All About your "Sag"
Ok so the title 'All about your "Sag"' may be a bit misleading. This is not an article about your balls. If you searched for "saggy balls" then you have opened the wrong article. For those that arrived here because they're interested in something called "Sag" and it's in relation to your dirt bike, then you're in the right place!
What is "Sag?"
Ok, I'll stop using the quotation marks now... Put simply, Sag is the measure of your bike's passive "squat" or Sag measured in two different ways: 1) Static Sag, which is a measure of how far your suspension springs compress under the bike's own weight and 2) Rider Sag, which measures the Sag with the rider on board.
Is Sag important?
Yes! This is another aspect of tuning your motorcycle for your individual needs. Every bike needs Sag. Why? Because on any surface, be it a road or a track with jumps, ruts, bumps and holes, there is essentially "bumps" and "holes". Your suspension compresses when it hits a bump and it extends when you hit a hole. The obvious corollary to this is that you suspension must be pre-set with some sag to allow for both eventualities.
Both the Rider Sag and Static Sag must stay within a specified range to ensure optimum bike performance however your exact settings will depend on track conditions and your riding style. Less sag can improve turning ability at the cost of slightly reduced overall stability. More sag can improve overall stability but may reduce turning performance slightly.
The relationship between Static Sag and Rider Sag is also important because it can provide an indication as to whether the springs that you're running are too stiff or too soft. A bike that does not have both Sag measurements within the recommended range will be unbalanced. If after setting the Rider Sag the Static Sag is more than your range in the rear, the spring may be too stiff for your weight. In this case, the spring is not compressed enough to allow the suspension to extend far enough on its own. A spring that’s too firm does not allow the rear tire to hook up under acceleration and passes more of the bumps on to the rider. If the Static Sag is less than your range in the rear, the spring maybe too soft for your weight. In this case, the spring required so much preload to achieve the proper race sag that it makes the rear end too high or even top-out when the rider dismounts. As a result, the weight transfer will be incorrect and the rear end will top-out under even light braking and on downhill sections.
Your Rider Sag should sit within a certain range, depending on the size of your bike, your rider weight, riding style, speed and skill level. As a general rule of thumb, the Rider Sag should be about 25% of travel for the front and about 33% for the back.
Static Sag is proportional to your bike's weight and size. For the front it is as little as 10mm for most bikes and for the rear, 10mm for small bikes up to 40mm for full size bikes. Bikes tend to carry more weight in their rear.
However there's this thing called "stiction". Stiction is the measure of the friction inside your forks or rear suspension "system". This includes the swingarm, linkages and shock absorber. Rear stiction is typically <5mm however for the front it can be as much as 30mm which his more than the recommended 10mm of static sag! So front Static Sag is pretty much unmeasurable! It is for this reason that we tend to concentrate our efforts on the rear which is a more reliable measurement. The rear ride height can be raised or lowered to suit your desired rake and the front generally left alone.
Ok, so what do I do with the front then?
1) Install a spring that suits your weight and skill level. 2) Check the Rider Sag; It should be around 30mm for a 50-65cc bike up to 50mm for a full size machine. If the Sag is not correct, you may have purchased the wrong spring. That said though, you can preload the spring with washers or spacers if required to decrease the Sag. Normal preload range is 3-15mm. Any more than this and your spring is too soft.
Rear Sag is where it's at.
Recommended Rider Sag (Rear)
Decreasing the Sag of your bike generally makes it quicker handling but reduces high speed stability. Decrease Sag on tight courses to improve steering accuracy and in muddy conditions to handle weight build up. Increasing the Sag of your bike usually makes it more stable but decreases front end traction and cornering ability. Increase your Sag on high speed and sandy tracks to improve stability. Too much Sag will cause your front end to be too light and deflect off bumps and too little Sag will cause your shock to be too stiff on bumps.
Changing Rider Sag can also correct a motorcycle chassis if it is unbalanced.
