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Tech Tip: Measuring Sag On Your MotorcycleAug 19, 2010
Getting the best performance out of your bike requires proper suspension set up, and setting suspension sag is the first step in this process. Motion Pro offers some great tools to help with this, like the Uni-Sag Telescoping Sag Scale and our Folding Sag Scale. Here are some tips on how to use sag scales like these.
There are two sag settings to keep in mind. Static sag is the amount of compression on the spring of the shock or forks when under a stationary load of just the motorcycle itself. Rider sag is the amount of compression with the weight of the rider and the motorcycle together. Too much sag means that the bike will bottom over bumps or feel too soft. Too little sag will mean a harsh ride or a “kicked out of the seat” feeling over bumps. Sag is also important in a “top out” situation when riding, like going over a rise at speed. Without some sag, the suspension will top out too early, and your tires could lose contact with the road or trail surface, resulting in a loss of control. Suspension damping also is dependent on sag, so if you change spring preload (to change sag), then the current damping settings (compression and/or rebound) will act differently. Therefore, sag is always set first, and then adjustments to rebound and compression are made to fine tune the function of the forks and shock.
Virtually all motorcycles have rear spring preload adjustments these days, and many have fork spring preload adjusters as well. Refer to your owner’s manual for what types of adjustments are available to you, and how they are adjusted.
The most straightforward method of sag adjustment is to concentrate on rider sag, which is the amount of suspension compression with the rider on board. Both front and back are measured in a similar manner. You have to start with a base measurement of the suspension when it is fully extended. For most offroad motorcycles, this is easily accomplished when the bike is on a work stand, with both wheels off the ground. Larger motorcycles necessitate an assistant to pull up on the handlebars to extend the forks, or lift up on the footpegs to extend the shock. Measure the length of the exposed fork tube to get your fork starting measurement, and measure from the rear axle to a fixed point on the rear fender or tail to get the shock starting measurement. Write the numbers down, so you don’t forget them and have to start over! No matter what changes you make to the preload, these numbers will always be your starting point.
Once you have the starting numbers, have an assistant hold the motorcycle upright while the rider sits on the bike with his normal riding gear on. Have the rider move up and down on the pegs to settle the suspension, and then assume the normal riding position. Then measure again at the same two points. This measurement is the rider sag.
Subtracting the rider sag measurement from the starting measurement will give you the amount of suspension movement from fully extended to the rider aboard. This measurement will vary depending on the type of motorcycle you are measuring. For off road and MX motorcycles, front rider sag should be 90-110mm, and rear rider sag should be 25-35mm. Street sport motorcycles will have much less sag, with the front at 20-30mm, and the rear from 15 to 25mm. Every bike is different, so refer to your owner’s information, or consult a suspension expert for advice.
The other measurement is static sag, where the measurement process is the same, but you are only concerned with the weight of the motorcycle itself without the rider. These numbers are always much less than rider sag, for obvious reasons. Contrasting rider sag to static sag can help you to evaluate whether the springs installed in your suspension are correct for your motorcycle and you, but that is a subject that will be explored in another article.
Motion Pro has two tools to help you set the sag on your motorcycle. The Motion Pro Uni-Sag Telescoping Sag Scale is designed to make the setting sag a simple one person operation. We also offer the less expensive Folding Sag Scale, which requires two people in order to accomplish this task. You can use a simple tape measure to set sag, but the UniSag and Folding Sag Scale offer much greater simplicity, accuracy, and convenience than a simple tape measure.
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