Ask any suspension expert which suspension setting is the most important, and they’ll probably tell you it’s sag. Get it right, and your bike tracks better, corners more confidently, and doesn’t bottom out when it shouldn’t. Get it wrong, and even your top-of-the-line mountain bike will ride like crap.
Despite its importance, most mountain bikers still rely on shock body O-rings and eyeball estimates to dial in sag. Slacker, a company already well-known in the moto world for its digital suspension tools, is now offering a more precise alternative with its new MTB Virtual Fender Mount. The tool promises repeatable, real-time sag measurements that are accurate down to the millimeter—and more importantly, ensures you’re running the ideal sag percentage for your bike and riding style.
Why the usual method falls short
Slacker founder John Casebeer says the traditional method of measuring sag at the shock body using an O-ring is inherently flawed. You can’t bounce the suspension to fully settle it, shocks can be hard to access, and even a 1–2mm measurement mistake gets amplified by the frame’s leverage ratio, notes a press release for the product. According to Slacker, that kind of error could result in a 4–6mm difference at the axle, which translates to a 3–5% swing in sag—a number that’s enough to noticeably change how a bike rides. And the percentage difference can be even greater for short-travel bikes.
How the virtual fender mount works
The Slacker MTB Virtual Fender Mount attaches to your seatpost or fork, with a rod extending out and aligning directly above the axle. A small loop guides a cable connected to Slacker’s digital measurement unit. Once in place, riders mount the bike in full gear and pump the suspension to settle it. Slacker measures the vertical distance the saddle moves downward, which can be used to calculate sag.
With the smartphone app or the optional Slacker Measurement Display, you can see percentage-based sag numbers, so you know exactly how close you are to your target settings. And because it measures at the axle, the data is a 1:1 representation of suspension movement and is not distorted by leverage ratios.
Does the axle path matter?
One question that comes up with axle-based tools is whether the rear axle path or suspension geometry could affect the reading. Casebeer says that’s generally not a problem. “The suspension is only compressing about 25–30%, so the tool remains aligned vertically with the cable,” he says. For high-pivot bikes with more rearward axle paths, “we recommend [aligning the cable] about one inch rear of vertical.”
You can use Slacker to measure fork sag, too
Slacker isn’t just for rear suspension—it can also be used to measure fork sag with the same level of precision. Using the included universal mount, riders can attach the tool to the upper and lower fork legs to measure front-end sag. The process mirrors the rear setup: strap on the mounts, position the loop over the axle, and view live readings through the remote or app.
Who is this for?
With a suggested retail price of $199.99 for the Slacker Digital Sag Scale and another $74.99 for the MTB Virtual Fender Mount (see below for purchase options), the system isn’t exactly cheap, especially if you’re only setting up one or two bikes. But for race teams, suspension tuners, or bike shops, the potential performance gains could easily justify the cost.
Whether you’re racing at the highest level or just want your trail bike to ride better, Slacker’s MTB Virtual Fender Mount could take the guesswork out of dialing in your sag.
- Slacker Digital Sag Scale: on sale for $179.99 at getslacker.com, $124.99 at Amazon
- Slacker MTB Virtual Fender Mount: $67.49 at getslacker.com, $74.99 at Amazon
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