Heres a guide I wrote a few months ago for another member.
In every modern sports bike fork, there are 3 key areas which make everything work. Damping, which covers both compression and rebound, and the spring. This is a quick, basic insight into how the damping works.
Damping is the means to reduce the size of oscillations in the forks when they hit a bump. Without it, the fork will act like a pogo stick and just continue to extend and compress after a bump. By using oil and slowing it as the fork compresses and rebounds, we can control the rate at which the fork absorbs the bump and returns to its original, static state.
Two different systems are used in both compression and rebound to control the oil flow-Shim stacks and adjustable sized holes, generally referred to as needle valves.
A shim stack is a small pyramid of very thin washers that bend when oil is forced though the small holes under the stack that are otherwise sealed. When the oil is forced through these holes, the amount it resists by depends on the shape of the stack, the number, diameter and thickness of the shims.
Shim stacks vary between forks and can be customised to suit the rider’s personal preferences.
So how do we translate this into setting up our bikes?
Before doing anything write down all your base settings and thoughts on what happening when you ride the bike.
First we need to know where everything is and what happens when we make changes.
Staring at the very top we have a little silver screwdriver slot that when turned adjusts the fork rebound speed. Rebound is the movement of fork travel upwards.
Next we have the preload adjuster. Preload does not effect the springs stiffness although it may feel like it. We adjust this to get the bikes sitting correctly or sag. There are two types of sag, static and rider.
At the bottom of the forks we have another screwdriver slot, this one adjusts the compression damping. Compression is the speed the forks compress.
Rebound Adjuster: The rebound adjuster consists of a shim stack and a needle valve. The needle valve adjusts the amount of oil being fed by the piston (fork) movement through the shim stack. To see what the adjuster actually does wind it fully but lightly in clockwise until it stops. This has now stopped the flow of oil so the forks cannot rebound. Push down on the forks and if the adjuster is working properly when would full in, the forks will struggle to extend. Now back the adjuster out to feel the first click. This click isn’t measured its known as click “0” Now screw the adjuster out counting every click after “0” until you come to a stop. Don’t force it to go any further. This is how many clicks of rebound adjustment you have to play with. Don’t worry if your mates bike has a click or two more than you its all down to how they are built and can vary slightly. Now with this fully out again bounce on the front end. The bike will quickly return, as there is now no control over this movement.
Now you have a feel for the rebound lets look at the next adjuster.
Compression Adjuster: This works exactly the same as the rebound adjuster with a shim stack and a needle valve. Only this time it adjusts how fast the forks return. Repeat the procedure above to get a feel for how the forks feel on full damping (needle fully, lightly wound in) to fully out. Remember to count the clicks out, with the fist one being “0”.
To complicate matters further on the later models we have high & low speed compression damping. The middle screw driver slot is low speed and the outer spanner fitting is high speed.
High speed and low speed refers to the shaft velocity in the shock absorber. It is not necessarily related to the speed of the bike.
Pre-load Adjuster: As stated earlier this does not alter how hard the spring is. All it actually does is as it says adds preload to a spring which takes away that first bit of travel. i.e. if a ten stone man sits on the bike and the forks compress 20mm and a 20 stone man compressor the forks 40mm. By adding 20mm of preload to the forks we have in effect added the 10 stone man. But when the 20 stone man sits on the bike it will still compress by 40mm. So the 10 stone man will think the spring is now harder but the 20 stone man wont feel any difference.
So lets now back this right off by winding the adjuster completely out (anti-clockwise). Measure the amount of fork leg showing. Now wind the adjuster fully in and again measure the amount of fork leg showing. We now have more showing as the bike is riding higher after adding preload.
Finally take the weight off the front wheel and measure the amount of fork showing. The difference between the measurement with the bike on its wheels / bike off its wheels is sag. You have to take the weight off the wheels not the wheels off the ground to measure sag. I use a paddock stand under the headstock to take the weight.
Now lets look at adjustments.
Adjustment: With everything wound fully (lightly) out we can start to measure and adjust. There are two crucial area’s here which we will look at.
Starting with the sag. Unless you are going to change the springs we are going to have to compromise here. The Daytona springs are too heavy for road use but perfect for racing.
Standard they run a 9.5K spring. I recommend a 9K for road and a 9.5K for racing. For best results you need a linear spring. But we have what we have so lets make the most out of it.
Ideally we would have a static sag of 25-30mm and a rider sag of a further 10mm.
