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How To Measure The Shock And Hardware Size On A MTB

October 21, 2020

When looking to upgrade the shock on your mountain bike, all the numbers and measurements can get a little overwhelming. In this blog, we are going to cover what all those numbers mean and how you can work out what size shock and what size hardware you need to suit your bike.

Before we begin

The fastest and easiest way to work all this out is to do a bit of searching online. Most good bike manufacturers will list detailed specs on their website or in the bike's manual, so you can save yourself a bit of messing around if you're able to look it up. You might need to dive into the bike archive or their support pages to find info for older models though.

Unfortunately, some of the larger brands still tend to be rather reluctant to share this kind of info, so it's still a good idea to know how to work it out yourself.

Got a Fox rear shock? You can use the 4 digit code to look up your shock info HERE

In this blog we will cover:

  • How shock sizing works
  • How to measure eye to eye and stroke length
  • Imperial vs metric sizing
  • All about mounting hardware
  • Bushing types
  • Shock mount types
  • How to measure your shock hardware

What you'll need:

  • Ruler or tape measure
  • Set of vernier calipers (preferably digital)
  • Shock pump
  • Allen keys

How shock sizing works:

Shock measurements refer to two factors — the overall length of the shock (measured from eyelet to eyelet) and how much it can be compressed (the stroke). If a shock is listed with the size 210x55mm this means it has an eye to eye length of 210mm and a stroke of 55mm.

Measuring eye to eye length

Simply measure from the centre of each shock eyelet to find the overall length of your shock.

Measuring stroke length

For coil shocks: Remove the spring and measure from where the shaft goes into the shock body to where the eye starts on the shaft.

For air shocks: In many cases, it's possible to get an accurate stroke measurement by simply measuring from the lip of the wiper seal to the end of the air shaft. Rockshox even make this super easy and engrave the eye to eye and stroke length on some of their shocks. However, some shocks have their travel limited internally with travel reduction spacers (not to be confused with a volume reduction spacer), so you won't always get an accurate measurement this way. The Float X2 on the Norco Sight in these images looks like it has a stroke of 55mm when measured externally, however, Norco has fitted it with a 2.5mm travel reduction spacer from the factory so the usable stroke is actually only 52.5mm.

The most accurate method for shock measurement:

  1. Measure the eye to eye length
  2. Attach a shock pump and slowly remove all the air from the shock
  3. Compress the shock fully and measure the eye to eye length again
  4. Subtract this measurement from the initial eye to eye length to find the usable stroke
    (ie. 185 - 132.5 = 52.5)


Note that I was 5mm off with the measurement in this image. Obviously compressing the shock, taking the measurement, and taking a photo was too many things for me to juggle at once

You'll probably need a helper to compress the shock while you measure it, so another method is to get to step 3 above, push the sag ring up against the wiper seal, then re-inflate the shock and measure from the lip of the wiper seal to the sag ring. This should give you the same measurement without having to juggle too many things at once.

It's important to note here that these numbers won't tell you how much rear-wheel travel your frame has. That's all down to the design and leverage ratio of your rear suspension. A 140mm and a 160mm bike could both use a shock with exactly the same size eye to eye and stroke length.

Also, keep in mind that two shocks that have the same eye to eye measurement could have a different stroke length. When upgrading, be sure to match up both the eye to eye and stroke length with your existing shock to avoid any issues.

Some frames do offer the potential to run a slightly longer stroke to eke out some extra travel (as people discovered by long shocking the Santa Cruz Hightower a few years back) but it's not something we would recommend unless you really know what you're doing. You're getting into quite dicey territory from a warranty standpoint if anything goes wrong so our official stance will always be to stick with the original manufacturer spec.

Imperial vs metric sizing

Imperial

Historically, MTB shocks have been measured in inches or imperial measurements. Specific sizes were often requested by frame manufacturers to suit the unique requirements (ie. fix the problems) of particular suspension designs. Over the years a bewildering array of shock sizes were created — all measured in inches, and fractions of an inch, with varying spring curves, and without any real logical progression in terms of dimensions or stroke length.

