Dirt Bike Suspension – Sag and Spring Rates
The factory can’t build dirt bikes custom fitted for every rider. A light rider tearing up the motocross track will want a bike that performs differently than for a larger rider playing on his dirt bike in the gravel pits. So dirt bikes are designed for the average rider, and for average riding.
Fortunately, checking your sag and spring rates is fairly easy, so you can check if you have what you need.
This is a lot easier if you have a buddy to help hold your dirt bike or a very understanding girlfriend or wife (or boyfriend or husband!).
If you set your clickers to a soft setting, you’ll get more accurate results.
Free sag or static sag is how much your dirt bike suspension compresses under its own weight. To find measure the sag, start by placing your dirt bike on a stand (or a crate, or wood blocks, etc) with the wheels off the ground. Measure the distance from the rear axle bolt to a convenient spot on your bike, straight up from the axle. You’re going to use this reference point again, so make sure you remember it, or mark it with a piece of tape or something. Record the measurement (A).
Now take your dirt bike off the stand. While that helpful friend is holding it upright, re-measure the distance from the axle bolt to the reference point. Record the measurement (B).
Subtract B from A, and that gives you the free sag. Ideally, you’re looking for about 8 to 10 millimeters for smaller bikes, and 25 to 30 millimeters for anything 125cc’s or larger. If you have access to a manual, check it for more accurate ranges.
Race sag, or laden sag, is how much your dirt bike sags while you’re on it – sometimes called rear ride height as well. To measure race sag, sit on your bike in a normal riding position and have someone re-measure the distance from the axle bolt to your reference point. Record the measurement (C).
Subtract C from A, to get your race sag. On average, smaller bikes will give you about 65 to 85 millimeters of race sag, and 95 to 105 millimeters for 125cc’s and bigger.
Too little free sag means the spring is too soft, and too much equals a spring that's too stiff. This may be the opposite of what you’ve been told or heard, but I’ve researched it a lot to try and get it right. (If you have definitive proof that I have it wrong, please email me – I’m more interested in GETTING it right than in BEING right).
To adjust the race sag you’ll need to adjust the spring preload. First, using a hammer and punch, release the top collar on the shock spring. Then gently tap the lower collar to compress the spring (more preload), or to release the spring (less preload). Lock the setting in place by re-tightening the top collar.