Demystifying Lower Leg Fork Service

If you happened to not be a cyclist, but read a lot of things written about bicycles, you would probably think that “servicing your lowers” was a critical part of the mountain biking experience. How often does this get talked about? When should you do it? Will you die if you don’t?

Myself, there has seldom been a time over the last 5 years where I haven’t felt like I’m neglecting at least one fork. It all came to a head this year (well…actually last year at this point!) when I had not one, not two, but three forks from three different brands that all were screaming at me to have their lowers serviced! Crazy times.

You’d think that such a fundamental thing to the experience of cycling would be easy to accomplish. Roadside lowers service booths, perhaps? And, in a sense, it is. Easy to accomplish, that is. Fox and Rock Shox all provide some sort of documentation on the process and most people that work on their bikes themselves should be able to follow along. Do not be daunted by the process. It can be messy, yes, but easy enough to complete.

You’d also think that sourcing the correct oil would be a trivial process. I mean, we’re all continually servicing our lowers so most shops should have an entire section dedicated to just this one task, right? But over the past year or so while trying to find the “correct” oil for my lowers, I’ve been on the following wild goose chases…all from reputable bike shops!

1) I tried to source some Fox 20 weight gold locally. Nobody that I phoned locally had any in stock. Ordered it online.

2) I tried to source some Fox Teflon Infused 5 weight locally. Nobody that I phoned locally had any in stock. One told me that they did, but when I showed up they tried to sell me R3 instead. When I explained to them that the damper side lowers oil mixed with the damper oil and it was advised that you really should use Fox Teflon Infused 5 weight, the only thing that registered was skepticism. I finally hassled our local Fox people to send me a bottle.

3) I went back to this same shop that tried to sell me the wrong Fox oil, searching for the Rock Shox recommended “Maxima Plush Light”. They now told me that they don’t sell fork oil, because they’d prefer to do the work themselves! I guess at least now they aren’t trying to sell people the wrong oil, so there’s that. But what oil are they using on their customers bikes?

4) I tried to source “Maxima Plush Light” from a few other shops. Nobody that I phoned locally had this oil. I walked into a shop and they recommended two other Maxima oils (neither “plush” nor “light”) or some Rock Shox 0w30. I sent a frantic text to my Rock Shox connection and he told me the 0w30 should be fine, so I bought that. As I left the shop, they gave me a volume recommendation that contradicted the official Rock Shox recommendation.

I live in one of the mountain bike capitals of the world (I would argue). I have people connected to major suspension companies that will answer my e-mails (occasionally). I care about doing this right (for the most part). Even with all of that at my disposal, it proved to be tremendously difficult to follow the manufacturer’s recommendations for a lowers service! And several shops gave me bad advice! Getting to the bottom of this felt like an important thing. Forks are an expensive, critical component of our bikes. It seems rather important to me to use the proper oils at the proper volumes but nobody else seems to really give a shit. So maybe I’m overthinking it? Maybe it really doesn’t matter and you can just jam whatever volume of whatever oil you have in there and call it a day?

Fox

I decided to reach out to a few manufacturers to hear what they had to say about things, and to try to separate fact from fiction. I started with Fox because they seem the most specific about things, with two separate and very definitive oil requirements.

The first thing that I wanted from Fox was confirmation on why they have different oil requirements for each side of their fork. My understanding was that the (correct) 5 weight in the damper side is doing double duty by slowly cycling through the damper, while the Gold 20 weight in the air chamber side is just lubing things up. But there must be more to it, right? For a more official explanation, Mr. Sean Estes of Fox provided the following:

The reason you want to use a specific oil is because part of the magic of Grip dampers is that they are self-healing, meaning they are designed to be able to ingest some bath oil into the damper and in turn be able to purge that oil back out to prevent pressure build up inside the damper that would otherwise lead to performance degradation or worse. With this in mind, the bath oil needs to be the same spec as the damper oil for this system to work as intended.

Taking things further, Mr. Brian Lampman, Engineering Manager at Fox provided the following.

The oil bath for the damper side is basically a percentage of oil required in a full damper and the 5wt damper oil is used inside and out. Grip X & Grip X2 have different bath oil requirements in a common 36 chassis because they have different oil volumes for the same given travel.

Air springs use a different oil spec and lower volume because of less chassis volume (capped inside the entire upper tube in most cases) compared to the damper side chassis volume (volume inside and outside the upper tube all around the damper). Less room for the oil allows it to get up the foam ring and wiper easily when cycling the fork.

This is sort of fascinating. Not only do you need to get the oil right, it really seems like you can mess things up if your volumes are off too! Mr. Jordi Cortes had thoughts on both of these subjects.