There is a lot of tech going on inside a shock and suspension manufacturers were often forced to squish all those complicated circuits and seals into shocks that weren't quite large enough, making it difficult to keep up with the ever-increasing performance and reliability requirements of long-travel bikes. The 7.875 x 2.25" (200 x 57mm) — one of the most commonly used air shocks on enduro bikes — was a particularly hard one to work with due to space restrictions compromising the placement of the internals, so its performance and reliability was never quite 100%.

There is also the small issue that most of the world outside of the US (and most of the bicycle industry) thinks in millimetres and it's much harder to perform complex calculations using the imperial system.

Metric

In 2016 (The day before April Fool's no less) Rockshox and a handful of other suspension brands announced the introduction of metric shocks as part of an industry-based initiative to create a more standardised and simplified approach to shock sizing. The new system wasn't just about suddenly deciding to use a different system of measurement though; it introduced a new range of sizes with more logical, evenly spaced increments between each size and a consistent air spring curve across the whole range. If a brand decided to run a different sized shock on their bike, they wouldn't have to redesign all their suspension kinematics as the spring rate would remain the same. The sizes were chosen in consultation with suspension and frame manufacturers to provide the most room to optimise the internals while making life a whole lot easier for frame designers, retailers, and consumers alike.

With metric sizing, eye to eye measurements increase in 20mm increments, and stroke lengths increase in 5mm increments (or 2.5mm with travel reduction spacers installed) which provides consistent air spring curves across all sizes. As part of the package they also revised shock hardware sizing to tidy up the mess of 80+ options to a much more sensible 18, while creating more freedom for the use of bearing or trunnion mounts for a better fit with particular suspension designs.

It's important to note here that imperial and metric shocks aren't interchangeable, and even though you'll often see imperial sized shocks listed with the measurements converted to mm (ie. 7.875x2.25" / 200x57mm), these aren't actually metric shocks. It's quite easy to spot these faux-metric sized shocks as they will normally have more random-looking numbers (ie. 200x57, 216x63) while metric sizing is much cleaner and doesn't hurt your brain so much to look at (ie. 210x50, 230x60).

Imperial sized shocks are still available for purchase but the intention is for them to be phased out over a 4-5 year window, so things will be quite messy in the meantime with a mixture of sizing standards on the market. You will start to see decreased availability over the next couple of years, but imperial sizing will still be supported as long as the demand remains. There is no need to hock your faithful steed just yet.

All About Mounting Hardware

Shock mounting hardware (often called reducers) does not come with the shock and needs to be purchased separately. A standard hardware setup consists of an axle that is pressed into the shock's eyelet, and spacers that slide over the axle to take up the space between the frame's mounting tabs and the shock to create a tight fit without any play. The hardware could consist anywhere between 3-7 pieces depending on the brand and age of your setup. Shock hardware can be the same on both ends, different sizes, or could be replaced by a Trunnion mount, a bearing, a yoke, or an open eyelet on one end. You'll need to know what you need before making a purchase so it's a good idea to remove your shock and check things out first.

Another benefit to come with the transition to metric shock sizing is a simplification of the sizing for shock mounting hardware. Metric shock mounting bolts come in 6mm, 8mm, or 10mm diameter and the hardware changes in 5mm increments. Once the old school sizing gets phased out it'll be simpler to work things out, and bike shops will find it easier to stock a full range of hardware sizes without having to resort to grinding and filing to remove those 0.1's and 0.2's of a mm to make things fit.

It is often possible to reuse your existing hardware when upgrading to a different brand of shock but different bushing types can vary in width and cause fitment issues when mixing brands. A mechanic in a workshop has the luxury of a trial and error approach to try out different combinations to find the best fit, but that's something we don't have the luxury of when shopping for bike parts online so it's better to get it right the first time.