Measure the full fork length (weight off the wheel/forks fully extended) = X
Put the front wheel on the ground and sit on the bike (in riding gear and position). Bounce the front a few times and allow to settle. Measure the amount of fork showing = Y
Sag = X – Y = Z
By adjusting the spring preload we are looking to get Z to equal 35 to 48mm (35 for smooth tracks 48 for road use).
Now we’re going to leave the front for the moment now that the sag has been set. We’ll come back to the other adjustments later.
Rear Shock Absorber
So far we have learnt what does what at the front, and its exactly the same at the rear.
Spend time tweaking the adjusting screws and counting the number of turns etc.
The only difference is the Compression adjuster is now at the top of the shock and rebound at the bottom. Later shocks have high & low speed compression adjustments. More of that later also.
Adjustment: Again starting with sag. Ideally we would like 5 to 15mm without rider and 30 to 45mm with rider.
To measure this we first need all the weight off the back wheel/shock and then the measurement is taken in a straight line vertically from the rear wheel axle = A. Its done here because if you went vertically down from the axle you are on the tyre contact patch on the ground, which is where all the travel is doing its thing. Draw a line from the straight edge onto your seat unit as a datum.
Again we are going to simplify and just work on the rider sag. Now sit on the bike and get a mate, wife, girlfriend, other person etc to measure from the axle to the datum marked on the seat unit = B. Its actually easier to put a datum on the seat unit and then draw across to the ruler for the two measurements.
Sag = A – B = C
By adjusting the collars on top of the spring we are looking to get 30 to 45mm (30 for smooth tracks and 45mm for road use). Note: the top collar is a locking collar and should be released before trying to adjust and then locked back up.
OK now you have the most important bits done, the sag. We can start to look at damping. I’d suggest unless you are feeling very confident you just put these back to where they was as the next bit is down to experience and feel. After years of practice and training I have a good feel for how a bike should be. This enables me to set a bike up onto base settings and then adjustments made based on rider feed back and watching the bike in action.
Right well you have continued to read on so lets start by going back to the original settings for the rebound and compression. With these set push firmly down on the motorcycle on the top yoke and seat unit and see how the bike reacts. You are aiming to get the bikes rebound the same speed front and back. This is very important to get them the same. Adjust on the rebound till front and back rebound at the same speed.
Now its time to go for a ride and start to get a feel for things.
Always begin with the basic settings recommended by the manufacturer. Always make notes, adjust in small steps and make only one adjustment at a time. Adjustments should be made with two steps (clicks) at a time. Adjustments should not be more than four steps from the basic setting.
By utilizing the adjustment possibilities you can test by trial and error, and learn how they effect your motorcycle. Always begin by test riding the motorcycle with all adjustments at their last ride setting. Choose a short run of varying character, ie, long and sharp bends, hard and soft bumps. Keep to the same run and adjust only one setting at a time.
Start with the rebound damping. If the motorcycle feels unstable, loose and rather bouncy then the rebound damping should be increased. Begin by turning the adjusting knob 4
clicks clockwise. Test run again and adjust 2 steps back if it felt too hard and bumpy.
If the motorcycle is hard and bumpy, especially over a series of bumps, then the rebound damping should be reduced. Turn counter clockwise 4 clicks, test run and make any necessary correction to 2 steps.
Compression damping The low speed compression adjuster affects ride height, smoothness over small bumps and grip. The high speed compression adjuster affects stability, firmness in depressions and fast corners. If the motorcycle has low riding position, the low speed compression should be increased. Screw clockwise 4 steps and test run again. If
this was too much then turn back 2 steps (counter clockwise).
If the motorcycle feels unsmooth over small continuous bumps or has bad grip, the low speed
compression should be decreased. Turn counter clockwise 4 steps. Test run and make any
necessary correction in 2 steps at the time.
If the motorcycle feels unstable in fast corners and has a tendency to bottom easily in depressions and chicanes, the high speed compression should be increased. Screw clockwise 6 steps and test run again. If this was too much then turn back 3 steps (counter clockwise). If it feels harsh and too rigid or has a tendency to hop under
braking, the high speed compression should be decreased. Turn counter clockwise 6 steps. Test run and make any necessary correction in 3 steps at the time.
When you have sufficient feel of the motorcycle you can make further fine adjustments. It is feeling and experience that counts.
Static Sag – this is how much the spring is compressed with the bike upright and no rider.
Rider Sag – this is how much the spring is compressed with the bike upright and rider including gear sat on bike.