by (Brian McCoy)
Well, it seems that you have 3 types of people when it comes to suspension;
1. Those who dont know a thing
2. Those who dont know a thing but claim they do
3. Those who know
Im probably somewhere between 1 and 3.. I know some stuff, and know its true and verified but dont really know as much as I should. Anyway, what little I do know should be of some help when setting up suspension for the street (used for a baseline). Maybe even some theory tossed in for comment.
Springs are the base for everything else. If you dont have the proper springs for your bike + rider weights, youre just wasting your time with all the smaller adjustments. In this field, I suspect that Race Tech has the best info (they have a sorta interactive profile of the rider and gives you the basic info on everything).
http://126.96.36.199/reference/springchartstreet.htm is the link for their spring rate vs. bike & rider weight chart though. Just dont forget that you need to adjust the rear aswell, because an unbalanced suspension will definitely not help matters.
A side thought on springs, when a suspension is working right they do not perform any altering of the ride. Basically, this means that progressive or multi-rate springs are an attempt to compensate for inadequacy elsewhere in the suspension. In the case of the stock VFR forks, its the lack of damping control, and the springs may indeed help, but proper forks and valving is worlds better than anything a progressive/dual rate spring could offer.
Damping rod style forks:
Most of the older forks are whats known as a Damping Rod style (the 86/87 VFRs, and the CBR600s until 93. They changed to cartridge in 94). In these forks, the damping is controlled by precisely drilled holes in a rod at the bottom of the fork. Thats great, works wonderfully except theres no adjustability without changing the sizes of the holes, or the weight of the fork oil. The reason the oil can change the damping characteristics is because of its viscosity. (Viscosity: a liquids resistance to flow). The higher viscosity oil, the more resistance to flow and the more damping both rebound and compression. Now, this would be adequate if all of the bumps we came into contact with were progressive too.. but thats not the case, and the main reason why cartridge forks with shim stacks work so much better.
With this style fork, youre pretty much limited to setting up the suspension for one specific area (highway, sport or some point in the middle where you suffer on both ends).
Cartridge style forks:
This is the new way to do things.. the best available telescopic fork design to date. With the availability to change slow, mid and high speed damping through the use of shims in shim stacks, you can ultimately derive the best all-around riding/suspension performance. Shims offer the ultimate in custom tailoring for suspension as you can set up a soft slow speed (able to soak up big potholes at 30mph and less) and a stiffer high speed (for better stick in the corners when out carving the canyons or racetracks).
The Cartridge Emulators from both Race Tech and Traxxion Dynamics are the intermediate step between a damping rod fork and cartridge type fork. Theyll give you the ability to adjust your compression and rebound damping and actually, as I think about it more you might be able to achieve effects very similar to a full on cartridge fork.
The twin chamber cartridge forks out nowadays are basically like the shocks that weve been using for years. About the only difference is the distance of travel, and the shock uses a nitrogen bag instead of air. Beyond that, it still has shim stacks that control the flow of oil on both compression and rebound.
This is probably one of the more overlooked parts to suspension. Now, everyone knows that raising the forks in the triples (dropping the front) gives better turn-in and less stability, while lowering the forks (raising the front) gives stability and harder turn-in. But a lot of times, this is achieved in a rather indirect manner by raising and lowering the ride height in the rear. While both ends are definitely linked and changing one has an effect on the other, its about a 5 to 1 difference. Say you want to increase turn-in and you try to accomplish this with rear ride height, youd have to make a movement 5 times as much as if you tried to accomplish the same task with the front end. And while doing that change, you also change the swingarm angle, which affects squat on acceleration. Squat is relatively important as if you have too little, the tire will spin up and you waste tires as well as get poor drive out of the corners. If you have to much squat, then the front forks extend, basically increasing the rake and youll likely run wide coming out of corners.
Obviously the rake and trail are important too.. but were pretty well stuck on what we have in that respect (unless you introduce custom triples that are adjustable, etc and if youre doing that, you likely know more about suspension than I do currently).
Its not really possible to give a blanket statement on tuning because everyone desires different results and has different personal variables. But I can give some guidelines that work for a base setting that you can figure out and work from.
Just like the springs, Sag is the base of everything thats to come (for tuning purposes). Its a rough adjustment of how much travel your bike uses, only to be fine-tuned with the other variables. You definitely dont want your bike to have no sag, as when you hit a hole (somewhere your wheel needs to compensate thats away from your bike), itll just end up jarring you as it falls in and hits the opposite edge. But at the same time, you dont want to allow so much sag as to take away from the majority of your travel in the compressed state. Its a fairly common thought that with the rider on the bike, in full gear, you should have about 30mm of sag. To set the sag, youll be changing the spring pre-load (accomplished with spacers in the forks, and by spinning the collar on the rear shock).
The ability to change this varies from bike to bike, but most have at least some rebound damping available. Compression damping is fairly self-explanatory.. it dampens how fast the shock/fork can compress. With cartridge style forks, you can change this with shims and shim stacks. This allows for a completely custom ride, with slow speed damping (hitting the end of your driveway at 5mph), mid and high speed. Rebound is similar in function, but works when the fork is extending back to the set sag height. You want the fork to use all of the travel without bottoming out on the compression stroke and come back to the set sag height, but not top out and then settle back to the proper height. Again, the same is true for the shock.
When all of your adjustments are made, you should have a 50/50-weight bias while sitting on the bike in gear, and in your normal riding position. Also, if you were to stand on the pegs and bounce the bike, the front and rear should compress and rebound evenly. This is balance, and is a necessary thing to have when youre finished tuning the bike. Without this balance, your bike will react in strange ways, possibly causing a bucking and weaving sensation as the shock and forks work against one another.
Oil weight, oil volume, springs, etc
These are the usual method for tuning a bikes suspension, and not really the best. Sometimes its necessary, like with our conventional damping rod style forks, but itll never offer you the performance that can be achieved with valving. The weight changes viscosity, which I talked about above. The volume changes the amount of air volume in the fork (which can be considered "low speed damping" as the air compresses first, before any oil is moved through any passages.) The springs are there to hold the bike up, and should be chosen for the bike and rider weight to do this task not to change the compression/rebound. And for things like the Anti-dive on the stock VFR forks, try to omit them and run all your suspension changes/tuning inside the fork. This helps keep the variables down, and will allow you better tuning success.
Hopefully this helps shed some light for those of you who dont know and want to.. and hopefully Ill be able to learn more and give some more in-depth reasoning behind the basic principals. Good luck.