Zeb, thanks to that more settled, ground-hugging and predictable feel. I did some back to back runs with the 190mm Zeb with a Secus and a 180mm Fox 38. The difference here was subtler than compared to the stock Zeb, but the I'd say the Secus-Zeb was a little better at maintaining contact with the trail when going light over rattly sections. This isn't so surprising given the force curves above. The 38 was a little more supple and comfortable on long runs though, so it's too close to say which was my favorite. I've not been able to test the Secus with the 38 yet, but I feel less need for it than with the Zeb.Longevity
I've had the Secus installed for a couple of months so I can't say anything about long-term durability, but it's worth mentioning that Vorsprung recommends servicing the IFP and seals in the Secus every 200 hours (see their website for instructions), so there is a little more work involved. My concerns about crash damage are minimal, because if you position it with just a few millimeters gap between the Secus and the caliper (as recommended), the unit is pretty well hidden behind the fork leg. Also, I have crashed hard onto the left hand side over a mat of wet roots, which caused some fresh pine shavings to get lodged in the bottom of the Secus, but no damage was sustained. Vorsprung offers a 12-month no-questions-asked crash replacement too, which adds more confidence.
Does what it promises to the spring curve.+
Noticeably improves traction, support and predictability.+
12-month crash replacement guarantee.
Inevitably, more mid-stroke support means more feedback in big-hit situations and softer beginning stroke will sit dynamically lower unless you increase travel.-
Not cheap, not exactly simple to install and requires some servicing.