Posted by Scott Lian on Tue, Oct 20, 2009 @ 06:01 PM
The most basic and vital suspension adjustments for your bike start with getting the right sag numbers. Sag is the amount of change in suspension travel your motorcycle experiences with the rider on board. The amount of sag dictates whether you have the correct spring rates, and the correct amount of pre-load (the amount a spring is compressed from it's natural state) placed on the spring. The springs used in your forks or around your shock are designed to operate under certain conditions, and often work best when under slight preload (compression), generally about 5-10mm. If your springs have too much preload, you will experience loss of initial compliance (when you brake or accelerate, the spring will be resisting movement as it is already compressed) which will sacrifice traction. Also if you have too much preload, the fork or shock will want to extend rapidly after being compressed, forcing the rider to rely heavily on rebound damping adjustments to counteract this, limiting your ability to tune rebound adjustments to accommodate track conditions and stability under acceleration and braking. In essence, you have fine tuning capabilities with your damping rates, if your springs are too stiff or too soft (under too much preload), your damping adjustments will be limited.
Now in order to find your sag, you must establish a baseline. When supporting the motorcycle with its wheels off the ground, the suspension will elongate to its maximum length, this is the basic measurement all sag numbers are measured against. It represents your total available suspension travel, and you want to make the most of it! As a rule of thumb, the amount of sag should be equal to 30% of your total suspension travel for street use, and closer to 20-25% of total travel for track use (which equates generally to 30-35mm (forks) and 25-30mm (shock) on most modern sport bikes).
The final number that works for you will vary by rider, track, bike, and much more... but it's pretty easy to get in the ball park with some simple measurements and adjustments.
To measure your sag, complete the following:
First support the front of the motorcycle and lift the wheel off the ground, measure the distance from the fork bottom to the outer fork tube (not the dust cap as this may flex or be removed on some machines). Another option is to measure from the fork tube to the bottom of the lower triple clamp, whichever you prefer, pick one and stick with it always! This is your total travel and we'll call it measurement "A."
Now, with the motorcycle upright, the suspension will compress a certain amount under it's own weight... this is called "free sag." To measure this, support the motorcycle on it's wheels and straight up and down (a front wheel chock can come in handy for this) and push on the triple clamp to compress the suspension and release, measure the distance from the fork bottom to the outer fork tube. Then pull up on the triple clamps and measure again. Take these two measurements (we'll call them B1 and B2), and find their average = [(B1-B2)/2]. This is measurement B.
To find your free sag subtract Measurement B from Measurement A and this is your free sag (free sag = A - B).
Static sag (also referred to as spring sag), is measured with the rider on the bike and in full gear for accurate measurements. When the rider is on the bike, the suspension will compress even more, this is called "rider sag." Rider sag + free sag = static sag.
To find your rider sag, support the motorcycle (either with a wheel chock or with a friend holding the bike) and have the rider get on the bike and into a racing position. Push on the triple clamp to compress the suspension and release, measure the distance from the fork bottom to the outer fork tube. Then pull up on the triple clamps and measure again. Take these two measurements (we'll call them C1 and C2), and find their average = [(C1-C2)/2]. This is measurement C, your rider sag.
Now add your free sag (measurement B) and rider sag (measurement C), and subtract from your total available suspension travel (measurement A) to find static sag.
Static Sag = Total Travel - (Free Sag + Rider Sag), or:
Static Sag = A - (B + C), this should be between 30 and 35mm for track use.
To complete your sag measurements, repeat the same process on the rear of the motorcycle. Popular reference points are the rear axle (either the center, or a repeatable spot on a rear axle nut, feel free to use a sharpie to make a mark on the axle or swing arm to make a reference point), and a point on the sub-frame of the motorcycle. If need be, place a piece of tape on the tail section, but the sub-frame is recommended as plastic can flex with the rider seated on the bike. Make sure whatever reference point you use is as close to directly above the axle as possible. Some bikes with single-sided swing arms (like a Ducati 916) require the use of a rear ride height tool, a bar mounted to the frame of the motorcycle that extends rearward. This helps provide consistent measurements, as the rear axle is located within an eccentric utilized for chain adjustment, and is not a fixed point as with double sided swing arms. In either case, maintain a consistent reference points top and bottom.
Rear sag measurements should be between 25 and 30 mm for track use.
If you are unable to achieve these numbers by adjusting the amount of preload on your fork springs or shock spring, measuring your sag will tell you that you need stiffer (if your sag is too high) or softer (if your sag is too low) springs. Without measuring sag, you will never know if your suspension is set up well for your weight. Once you have the correct springs on your bike, and achieve sag numbers within the range discussed here, you are now ready to start adjusting your compression and rebound damping settings, which will be discussed at a later date.
Remember, the correct sag numbers for YOU will vary depending on the type of bike you ride and your individual riding style. These recommendations are intended to help you get a baseline, but it's up to YOU to dial it in!