Your riding style can dictate the specific Sag that will work for you and your bike. For example, older riders tend to sit down more and so tend to run less Rider Sag to keep the bike on an even keel.
How to Measure and Adjust Rider Sag
Before you check the Static Sag, you must first check/set the Rider Sag. Why? Because the Rider Sag settings can effect the Static Sag. The amount of preload that you dialled into the rear shock to achieve your desired Rider Sag will effect your Static Sag.
1) First put the bike on a stand and ensure that the rear wheel hangs freely off the ground.
2) Using a tape measure, measure the distance between the unloaded rear axle and a point as close to vertical on the rear fender. Mark this down as Dimension A.
Pic: Unloaded Sag Measurement
Pro tip: Measuring your Sag from the same points each time is critical to an achieving an accurate Sag measurement. Usually the point where the rear fender meets the side cover is a good point to measure. Depending on the bike though, to ensure that you measure from the same point each time, mark the fender with some tape or a permanent marker.
3) With the bike on level ground, measure the loaded dimension from the same points as before except with the rider onboard. Record this measurement as Dimension B. You may need assistance with this step. Someone to hold your bike and another to perform the measurement is ideal. For best results, wear all your normal riding gear. We recommend at least wearing your boots.
Pic: Rider Sag Measurement
4) To calculate Rider Sag, use the following formula:
Rider Sag = Dimension A - Dimension B
5) To adjust the Sag, using a long punch and mallet, gently tap the locking ring on the shock absorber loose. Spin it several turns away from the adjusting ring.
Pro tip: Avoid using a screwdriver or other sharp object as this will damage the ring. The proper punch will have a round, flat end.
6) Next we need to adjust the preload on the spring. Turning the spring clockwise will tighten the adjusting ring at the top of the shock absorber and increase your preload. Conversely, turning it counter-clockwise will loosen it and reduce preload. The spring should be able to be turned by hand from its base but before you start adjusting the preload, mark the spring with some tape or a permanent marker. This way, you can count the number of turns for each successive measurement. As a general rule, 1 turn will increase or decrease the Sag by about 3mm. So for a 10mm adjustment, turn it about 3 times in the desired direction.
7) Repeat steps 2-6 until you achieve your desired Rider Sag.
How to Measure Static Sag
1) This measurement is taken with the bike still on level ground, except without the rider onboard. First press down sharply on the rear of the bike and let it spring back into a neutral position. Measure from the same points as before and record this measurement as Dimension C.
Pic: Static Sag Measurement
2) To calculate Static Sag, use the following formula:
Static Sag = Dimension A - Dimension C
Ok so what if I can't get my Static Sag and my Rider Sag to both be within the recommended range?
This part can be hard to get you head around. If your spring is too soft, you will have to preload it more to achieve the desired Rider Sag. This means that the Static Sag will be low as the spring will have an excessive preload. Conversely, if your spring is too stiff, you will have to release preload to achieve the desired Rider Sag meaning that the Static Sag will tend to be too high. So, a soft spring will initially ride high in the rear because it needs to be preloaded more than a stiffer spring. Conversely, a stiff spring will have a lower initial ride height but will feel more firm on bigger bumps because of it's rate.
|Static Sag Measurement
|< (less than) Specified Range
|Spring is too soft - install a stiffer spring
|> (greater than) Specified Range
|Spring is too stiff - install a softer spring
Static Sag Condition
Remember, you must set your Rider Sag first in order to be able to ascertain the resultant Static Sag. Your Static Sag should measure 10mm for small bikes up to 40mm for full size bikes. Now you know how to pick the right spring for your bike!
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So, if you followed all the above steps and selected the correct springs for your bike, you will have set your bike up for optimum performance with neutral handling characteristics and you can then fine tune things like compression and rebound damping. Be sure to check out our other suspension related blogs including how to set your rebound and compression damping and an interesting article on the effects of changing the oil height and viscosity in you forks. Happy reading!