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SERVICE
Suspension nomenclature
Would you please do me the favor of explaining the meaning behind the terms preload, rebound, damping and any other such word having to do with the suspension of my motorcycle? And how do they all work together?
I own a 1986 VF500F Interceptor, and in almost every road test done on my bike, the attitude toward the suspension seems to be the same: The bike is undersprung and underdamped. How can I correct this, and what modifications would be most cost-effective?
Chris Kleffman Simi Valley, California
In the beginning, there was the bump . . . and hitting one of them on a motorcycle tied rigidly to its wheels was painful. Not only would the wheel be forced upward, but so would the rest of the bike, and you. Jarring, to say the least.
So between wheel and motorcycle chassis are placed springs. A wheel now can be forced upward by a bump, but only a fraction of this movement is transmitted through the spring to the rest of the motorcycle. The give, or compliance, of the spring pro vides a softer ride.
But springs by themselves aren7 enough; with only springs, your motorcycle would hit a bump, and then bounce up and down until friction at the swingarm pivot and elsewhere in the suspension system slowly absorbed, or damped, the bouncing.
Since bouncing for several minutes after hitting a bump isn 7 desirable, suspension designers add a controlled amount of resistance to movement in the suspension system. Because it serves to damp-out bouncing, this resistance is referred to as “damping. ” In practice, damping is usually obtained from pumping oil through small holes inside a shock absorber.
The damping, or resistance to movement, that shock absorbers offer depends on several things. It may vary depending on whether the shock is being compressed (compression damp-
ing, as when a wheel is being forced upward by a bump), or extended (rebound damping, as when a wheel is rebounding back down after hitting a bump). It may also depend on the speed of the movement; generally, shock absorbers offer more resistance as the movement speeds up.
Motorcycle handling is dependent on damping being matched to springing. Too little rebound damping, and a motorcycle will bounce too much after a bump as the springs overpower the shocks. Too much rebound damping and the wheels won 7 be able to move fast enough to stay in contact with the ground; now it's the shocks that overpower the springs. Too much compression damping, and a shock will simply not respond to a bump: the ride will be stiff, just as if the springs were too hard. Too little compression damping, and a wheel may be bounced into the air and leave the ground; in this case the bump is overpowering both the shock and the spring. Hopefully, though, a motorcycle manufacturer will have damping set about right for most machines. If not, a different shock may be required.
Spring preload is another factor affecting suspension performance. Essentially, it's a way of setting motorcycle ride-height by putting an initial load into a spring before installing it on a bike. The more the spring is collapsed when installed on a shock, the less it will have to collapse to carry the weight of the bike. With adjustable preload, it's possible to vary the initial collapse so ride height can be kept
constant while rider weight differs.
Finally, a VF500F isn't undersprung or underdamped for most street riding. Onl y on a racetrack might it really need stiffer springs and a higher quality, more precisely damped shock; in that case, we'd suggest contacting an aftermarket shock company, such as Fox or Works Performance, and getting specific recommendations from them.
GSX-R1100 tires
I am the proud owner of a GSXR l 100 Suzuki. This machine is everything they say it is, but its tires are not. It came with Dunlop radiais, which I think are an incredible tire at speed, but I’m not satisfied with their performance during slower, everyday riding. They just don’t feel as stable or surefooted.
I would like to know if the rims are designed specifically for a radial tire or if I can mount a set of Michelin Hi-Sports or another type of bias-ply tire.
P. Watson
Kelowna, B.C., Canada
GSX-R 1100 wheels use the industry-standard MT-profile rim, and Hi-Sports or any other tubeless biasply tires can befitted. (Hi-Sports may be the most common tire used in GSXR Cup series road races.) But that may not solve low-speed steering problems on the GSX-R 1100. On our test bike, we traced low-speed steering imprecision to an overly stiff steering damper. Removing the damper while keeping the Dunlop radiais solved the problem completely.
Car coils on a Four
A couple of weeks ago I came up with an idea that I thought for sure would work on my Kawasaki KZ650SR. Through my employer, I purchased two 40,000-volt Mallory automotive-type coils that use external ballast resistors. Because bike coils fire two cylinders at a time, I made a Y-type connector that would plug into each coil at one end and hold two 8mm wires at the other end. I clamped the coils underneath the frame rails just under the gas tank. I then mounted the ballast resistors underneath the old coil brackets and wired accordingly.
My problem is that when I tried to start the engine, all it could manage to do is to backfire through the exhaust. My attempt to bypass the resistor proved fruitless also. What am I overlooking? Am I wrong in assuming that all standard ignition systems basically operate on the same principle? Please enlighten me.
Scott E. Parmer
Sandusky, Ohio
Your Y-terminated car coils and two-lead bike coils don 7 operate on the same principle. One of the leads on the bike coil is a ground, and both plug gaps must ionize and fire to complete a circuit. On your set-up, as soon as either plug fires, all the energy stored in the coil will dump through that plug; the other one won't fire. In other words, the plugs attached to the bike coil are wired in series, and with your setup they're in parallel.
The simplest fix is to re-install your original coils, or to purchase heavyduty dual-lead bike coils, such as those offered by Accel.
