The Definitive Guide to MTB Shock Compatibility: Will It Fit?

Rear shock compatibility is notoriously one of the murkiest waters to wade through in the mountain bike world. Unlike forks, where you mostly just worry about travel and wheel size, rear shocks require a harmonious marriage with your frame’s geometry and kinematics. Get it right, and you unlock a whole new level of performance. Get it wrong, and you could be looking at a very expensive paperweight — or worse, a cracked frame.

Let’s break down exactly what you need to know about eye-to-eye lengths, stroke measurements, and the often-overlooked hardware nightmares.

The Golden Rule: Eye-to-Eye Length

First things first: you cannot simply guess this number. The eye-to-eye length is the distance between the center of the top mounting hole and the center of the bottom mounting hole when the shock is fully extended.

Think of this as the chassis of your suspension. Your frame was designed around a very specific static length. If you install a shock that is too long, you’ll steepen your head angle and raise your bottom bracket, ruining the geometry the engineers spent years perfecting. If the shock is too short, you’ll slacken the bike out and potentially cause the rear tire to smash into the seat tube before you even start moving.

The Metric Shift
The industry has largely shifted from Imperial measurements (like 7.875 x 2.25") to Metric sizing (like 210mm x 55mm). While this aimed to standardize things, older bikes still use Imperial.

• Pro Tip: Do not try to convert Imperial to Metric to see if it’s "close enough." A 7.875" shock is 200.025mm. That is not the same as a 200mm metric shock. Precision is the name of the game here. Stick to exactly what your frame manufacturer lists.

Stroke Length: The Distance of Travel

The second number in the equation (e.g. the "55" in a 210x55 shock) is the stroke length. This represents how much the shock shaft can compress before it bottoms out.

This is where things get a little spicy. While the eye-to-eye length is non-negotiable, riders sometimes toy with stroke length — a practice known as "long-shocking." Increasing the stroke length (while keeping the same eye-to-eye) effectively increases your rear wheel travel.

The Danger Zone
While squeezing an extra 5mm of stroke out of your shock sounds appealing for that endless-travel feel, proceed with extreme caution. Your frame has physical limits. If the shock compresses further than intended, specific parts of the rear triangle might contact the seat tube or the saddle might buzz the rear tire at full compression.

Always remove all air (or the coil spring) from a new shock and cycle the suspension through its full travel to check for clearance before your first ride. If hard parts touch hard parts, you’re gonna have a bad time.

Mounting Standards: Standard vs. Trunnion

There are two different mounting styles — standard and trunnion.

• Standard Mount: This is the classic style we’ve seen for decades. There is an eyelet (a hole) at both ends of the shock. You press bushings and mounting hardware into these eyelets, and a bolt runs through them.

• Trunnion Mount: This is the newer, cleaner looking option found on many modern frames. The shock body itself has two threaded holes on the side of the head, allowing it to bolt directly to the frame or linkage. This reduces the overall length of the shock assembly, allowing designers to squeeze longer shocks into tighter frame spaces.

These are absolutely not interchangeable. If your frame takes a Trunnion shock, a standard eyelet shock simply won't fit, no matter how much grease and enthusiasm you apply.

The Hidden Headache: Mounting Hardware

You’ve got the right size shock. It’s the right mount style. You go to install it, and... the gap in the frame is 3mm wider than the shock eyelet. Welcome to the world of mounting hardware.

Every frame manufacturer uses different spacing for the shock mounts. You might need a 22mm wide spacer kit with an 8mm bolt hole (M8x22) for the top, and a 30mm wide kit for the bottom.

When buying a new shock, particularly from the aftermarket, it often does not come with this hardware because the shock brand has no idea what bike you own. You will need to:

1. Measure the inner width of your frame’s mounting tabs (or check the spec sheet).
2. Measure the diameter of the bolt.
3. Order the specific bushing and spacer kits to match.
4. Get the right hardware for your shock. Generally speaking, it's best to stick Fox hardware with a Fox shock, RockShox with RockShox etc.

Pro Tip: Fox and RockShox hardware are interchangeable — you just need the correct bushing. Fox uses a tan bushing, RockShox uses a silver one. If you swap the bushing inside the shock eyelet, the rest of the hardware (spacers, reducers, bolts) can be mixed and matched. RockShox bushings do require a specific removal tool, but once you get them out, everything cross-fits without issue.

Clearance Check: Piggybacks and Bottles

Performance shocks, especially those geared toward enduro and downhill, usually have a "piggyback" reservoir. This extra canister holds more oil, helping to manage heat on long, demanding descents so your damping remains consistent.

That said, piggybacks are bulky. On smaller frame sizes or frames with complex linkage designs inside the front triangle, a large reservoir might physically hit the down tube.

For example, a RockShox Vivid Air has a massive air can and reservoir compared to a RockShox Deluxe. Consider this before upgrading to a high-volume air can shock. Poke around online to see what others are doing or call the manufacturer to confirm compatibility. If it looks tight, it probably is.

A Note on Internals: Damping Tune

Finally, compatibility isn't just about physical fitment — it's about performance fitment. Frames have different "leverage ratios." Some frames require a lot of force to move (low leverage), while others move very easily (high leverage).

Shocks come from the factory with an internal "tune" for both compression and rebound.

• If you put a shock tuned for a high-leverage frame onto a low-leverage bike, it might feel harsh and over-damped.
• Conversely, the wrong tune can make a bike feel wallowy and unsupportive, blowing through travel too easily.

Check your frame manufacturer's recommendation for shock tune. If you are buying second-hand, this is critical, as that shock was likely tuned for a completely different bike.

Frequently Asked Questions

Can I put a coil shock on any bike?

Not necessarily. Coil shocks are linear, meaning the resistance stays the same throughout the stroke. Air shocks are progressive (they get harder to compress at the end). If your frame's suspension design is very linear, putting a coil shock on it will cause you to bottom out constantly and harshly. You need a frame with a "progressive" leverage curve to run a coil safely and effectively.

What is a "stroke reducer"?

This is a plastic or rubber spacer clipped onto the damper shaft inside an air shock (or under the bumper of a coil). It physically stops the shock from compressing fully. Manufacturers often use the same shock body for multiple sizes (e.g., a 210x55 and a 210x50 are often the same shock, just with a 5mm spacer in the latter). You can sometimes remove or add these to change travel, provided you respect the frame clearance rules.

How do I measure my current shock?

Don't trust the sticker if it's worn off. Take the shock off the bike. Measure from the center of one eyelet to the other (in millimeters) for the length. For stroke, measure the exposed shaft length — but be careful, as some shocks have exposed shaft that isn't used. The most accurate way is to look up the serial number on the manufacturer's website.

The Skinny

Upgrading your shock is one of the most transformative changes you can make to your mountain bike. It can turn a nervous, chattering ride into a planted, traction-grabbing machine. But the landscape of compatibility is a minefield.

Take the time to verify your eye-to-eye, confirm your stroke, identify your mounting standard, and hunt down the right hardware. When in doubt, call the frame manufacturer. They want you to enjoy the ride just as much as you do.

Get the numbers right, bolt it on, and get ready to feel your bike come alive in ways you didn't think possible.