Suspenion Tuning

The Black Art

One of the most commonly asked about subjects is suspension. As with anything to do with riding motorcycles, there are usually simple explanations for everything that happens, making it a matter of just taking the time to analyze each individual action and the resulting consequence. To assist in making this process possible in the ‘black art’ of suspension.

Firstly let’s discuss what suspension actually does. Simply put, the suspensions job is to keep the wheels on the ground. (Giving you a nice comfortable ride is very much a secondary consideration!) This means that how the suspension works will directly affect arguably the most important concern when riding a motorcycle - traction.

We have already discussed how the way you ride can affect the suspensions operation, simply by holding on too tightly and putting too much input into the bike, you are not allowing the suspension to complete it’s task. The end result of not allowing the suspension to fulfill it’s role, is that traction is affected, and the bike will tend to run wide. It will also amplify any bumps or deviations in road surface by transferring the force back and forth between your body and the bike. (For more detailed information on this, either see “Keith’s Corner” at the California Superbike School web site - http://www.superbikeschool.com - or read chapters 7 to 11 in “A Twist of the Wrist II” by Keith Code.)

No amount of changes to the bike will overcome the problems associated with the rider doing something to the motorcycle which interferes with the way the suspension is supposed to work. So, riding problems aside, how does it all work?

There are several components to the suspension system of any motorcycle. Each one has a specific role and design parameters, and each will be effected in some degree by adjustments to the other components. There are two major operations of any set of forks or shock absorber. They are; Springing; and Damping.

The spring has two variables in it’s operation, one is pre-load, the other is the spring rate. Preload is adjusted by changing the fitted length of the spring thereby changing the amount of initial force needed to begin movement of the spring. Changing the preload also affects a very important part of the basic set up which is ‘static sag’ (we’ll talk a little more about that in the next issue). The spring rate on the other hand can only be adjusted by changing the spring. It determines the amount of force needed to keep the spring moving which can be lesser or greater depending on the amount of coils and the thickness of material used for the spring.

The damping’s job is simply to control the springs’ speed of movement. There are two basic areas of this; the first is compression damping, which deals with the rate at which the forks or shock can compress or dive; and the rebound damping which deals with the rate at which the suspension can extend or raise.

Without damping to control the spring, it would be free to continue moving the force backwards and forwards along it’s available travel, which would be substantially more than you want on a motorcycle. (If you have ever jumped up and down on a pogo stick, you’ll know what a spring feels like without any damping!)

So, this means you have four totally different ways in which to change your suspension. When you consider that most modern sport motorcycles have around 12 or more damping adjustments, 12 or more rebound adjustments, 'endless' preload front and rear, along with a plethora of different rate replacement springs available, you now have literally thousands of possible combinations! Add to this the fact that the front needs to be balanced with the rear, and it’s no wonder most riders are confused about what to do with the suspension!

Setting the sag

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 it’s 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 center stand isn’t fitted) while you do the measuring.

Once you have this distance, stand the bike upright, and allow it to sit under it’s 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 it’s 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

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 times 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 times 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 effect 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.

Measuring the travel

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 it’s 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.

Damping

Now that we have the springing set correctly, it’s time to control the spring movement with the damping. Please note — if you haven’t undertaken the process of setting the sag, doing the “One G Test” and measuring the travel, you won’t find the information on damping of much value. If you’ve missed the AMCT issues with this information, copies are available by contacting the Australian Superbike School, tel (03) 9792 1322, fax (03) 9792 1075

Down to business... there are only two directions the suspension moves, up and down. When the suspension is diving or compressing, it is termed the ‘compression’ and when it raises or lifts it is called the ‘rebound’. The damping's job is to control the speed of both the compression and rebound of the suspension, thereby controlling the speed the wheel can move up and down. This decides what sort of contact (if any) your wheel with have with the ground, thereby affecting the tire traction to the road.

It accomplishes this hydraulically, by transferring oil from one reservoir via a piston with a series of valves to another reservoir. By adjusting the damping you are changing the amount of oil that is able to go through the valves (the ‘flow rate’) thereby changing how quickly the suspension can move. The larger the hole in the valve the faster the suspension will travel, the smaller the hole in the valve the slower the suspension will travel. Similarly, the thinner the oil in the suspension, the faster the travel, the thicker the oil, the slower the travel.,

On most modern motorcycles there are adjustments for both the compression and rebound damping both front and rear, along with the ability to change the oil weight. Before delving into the specifics of setting up the damping, it is important to realise that both ends of the bike will effect each other in their set up. Meaning your front forks may be set up perfectly, but if it is mismatched to the rear in either the springing or the damping, it will be less effective.

