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Ever since the beginning of full suspension, shocks have been on an evolutionary path alongside frame design. Over that time span, frame designs have varied greatly from the wildly experimentative ’90s to a more methodical approach.
Generally speaking, each generation was better than the one before it, and shocks of all types played nice with most frames. In recent years, something seems to have changed. Shocks have seemingly gotten better, but have frames?
For a very long time shocks were typically connected between the front triangle and rear end by some sort of linkage. There were exceptions, such as the Amp Research and Turner Stinger that drove shocks directly off of their chainstays, but most used a linkage of some sort to drive the shock. Then came along a structure called a “clevis.” Some also refer to it as a “strut” or “yoke mount,” but we will call it a clevis from here on out.
This wishbone-shaped structure attaches the shock to the chainstays or linkage going around the seat tube. It is probably not a coincidence that the clevis came along when the demand for dropper seatposts escalated. This created the need for uninterrupted straight seat tubes, so it makes sense to simply go around the seat tube with a clevis instead of attaching a rocker link or interrupting it. Plus, it’s a simple and fairly lightweight way to connect the rear end of the bike to the shock.
Around the time of the clevis’ rise to popularity came a seeming increase of shock failures, and they appeared to be happening with certain bikes. The first clue as to what was going on came when shock manufacturers quit recommending their coil shocks for some models, i.e., bikes with clevises. And, some of these bikes that came spec’d from the manufacturer with a coil shock were replaced with air shocks and told that they could no longer run them.
The reason a clevis can be problematic with some shocks is simple leverage. Depending on the length of the clevis, side-load leverage on a shock can nearly double. Long clevises are worse than short ones. It effectively lengthens the shock’s shaft and the forces placed upon it. So, the clevis is bad, right? Well, yes and no.
Shocks are designed to primarily accept compression and tension forces, but shock manufacturers know that there will be some side or lateral loads as frames flex. All shock shafts ride on bushings instead of just seals for this reason—to handle lateral loads. The reason some coil shocks can’t handle the lateral loads created by a clevis is that they have smaller shafts and less bushing surface area to distribute these loads. Air shocks have much larger shafts and bushings to go with them, so naturally, they take these forces much better.
Fox and RockShox both have coil-shock-compatibility guides on their websites outlining what bikes are incompatible with their shocks, as well as maximum clevis lengths for particular shocks. Most bikes listed as incompatible are older-generation models, so it seems as though manufacturers may be learning and improving. Fox says there are length limitations due to the side load that strut/yoke mount or clevis designs can have on coil shocks due to the smaller diameter of the shaft.
Just because an air shock can handle a long clevis doesn’t mean it’s good for it. The same side loads that are detrimental to the coil shock can also wreak havoc on an air shock. “Those bikes will chew up an air shock. It just takes longer,” says DVO CEO Bryson Martin. “You’ll see wear on the outside of the shaft, the internal bushing will wear, internal parts will wear, but generally you won’t have a catastrophic failure.”
Members of MBA’s wrecking crew have even ovalized the shaft of an air shock under really hard riding, so just because air shocks are tougher does not mean they are immune to abuse. “We would like to take the stance of saying that a long clevis should not be a part of bike design,” says Martin.
It might seem like we’re picking on the clevis, and we are; however, it’s far from the only thing that’s making life hard on shocks. Other design elements and factors can shorten the lifespan of a shock. Poor frame alignment can lead to premature wear on a shock as it gets constantly side-loaded as it cycles. Most frames are good in this regard, but it’s not uncommon to occasionally get one that requires a little finesse to get the shock installed because things are not lining up quite right.
“It’s always tough being the manufacturer, because there’s all these different elements that’ll lead up to a shock failure,” says Martin. “It’s not necessarily an issue with assembly or a manufacturing error. When we look at a bike, we know exactly what’s gone wrong. Generally, it’s bad frame alignment, bad bushings or bad suspension design.”
