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.
|The Secus does what it says on the tin. It significantly improves tracking, predictability and reduces brake dive, but this inevitably comes with small downsides; chiefly it's less forgiving on large bumps. I also found that going up in travel was the best way of maintaining ride height while enjoying the full benefit of the soft-beginning stroke, which is something to consider. The benefits and trade-offs may vary with other forks, but with the RockShox Zeb I tested it's a good option for aggressive or stronger riders who are happy to take a bit more big-hit feedback in exchange for more predictability, support and ground-hugging grip. —Seb Stott|
Yeah, I don't know about you but this sounds like a very expensive cock ring.
And what kind of pseudo-science BS is "support"?
I understand your point, and believe that some, have the ability to distinguish a 400lbs spring from a 425lbs, but...
"Normal" riders don't check tires pressure on day ride.
"Normal" riders don't adjust air spring... they simply did it a few months/weeks back
"Normal" riders, don't change rebound or compression settings to not "destroy the perfect setup from factory!"
Yes setting suspension on a coil can be expensive", but from what I check, there aren't too much complaints about bike prices. Coil spring is "peaners"...and you can resell coil springs that you don't use anymore.
If you can live with 50lb gap or not depends on your trails really. Typically I spend winter season on more bikeparky trails, with jumps and dirt. Every spring I get back to chunky stuff in the mountains I feel like my teeth would fall off on the first ride. Then I turn the spring by 25lbs, open up rebound a bit, get -5PSI on the fork and suddently I ride a different bike.
But if it makes you feel better then half the game is feeling good - air is for the “now” coil is for ever and will always be the forever , I went back to air cos it was better , said no one ever
Point is, constant tinkering with a million combinations isn't going to be better than a bit heavier well thought out fork.
The trouble is nowadays people expect their enduros to perform like dh rigs downhill but be as versatile as a trail bike. Don't get me wrong I use my sb165 for everything but I bought it with the intention of why Enduro bikes initially came about, to be a dh bike you can pedal uphill. Not to be a trail bike that will pedal up like a 22lb Scott Spark and descend like a V10.
The increased sprung mass does big favours downhill to the suspension performance and is negative up hill, so the ideal do it all bike that can do everything great can't really exist as in one instance weight favours you and in another it's a negative... Unless ofc you talk e bikes which both go uphill easily but have a high sprung mass also.
EDIT: Zeb 29 comes in at 2.32 kg as well
For sure the lubrication side of things needs regular attention in air suspension, that will probably always be a downside unless we start using airbags instead of pistons.
Also, say your spring rate is correctly set up at say 100 psi on your pump, when reinflating to 100psi with the same pump your springrate is still correct for how you had it set up (even if calibrated equipment reads that at 108psi, your pump will inflate to actual 108psi again dispite showing 100psi).
@seb-stott curious on your take of IRT vs the fork in this review
I wouldn't know, my list is longer than my hair.
Now, having read this and considering the principles Seb explains are correct regarding spring gradient, can someone explain the rationale behind RockShox's recent move to a smaller negative spring/equalisation at zero travel? It feels counterintuitive to me.
I bought the C1 but havent installed it, I think really I just need to go up 10mm with the B1 and it would be the best but idk if they exist anymore.
1. Your fork initial length is set by your fork travel (for a given model and wheel size). This initial fork length gives you your fully extended geometry.
2. The fork's ride height is then determined by the initial length minus the sag.
3. The sag is determined by the spring rate curve.
4. The way the fork behaves on the trail, in terms of its ability to suspend the bike from the wheel properly, is also determined by the spring rate curve. The Secus and Luftkappe are both intended to soften the initial travel, which is why you feel like it moves through the initial travel more easily - because that's what gives you the initial compliance.
Now if you want a certain (quasi-static) ride height, and your fork is a certain travel, then your hand is being somewhat forced in terms of the spring rate curve you end up with. You're basically forced to change your suspension behaviour, quite possibly in a way that you don't like, to make a geometric adjustment.
But if you subsequently don't like the on trail behaviour of the suspension (eg initial stroke feels harsh), adjusting the spring rate curve (either scaling it up/down by altering pressure, or changing the curve's shape with a Luftkappe, Secus or Smashpot for example) now also changes your geometry and fork ride height.
If you had an independent length adjustment for axle to crown or axle to handlebars then you could adjust that out easily enough, like for example on a dual crown fork you'd just move the stanchions in the crowns.
With a SC fork, raising/lowering your bars is usually the easiest form of this, but it doesn't affect BB height or head angle, and it does affect the bike's fit somewhat. Or you can do what Seb did, which is to adjust the travel to get the ride height you want (20mm is pretty extreme though) - but now you've got a spring rate that may or may not be matched to the amount of travel you have, and then many riders will feel like they're not able to "get full travel" in spite of everything else because the o-ring doesn't end up at the crown.
