19 Apr 2016


Sag, and why it’s so important
Otherwise known as static sag, it’s the amount the forks compress under the weight of the rider – usually expressed as a percentage. For instance, if your 100mm forks compress 20mm as you sit there, you’re running 20 percent sag. By setting it right, you’re doing a few key things. Firstly, you’re tuning the spring rate so it’s appropriate to you, and can support your weight. And secondly, you’re giving the fork room to work and provide traction.

Your front wheel needs to drop into holes for the tire to stay in contact. With inadequate sag, it can’t. If it’s in the air – maybe only by millimetres but multiple times a second – you have no grip! It might feel fast running your forks super-hard with very little sag, but it badly compromises their ability to create grip and stability. If you’re after pedalling efficiency above all else, you’re better off switching to a rigid fork and taking the weight benefits too.

Setting up your fork is important, along with other components – check out the spec and setup on Russell Stevenson’s bike, featured here

Setting the sag
Chances are your forks use an air spring, and adjustment will be easy – you do it with a high-pressure pump. You may, however, have coil forks. If you do, and they’re either way too soft or too hard (measure the sag to find out), you’ll need to fit a different rate spring. These can be bought from various suspension tuners, and are usually colour coded.

To set sag…
• Climb aboard (in full riding gear) next to a wall, stand up and bounce a few times until the suspension settles. Don’t hold the brakes. Balance as close to upright as possible, so no weight is supported on the wall. Just stick out an elbow as a prop.

• Carefully adjust the telltale (use a ziptie if there isn’t one – fit it ‘backwards’ so it doesn’t ratchet shut). Then, equally carefully, get off the bike without pushing down.

• Use a ruler to measure from the fork seal to the bottom of the telltale to the millimetre. Divide that number by the total stroke of the fork and multiply by 100 to find the percentage. If your fork is 100mm, it’s even easier – the number on the ruler is the percentage.

• Adjust pressure in 5psi increments. For XC, 20 percent is a good place to start, and you’re unlikely to want to go as much as 5 percent either way.

• For long-forked hardtails (130mm or more), start around 25 percent.

Damping – rebound
Now you’ve got the right spring, you must control its bounce. Most forks feature rebound, which controls only the spring’s re-extension. You want it slow enough so it can’t fire the wheel back off the ground, pogo stick-style, but not so slow it can’t re-extend before the next impact – if it can’t recover, the fork gradually packs down until you run out of travel.

Turn your adjuster fully and count the clicks, then return to the middle. From there, adjust until you see/feel them return with a noticeably (yet subtly) slower motion than you can compress them. You should be in the ballpark now – ride this base setting and adjust one click at a time until they feel right. Your rebound can be wrong if it’s way out, but right is quite personal.

Note that rocky, choppy terrain needs faster rebound than smoother trails or ones with bigger, but more isolated impacts.

Damping – compression
Compression damping controls how rapidly your fork can compress. The most sophisticated forks break this into high and low-speed damping. Note that those refer to shaft speeds, not the rate you’re travelling! Anti-bob, ‘platform’ systems work by damping low-speed inputs (your pedalling) while leaving the fork free to move at high shaft speeds (impacts).

A touch of compression damping stops the fork using too much travel under hard braking and pedalling. Again, you can go much too far and affect performance, but the ‘right’ setting is heavily influenced by rider preference.

Correct fork setup on your hardtail gives you confidence – and check out next week’s tips on full-sus setup.