When we work on oil development, we work with a supplier to hit specific requirements. Lubrication and damping are two factors but compatibility with parts is another. Many oils can actually degrade rubber or cause seals to swell and lose function. There are definitely cross compatible oils but that can be a crap shoot. Just buying expensive stuff doesn’t necessarily mean it works. For that reason it’s probably best to stick to the factory oil unless you or someone you trust has firsthand experience. Volumes are critical as well. You’re balancing positive, negative and lower leg volume which can drastically affect your ride. Too much oil and your spring curve will get extremely progressive and you may get some leakage from excess pressure. Too little and you’re getting a more linear spring but also not lubricating the internals properly and in the case of a recirculating damper you’d run the risk of oil loss.

So, straight from the horsiest of horses' mouths, hit your volumes and use the right oil.

Another thing that I was really interested about was crush washers! I asked Jordi just how important it was to replace your crush washers each time you crack your fork open, and for any hot tips on servicing your forks.

I think we’ve all recycled crush washers but they do serve a purpose. Similar to oil drain washers, you really do want that thickness to prevent metal on metal contact and ensure proper torque. As for tips, plan it out, run through each step before doing it, don’t allow any sharp objects anywhere near your work and use the right tools. I’ve seen people use the rebound cover as a tool to hammer on to remove the damper from the lowers. That usually doesn’t end well.

Formula

After talking with Fox, I felt pretty good about my strategy of using the right oil and following the manufacturer’s instructions. Maybe this is just a Fox thing though? Maybe another manufacturer will have a differing philosophy. Since I already had the Formula folks on an e-mail chain, I posed some similar questions to them about their forks. May as well take advantage of a captive audience, right?

The advice given by Formula wasn’t much different than from Fox. They also have two oils, one damper oil and one bath oil. But beyond that, they seem a lot more easy going about the process. Baptiste Del Monte at Formula had these things to say.

We do offer two oils, OJO1 for the hydraulic cartridge and FX for lubricating the stanchions, as well as inside the air chamber to lubricate the quadring. The properties of the two oils are different: the one for the hydraulics is stable even with varying temperatures and offers good wear resistance. On the other hand, the oil for lubricating the stanchions is not suitable for hydraulics, just as hydraulic oil is not suitable for lubrication. The stanchion oil is more viscous and contains specific additives to reduce sliding friction. If you can’t find our oil, OJO1 (in the cartridge) can be replaced with Kayaba 01. FX (stanchion lubricating oil) is very similar to Fox Gold 20WT, so that can be used instead. You should never use low-viscosity oils with few anti-friction properties for the stanchions, otherwise, you risk poor smoothness, jamming, and lack of fluidity on rough terrain

While not outright encouraging experimentation, Formula did seem okay with mixing things up a little bit in their damper.

It is not recommended to use oil that is too viscous in the cartridge, as this could lead to pressure imbalance problems, since the cartridge is designed to work with OJO1 or possibly with less viscous oils. In the colder months, especially when temperatures drop below zero, there are two ways to improve performance. First, you can use a less viscous oil in the hydraulic cartridge, such as Formula brake mineral oil, to make the hydraulics more fluid and responsive. Second, you can modify the lubrication of the stanchions by mixing FX oil with OJ01, resulting in a combination that improves smoothness without compromising performance. However, this is a modification only for users who will change the oil again in warmer months, not for those who disassemble the fork once a year or less.

After that, I asked them about any concerns with oil volumes, or with bath oil mixing with the damper.

Getting the correct volume in the hydraulics is essential to ensure the cartridge functions properly, otherwise, emulsions may form and compromise performance. As for the stanchions, too little oil reduces lubrication, while too much increases internal pressure, with the risk of pushing out the dust seal. Changes to these volumes will not affect the characteristics of the fork.

Rock Shox

My final port of call was with Rock Shox. I had numerous casual conversations (fun!) about fork oils with a few different Rock Shox people and I knew that they had similar advice to Fox on most topics. However, what I really wanted to know was why Rock Shox had so many different available oils and which of them were cross compatible. Unfortunately, I wasn’t able to nail down an official response from Rock Shox.

What I learned

The conclusion that I took home after all of these conversations was that I hadn’t been overthinking it. I did some crazy shit with suspension forks back in the day, running fast and loose with oils and volumes. In this day and age though, if you think you need anything other than the stock oil at the stock volume, what you probably actually need is a suspension tuner, not a creative servicing solution. With the wisdom I now possess as a rule following middle aged man, my advice for servicing your lowers is simple.

1) Buy the right oil.

2) Follow the manufacturer’s instructions.

3) Dispose of your oil properly. In BC, it’s easy. Any BCUOMA depot will be able to handle it. If you aren’t sure, search for “gear oil disposal” using your favourite disposal app. Further instructions can be found here.

That’s it! Happy servicing.