DIY suspension adjustments

[wordsmith]

Using the information in the Owner’s Manual I’m going to try to do the necessary suspension adjustments myself, to give a less-harsh ride. At around 80kg fully kitted I don’t present much of a challenge to the suspension, but it still is pretty mediocre.
 
So if I may, some questions for the suspender-literate among us: not really understanding the difference, in practical on-the-road terms between [strong]rebound[/strong] and [strong]preload[/strong], which should I start with?
 
And – of course I’ll make careful notes of every adjustment so that if necessary I can return to base settings, but is there anything else I should know or do?
 
On my previous Tracer the dealer made some changes that were reasonably successful, and if I can reach that state of affairs again I’ll be happy.
 
All and any thoughts welcomed.
 
[em]Wordsmith (’39 model) – Brisbane, Queensland, Australia[/em]
 

[wordsmith]

Thanks, piotrek: I'm awaiting further inputs from others (hopefully) for guidance before I set-to with spanners. But unless I have mis-read your own adjustment (preload now 10mm from top of nut) and how that chimes with the handbook, your adjustment may - as I see it - have [em]stiffened[/em] the spring preload. Book says the shorter the distance from the top of nut the higher the spring preload. Yours, at 10mm, seems to be less than the factory setting of 16mm (which my bike is). Not wanting to start WW3 - just still a bit confused! But thank you - any clarification? Have I indeed misunderstood? I was hoping to use your settings as a starting point since we are not too far apart in body weight..
 
W

[pattonme]

harshess is a function of oil flow and not preload. Preload "positions" the suspension along the linear axis of movement. To contrive an extreme example, let's say that at "zero" preload (knob fully CCW) the fork rests 3 inches from fully squashed but with 6 turns of preload that increases to 4 inches. Said another way at "zero" preload you only have 3 inches of effective travel but at 6 turns you have 4" worth to absorb road imperfections.
 
No matter how much preload you put into the system (assuming not in gross excess of topping out) it still takes the same amount of weight to compress the spring a given unit of distance.
 
The rebound circuit on the FJ is reasonably decent. The comp circuit is pretty much hopeless and can not be changed unless you tear it apart and change the shim stack. There is some benefit from changing oil viscosity but in the time you spend chasing this formulation or that, you could have fixed the problem with shims and piston. You want the rebound to be as fast as possible without pogo'ing. You want at most a single bounce when doing the "shove test".
 
You put in as much or as little preload into the forks to achieve the desired sag range or the best compromise. For these forks I would aim for 38-43mm of rider sag and ~25 of static sag. If your springs are too weak you'll need a lot of preload to hit rider sag but have too little static. If your springs rate is too high you might hit static but you won't get rider.

[paulsmith]

Mis-information regarding motorcycle suspension is legendary and I am afraid that this site is no exception.
The most fundamental issue is that suspension is not about comfort, it is about handling. It is just a pleasant coincidence that a good handling bike is usually much more comfortable then a bad handling one.
 
The purpose of suspension is to keep the wheels on the road. The reason that this is important is because you can not brake, accelerate or steer if your wheels are not on the road. Things trying to push your wheels off the road are bumps, imperfections, dips, potholes, braking, accelerating and cornering.
 
The wheels have a range of motion that they can travel through before the rest of the bike must move. The normal range (known as 'static sag') is 30/70 (or sometimes one third/two thirds). That means that when the bike (and its payload, ie you, your pillion and/or your luggage) are not doing anything else, the suspension is using 30% of its travel. This leaves the wheel up to 70% of its travel to move up if pushed by a bump, or 30% that it can move down to fill a hole. This adjustment is made with the spring preload. A heavy rider with a pillion will need a different amount of preload to get the same sag as a light solo rider. If the weight is too much for the spring, or the spring is too strong for the weight, or if the preload is not correctly set, then the sag will not be in the required range, meaning the wheel wont be able to move as far before the bike has to move. Despite what you will read elsewhere, preload doesn't do anything else except adjust the sag.
 
A spring is like a pendulum, once you get it moving, you need to apply a force to stop it again. When suspension moves, oil is forced through a valve which dampens the movement by resisting the flow of oil. Rebound and compression are one way valves so you set different amounts of resistance when compressing the spring and when expanding (rebounding) the spring. Additional valves can be added to control high and low speed behavior separately. 
 
The amount of dampening you need is dependent on the weight on the bike and on how you ride. If you have too much, the spring can not return to its neutral position quickly enough to absorb the next shock, so the ride will feel harsh. If you have too little, then the spring will continue to bounce for longer then is comfortable. This affects you for example when you brake into a corner (compressing the front spring). As you come off the brakes, the spring will extend making the forks longer and opening your turn, but without enough damping, they will contract again, shortening the forks and making the turn tighter, they will then extend again, and so on.
 
 

[pattonme]

while your intent is laudable, it's really not all that useful. Some people are more careful than others about measuring or paying attention to stiction. Some people like 'bouncy' others like it 'stiff'. The stock springs are progressive so chasing sag causes some to require a visit to their nearest mental hospital.
 
If the "Internet says" for a 180lb rider set the preload rod with 4 lines show to get a sag of 40mm - true that seems useful on it's face. But unless you go and actually measure and confirm, you're just blindly trusting strangers. By the time you measure you'd have spent the time to figure out your own "lines showing" setting anyway.
 
