Service

SERVICE

PAUL DEAN

No wiggle room

Q I love my 2007 Suzuki Bandit 1250, but it does one thing that I do not like. If I take my hands off the handlebars, the front end will start to wiggle back and forth, and if I don’t grab the bars right away, the wiggling gets worse. I don’t know if this is dangerous, but it scares me. I previously had a Honda 599, and it did not act this way. Is this wiggling normal for a 1250 Bandit? Tina Sherman Overland Park, Kansas

I can’t tell you with certainty that such behavior is “normal” for a 1250 Bandit, but it does occur on many different motorcycles and under a wide range of conditions. Some bikes do it consistently, regardless of the circumstances. Many others do it only after the front tire wears a bit, while some only act this way when fitted with certain front tires. Bikes that have only a single front disc brake often do it because the mass of their front ends is not equally distributed about the steering axis, while many other bikes never do it at

all. Other factors—road surface conditions, steering-head bearing adjustment, swingarm-pivot freeplay and tire pressure, just to name a few, can also cause hands-off front-end waggling.

This behavior usually is triggered by the smallest of road-surface irregularities that hit the front tire’s contact patch a little off-center, just enough so to divert the front wheel a fraction of a degree from its straight-ahead position. Normally, when you’re holding onto the grips, your arms serve as dampers that absorb such movements. But when you release the grips altogether, those little deflections can quickly grow into the kind of wiggles you describe. The selfcentering effect of front-wheel trail tries to return the wheel to the straight-ahead position; but between the considerable inertia of the entire front end (fork assembly, wheel, rotors, calipers, handlebars, hand controls, etc.), along with the elasticity of the tire around the contact patch and the energy infused into the process by the speed of the motorcycle, the trail’s attempt to straighten the wheel causes the front end to go past

center in the opposite direction. This, in turn, causes trail to induce an even more forceful attempt to re-center the wheel, but those same forces swing it farther past center than before. This process of over-correction continues with everincreasing amplitude until it becomes a full-blown front-end wiggle.

I’ve never tried to rectify a hands-off wiggling condition on a 1250 Bandit, so I cannot offer any conclusive suggestions other than for you to assure that all of the adjustment and maintenance factors I mentioned earlier (tire pressure, steering-head bearing adjustments, etc.) are in proper working order. If all those factors pass muster, the only other suggestion I can make sounds like a bad Henny Youngman one-liner:

Tina: “It wiggles when I do this.” Paul: “Don’t do that.”

Under the gyro-scope

Ql’ve got a question about motorcycle steering. I think I have a decent understanding of rake and trail, and I know that countersteering is what makes a bike turn. But every so often, I read or hear someone mention the “gyroscopic precession” of the front wheel, and that confuses me. Could you please explain what this term means and the effect, if any, it has on the way a motorcycle steers? Carl Fletcher Marietta, Georgia

A Great question, but my answer is one that will prompt a number of engineers out there to write and complain that I did not explain the full context of the matter. But precession is a complex subject and I only have so much room to work with here, so I’ll have to keep it simple and endure the wrath of those who believe that my response came up short.

Gyroscopic precession is a term that describes the way in which an object, usually a disc of some sort spinning on an axis (a motorcycle or bicycle wheel, a flywheel, a helicopter rotor blade, etc.), reacts to changes in the plane of that axis. The basic rule of precession states that when an outside force changes the plane of the axis, the resultant reaction will take place 90 degrees of rotation later.

You can witness this reaction by spinning a bicycle wheel while holding its axle (the axis) out in front of you in both hands, parallel to both the ground and—if you’re in a room or a garage— the wall directly ahead of you; and we’ll assume that the wheel is spinning clockwise when viewed from the right, just as it w ould on the bicycle. If you move the axle forward, backward, up, down or in any direction that keeps its axis in the same plane (parallel to the ground and the wall), you feel no resistance and the spinning wheel remains vertical. But if you tilt the axle by, say, moving its right end upward, you would think that the top of the spinning wheel w ould just lean to the left; but it doesn’t want to do that. Instead, as you try to raise the right side of the axle, you feel a resistance while the front of the wheel—which is 90 degrees of rotation later—turns to the left. You get the same sort of response no matter how or in which direction you alter the plane of the axle: The reaction takes place 90 degrees of rotation from where the input occurred.

Anyone who has played with a toy gyroscope has seen this phenomenon in action. It’s a common trick to balance one end of a spinning gyroscope’s axis on a taut string and then watch what happens. The gyroscope will try to fall ■■■■■■■pppwwippiii in one direction or another, but as soon as it starts to tilt (which changes the plane of its axis), it swivels 90 degrees in the direction of rotation. But that swiveling is another axis-changing motion that is answered 90 degrees later, as is the next, and the next, and so on. As a result, the gyroscope wobbles around in a circle on the string but never falls off until it stops spinning.

