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AASQ #98: Can I switch the spring in my fork from coil to air?

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We know, there’s no such thing as a stupid question. But there are some questions you might not want to ask your local shop or riding buddies. AASQ is our weekly series where we get to the bottom of your questions – serious or otherwise. This time we discuss swapping the fork spring from coil to air! Hit the link at the bottom of the post to submit your own question.

Can I switch the spring in my fork from coil to air?

SilverFish UK: Probably not on its own. Focusing on the Marzocchi Z1, the only current fork with a coil sprung option, and the post-2016 Fox 40, the next most recent fork which had a coil sprung variant….

marzocchi z1 coil conversion kit suspension fork mtb enduro

The inside wall of the upper-tube (stanchion) is used as a sealing surface for the air piston seal to work on. A coil spring, when compressed, will normally move enough to touch the inner wall of the upper-tube and leave marks and scratches; this leaves a surface which is not good enough for an air seal. You are likely to suffer air leakage past the piston.

Replacement of the spring side upper-tube (Fox 40 fork) or crown steerer assembly (Z1) would be needed to complete the conversion to air.

silverfish uk fox suspension servicing tuning centre fox40 dh fork

However… if luck smiles on you and maybe the fork hasn’t been used much at all then if, with very careful inspection of the inner upper-tube wall surface (hold it to the light and roll it around as you look) you can see no marks or scratches, you could fit the appropriate air shaft, top cap and volume spacer arrangement as the upper tubes are the same in coil and air sprung versions of these forks. Be very sure of your assessment though as it could prove more costly than anticipated!

TFTuned: This is a good question, and really quite timely because it allows us the opportunity to give proper consideration to a question which, until recent years might have been answered simply with… “why on earth would you want to?”

One of the more significant developments in bicycle suspension that we’ve witnessed over recent years is that of the air spring.

The user friendly air volume adjustment options that many manufacturers have adopted, combined with large negative air volume relative to positive, has improved the performance of air sprung forks enormously.

With the relatively recent crop of new bike parks emerging all over the country, and the corresponding increase in our mountain bike fraternity, we’ve been able to enjoy watching a ‘new breed’ of mountain biker for whom small bump compliance and grip over technical terrain aren’t such big considerations as a light weight, robust fork chassis and an increase in resistance from the spring for big impacts…

Until really quite recently, the ‘standard’ bike park jump line that a vast number of riders are now comfortable with tackling, would have been limited to very few ‘elite’ freeriders and available to view only on a DVD or in a magazine.

vorsprung secus fork coil like performance with air spring
Check out Steve Fisher’s review of the Vorsprung Secus here

There’s room for development yet, and as we witness clever aftermarket options from the likes of the Vorsprung and their ‘Secus’ with variable shaft position sensitive negative air volume, it’s conceivable that mass produced air sprung forks are likely to develop even further in the not too distant future.

Despite the apparently numerous advantages of air springs, we’re still of the opinion that there’s nothing on earth to offer the sensitivity, compliance and control of a good old fashioned coil spring, so it’s important to give some thought to your requirements before making any purchase, rather than living with an inappropriate spring medium. We’re here to help with that kind of thing, by the way….

2019 Ohlins RFX36 enduro mountain bike suspension fork
The Ohlins RXF36 enduro mountain bike fork can switch between air and coil springs

So, going back to the original question of whether you can change from coil to air: for the vast majority of suspension brands that we’re familiar with, the simple answer is no… sorry.

With the exception of the Ohlins RXF 36 coil, where the air spring is enclosed within a separate cartridge inside the fork leg.

suspension fork internals smooth surface inner stanchion perfect seal

Other manufacturers tend to use a larger piston which seals against the inside surface of the fork leg. The air leg is ‘honed’ on production to provide an immaculate (and fragile) air sealing surface which would be damaged if abraded by a moving coil spring. That air leg needs to be perfect if the air spring is to perform reliably:

Thankfully, the vast majority of manufacturers offer an air sprung version of their fork, on account of the light weight and adjustability that only an air spring can offer.

Vorsprung Suspension's Smashpot coil conversion kit
Convert your air sprung fork to coil with Vorsprung’s Smashpot bottomless coil conversion kit

More often than not we’re approached by customers wanting to go from air to coil, and with the increasing popularity of e-bikes, the coil spring is just as relevant for mountain bikers now as it’s ever been. Thankfully, we have some great options for these customers, like the coil conversion kits from Vorsprung and PUSH industries for Fox 36 and RockShox Pike forks.

JTech Suspension: It very much depends on the brand and model of fork you have. The biggest sticking point is usually that the coil spring will have made some tiny scores on the inside of the stanchion, where it’s common for the air spring piston to seal against. This will cause a transfer of air, which isn’t something you want!

j-tech suspension workshop bespoke suspension tuning services

It would still be possible, but the stanchion/CSU would need replacing also, which isn’t going to be cheap. There are forks that use a cartridge system – for example, Ohlins RXF forks. This means the air spring is a complete cartridge, that doesn’t rely on the surface of the stanchion for sealing. These forks can be changed from coil to air very easily, without any problems. For an exact answer, we’d need to know the make, model and year of your fork.

j-tech suspension workshop bespoke fork tuning services

SR Suntour: This question on our side relates to two models, where we offer both coil and air spring solutions:

  • XCR series
  • Zeron series

sr suntour zeron fork coil version exploded diagram

Both models are coming from Taiwan. Actually, if you have a bike that comes with a Zeron or XCR32 coil version, you can basically convert from Coil to Air. But, for this you would have to replace the complete upper assembly (which is machined and receives an anodized coating on the Air version of the ZERON, for example) and get the other specific Air parts (piston, support tube assembly etc…).

sr suntour zeron air srpung fork can be converted to coil spring

In the end, only the bottom case assembly and oil cartridge remain the same. Here we provide two illustrations of the parts that need to be switched for converting the forks from Coil to Air (highlighted in green).

In any case we recommend that riders contact a certified dealer from our dealer list or the distributor representing SR Suntour in the corresponding country for proper assistance.

Thank you very much to Tim Williams at SilverFish UK, Paul at TFTuned, Arnaud Hacquard at SR Suntour and James Williams at J-Tech Suspension for contributing to this week’s Ask A Stupid Question!

Got a question of your own? Click here to use the AASQ form to submit questions on any cycling-related topic of your choice, and we’ll get the experts to answer them for you!

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3 years ago

Just buy a Formula fork and you can switch from coil to air and vice versa.

3 years ago

Tyler and co: first and foremost, HAPPY HOLIDAYS!

Now for the whiny part: these new Genesis ads make the articles almost unreadable on an iPad. It makes for a glitchy, obtrusive, and obnoxious browsing experience. Just wanted to give your team a heads-up.

2 years ago

Actually, Suntour Zeron have plastic sleeve insert in the left stantion to prevent the spring from touching and scratching. So you are wrong, Zeron can be convert to air if the insert is removed.

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