Good examples are bikes with a trunnion mounted on top and what is often called a hard mount on the bottom. They’re running the shaft that holds the lower pivot through the lower shock eyelet. “There’s no bushing or anything, so it’s really hard mounted top and bottom, and that’ll chew up shocks,” says Martin. “The trunnion was the dumbest solution to gain a couple more millimeters. It’s like coming up with a solution to a problem that will cause 10 times more problems. In a perfect world where you don’t have any forces other than up and down, it’s going to work fine, but that’s not what we see in real life.”
Fox’s maximum strut-length chart backs this up, showing shorter recommended lengths for trunnion mount shocks than the Igus bushing eyelet style. “Trunnion mounting styles transfer energy directly from the bike’s suspension design into the shock head without a bushing, so it does mean that the kinematic design, tolerances, and deflection can play a bigger part in direct function and wear of the shock over time,” says Fox PR and Content Marketing Strategist Matt Hornland.
Igus bushings are those light-tan-colored plastic-like bushings that are often found at each end of a traditional shock eyelet. These bushings are the bearing for the shock to pivot on, and they also provide a little bit of give or flex laterally. “In a good design there aren’t many side loads, and what there is, Igus bushings deal with quite well,” says Hornland.
Even the best suspension design in a perfectly aligned frame can destroy a shock if its pivots are not properly maintained. As pivots wear and create lateral slop in the system, side loads are introduced to the shock just like a misaligned frame. If you ride your bike in this state long enough, you’ll destroy your shock.
Martin likes to use the analogy of driving a car with worn-out bushings and ball joints with a front end that’s rattling. You can still drive it, but you can’t blame your shocks if they go out, because your front end is worn out. “That’s what your bike is doing,” he says. “Look at your pivot bearings and bushings, grab your rear wheel, and if it’s clicking back and forth, that’s gonna affect your shock.”
The good news is that there seem to be fewer new bike designs with long clevises. Shocks are improving to deal with the forces that frames put on them, too. For example, RockShox says that its 2023 Super Deluxe Coil and Deluxe Coil rear shocks were designed from the ground up, partly due to the additional stress clevis designs can put on coil shocks. These coil shocks are compatible with all frames on the market. Fox says that its shock designs have evolved with more durable bushings and seals, improved testing, iterative design, and stronger materials while retaining the weights, adjustability, and smoothness.
DVO gave us a sneak peek at a pre-production air-shock design with a much larger shaft, thicker walls and spherical Igus bearings at both ends. These spherical bearings isolate the shock from lateral forces and could be another big step forward in shock longevity. “At the end of the day, we just need to make our stuff beefier and heavier,” says Martin. “We’re just going forward with it because of all the wear and tear we see, and all the crap out there as far as long clevises and bad suspension designs.”
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RockShox Flight Attendant Review: Long-Term Test of Game-Changing XC Suspension
Feedback Sports Velo Hinge 2.0 Review
GIANT PROSHIELD MENS MTB JACKET REVIEW – BREATHABLE RAIN AND COLD PROTECTION
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Video: ‘Process Perfected’ With Tayte Proulx-Royds
ASK MBA: Can I Use Six-Bolt Brake Rotors on a Centerlock Hub?
ASK MBA: WHAT’S THE BEST WAY TO GET SPONSORED?
ASK MBA: WHAT ARE THE BEST INEXPENSIVE FLAT PEDALS?
ASK MBA: DOES MOUNTAIN BIKE APPAREL REALLY MAKE A DIFFERENCE?
DIAL IN YOUR MOUNTAIN BIKE’S HANDLING AND FIT WITH THE SCIENCE OF STEMS
ALL ABOUT YOUR MOUNTAIN BIKE’S MOST IMPORTANT COMPONENT: TUBELESS RIM TAPE
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polyurethane bushings BIKE CHECK: AARON GWIN’S CRESTLINE DOWNHILL RACE BIKE