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"Motorcycle Cruiser", October 1, 2012

JACK, SQUAT AND PULL

BY MARK ZIMMERMAN

One of the problems associated with cruisers is their predilection for soft suspensions. The OEMs like to fit cruisers with supple springs and light damping primarily because it makes for a comfortable ride, and since most cruiser owners aren’t concerned with razor-sharp handling, any shortcomings in that department are generally overlooked or simply cured with aftermarket springs and shocks.

The problem with soft suspension, outside of obvious things like willowy handling and reduced ground clearance, is that the bike changes attitude during braking and acceleration, a phenomenon that can be exacerbated by certain inherent design characteristics — in particular, the interaction of the final drive and the swing arm.

Although it may seem counterintuitive, whenever we apply power to the rear wheel, the pull of the chain or belt (they’re one and the same for the purpose of this discussion) tries to drive the rear wheel under the motorcycle. This extends the swing arm, causing the rear of the bike to rise. Allow me to point out that shaft-driven bikes are subject to the same problem; in fact, because the pinion tries to climb the ring gear whenever power is applied, it’s quite exaggerated compared to your average chain-driven bike, which is why some manufacturers, in particular BMW and Moto Guzzi, have come up with some pretty fancy rear drive/swing arm linkage systems to counteract the effect.

This action, commonly referred to as “jacking,” occurs because the drive chain normally works at some slight angle to the swing arm. This produces a force that causes the swing arm to move upward at its pivot point, or downward at the rear axle if you’d prefer, which in turns lifts the rear of the bike.

If you doubt this happens, you can prove it anytime you let out the clutch at a stop, especially if you hold the front brake on. Granted some bikes “jack” worse than others; the location of the swing arm pivot in relation to the countershaft sprocket, the spring rate and even the swing arm length and final drive ratio all play a role in how hard the bike jacks, but to some degree it’s always present. Stiff springs and well thought-out frame geometry reduce a bike’s tendency to jack, and at one time there were some clever aftermarket chain-tensioning devices claiming to minimize the effect by ensuring that the chain stayed parallel to the swing arm.

Let’s hold those thoughts for a moment and discuss “squat.” Whenever a motorcycle accelerates, its weight shifts rearward; it squats down so to speak, and this can become a problem whenever the bike accelerates out of a turn, particularly if the motorcycle is powerful and its rider bold.

What happens is this: As a rider winds the throttle the bike accelerates forward, transferring a portion of its weight toward the rear of the motorcycle. This causes it to squat at the rear wheel, which takes weight off the front tire. As the tire unloads, grip is reduced, so the front end begins to slide or “push” toward the outside of the turn. We respond by either increasing the steering angle, backing off on the throttle or, if things go too far, by running off the road.

Besides minimizing jacking, stiff springs also reduce squat and push, but unfortunately can make the bike less comfortable to ride and may create other problems. For instance, because overly stiff springs can prevent the suspension from compressing over bumps, the tires may literally hop off the ground, compromising control and making it difficult to hold a line through turns. Granted that’s an extreme situation, but it happens on a regular basis. So outside of riding slowly through the fun parts, what can be done to counteract the unpleasant side effects of squat?

Well, in many instances, we can use the swing arm’s tendency to extend under power to neutralize it. As the bike starts to run wide, feeding in a little more power will cause the swing arm to extend, raising the rear of the bike. This transfers weight back onto the front wheel, traction is recovered, and the bike turns more sharply.

Yes, you can overdo it. Overloading the front end, whether by applying too much throttle, providing too much steering input or by misjudging the amount of available traction, is just as bad as not loading it enough, but ironically the easiest way to regain traction when the front is overloaded is also to apply some throttle. In this case, the object is to transfer weight to the rear of the bike, which relieves some of the load on the front tire and allows it to find some grip. Admittedly, the maneuver takes a fine hand and a finer sense of timing but with practice, anyone can learn.

Motorcycles, like everything else in the universe, are subject to laws of physics. Obey them and you’ll never get into trouble.

 

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