My recommendation there (which is in line with what your current approach seems to be) is to get your suspension feeling the way you want it to feel on the trail first, then adjust the axle-to-bar length via whatever means you prefer (travel or just stem/bar adjustment).
I think It may be subtle in well supported in bike park type terrain but won't that extra force required at the start of the travel mean less traction over myddy, off camber, awkward tracks, flatter turns and natural terrain (enduro/trail riding) when compared to the 38 and zeb+secus curve?
I feel it's strange that so many people don't realise what sag is for. It's like the forks were dumbed down at the cost of performance.
One thing that stood for out me from your testing was the fact you went on to add more travel (more sag?) to ajust the geometry and mostly didn't use the (expensive) damping options. Does this mean you liked how the necessary force to start movement was reduced but maybe not so mutch the finishing spring force (too hard)?
Novyparts released in 2007, a piston for this, for the Rockshox lyrik mk1 and the Rockshox totem and the Boxxer.
He continued with the following forks: Sektor, Revelation, Pike, Lyrik, yari, all Boxxer, Zeb.
In addition it is Bos engenering, who with his experiences in motorcycles and in car racing, released the first forks with very large negative volume and with high pressure, long before Rockshox or Fox, with the first Bos Idylle Pro (also the first fork pressurized!)
Regardless, it did not do exactly what the Secus does. Enlarging the negative chamber is only one part of that.
Either you get a progressive air spring, where the decision is for a soft initial and hard ramp-up end stroke. On the neg side you get a fork that's still quite plush in the mid-stroke, i.e. it sits deeper in the travel than you may wish.
Or you get a linear coil spring, where you dont have that mid-stroke valley. On the coil side of things you just get to decide, whether you want big hit compliance or soft initial stroke. I must say, for DH purposes, a stiff coil is still the bees knees, who cares for a soft initial stroke, when charging hard.
With respect to this product, great innovation by Vorsprung as ever. And it moves the air spring into the direction of a coil-like rate. IMO the above generalisation still applies.
You may be able to eke out a more coil-like fork at a slight weight advantage with this system.
How's this for armchair engineering?
You're right that there's no free lunch (at least, at any one small region in the travel). If you want more support, you have a higher spring rate, which delivers more force within that region. If you want more compliance in a given section of the travel, it's also going to push through more easily.
However, you can look at where in the travel most sharp impacts are beginning (usually, though not entirely, early in the travel), and where in the travel people are asking for more support (usually under braking or big impacts, so deeper in the travel) and try to match the respective spring rates to the most common demands on that part off the travel. It won't mean it's perfect everywhere, because if you're say 50% of the way through the travel and have a high shaft speed impact beginning at that point, it'll be quite a bit stiffer than a more typical air spring rate would have been, but that happens pretty infrequently compared to hitting square edges beginning at say 15% travel. Conversely, trying to get support under hard braking on steep terrain isn't usually happening at 15% travel, but you may well be at 50% travel there, where the stiffer (relatively to a standard air spring) spring rate is beneficial.
It's fairly evident to anyone who's ridden a coil that the linearity typically works well for small to medium bump compliance for the majority of the travel, but as you point out, you run into significant compromises with excessive movement or bottoming on bigger hits there, which is usually a deeper stroke issue.
The Secus was designed to mimic the behaviour of a coil spring in terms of overall spring rate for the first 2/3 of the travel (though you still have air spring friction and the pneumatic damping effects) but also allow you an appropriate amount of ending stroke ramp. By appropriate, I mean the majority of riders on the majority of terrain should be able to find a setup that allows them to use full travel on occasion, without the overall ride quality being too soft or too firm.
There's also the issue of variable rider weight - riding with a pack, or even bikepacking, and still being able to set up the suspension to work optimally. Not easy with coil at all. Again, an air-air system works better here. Admittedly it's a minority use case.
Didn't Intend's Bandit aim to integrate much of what the Secus does within their asymmetric fork?
In the world of dual spring forks there are also the dual chamber air springs, just for the sake of arguement. Manitou Mezzer pro IRT, DSD runt drop in cartridge (for many fox, rs forks), and I guess the EXT Era is also in this category as well, as I uses both a dual air chamber, and a small coil spring, but its also a $2k fork.
Something worth noting, however, is that I'm actually having to run about 20% LOWER pressure than I used to without the Secus to achieve proper sag and support. I'm currently at 69psi (nice) with two tokens. Despite these major differences, I'm still experiencing all the benefits @VorsprungSuspension claims on the box. Maybe I got a "bad" apple that's somehow still amazing? Perhaps they can chime in here...
