MOTORBIKE SETUP: Suspension No.5

Suspension - 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 previous articles, refer to the links below.

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 tyre's 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.

There are no hard and fast measurements with which to set the damping on the suspension and most riders don't know what their bike is really doing when they're riding it. So the question must be asked...how do you know when it's right? And how do you know when it's wrong?

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 tyres, 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 tyres 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 neighbour...

Good luck with your riding...

Steve Brouggy is the owner of the

Contact:
2 / 76 Rushdale St
Knoxfield, VIC 3180
Australia

Phone: 1300 793 423
+613 9763 3338