How to set suspension sag

What is sag?

Loaded sag is the name given to the amount of suspension travel used up when the bike settles with a rider on board. To alter the sag you need to adjust how much the springs in the forks and the spring in the rear shock are compressed (also called altering the springs’ tension or altering the pre-load). As a very broad guide a road bike should have 25-30mm of loaded sag at the front and 20-22mm at the rear.

Why bother?

Adjusting the sag to match your weight will ensure your bike's suspension works in its optimal range. This will improve ride and handling and boost your confidence as well as making the bike safer to ride.

Don’t do it if…

You are particularly rubbish at very simple maths, or own a bike that doesn’t have pre-load adjustment – you can but it’s very specialised and includes new springs, fork oil and money.

Stuff you’ll need

A friend, a socket that fits the front fork pre-load adjusters (though some bikes only have facility for adjustment by spanner), and a C-spanner to turn the rear spring’s pre-load ring. Check the bike’s toolkit and if there isn’t one you’ll have to visit a specialist tool shop. You’ll also need a small cable tie, a tape measure, a notepad and a pen.

What can go wrong?

The worst that can happen is that you get completely confused or ‘lost’ with which way you need to adjust the pre-load. As long as you have an owners’ manual, or have access to the stock settings (dealer can supply), you can revert to standard and start again.

Anything else?

This guide shows you how to set sag for a solo rider. If most of your riding is two-up or with luggage, you’ll need  more pre-load at both ends (especially the rear) to compensate.

1. Before you do anything, check that the bike is on stock suspension settings. Second-hand bikes are particularly likely to have been mucked around with. Revert to the settings given in the owners’ handbook, or from a dealer. The reason for this is to have a solid base setting to start from to reduce the chance of confusion.

2. Starting at the front of the bike, fit a cable tie around the slider part of one fork leg (the chromed bit). With the aid of a friend, pull the bike over on its sidestand to lift the front wheel clear of the ground so the suspension ‘tops out’. Slide and butt the cable-tie against the fork seal.

3. On upside-down forks (pictured) measure the distance between the fork seal and the point where the slider joins the lower casting (the mount for the wheel spindle and brake caliper). Note the measurement and call it ‘A’. On conventional right-way-up forks measure from the fork seal to the underside of the lower fork yoke instead.

4. Sit on the bike in your normal kit and riding position while a friend balances it (without applying force up or down). The cable tie will move as the forks compress. Measure from the fixed reference point on the casting to the top of the cable tie (or on right-way-up forks, from the bottom of the tie to the fixed point on the yoke). Call this distance ‘B’.

5. Your sag value is simply A minus B. If you have too much sag, say 40mm, you need to increase the fork tension/pre-load by turning both pre-load adjusters in/clockwise. Start by increasing pre-load one turn on each adjuster and then repeat the measurement procedure until the ideal sag figure is achieved.

6. To measure the rear suspension’s sag, lift the rear of the bike until the rear wheel is off the deck. Measure vertically from the centre of the rear wheel spindle to a fixed reference point on the chassis. On the ZX-10R used here we used the bottom of the bungee point. Make a note of measurement ‘A’.

7. Again sit on the bike as if riding normally. The bike has to be supporting all your weight to get an accurate measurement, so feet must be on pegs. Try parking next to a wall and balancing yourself with an elbow while your pal does the measuring. Again measure from the centre of the spindle to the fixed reference point for measurement ‘B’.

8. Subtract measurement ‘B’ from measurement ‘A’ to get the rear sag figure. Now it’s just the simple matter of adjusting the amount of pre-load on the rear spring (or springs if it’s a twin-shock set-up) and re-measuring until A minus B is in the right range. More pre-load will reduce sag, less pre-load will add to it.

9. Rear pre-load adjusters can differ, but the principle is the same – by reducing the length of the spring you increase the tension in it. On the ZX-10R you need to use a
C-spanner to slacken off the upper locking ring first. More often than not, it’s easier to use a hammer and drift to crack it loose.

10. Back off the locking ring by hand to give more access to the adjusting ring. Fit the C-spanner to the adjusting ring and turn clockwise to increase the pre-load, or anti-clockwise to reduce it. Adjust by one to two turns at a time then re-check the sag. Go steady as C-spanners are notorious for slipping off.

11. Once you’re happy with the sag figure, tighten the locking ring to the adjusting ring, being careful not to move the adjusting ring while doing so. Two C-spanners are ideal, otherwise hold the adjuster ring with the C-spanner and get your friend to tap the locking ring against the adjuster with a drift and hammer.

12. Push up and down on both ends of the bike to ‘settle’ the springs and re-check the sag figure. Take the bike for a test ride to assess the improvement in ride quality. If you haven’t been able to get the sag figure you desire then the chances are different springs (with a harder/softer spring rate) are needed from aftermarket suspension specialists.