We are testing metric and trunnion shocks on some of our full suspension bikes as part of our incremental product development program.


Why would the world be moving to Metric and Trunnion mount shocks?


These two are very different concepts, but introduced at the same time, and thus adopted simultaneously. They don't need to be used hand in hand, and you can get Metric shocks that are not trunnion but unfortunately not Trunnion mount shocks that is non-metric. Normal eyelet mount shocks and imperial sizing will be available from all suppliers as long as it stays relevant to riders.


Let's start with the basics; Metric.

Metrics is a measurement standard used across Europe - millimeters instead of inches as measurement.

So what this means for shocks; is that for the eye- to- eye and stroke length that has been used for shocks, going forward, will be standardized and consolidation into a few set sizes, specified in millimeters. This also applies to mounting hardware, making for easier shock choice, fitment and servicing, as well as sourcing replacement hardware. Fox is quoted as saying that metric is a fit issue, where Trunnion is a performance update.


Whom is playing along?

Fox, Rockshox, CaneCreek and X-fusion have jumped on board, to name but a few. This means that there is no shortage of choice for frame designers.


So does metric actually make a shock any better?

Yes - with the new standard also comes a new shock design, so even if not as monumental as trunnion, there are performance benefits with metric shocks.


Can I pop a metric shock in my pre-metric days frame?

Unfortunately not - maybe in select cases there's a botch that will fit, but unfortunately the new standard is not supposed to be backwards compatible.


Here's a table with the new available metric sizes for shocks.

So then on to Trunnion mount shocks.

Trunnion mount refers to a shock that is mounted (the shock hardware goes) through the upper part of the shock body, and not what has been the norm, through external eyelet mountings. It saves space among other positive attributes.  Trunnion is designed to be installed in conjunction with a set of frame/rocker mounted bearings at the connection.


Why is space an issue? 

Space can be a two part problem, in modern frames and with modern suspension platforms, space inside the frame for shock can be limited and thus limiting in terms of choice, resulting in a less than ideal shock purely due to size constraints. This valuable space is also needed for hydration, where a rider wants a bottle cage inside the front triangle, where some suspension designs and shocks just does not allow for the clearance. This is especially notable on XS, small and sometimes medium frames, where large are not excempt from this issue, but naturally have more room and thus less prone to it. 

The second part of the space issue is inside the shock itself.  The more bushings, air volume and assembly parts needed for proper operation needs more space - to fit everything in, that's a lot of tech in a very small package, sometimes at the cost of performance.

Moving to trunnion gives the suspension internals more room. There’s more room for bushing overlap, and so there’s less binding under load - meaning it is laterally stiffer, and the seals takes less strain, meaning they work better too. More importantly, though, the increased length (also now in metric sizing) allows for better, more controlled gas compression.


What's gas compression?

Gas compression refers to how a shock works. When a shock is compressed, the air inside the shock reservoir is compressed, compression and decompression of this air (gas) is what a shock does - and how it handles these two events when force is applied to it (and how to adjust the rate of these two events) is what makes a good shock. OK, its a bit more technical than that, but that should do for now.


What else?

The shocks can carry a larger air volume, making them more supple and responsive. Since the internal volume of the shock is bigger, a larger volume of air can be used, meaning that the shock has a bigger air "bumper" and since the bigger air volume handles pressure easier, you can run lower pressures, meaning the shock will remain cooler for longer and also retain big hit compliance and small bump sensitivity when it has been setup for the correct sag without feeling harsh.


Is it worth it?

The short answer is yes. Frames can be made smaller, meaning more stand over height, better durability, less friction and most of all better performance.