The resistance you felt when you changed the plane of the spinning bicycle wheel is what causes the wheels of a motorcycle to act as gyroscopes that give the bike much of its stability. But when the rider moves the handlebars to, say, the left, that action tries to steer the front part of the front w heel to the

left. Because of precession, however, the reaction to that input takes place 90 degrees of wheel rotation later, down near the bottom of the front wheel. But the wheel is tightly clamped to the fork, and the fork is tightly attached to the frame, meaning that the wheel can’t just lean on its own, independent of the rest of the motorcycle. So, as the bottom of the w heel tries to lean to the left, precession also causes the rest of the bike to w ant to lean to the right. Given the masses involved, this reaction is rather slight, but whether turning left or right, it works in harmony with countersteering to help the bike lean in the desired direction.

As I implied earlier, there’s a lot more to precession than this, but I hope my relatively brief explanation has cleared up some of your misunderstanding of the subject.

Candid Cameron

Valentino Rossi was involved in a vicious highside at Mugello this past June, and it appeared to me that the traction control system on his Yamaha failed to interfere in time to prevent the accident. Do you have any theories regarding why this happened? I thought traction control was supposed to prevent this type of incident. Mark Lancaster Atwater, California

A A traction control system is no less fallible than are ABS systems, which only recently have been able to equal in stopping distance what an expert rider can accomplish. Traction control, again like ABS, works by a series of mistakes. The rider applies throttle until rear tire slippage exceeds a set value, whereupon torque is reduced slightly by ignition retard or fuel cuts, then restored. This is inefficient for the same reason that trying to control wheelies or wheelspin with the throttle in drag racing is inefficient: You lose time at each throttle reduction.

Therefore, more recent systems attempt to forecast the actual throttle-angle profile for each corner, based upon the many measurements of track friction coefficient made by the torque-control system. Each such “measurement” (actually, each cycle of traction-control operation) is then paired with its GPS position and stored in memory. Later, these data are connected in software to create a predicted throttle-angle profile. I believe that in 2007, Ducati had such a system while Yamaha was still relying on simple traction control for most of that season. In 2008, both companies had such systems, but Honda apparently did not (Honda at the end of last year hired Andrea Zugna, Yamaha’s software writer who had created its system).

As you can see, then, this has been a process of working ever-closer to the limit. That’s a dangerous place! Also, as Jorge Lorenzo has observed, “If you go into the corner too fast, traction control cannot save

you."

Kevin Cameron

Gone fishing

QWhen changing the oil recently on my 2007 Kawasaki Vulcan 900,1 dropped the oil plug into the drain pan. I fished the plug out of the pan and reinstalled it, but when I emptied the pan a few days later, I found the aluminum washer that usually is on the plug. This bothered me so much that I ended up draining the oil that had only 800 miles on it just to replace that washer. Was I justified in doing so? How important is that aluminum washer? Is it for thermal contraction and expansion or for improved sealing or did I not need to worry about it at all? Marcel Thomas Hobart, Indiana

A If you tightened the plug properly, chances are that nothing terrible would have happened. The plug probably would not have fallen out and oil probably wouldn’t have leaked. But is “probably” good enough for you? It’s certainly not for me. Isn’t the assurance that you won’t literally hit the road in your own spilled oil or wind up with an expensive engine rebuild—or both—worth the cost of a few quarts of oil and a little bit of labor on your part? The washer is made of soft aluminum to serve as a reusable gasket between the plug and the engine case; and since aluminum has a greater expansion rate than steel, the washer “grows” in thickness slightly more than the plug expands as the engine heats up, helping to keep the plug tight. So, congratulate yourself: You did the right thing. □

Recall Roster NHTSA Recall No. 10V303000 Manufacturer: Kawasaki Motors Corp., USA Model: 2010 Z1000 Number of units involved: 1161 Problem: Under severe riding conditions on certain of these motorcycles, the leftfront brake hose may contact the brake rotor during extremely heavy front-brake application. This could cause brake fluid to leak, creating the potential for a crash. Remedy: Dealers will inspect the front brake hose and replace it if required. Additionally, the brake hose position will be adjusted to prevent contact with the rotor. Owners not receiving this free remedy can call Kawasaki at 866/802-9381.

Got a mechanical or technical problem and can’t find workable solutions in your area? Or are you eager to learn about a certain aspect of motorcycle design and technology? Maybe we can help. If you think we can, either: 1) Mail a written inquiry, along with your full name, address and phone number, to Cycle World Service, 1499 Monrovia Ave., Newport Beach, CA 92663; 2) fax it to Paul Dean at 949/631-0651; 3) e-mail it to cw1Dean@ aol.com; or 4) log onto www.cycleworld. com, click on the “Contact Us” button, select “CW Service” and enter your question. Don’t write a 10-page essay, but if you’re looking for help in solving a problem, do provide enough information to permit a reasonable diagnosis. Include your name if you submit the question electronically. And please understand that due to the enormous volume of inquiries we receive, we cannot guarantee a reply to every question.