Ever since the inception of the full suspension mountain bike, frame manufacturers and suspension manufacturers have worked hand in hand to smooth out whatever ridiculousness we decide to skid our bikes down. It’s not an easy job, and one of the least exciting parts of it is figuring out the most effective way to attach a rear shock to a frame in a way that’s cost effective, reliable, and easy to understand for mechanics around the world. We’ve had a myriad of shock mounting standards in the last few decades, but now the industry has settled on metric shock sizing in the most recent decade. With metric sizing, the trunnion mounting standard was born, serving as an alternative to standard shocks when frame manufacturers are designing their latest and greatest rear suspension apparatus.
To put it simply, trunnion refers to the mounting style used on one end of the shock. It uses bolts on the sides of the shock body to attach directly to your frame or linkage, and a trunnion shock will have a trunnion mount on one end with a standard eyelet on the other end. In contrast, a standard shock will use standard eyelets on both ends of the shock. Unlike a standard eyelet, a trunnion mounting interface will not use any mounting hardware, but will typically use some proprietary spacers between the frame/linkage and the shock. It’s just an alternative mounting style that gives bike companies options when figuring out the optimal way to package their rear suspension design.
Why Trunnion?
That’s a great question, and the main benefit that trunnion mounting gives to frame designers is a more compact package. Compared to a standard shock, trunnion shocks are 25mm shorter in total length while providing the same stroke length. Because the mount is directly on the shock body rather than sticking out further, you can pack all that punch in a slightly smaller footprint. This snubnose design means designers can shove shocks into places they wouldn’t be able to otherwise, allowing an engineer's creative juices to flow freely rather than being constrained to just one mounting interface option.
When metric shocks launched in 2016, folks largely moaned at another mounting standard hitting the market, especially when these shocks were actually longer than the imperial shocks than they were replacing. In the long run, metric shocks have certainly simplified shock purchasing in the aftermarket as well as shock purchasing for product managers speccing OEM bikes, but it’s undeniable that metric shocks with standard eyelets are longer than imperial shocks with similar stroke lengths. As a concession, trunnion shocks were introduced to give a shorter length option in case that was truly necessary to jam a bike’s linkage together.
Can I upgrade to Trunnion?
No, neither trunnion shocks or standard shocks are better than the other. There’s nothing more to it than a different mounting standard for your shock, and you must use the style of shock your bike was designed for. It’s like buying a 29” tire for your 27.5” bike because you heard they were better. That’s just never going to work, and you need to stick with the style of shock your bike was designed for. If anything, trunnion shocks are arguably an inferior design to standard shocks. While they manage to pack all that punch into a smaller footprint, they’re more susceptible to failure from the side loading and twisting forces that are continuously being sent through a mountain bike frame. Trunnion shocks aren’t wildly unreliable, and I should be clear that this results in a very marginal difference in overall reliability that also depends heavily on the frame design that the shock is installed on.
How do I make sure I buy the correct style of shock for my bike?
Luckily for us, as long as you get the correct size of shock for your bike, it is guaranteed to be designed around the correct mounting standard. Trunnion shocks and Standard shocks aren’t manufactured in the same lengths, so there’s no concern of getting the incorrect shock if you’re getting the right size of shock. For example, this Transition Sentinel uses a 205x60mm shock, and all shocks in 205mm length have a trunnion mount on one end, and a standard mount on the other end. If you need help figuring out exactly what size of shock you need, check out our Mounting Hardware Database, that includes shock specifications in addition to mounting hardware specs.
We carry a wide range of shocks in both trunnion and standard configurations, check them out below and please let us know if you have any questions about getting the right shock for you and your bike!
