Part 2. Setting the sag
As mentioned in part one, setting the static sag of the motorcycle is the starting point to getting your bike in the realm of using the suspension correctly. As you set the sag you will uncover some basic flaws in your settings that can be arrived at no other way.
The static sag is set with the bike under its own weight, and is backed up by two further tests; the One G Test (with rider and normal weight load in place); and measuring the amount of suspension travel used while riding. To get the springing rates correct for your motorcycle, this process must be worked through in sequence. If you decide to miss one of the steps or allow the settings to fall outside the guidelines given, you’ll be missing at least one or more ingredients to getting your suspension working the way it’s designed to.
So, let’s discuss exactly what the static sag is — Static: motionless, inactive. Pertaining to bodies at rest or equilibrium. Sag: curve downwards under pressure. Give way. Hang loosely — so we are in affect measuring the “curve downwards of a body at rest”. The exact measurement for what to set the static sag at will differ from bike to bike, rider to rider, and will vary depending on how the bike is to be used. There are however some fairly stable guidelines on where to start.
First you need to know how much travel your suspension has by extending it fully and measuring in the direction of travel. On the forks this is easy as you simply measure from the top part of the lower fork leg, to the bottom of the triple clamp.
On the rear, you need to pick two points that are at each end of the travel. For example, the grab rail and the axle. Unload the suspension fully by taking the weight off it and measure as shown in pictures 1 and 2. This will give you the unloaded distance.
To get the fully extended length of the suspension it’s important that you take any sag out by lifting whichever end of the bike you are measuring. If possible it’s best to actually lift the wheel off the ground when doing so, to ensure you are getting accurate measurements. This can be done by having someone balance the bike of the side stand (if a centre stand isn’t fitted) while you do the measuring.
Once you have this distance, stand the bike upright, and allow it to sit under its own weight, measuring from the same points. It’s best if you give the bike a couple of bounces up and down, then allowing it to settle before measuring. This guarantees the suspension is settled to its true sag. The difference between these two measurements is the static sag.
The rear sag should be somewhere between zero and 10 millimeters and the front between 20 and 30 millimeters. As mentioned earlier, there will be some differences from bike to bike and rider to rider, depending on how you are using your bike, but it should always fall within this range. For example, during racetrack use you will find that firmer is better (within reason), whereas on the open road that is not always the case
The One G Test backs up what we have already done in setting the static sag of the motorcycle, and takes into account the particular weight load that is going to be carried and the way the bike is going to be used. This is the part that most riders forget...so listen up!
To be effective, the measurements need to be done with the normal weight load (i.e. the rider) in place. If you are a courier for example and carry a reasonable weight on the back of the bike for a large percentage of the time, or you only ever ride with a pillion in place, it would be a good idea to simulate that situation for the purpose of measuring. If you were planning a touring trip with baggage, but still wanted the bike to handle at it’s best for the roads you plan to ride, you could reset the static sag to suit the weight load carried for that period of time, then change it back to the way it was once the trip is over.
To do this correctly you will need a measuring tape and three helpers. Two to balance the bike with you in place, while a third person measures the sag. First, you need to know how much travel your suspension has by extending it fully and measuring in the direction of travel, just as we did to set the static sag. On the forks this is easy as you simply measure from the top part of the lower fork leg, to the bottom of the triple clamp.
On the rear, you need to pick two points that are at each end of the travel. For example, the grab rail and the axle. Unload the suspension fully by taking the weight off it and measure as shown last issue. This will give you the unloaded distance.
Once you have this measurement, the rider (and any other normal weight load) should be put on the bike as per the pictures shown, with helpers holding the bike front and rear. The rider should then bounce the bike up and down a few times to free up the suspension and put it in the position it would normally sit in with the rider in place. The designated ‘measurer’ should then measure from the same points the first (unloaded) measurements were taken.
The difference between these two figures (the sag) should fall somewhere in the vicinity of 25-35mm front and rear. If you find your bike is either less or more than this, simply adjust the preload either harder or softer using the preload adjuster, until it falls within this range.
Often time’s bikes that have done a few kilometers will sag way beyond ideal settings, as will new bikes sometimes. As a matter of fact I have measured brand new sport motorcycles that have never been ridden and found they have more than 50mm sag front and rear! As you make adjustments you may find that you get to one end or the other of the existing springs range. If this happens, it means the next spring available (either heavier or softer) is probably going to be on your shopping list.
Often time’s riders try to make up for a lack in one area of suspension by adjusting another. In this example, if the bike were still too soft after the preload was adjusted fully, you could turn up the compression damping to try and slow the speed of movement downwards. This would begin complications that could severely affect the suspensions ability to function. If you were to arrive at this point, speaking with an expert (as opposed to “pub experts”) is really the only course of action.
Next issue we’ll talk about the final point in getting the springing right in your suspension, and start to figure out what to do with the damping.
The final part of setting the springing (as we’ve been discussing the last two issues) on the suspension is to measure the amount of travel used while riding. It’s important to do this final step rather than rely on how the bike feels. Often our perception of what the motorcycle is doing is quite different from what is really happening.
This final step is probably the most simple of all. To measure how much travel the suspension is using it’s just a matter of putting a cable tie (sometimes referred to as a zip tie) on the shaft of the forks and the shock. In this way you can see how much of the available travel is being used by the location of your cable tie after the bike has been ridden.
To ensure accurate results, you should only measure the amount of travel being used after riding in your normal environment. Meaning that if you do mostly ride days and sport riding, just going around the block isn’t going to give you a very accurate idea of what the suspension is actually doing!
If you find your cable tie is pushed up against the end of the shaft, chances are your suspension is too soft. On the other hand, if your cable tie is only half way along the suspensions travel, chances are your suspension is now too hard. The ideal scene is to have the swept area (the total amount of travel used) to be only slightly smaller than the total travel available.
By using the three measurements we have discussed over the last two issues of AMCT, you now have the tools to set the springing correctly. As you change the preload to suit one of the settings, you may find that it makes the suspension fall out of the ideal range in the other areas. This tells you that your spring may not be the right one for your application. For example. Lets say you did everything right in setting the sag and the one G test, but your suspension is bottoming out (using too much of its available travel).
This would tell me that your style of riding requires a harder spring set with less preload. This spring would require more force to use all its travel, but less to start it moving. Meaning that you can still get the sag figures correct, while using the right amount of available travel.
There are far too many possible variations for us to cover in this short amount of time, but at least now you should be able to recognise whether or not your suspension is working within the range it was designed to work.
"The price of apathy is to be ruled by evil men"
Plato
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