We highly recommend getting new hardware to match brands when purchasing a new shock to avoid any potential fitment issues that could cost you time off your bike!

Bushing Types: With a few exceptions (Cane Creek non-inline shocks have a 14.7mm eyelet and some older ones are 16mm), most modern shocks from the main brands have eyelets with a 15mm inner diameter and a 12.7mm (1/2") width. A bushing is normally pressed in to provide a buffer between the eyelet and the hardware and wear over time so the shock eyelet doesn't have to. They are an inexpensive part that is easy to replace.


Inner and outer dimensions of a standard 15mm bushing

Some shocks come with a metal bushing with a PTFE coating while others come with a beige coloured polymer bushing. On some shocks, the bushing will be exactly 12.7mm wide and sit flush with the eyelet, while others — most notably Fox — use a flanged polymer bushing that extends outside the eyelet. That extra width is what can cause problems if you try to mix and match hardware and bushing types between brands.


A standard DU bushing (L) and one side of a flanged Fox bushing (R)

Types of shock mounts


From L to R: Bearing Mount, Trunnion Mount, Yoke

15mm open eye: Literally just an eyelet with no bushing or hardware installed. Frames requiring this setup will come with their own proprietary hardware. If you buy a new shock you will need to remove the bushing from the eyelet before the shock can be installed.

Trunnion mount: Popular on frames with vertically mounted shocks. A Trunnion mount has a threaded hole on either side of the shock body which shortens the overall length while maintaining the stroke length. A shock with a trunnion-mount is 25mm shorter than the standard mount equivalent — allowing for reduced standover height and greater flexibility with shock placement.


Standard mount (L), Trunnion mount (R)

Bearing Mount: Some suspension designs that produce a large degree of rotation on one mount can benefit from using a sealed bearing in place of the standard bushing/hardware setup to reduce friction and provide a more supple feel. Manufacturers can choose to have this on the shaft end or the body end — whichever end has the most rotation.

Yoke: A yoke is a strut that extends from the linkage to attach to the rear of the shock. Yokes are often found on Specialized FSR frames but also other brands such as Devinci. Due to the proprietary sizing found on Specialized frames upgrading the shock has only really been possible by purchasing a BikeYoke aftermarket to allow you to fit a standard-sized shock. Thankfully 2019+ Specialized models use standard metric sized shocks and they've done away with their proprietary sizing (hooray!)

If you're a Specialized owner and need a BikeYoke to upgrade your shock, CLICK HERE

How to measure your shock hardware

For the most accurate readings, you'll need a set of digital vernier calipers for this as they will give you a repeatable and accurate measurement. You'll need to be accurate to within a 1/10mm so a ruler or tape measure isn't going to cut the mustard.

Note that the shock mounting bolt comes with your frame and doesn't come with the hardware. If you've managed to lose this you'll need to contact your frame manufacturer (or a brand dealer) to source a new one.

Shock hardware is labelled first by the width of the hardware axle, and then by the diameter of the mounting bolt. 21.8 x 8mm hardware would be 21.8mm wide and take an 8mm bolt.

To work out your shock hardware sizing you will need to take these two measurements:

  1. The width of the hardware axle, or the inner width of the shock mount tabs on the frame.
  2. The outer diameter of the shock mounting bolt, or the inner diameter of the bolt hole on the hardware.

Measuring the width

Measuring the bolt diameter

Hopefully now you're skilled up with everything you need to know to upgrade the shock on your bike. If you're still stuck and need a hand feel free to get in touch with our team at help@mtbdirect.com.au or hit us up on live chat on the site and we'll do our best to get you sorted.

Meet Rob!

Rob grew up in Canberra but recently moved to Bright in Northeast Victoria to be close to some of Australia's best MTB trails. He's been into racing MTB's since his teens and has learned a thing or two about bikes along the way. His current steed is a Norco Sight C2 and he's looking forward to hitting up all the rounds of the Victorian Enduro Tour this season.