HSC - fully open
LSC - 8 clicks from open (right in the middle)
HSR - 2 clicks from fully open
LSR - 3 clicks from fully open
I wouldn't say you have a bad apple at all. I've seen many riders in the forums who like it with less pressure. Just depends on your ride preferences. I have found that as I gain better cardio and strength the firmer settings have been more appealing as I ride harder. I haven't played with anything for months and seem to have found the sweet spot. Overall, this really transformed the ride experience. Bike mod gold if you have a Fox or RS fork.
That can't be a con. "Mid-stroke support" has been a damn golden goose holy grail buzz word for waaaaay too long. So many reviews can be found saying X is better than Y because X has more mid-stroke support.
But apart from that, I echo the review, does what it says on the tin, more supple for tracking and more support.
Sram, I will tell you....you are good at the game you play!
This is the perfect summation
A small side note: you don't need a shaft clamp nor heat gun to remove a foot nut from RS air shaft, there's a 8 (or 10) mm socket slot on the air piston.
As I moved to a MegNeg on my own privateer 161 just recently, I had some issues with balancing front (Lyrik ultimate) and rear (stock shock with MegNeg). I like the feel of the MegNeg, but it's in my opinion, on this bike, not the best companion to the Lyrik. So, I filled the MegNeg negative chamber with 4 bands. That seems to be a decent compromise. Based on your review, getting back to more negative Volume in the rear would lead to bying a 38 or a Secus. Thanx for that ;-)
The Luft Fusion does not increase the lower leg volume but increases the positive chamber.
One thing I'll say about the Zeb is that I found it VERY good on initial small bumps from new, basically it felt as good as a coil, but with about 70 hours on it now it's clearly due a lowers service and is a bit more pingy.
Every time I get to 50 hrs on a fork (about once per month) I think to myself "Is it really working any worse? Do I really need to service the lowers and change the oil?". I prevaricate for a day or two and then luckily get a rainy day where I think I might as well be tinkering in the workshop.
About five minutes into the next ride, post service, on my 'new' feeling Zeb or Lyrik (depends on which bike is due) I am shaking my head and wondering why I left it any hours past the 50 hr mark at all!!
And I go through this almost every time - so I must be a slow learner.
The upside of the RS 50 hr service is it is just a clean out and fresh oil, one doesn't have to replace foam rings and seals, which makes it simple and relatively economical.
That 50hrs comes round very quick y'know - getting ProBikeGarage has really been an eye-opener.
The Secus is awesome engineering.
But which air springs have a drop off in spring rate/stiffness through the travel? Isn't that _the thing_: that an air spring naturally ramps _up_, non-linearly?
Did not realize there was a valve in there, explains a lot more! It's kind of a position-sensitive spring system?
Again, awesome engineering!
But y'all been telling us forever that steeper head angles make it easier to turn... even too easy; enough that it's been in the "Cons" column on many a review: both as steep making it twitchy or providing "telepathic turns", and slack making it take more effort and be slow to turn in.
Now all of a sudden steep means harder to turn in? Difficult to trust reviews when it goes back and forth like this.
"The best you've ridden is the best you know" - Paul Thede, Race Tech
Running a Super Deluxe coil too (cos i blew up my Fox X2)
He modifies the air spring piston: he does a specific air piston tailored for your weight.
The ratio between negative and positive is different and also he modifies the location of equalization between the two so you don't have the problem that you had with the secus where it sits lower in travel !
And all this only costs 89€...
Of course the air spring is tailored to your weight because to mimic a coil spring you have to have different negative to positive ratio for different weights !
But, yes i never want to work worldwide, i always prefer to do taylor-made, be small, be free and not part of bike showbiz. It's why my products are open-source and not registered.
And i'm really happy for your buziness. I think Suspension and particularly Mtb suspension need something like you to explain the other side of big brands...with 5 engineers and 50 guys for marketing
1. The Secus body can rotate to contact the brake caliper for support in the event of an impact
2. The Secus can also safely flex upwards to contact the fork lower leg for support, so it's harder to damage than it might initially appear.
3. It's a lot less exposed than your derailleur!
4. We offer a 12 month crash replacement guarantee - if you damage it in the first 12 months, we'll get you whatever replacement parts you need free of charge.
The TAC SFF (Triple Air Chamber, Separate Function Fork, meaning damping in one leg and spring in the other) operates a bit differently to the Secus, although I can see the aesthetic similarities! TAC does the equivalent of pressurising the outer tubes (the Secus doesn't, it just expands the effective volume of the lowers, which in this case are the outer tubes) and is otherwise a two-chamber (positive and negative) non-self-equalising air spring, ie you have to manually set three chambers instead of one. The "inner" and "outer" positive adjustments on the TAC function in parallel, not in series like say IRT or other twin-positive-chamber MTB air springs, or like the twin negative chambers in the Secus.
If you were to compare the Secus directly using the same kind of nomenclature, it'd be a four chamber, self-equalising air spring with one of the chambers initially at atmospheric pressure.