Suspension setup, by Tak

Well, the most difficult part of building a special is finishing off, to make it work sweetly, in harmony. It takes time. It takes patience. It takes faith. It takes certain level of understanding in mathematics and physics. It takes artistic sense. The one (special builder) must have concentration, courage, that goes without saying. But more importantly,... the one must be both a mathematician and a poet at the same time. Working on your own project is to philosophers and warriors, buying a finished product is to accountants and merchants. Sensitivity is an important thing. (though it doesn't make you a practical person...)

Here's the BIG (useful) secret about setting up hybrid super bikes :-) I can give you an idea, but, hey, there's absolutely no substitute for experience here. So be prepared for a lot of setting change&test ride, and here we go...

1) Chassis attitude
Set the ride height (both front and rear). Remember that if you lower the front, you need to RAISE the rear. Ride height set includes preload adjustment. Then, set the damping adjuster at FULL SOFT and test ride it. Concentrate on how the bike steers. Continue this until you get the bike steers as quick as you want. If the bike starts "falling over( you have to force the bar to avoid capsize ) then, you have gone too far, go back to previous setting. (If you are a good rider, you can feel 5mm difference in trail.)

2) Compression damping
What? Isn't compression damping the last thing to adjust? Well, yes and no. If you ask your local MC shop mechanics, they would say so. But when you do major tampering to your suspension ,then that's a different story. The idea is to get the best squat as the bike loaded up coming out corners and under power rear), and to get the best control of front end dive under braking. Also to set the damping force to suit the bumps in critical areas such as corner entry (which is normally braking zone, too), corner exit, etc. Start from FULL SOFT and test ride it, then ,increase the setting one click at a time. Pay attention how it reduces chassis upset. How does front end dive change? How does rear end squat change? How does it feel when hitting the bumps? As you increase compression damping in the forks, It slows down front end dive under braking, reduces mushy, vague feeling. Going too far, then, the ride becomes too harsh, front end kicks off when hitting the bump, and you get the judder under braking. As you increase compression damping in the rear shock, It stops the bike running wide from mid-corner toward exit. (because it reduces rear squat), makes cornering steadier, gives you more confidence to use the throttle to turn the bike. Going too far, the ride becomes too harsh (you'd feel medium sized bump directly) and loose the traction and the feeling of control. Continue test ride& add compression damping until it shows aforementioned symptom of too much damping force. Now you know you have gone too far. Back off one or two click. Now your compression damping is set. Next thing is rebound damping.
 

What I did...

3) Rebound damping
The idea is, to tame the pitching motion (caused by weight transfer) , bounciness. In other words, to tighten up the chassis, eliminate the "floating feeling". This will give you a better, more stabilized platform. Set the rebound damping at FULL SOFT, test ride and return. Add one (forks) / two or three (rear shock: depends on the shock) click then test again. As you increase rebound damping in the forks. You get more traction "feel". Front end shoots up less when you release front brake. It stops shaking the head when hitting the bump(s). The bike wallows less. You have better feedback from the front end, makes handling more precise. Going too far, forks get packed (too much rebound damping force preventing the springs extending before the next bump arrives to compress it) Ride becomes harsh. And you start losing the feel of control. As you accelerate hard on bumpy road, front end feels vague , wiggle ,because front tire not staying contact with the ground.
As you increase rebound damping in the rear shock, It will reduce mushy, vague feeling. You feel getting more traction. The bike stops wallowing/waving. Stops "pogo" action. Stops pushing front end (running wide) when you opened the throttle coming out the corner. There's less pitching motion under deceleration, after the bump, and corner turn-in. The bike has better control. Going too far, rear shock gets packed. The ride becomes harsh. The bike becomes skittish. Rear end hops / skips under hard deceleration. Feeling of control is lost. Continue test ride& add more damping until the bike shows aforementioned symptoms. Now you know you have gone too far. Back of bit by bit until you get it balanced. By this time, you already know that front and rear are kind of tied together. Less front rebound allows weight transfer to the rear under acceleration, less rebound in the rear allows weight transfer to the front under deceleration. Keep fine tuning until you think it feels right. If you run out of adjustment range ,or the best setting is close to the end (one click off from max, for example) then you need internal modification. (re-valving.)

The most important difference between the stock bike and mine is *how it feels*. The stock bike would sink and lift and still be gently bouncing up&down several hundred feet past the bump, by which time more bumps would add their ripples to the pool, turning the road into an endless sequence of lurches. Traction and ground clearance varied as the bike rise and fall and the steering was best described as vague. In exchange of a lot of hard work, mine stopped all of this. As each bump moves the wheel, the suspension grabs it and, gently but firmly put it back again. Much better ride, much more confidence, the handling is quick and precise. It goes where I point it. Suspension work can take a long time to get it done right. You may get lost at one point . All because this is an self-educating process. But it pays. When you finally get it right, even the same old piece of road has now different meaning. If you love the corner, it's worth the trouble.