The difficult thing when adjusting the damping is that there are no rule of thumb measurements that are easily taken and compared like there is with the springing. In fact unless you have state of the art data logging equipment and a shock dyno, the decision of how to change your bike will fall somewhere between the observation of a technician (whoever you talk about suspension with) and the feedback given by the rider (you). This is where riders get lost in trying to correct a handling problem, and place the whole process of correcting suspension into the too hard basket.

The true difficulty here is that we now open up the whole subject to the different perspectives given by each of these individuals. It is no surprise that when data logging (sometimes referred to as telemetry) first appeared on racing motorcycles, the rider almost always wanted to go the opposite direction in adjustments to what the data logging system suggested. Showing conclusively what the experts have known all along...very few riders actually know what their bikes are doing while they ride them! Racing has now become much more of a precise science since the advent of such systems, to the point where a world championship level team wouldn’t be able to function effectively without it.

What this means to you and I is that we are now dealing with an area that requires some expertise to get right, along with an ability to observe what the bike is doing while being ridden. If you follow the guidelines and examples in the coming issues we can at least give you some idea of what your bike may be doing, and hopefully help you chose a way in which to fix it.

I've seen suspension technicians bounce a bike up and down and make a decision as to what changes to make to the suspension. Mere mortals like ourselves may observe this and, seeing what decision they arrive at, start to base similar decisions of our own on this information. The problem is that we haven't had the technical training they have.

We probably haven't bounced a couple of thousand bikes up and down before, feeling for what the suspension is doing either. Therefore, chances are we haven't gained a 'feel' for what good damping should 'feel' like on a stationery motorcycle.

It should also be noted that this is usually not the only source of information that the technician will base his decision on. As a matter of fact, to be as accurate as possible, they will use several sources of information. So in combination with 'feeling' the bike by bouncing it up and down, they will look at how the bike is wearing the tires, if at all possible (especially in a racing situation) the technician will try to observe for himself what the bike is doing, along with getting the riders perspective.

Obviously if you are not racing and you come to a suspension expert, they will probably not be able to observe what the bike is actually doing, and will have to base their decision on the other factors. This should be enough for someone who has the skills to decipher the sometimes incomprehensible babble that we (as riders) offer, and what he 'feels' the bike doing.

Our job as riders then is to give the technician accurate information. In other words we need to observe what the bike is doing and report it. If you are going to be your own technician, that's fine, just recognise the different roles you play. On the bike you are the rider, gaining the information needed to base a decision on. Off the bike you are the technician and your job is to take that information and base a decision on it. Don't expect every decision to be correct. Chances are you'll have to repeat the process of changing these 'hats' more than once to get it right.

So...what exactly are we looking for? The damping can only be one of three things. Too hard. Too soft. Or, just right! To help you understand what effect each of these have let's look at them individually.

Too hard - as a rule, if the damping is too hard, the bike will feel unstable. If it's too hard in the compression damping the suspension will not comply with bumps, as the transferring of the force is too slow, meaning the wheel will tend to skip over the top of the bumps rather than follow the contour of the road surface. If too hard in the rebound damping the bike will tend to 'pack down' or 'squat'. This is caused by the suspension not allowing the bike to raise back up to it's original position quickly enough. The end result of this is sometimes referred too as suspension "chatter." Although the bike will feel unstable, it will tend to feel quite 'stiff' or 'taught.'

Too soft - the interesting thing here is that if the damping is too soft, the bike will also feel unstable. Only this time, rather than 'chattering' it will tend to feel 'sloppy,' and possibly 'wallow' in corners. (wallowing means that the suspension will oscillate without anything seeming to start it off, like bumps for example) It may also feel a little like a pogo stick, continuing to transfer the force up and down the length of the travel. This is particularly noticeable on bikes that have done a lot of miles.

Just right! - if the damping is working correctly, you probably won't notice anything! The suspension will keep the wheels moving up and down as required to keep the tires on the ground and the bike stable.

Who was it that said "the more I know, the less I understand"? The interesting thing about reading information on a technical subject like suspension is that it will tend to raise more questions than it answers. I'm sure that over the last few issues we have been focussing on the subject of suspension, that has happened for you. That's okay. In fact that's what's supposed to happen.

It's now up to you to use this information and apply it. Doing so will answer some of your questions, and the ones it doesn't need to be directed to someone who knows what they are talking about. And...just in case you're wondering...that's probably not your brother in law or next door neighbor...