The only reason that Magazines persist in printing clicker settings is because their readership can't be arsed to learn a modicum about suspension nor take the time to take basic measurements, nor approach changes in a scientific fashion. As Arthur C. Clark once said: "Any sufficiently advanced technology is indistinguishable from magic." except suspension isn't that advanced.
 

[pattonme]

By 'scientific' I mean change 1 variable at a time and use a repeatable testing circuit. When I got my first ever fully adjustable shock (Wilburs 641 series) I spent 2 days and about 3 hours riding the same street segments and parking lot sections at the same range of speeds to test what a turn on this or that adjuster did. When I took the bike to the track I rode the first 2 sessions without touching a thing to see if the observations were imagined.
 
Tuning should be done in 1 turn increments at first. Then go down to 1/2 turns. Then offset by a click and then do 1/2 turn either side. Mere mortals are hard pressed to detect much change at 1/2 turn increments.

[paulsmith]

Well now you know why there is so much mis-information out there. Some people just don't want to know and others just want you to believe it is much too hard so they can seem like 'experts'.
 
There is one simple objective step, setting the sag. All you need is a friend and a tape measure.
And there is one simple subjective step, setting the damping to suit your taste. That involves turning a knob and if you don't like the result, turning it back again.
If you want to call this rocket science, then it is the sort of rocket where you drop some 'Mentos' into a bottle of cola and run away laughing.
 
Whether you want to do this once every time you get a new bike or once every time you ride out is your choice. I choose to do it once every six months or so and then once before I go on a tour with my missus and/or luggage.

[paulsmith]

... Tuning should be done in 1 turn increments at first. Then go down to 1/2 turns. Then offset by a click and then do 1/2 turn either side. Mere mortals are hard pressed to detect much change at 1/2 turn increments.

Bull!  
Rule 0. Set the Sag.
Rule 1. Stop when you cant tell the difference.
Rule 2. Write down your starting point.
Rule 3. Halve the remaining range.
Rule 4. If you don't like what is happening, go back to your starting point.
Rule 5. Same as rule 1.
Rule 6. Write down what you did.
 
The process I use is pretty simple and works on the road and the track. There are others ways to do it but this works for me.
Step 0. Set the sag. Nothing else will work right until this is set. And remember, close enough is good enough. If you have set it before (and remembered to write it down) then you already know the target height you are looking for so it is just a case of checking you are there. 
Step 1. Pick a test route. It should include the type of roads you typically ride. There is no point in setting your suspension for a dirt track if you only ever ride on motorways and vice versa. I prefer a route in a figure eight so I can bail out early if I have fecked up.
Step 2. Ride the route a few times. This is because you will get used to it and come to anticipate the bumps and imperfections and this will change your sensitivity to them. You don't want to think an adjustment is better or worse just because you knew where the bump was and braced for it. 
Step 3. Record your starting point. Count the clicks to max. Then count them back to your starting position. Then count the clicks to min and the clicks back to starting position. Doing both only takes slightly longer but protects you from failed adjusters that don't stop at the limit.
Step 4. Pick one adjuster and put if half way to the further limit. For example, if your rear preload has twenty clicks and you are on seven, then 20 is further away then 0 and half way between 7 and 20 is 14. Ride your test route. The change is clearly better, clearly worse, or not noticeable.
   If it is better, then your starting position becomes your new limit. ie you are now looking between 7 and 20.
   If it is worse, then your current position becomes your new limit. ie you are now looking between 0 and 14.
   If the change isn't noticeable, you are done. Time to move on to the next control.
   If you don't like what is happening, put is back to the starting position and move on to the next control. You can always come back later if you want to.
 
The only difference between you doing this and Valentino Rossi doing this is that the Dr. is able to notice changes that you can't.
If you really want to be like the Dr., get a mate to make the adjustments and you tell him better/worse/didn't notice. He should send you out with no change a few times to check if an improvement is real or imagined.
 
 

[wordsmith]

There is one simple objective step, setting the sag.  

I thought that 'sag' is what happens to a woman's breasts or a man's belly as each gets older. In a motorcycle context - please explain!
[em]Wordsmith[/em]
 

[pattonme]

@wordsmith, you're kidding, right? Left off the sarcasm tag?
 
http://racetech.com/articles/SuspensionAndSprings.htm
 

[bgigharbor]

There is one simple objective step, setting the sag.  

I thought that 'sag' is what happens to a woman's breasts or a man's belly as each gets older. In a motorcycle context - please explain!
[em]Wordsmith[/em]

I there were a dislike button I'd use it for this post. 

[wordsmith]

@wordsmith , you're kidding, right? Left off the sarcasm tag? 
http://racetech.com/articles/SuspensionAndSprings.htm

Thank you - but nuclear power stations come with less comprehensive instructions, I'm sure!  [em]W[/em]
 

[duhs10]

Ok dumb question....
 
How do you know the maximum travel of your suspension in order to set the sag? Is it from the seal (with no load) to the point where the fork would physically contact the lowe mount?

[pattonme]

read the spec sheet for one. But in this case there is a fluid lock and mechanical stopper that will hit before the seal crashes into the bottom.

[captainscarlet]

Ok dumb question.... 
How do you know the maximum travel of your suspension in order to set the sag? Is it from the seal (with no load) to the point where the fork would physically contact the lowe mount?

It's in the manual, 137mm at the front and 130mm at the rear.  That gives you rider sag values of 41mm and 39mm respectively.  As none of us on this forum are called Marquezpedrosalorenzorossi, let's call it an even 40mm across the bike; nice easy number to remember. 
CS