Editorial
Coil Forks VS Manitou IRT Air
More Coil Than Coil
Coil, coil, coil. It seems to be the only thing bike nerds want to talk about these days. I love coil shocks (on bikes that work well with them) but with rare exceptions, like the expensive PUSH ACS3 coil conversion, I'm highly skeptical of the roaring return of coil sprung forks to mountain biking. Frankly, outside of suspension companies taking on a massive inventory expense to offer variable rate springs, which isn't going to happen, I can't think of a 'dry' coil fork I'd buy.
I simply have no desire to ride a linear spring rate after experiencing the easy tuning, support and progressive feel of a good air fork. What about using the damper to tune the performance of a coil sprung fork? Yes, compression damping will hold up a rider during hard braking events but more adept suspension tuners than myself will point out that the damper is just slowing the fork's motion and too much will create a harsh ride and result in a loss of traction.
"But. Moto!" is of course the standard rebuttal. What about moto?
Manitou offers two high performance air systems for their Mattoc & Dorado Pro forks. Increment Volume Adjust (IVA) uses a simple positive and negative air spring arrangement with tokens to change volume. Infinite Rate Adjust (IRA) introduced a second air valve to allow infinite adjustment of mid stroke support - a system that is gaining popularity in mountain bike forks.
In a coil sprung dirt bike fork, oil height is used to adjust the suspension's mid-stroke rate. Increasing oil volume shrinks the air volume inside a stanchion tube. The oil cannot compress as the fork telescopes so the air volume controls the mid-stroke ramp of the coil spring.
Anyone who has attempted to maximize the performance a classic Marzocchi fork - Shiver, Monster-T, Super-T, Z-1, etc - for aggressive riding has likely played with balancing oil height and spring rate to get the correct mix of small bump, full travel and mid-stroke support.
Sometimes the companies on the fringes are producing some of the most interesting products.
Ed Kwaterski is Manitou's head suspension engineer and before that he worked for Showa designing, setting up and supporting power sports suspension products. Features like Manitou's adjustable hydraulic bottom-out derive directly from his past experience as does IRT. In layman's terms, the main air spring is the initial spring rate and the IRT spring is the equivalent of changing oil height to adjust the spring's mid-stroke rate.
Ohlins, Formula and now BOS are making air forks with similar systems with adjustable air pressure, self balancing negative air and air adjustable midstroke rate.
IRT Ferdanerds
40 Reasons For Air Forks
I was working at SuspensionWerx, a suspension service shop, when Fox released the first air-sprung Fox 40 in 2013. The 2014 40 inherited the inverted RC2 damper released in 2012 and had an all new chassis that was lighter and less stiff than the previous nine model years.
Riders were hungry to purchase one. The question I kept hearing was "if I don't like it can I install a coil spring?" and the answer was "easily." Not a single rider chose to go back to coil after riding the air system.
The first generation Fox 40 air used a long coil negative spring (rather than the current self-balancing system) and a token system more similar to Manitou's IVA than the current token system. Most riders started out with their forks set very linear - as "coil like" as possible - and were quickly back to try more progressive settings that are more similar to the performance of a stock 40 today.
Born again coil suspension may be the flavour of the week but considering how good air systems have become, it'll be interesting to see how sweet they taste over the long term.
Following Up
I'm currently reviewing a Manitou Mattoc Pro (originally the Magnum Pro) and running both the set-and-forget IVA and highly tuneable IRT air systems back-to-back in the same chassis.
In the mean time please check out my teardown and Stache followup. Thanks to SmithTech, the local distributor and service center, for the air system swap and setup advice.
Comments
Lowcard
7 years, 2 months ago
I had a Manitou Mattoc with those internals last year and it was stellar. There were some issues with air seeping past a seal then effectively reducing the travel on harder hits, plus the reverse arch...ugh. The bottom out control was amazing and in my mind the best feature of the fork.
So far the closest air fork to coil feel has been DVO's Diamond with its Off The Top adjustment. As long as the dust wiper is lubed up, and your OTT is dialed in, there is not a smoother, more supple air fork on the market.
