A little over a year ago, my friend became coil-curious. We were on a ride, and he saw another mountain biker riding the same bike but with a coil shock. As we climbed back into his truck, he made up his mind—he would buy a coil shock.
He researched coil shocks for a few weeks, speaking of how plush coil-sprung suspension rides and its simplicity as if he were trying to convince me to make the switch. All reasons aside, we all know why he wanted a coil shock—they look cool.
So, after a handful of weeks, my buddy pulled the trigger on a coil shock he found on Facebook Marketplace. He carefully ensured this new shock would fit his bike, confirming the eye-to-eye measurement and stroke length. He purchased the shock, brought it home to install, and realized he had made a terrible mistake.
The coil shock he’d purchased was a trunnion mount. His bike frame was not.
My friend, who was somewhat new to mountain biking, was unaware of the two different shock mounting systems: standard and trunnion. Fortunately, he resold the coil shock, recouping his initial loss. Sadly, he has yet to experience the plushness of coil-sprung suspension.
Why are there two different styles of shock mounts in the first place, embarrassing my friend so? What is the difference between trunnion and standard shocks, and why have more brands been using the trunnion option?
Naturally, I scoured the depths of the internet, and reached out to some industry experts to find the answers to these very questions.
Important factors and the big difference
Regardless of whether a shock is standard or trunnion mount, both styles have some of the same essential factors. There are two crucial measurements on either: eye-to-eye and stroke length.
The eye-to-eye measurement is the overall length of your shock, measured from eyelet to eyelet. These eyelets are on either end of your shock and are where the shock, using mounting hardware, attaches to the frame.
Frames are designed to fit a specific overall length of shock. Using a shock with an eye-to-eye measurement that is too long won’t fit, and one that is too short will change your bike’s geometry and suspension characteristics and will likely damage it.
The other important measurement is the shock stroke. Shock stroke is how much your shock compresses—essentially, it is the measurement of the shock’s stanchion. To be clear, this isn’t the same as how much travel your shock has.
For clarity’s sake, let’s acknowledge that stroke and eye-to-eye length aren’t directly correlated and vary from bike to bike. Just because two bikes have the same rear travel number doesn’t mean they will have the same eye-to-eye measurement or stroke length for their shock.
A third factor, where my friend didn’t do his research, is the most significant difference in trunnion and standard shocks. A standard mount shock has eyelets on either end of the shock, while a trunnion mount shock mounts directly to the linkage through the shock body. This direct-mount happens at the top of the shock, with the opposite end of a trunnion shock having a traditional eyelet.
A trunnion shock also has less mounting hardware than a standard shock. Each standard mount shock has different mounting hardware for each eyelet, depending on the frame. A trunnion shock eliminates half of the hardware due to the body of the shock using the frame’s linkage bolt to mount to the linkage.
This also means that the linkage on a frame that uses a trunnion mount shock is much wider at the mounting point.
Why have trunnion shocks become more prevalent on mountain bike frames?
There are a lot of needs when it comes to mountain bike frame design. We want long-travel bikes that can hold a water bottle. That means putting a larger shock and a bottle cage in an already crammed front triangle.
But not all trunnion shocks are on the longer travel side. Steve Matthews of Vorsprung Suspension in Whistler, B.C., said that while he doesn’t have exact figures, he sees trunnion shocks on long, mid, and short-travel bikes.
The trunnion mount’s advantage is fitting longer shocks into tighter areas. However, Matthews did mention a stipulation. “This is usually only relevant if the shock is vertically oriented in front of the seat tube and the bottom of the shock has to be a fair way up away from the bottom bracket,” Matthews said, using the example of fitting a shock with an e-bike motor.
Matthews also mentioned that trunnion shock mounting hardware tends to be much more standardized than “standard” eyelet shocks. “[Trunnion] is also a standardized size—54 mm wide and M10 bolts—rather than the literally 100 different ‘standard’ sizes of shock hardware.”
Trunnion-mount shocks not only make frame designers’ jobs a bit easier but suspension manufacturers like them as well. Since the overall eye-to-eye length is incorporated into the shock body, suspension engineers have more room internally for all the features they’d like to include.
Downsides to trunnion mount shocks
While trunnion-mounted shocks give frame and suspension designers a little extra room to work, they can have issues. One such issue has to do with how the trunnion shock is mounted.
With the trunnion shock mounted through the shock body itself, the shock has far less flex. This can lead to added stress at the shock mounts and in the shock as well. “Excessive rigidity puts a lot of side load on the shock when the frame flexes,” Matthews explained. “In our experience, this leads to a lot of premature shock failures.
The frame, shock, or both could be damaged in extreme circumstances. In most cases, however, this will result in more stress on a trunnion shock, which could mean more frequent service intervals.
