In writing this I'm assuming my reader has a basic understanding of motorcycle technical jargon , As in compression damping controls the decent and rebound the return and preload is the initial pressure on the spring ( winding it up will not increase the rate of the spring ).Suspension is supposedly a black art that no mear mortal will ever understand.My experience is only of production based bikes .Pure race bikes tend to have adjustable geometry .Though adjustable yokes and swinging arms are becoming more available for production bikes.The best starting point is a bike that has a well sorted geometry to start with ,so a fair indicator of this is a bike that's popular for super sport racing such as R6 .I found that it was actualy fairly simple to arrive at some pretty good settings without even riding the bike if you followed a few basic rules on laden sag .Mind you I had some experienced advisors .
The first thing is to start with good kit .I always use Ohlins rear shocks as they seem to represent good value for money ,there are of course many more to choose from .The basic idea is to have a shock that gives consistant results ,so the damping oil viscocity does'nt vary to much with heat ,the oil does'nt cavitate as it passes through the valves ,seals do'nt cause stiction and you can go on and on ,but thats why you buy a good quality shock because all the clever stuff's been done for you.The front forks tend to be a bit more restrictive as most of us can only afford to modify the standard items ,this can be done by a specialist (who if the bike is commonly raced will know which spring you want )or you can use these basic rules to find (usualy) a slightly heavier rate spring ,Add some thicker oil (oil vscocities as stated by manufacturers vary tremendously so always stick to the same brand when testing for damping rates 10/15w), leave an air gap with the fork collapsed of 130mm and you'll be in the ball park .Obviously having the damper valves modified by a specialist will improve on this , but I tended to wait till we ran out of damping adjustment then I new which way they needed to go.
Once you've bougth you're shock you need to check out the rear spring rate , this is done by measuring the length of the chrome part of the damper rod with the shock fully extended ,you should ignore the bumpstop rubber on the shaft .Having done this what you are looking for is when you sit on the bike and the spring compresses you should use a third of the travel of the damper rod .If this is not the case adjust the spring preload until it does use a third of the travel .Next you need to check the unladen sag , this is the amount the back of the bike drops buy when stood up,so you'll need to get a tape measure , get a friend to lift the back right up and measure between a fixed point on the swinging arm and another on the tail unit ,then let the bike stand on it's own and measure again , the difference between the two is you're unladen sag.This measurement varies from light bikes to heavy .Lightwieght 250's will be topped out with no sag , a heavy 1000 may have 25mm , most production based bikes have 10-20mm.You must have this unladen sag after checking the travel and setting the preload .If you have no unladen sag then your rear spring will be to hard , if you have an excessive amount then the spring is to soft. Most shock suppliers will go on the safe side and give you a spring that is to hard . This will result in tyre tear and instability under braking seem to be the most noticeable affects .
Pretty much the same set of rules apply to the front end ,the unladen sag at the front forks is usually about 25mm on most bikes.As far as damping adjustment goes ,you want just enough to calm the spring when you bounce on the bike .you should not be able to see it slow the suspension . This should give you a good starting point the rest is down to personal feel .There are so many variations.