However, after riding my buddy's Fox with the Push coil conversion, well that pretty much ruined me for air forks. When he can ride all of Martha Creek without stopping and his hands feel fresh at the bottom, you know the fork is the cause of that. Air suspension is really good, but its still not as good. Long descents like that and air suspension changes when it heats up. Coil suspension doesn't, or it takes a lot more heat to make it perform worse. As long as you can control the progression of the travel then for performance sake, coil suspension can't be matched.
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Andrew Major
7 years, 2 months ago
Completely agree re. hydraulic bottom out. Every high performance fork should have that feature.
I’ve had a good experience with an DVO Emerald Boost fork but would not have included it in my top three in the category. I would certainly take the opportunity to revisit my experience.
I purposely carved out a hole for the PUSH coil system as in my brief experience it works exactly as advertised, has many spring rates available and is highly adjustable. To my knowledge none of the coil forks coming online now have similar rate tuneability? It is a good example of how progressivity of a coil fork can be adjusted without adding a ton of oil. Expensive, but I hope they make them available for for fork chassis.
One reader brought up adding oil to a coil fork (Ohlins as an example) to control rate but without a proper oil seal I’m sure it would just purge out the dust wipers under pressure.
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Kos
7 years, 2 months ago
Do the new coil forks still make a racket like my old Fox Vanillas? Great forks, but lots of noise from the spring contacting the inside of the stanchion tube. I remember fooling around with shrink wrap, etc. without ever quite achieving complete success.
As far as air vs. coil and moto goes, the AER 48 air fork on my KTM is the best stock dirt bike fork I've ever owned, but a good stretch. Even a bit better than Yamaha's previous top of the heap SSS stuff.
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Andrew Major
7 years, 2 months ago
Spring guides have come a long way (last gen coil 40 was pretty quiet) but everything I’ve seen is still using a wrap.
Very cool re. AER 48. Is it a air spring with a coil negative?
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Eric Taylor
7 years, 2 months ago
The fork that interests me is the new MRP ribbon coil. It's 4.6 lbs and has their ramp control system. With a decent weight and being able to make it progressive, I honestly don't see why I would pick the air version over it. Yah set up is easier, but then you have to keep resetting it up where as after you get the right spring weight, the coil is set and forget for the most part.
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Andrew Major
7 years, 2 months ago
Unless if they’ve changed it for the Ribbon Coil, the Ramp Control system is only affecting the end stroke ramp/support of the fork so it’s still relying on the damper for midstroke ramp with a linear coil spring?
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David Max
7 years, 2 months ago
Wouldn't this apply for the Push ACS3 coil conversion that you mention in your introduction as well? Or does the the air bump stop feature provide mid stroke support on top of providing ramp up to prevent bottoming? There certainly seem to be a lot of very positive reviews of it over on MTBR http://forums.mtbr.com/shocks-suspension/push-acs-3-coil-conversion-kit-1050590.html
While we're at it, do you have any plans to review the ACS3 conversion at some point?
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Andrew Major
7 years, 2 months ago
Hi David,
It’s a great point and in anticipation I reached out for some answers on the question of the Push systems rate adjust - haven’t heard back yet.
I’ve only tried it briefly and it certainly doesn’t feel overly linear even with the compression damping open.
Cam is currently in the process of reviewing the PUSH system.
Thanks!
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David Max
7 years, 2 months ago
Thanks Andrew,
I just wanted to add that I've really been appreciating your thorough and thoughtful reviews. It's the wrong thread and I don't want to divert this one too much, but your teardown on the Quadiems has me salivating over them a bit. If they live up to the hype performance wise I just might dig up some cash to spring for the extra shiny ones at some point. I know you like them a lot, but I've struggled to get balanced piston movement on the MT7s that I have on my Nomad and the idea of fully rebuildable, modern brake is really appealing to me.
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Andrew Major
7 years, 2 months ago
Hi David,
The Quadiem is a great looking and well made brake for sure. I have a handful of rides on them now and I'll definitely be trying different pads and possibly rotors - better with full break in but the friction just doesn't match the pressure at the lever - but otherwise really impressed.
I have a lot of experience with setting up Magura's brakes so I may have some insights into the process of getting them to run sans drag that you haven't considered. If you're so inclined fire me an e-mail ([email protected]) and we can compare notes.
