Measuring sag can be a total pain – whether that's because your fork or shock doesn't have sag guides etched onto the stanchion or the coil spring is obscuring your view.
Maybe you're working on your bike alone and don't have as many limbs as an octopus. Regardless of the reason it's tricky, getting it right is important.
Cue the Slacker MTB Pro Kit sag measurement tool.
Developed by John Casebeer [cheers!], who was on a quest to improve his MX bike's setup, it's been expanded to work in the world of MTB, co-developed by MTB suspension guru Rulezman.
The MTB Pro Kit retails for $374.95 / €397 and includes everything you need to perfect your bike's sag setup.
Stowed in its own fancy case is the V5 Digital Suspension Tuner, a virtual fender mount (the metal bar essential for measuring wheel sag correctly), a wireless remote and a smartphone bar mount. There's also an app.
The correct sag measurement
Rear suspension sag is most commonly calculated by measuring how much the shock's shaft compresses into the shock's body, or how much the eye-to-eye measurement changes between full travel and compressed to sag.
But this isn't the best way to do it, according to Slacker.
The leverage ratio of the rear shock – which describes how much the rear shock moves compared to how much the rear wheel moves – can change through a bike's travel, usually
Bikes with progressive suspension kinematics have leverage ratios that reduce as they compress. It may start at 2:1, but could reduce to 1:1 towards bottom-out.
A 2:1 leverage means a bike with a 60mm-stroke shock would have 120mm of rear-wheel travel and, as the leverage range changes, the relationship between shock stroke compression and travel does too.
Therefore, 30 per cent of your shock-stroke's sag won't be the same as 30 per cent of your bike's rear-wheel sag.
The Slacker is claimed to measure wheel sag rather than shock sag, potentially improving your bike's setup.
How does it work?
The digital suspension tuner mounts to the rear wheel axle, while the virtual fender mount attaches to your seatpost and has an arm extending rearwards over the wheel.
A wire cable extends from the digital tuner and attaches to the virtual fender. The amount the cable then retracts into the tuner measures how much sag your bike is running.
This is then displayed on the wireless remote or smartphone app.
However, it's crucial all the components are mounted correctly for the measurement to be accurate.
There's more
The tool can be used to measure fork sag, too.
Again, it attaches to the wheel axle, but this time the wire cable fixes to the fork just under the crown.
Then, as you weight the bike and compress the fork, the cable retracts into the tuner, in the same way does for the rear suspension.
And, hey presto, you've got your sag measurement.
A word of caution
However, sag measurement isn't the be-all and end-all of bike suspension setup. It can vary and fluctuate depending on a host of variables that are tricky to manage.
Body position on the bike, how much kit you're wearing and day-to-day weight changes, ambient weather conditions (affecting spring pressures), the slope of the ground you're measuring sag on, plus a lot of things, contribute to the measurement.
Sag is a great starting place to begin suspension setup, but you shouldn't get fixated on chasing specific figures if your bike already feels good.
The Slacker is still a great tool, especially if you struggle to get your bike set up on your own, or want to geek over the minutiae.