 

Truth, Myth, and FAQs. Here are some useful "semi-secrets"...
 

Steering geometry:
 

Q : People keep on telling me I'm gonna have to keep the same rake angle, or  drop the rear end to compensate front end drop. (They tell me I'd get "very twitchy" handling otherwise.) Is this true?

A: Exactly the opposite is true. A lot of people misunderstand this. When you fit shorter forks & a 17" wheel from a modern sport bike (thus dropping the front end), You will need to *RAISE* the back end. If you lower the back end (to compensate front end drop, mistakenly) what you get is an absolute dog to go around the corner, which is slow to turn in initially, then run wide toward the corner exit.

Q: Then, why people say that? What about rake angle? Less rake angle=less stability, right?

A: Common misunderstanding. A big rake angle does not in itself confer stability. Trail does this.   The relationship between rake, offset, and trail is Trail = [ tangent (rake angle) X tire radius] - [offset / cosine (rake angle)]
Here's an example.
 

Stock ZX750 E2:     rake 28 degree, trail 117 mm
Stock ZX-9R  B2:    rake 24 degree, trail  93  mm

ZX750E offset : 51mm
ZX-9R    offset : 39 mm

750turbo triple has much more offset than ZX-9R's. (Do you understand what this means? If you do, fine. If you don't , think again before starting the project like this.)

I set-up my bike (after 8 months of testing) at ZX-7R like 25 degree.

On my bike, this set up gives (tan 25 x 304)-(39 / cos 25)=98.7 , about 99mm trail the bike is perfectly stable and safe to ride anywhere. So long as you have 95-100mm trail , you are OK regardless rake angle. See figure below. (Incidentally, ZX-7&7R(93 onward) have rake/trail  25 degree/99mm)
 
 

Q: So what makes quick turn in then.

A: Quicker/slower  turn in is a matter of *rolling moment*. To make it quick, you will have to raise the back of the bike, NOT lowering the front end. (They may look like doing the same thing to the steering geometry but DO NOT have  the same effect on rolling moment. 'Cause one will lower the CG, the other will raise the CG.) Just raising the rider's sitting point or foot peg position can make it quick, too. (You get more leverage to initiate leaning , you know.) If the bike tends to run wide from middle of the corner to exit, it's a good sign of lack of rear ride height.
 

Q: What will happen if I just lower the front end (just fitting the new front end)?
 

A: You will reduce the amount of trail, which means reducing the steering's self-centering effect. Thus, you are losing stability. This does NOT make your steering quicker, it simply effects "yaw" , and remember, motorcycle's cornering process is a combination of "roll" +"yaw" (and sometimes + "pitch" ). Because of lowering the Centre of Gravity (CG), the bike will be slow to turn in . Then, because of not having enough rear ride height, the bike tends to run wide from mid-corner to exit. The more throttle you open the worse it gets.
 
Speedometer
 

Q: When I am switching to  a 17-inch wheel, do I need to recalibrate the clock?
 

 A: As long as you use speedo-drive from 17-inch wheel, you don't need to recalibrate your speedo. Typical Japanese motorcycle speedo shows 60MPH when cable is rotating at 1000RPM. ZX-9R speedo-drive with GPz cable and clock  work just as well as stock set up. If you want accurate speedo, you can use bicycle computer.
 
Changing wheel only
 

Q: Is it possible to change only wheel? I think it would be easier...
 

 A: Want to keep your stock forks? You can make 17-inch wheel from other bike to fit into stock GPz fork. (It was my original plan.) But actually it's more work than swapping the entire front end. Also you are more prone to screw-up steering geometry. (To set up front end properly, you will need much less offset than stock set-up.) Incidentally, if you buy a set of 17-inch wheel from Performance Machine, it'll cost you Front 1610-4xxx 17x3.5 $659 Rear  1666-4xxx 17x4.5(or 5.5) $780 ($800 for 5.5") Way too much money for me. Of course it's a lot easier option since they are made to fit. But to me this is an option for people who have more than enough money and less than enough imagination... (I got ZX-9R whole front and rear end for less than the price of a set of PM wheels. And I have USD fully adjustable forks, 4-pot calipers and floating discs, etc, etc.)
 

Swingarm
 

Q: I am thinking to fit a shorter swingarm. Short wheelbase helps cornering ,right?
 

A: If you reduce swingarm length, you will suffer cornering stability. The bike will not trace the line, and will be unstable throughout the corner. A lot of people misunderstand that longer wheelbase helps high-speed stability. It doesn't, gyroscopic-effect does it.  What it really does is to keep the front end planted.
 

Fitting a shorter arm AND 17-inch rear wheel creates another potential problem. To raise the rear ride height, you have to increase swingarm droop angle. With the shorter arm, this angle becomes greater. Too much swingarm droop angle induce tail slide. Also it makes the bike *jerky* with throttle on/off.
 

Tak (no longer active on the forum)