Thanks for the props - I really appreciate them. And thanks for reading & engaging.
Andrew Major
7 years, 2 months ago
I spoke with JamesOC @ SuspensionWerx who has installed a number of PUSH ACS3 systems - along with re-tuning fork dampers to work better with them.
The air bump stop only affects the end stroke ramp/support but the reason the system doesn't feel linear - has such a notably different midstroke rate compared to how it initializes - does come down to air volume inside the stanchion. The PUSH spring, spring guide and air bump stop system take up so much space there's limited air volume just like in a coil/oil moto fork.
He tunes the midstroke rate by varying the amount of oil in the spring side of the fork within a ~25cc range.
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David Max
7 years, 2 months ago
That's a really neat insight, thanks for digging into it! It sounds like a nice and simple, albeit expensive approach, to being able to adjust all the spring rate throughout the stroke. Next time, I'm going to be wrestling with my brakes I'll drop you an e-mail and hopefully learn something new.
Cheers!
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mrbrett
7 years, 2 months ago
Progressive rate springs are extremely expensive to manufacture, and given the small production runs and numerous rates required for a mountain bike application I can see why we don't have any.
However, I don't understand why we don't have more than one spring rate on a fork or shock, like one might find if building a rock crawler or buggy. It's a common setup to run two spring rates, with a slider that moves along the shock body that allows one rate for high shaft speed impacts and one for small bump sensitivity, or also to tune ride height (sag).
(ignore the triple rate setup on the right in the picture, a tender spring is a whole different thing altogether)
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Andrew Major
7 years, 2 months ago
That’s an interesting question that I’ll ask around about.
I mean that’s how my Type-3 spring Rock Shox Judy worked in ‘98.
Technically it’s how the ‘97 Type-2 worked too... except the primary spring was an elastomer.
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David Max
7 years, 2 months ago
To take this a step further since we're nerding out already, I've been wondering whether we'll see the introduction of carbon composite springs in MTB at some point. Essentially, they are stacks of individual carbon belleville washers that would allow for rate tuning throughout the stroke by swapping out individual disks. Without having done too much research on them my suspicion is that the space constraints inside a MTB fork are such that it might not be possible to make use of them, but it seems like they could easily be used effectively for rear shocks.
MW industries, one of the main companies that manufactures them, is a subsidiary of Hyperco, the company that supplies springs for Push. I did ask Darren about them years ago in a MTBR thread and he implied that it was something they were looking into, but it doesn't seem like it ever went any further than that. If you are talking to folks down at Suspension Werx about funky, progressive spring systems it would be great to hear what their thoughts on them are.
More Info:
https://www.vitalmtb.com/photos/features/INTERBIKE-Part-2-Some-of-the-Latest-and-Greatest-for-2015,8119/MW-Industries-Carbon-Composite-Bellows-Shock-Spring,82137/bturman,109
https://www.mw-ind.com/product-types/composite-springs/
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mrbrett
7 years, 2 months ago
Did not even know such a thing existed!
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Andrew Major
7 years, 1 month ago
I assume cost is the biggest issue but there could also be compatibility with clearance issues for some shock reservoirs?
It’s very different looking which I think would make it a hard sell initially.
There’s also the issue with something being so tuneable riders are infinitely more likely to end up running sub-par settings. Saw this a lot with CCDB shocks before CaneCreek had their awesome base tune library running.
That then comes back to cost because without enough buy in because of complicated setup, appearance and compatibility it’s not worthwhile to manufacture for MTB applications.
When I first saw the ‘Bevel’ shock on Vital I was wondering if it could stroke enough (compress enough) to match shock dimensions (can it fit on an 8.5” eye-to-eye shock and compress 2.5” for stroke). If that was an issue maybe as Metric catches on we’ll see it again?
I’ll ask James & James at SuspensionWerx for an opinion next time I talking to them.
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mrbrett
7 years, 1 month ago
James & James sounds like a Victorian era legal firm.
