Suspension school · Suspension Basics

Sag: What It Is and How to Measure It

The one measurement everything else is built on, and how to get it right first time

Sag is how far your suspension settles under your weight when you sit or stand on the bike in a normal riding position. It is written as a percentage: a 160mm fork that compresses 32mm under you is at 20% sag. Every other adjustment on the bike assumes this number is roughly right, which is why a good suspension tech always checks it before touching a single dial.

Why it matters

Suspension has to work in both directions. The wheel needs spare travel to soak up hits, but it also needs room to extend downwards into holes and dips so the tyre stays pressed to the ground. Sag is what creates that downward reserve. It also sets your ride height, so it quietly controls the bike's geometry. Too much sag at one end and the angles change, and suddenly it steers like a different bike.

The targets

• Fork: 15 to 20% of travel. This is the range Fox, RockShox, Marzocchi and SR Suntour all publish.
• Rear shock: 25 to 30% of shock stroke, per Fox's official guidance.
• Within those ranges, the firmer end gives support and efficiency, the softer end gives grip and comfort. XC riders tend towards the firm end, enduro and eMTB riders towards the soft end.

The front runs less sag than the rear because most of your weight sits towards the back of the bike, and because the fork has to hold you up under braking and on steep terrain. A fork at 30% sag feels lovely in the car park and horrible on the first steep chute.

Percent of what, exactly

Fork sag is measured on the stanchion and the maths is simple: 20% of a 160mm fork is 32mm. Rear sag is measured on the shock shaft, and here is where people trip up. It is a percentage of the shock's stroke, not the bike's rear wheel travel. A bike with 150mm of rear travel might use a shock with a 62.5mm stroke, so 30% sag is about 19mm at the shock. Aim for 30% of 150mm instead and you will be riding around at nearly full squish, wondering why the bike feels like a sofa.

Measuring it properly

• Kit up exactly as you ride: helmet, pack, water, tools, pads. On most riders that adds 3 to 5kg, and the suspension does not know the difference between you and your backpack.
• Open any compression levers or lockouts so the suspension can move freely.
• Push the rubber O-ring on the stanchion, and the one on the shock shaft, down against the wiper seal.
• Lean lightly against a wall or have a mate hold the bars, then climb on. Stand centred on the pedals in your normal descending stance for trail, enduro and eMTB setups, or sit in your climbing position if you are setting up an XC bike.
• Bounce once, gently, then let the bike settle and hold still for a few seconds.
• Step off as smoothly as you can manage. No push-off, no hop.
• Read how far each O-ring moved, then divide by the travel (fork) or the stroke (shock) and multiply by 100.

Do it two or three times. The readings should agree within a couple of millimetres, and if they do not, you are probably stepping off with a bounce.

The mistakes nearly everyone makes

• Measuring in jeans on a Tuesday night, then riding at the weekend with a full pack.
• Sitting down for the measurement on a bike you mostly descend standing up.
• Leaning half your weight on the wall arm, which unweights the bike and reads soft.
• Forgetting a lockout or firm mode is switched on.
• Using rear wheel travel instead of shock stroke for the rear calculation.
• Measuring with the shock pump still attached, which changes the spring slightly and blocks the O-ring anyway.

The eMTB difference

An eMTB weighs roughly 20 to 27kg against 13 to 16kg for an equivalent analogue bike, and that extra mass sits low and central, with the motor and battery loading the rear in particular. The sag targets do not change, but the pressures needed to hit them do. RockShox prints 'for eBike add 10psi' on its fork legs, and Fox publishes separate E-Bike+ pressure charts, which work out at roughly 9psi extra on a 38. Set sag by measuring, not by copying a mate's analogue bike.

Last thing: sag drifts. Air seeps slowly past seals over the weeks, your kit changes with the seasons, and winter riders usually carry more. A two minute check once a month keeps the whole setup honest.