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another_waki
7 years, 1 month ago
While I can see how many who test forks side to side could go for air, it’s not that simple. Some dampers are better than others with speed sensitivity and will let oil through on square edge hits. For comparison RS RCT3 Charger system is not as good as Fox Fit1- Fit4. Same for rear shocks, a well tuned CCDB will be extremely precise with LSC. Every single RS shock is a joke. That means that there are systems that actually offer plush and stable at the same time. Every single owner of Öhlins fork I know, and those are mostly good riders, takes coil any time. There is no help for RS Sektor coil though... there is air and there is air, same with coils. Manitou coil forks of the past... let’s not mention that. Marzocchi never had a suspension fork with decent LSC damping. I could eventually romanticize RC3 Ti series.
Then there is the clock for what it’s worth. And I know from Two horses mouths that forks setup by them for World Cup racers use coil, because they are just faster. At the end of the day, Aaron Gwin could win on a shopping cart. The whole coil vs air thing is as silly as clips vs flats and the wheelsize mess
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Andrew Major
7 years, 1 month ago
Regarding World Cup how many of those racers are using progressively wound coil springs? I don’t pretend to be intimate with the dark arts of WC DH bike internals but that is certainly a thing at that level - custom springs.
I’m not discounting the benefits of a great damper - I specifically mention the benefits of Manitou’s hydraulic bottom out - but my point is exactly that relying on ‘too much’ LSC is a crutch for having a progressive spring.
Ohlins twin tube damper is fantastic and there the HSC adjuster comes into play differently than most forks. I haven’t had the opportunity to take one apart re. how much air volume is in the spring side but it may be between oil and guts it is acting similarly to how JamesOC explains the PUSH systems performance. My experience with the Ohlins air system was excellent so it sounds like I definitely need to ride the coil version.
I should have specified that I was looking specifically at high end forks.
Cheers,
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nick bitar
7 years, 1 month ago
I may have the chance to pick up an ohlins fork at a decent price. I assumed that I was going to get the coil as it's the latest and greatest but perhaps the air is the way to go?
I think that my local terrain works pretty well with a slightly more linear rate then is currently fashionable (desert terrain with lots of mid size bumps and constant vibrations at the handlebar, not a lot of really big compressions).
But it's going to be on an all mountain hardtail that I'm guessing would suite a progressive system that doesn't dive too much to maintain geometry.
It's an awesome choice to have to make!!
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Andrew Major
7 years, 1 month ago
Great problem to have.
I haven't ridden the coil but the RFX air forks are some of the nicest I've ridden. In my experience it is just best to ignore the base settings. They work just like IRT except the gap between the main rate and mid-rate pressures is much larger.
The Ohlins open dampers are excellent.
AM
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Poz
7 years, 1 month ago
Looking forward to the IRT review.
Have a Mattoc and find it fantastic but the constant desire to tinker has me looking at the IRT as that next thing.
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Andrew Major
7 years, 1 month ago
Definitely the easy choice for the tinkerer. Everyone I've talked to who has ridden both systems prefers IRT on the trail. With base tunes the starting setup is only marginally more complicated and it's sweet that there is no token-changing disassembly required for fine tuning.
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Mammal
7 years, 1 month ago
Agreed.
The one thing I found with mine, was that it was well-served to re-balance the IRT/Main chambers from time to time (every 4 rides or so). I find that the air ratio very sensitive to changes, and I think there may be a small amount of seepage between the two over time. I could never notice much of a difference with the pump readings, but could definitely feel the difference once I re-balance and ride.
I'm interested if you might find this over time, as well... I noticed this most when it was toward the point of needing service badly, when friction is at a max, and you are trying to squeeze out all the sensitivity that you can.
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JT
7 years, 1 month ago
Have you contacted Manitou about it? Had the same problem with mine, but there was a redesign to the air piston. New version traps the o ring between two glide rings. Easy install (if you have their tool kit), and I haven't had an issue since.
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Mammal
7 years, 1 month ago
I think I have a relatively new version, but it's not the boost casting... I'll ask my good buddy (Zac, mentioned in the article). He'd have access to all the bits. Perhaps you're on to something there.
As an aside, I just had the low-friction seals installed, and they're amaze-balls. Small bump is so slick.
Thanks.
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Poz
7 years, 1 month ago
Excellent. Will need to put an order in before I do the winter teardown and rebuild.
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