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Weight watcher
Paul Dean
I have a question that has probably been asked before, but here goes anyway: How do you get the dry weights you publish for your testbikes? You say that Honda’s new CBR954RR weighs 406 pounds while Honda states that its weight is 370. You also show the 2002 ZX-9R at 440 pounds and Kawasaki has it at 403. What gives? Joey Prado San Antonio, Texas
We employ a clandestine, highly complex means of calculating the weights of all our testbikes: We weigh them. On a scale. A certified one, even. But how the manufacturers arrive at the absurdly inaccurate weight figures they publish is one of the great mysteries of our time. All I know for sure is that the claimed weights are always much less than the actual weights. During my 29 years in the motorcycle-magazine business, I don’t recall ever scaling a bike that weighed as little as its maker claimed.
I did have a representative of a Japanese manufacturer once tell me that although he wasn’t 100 percent certain, he thought his company published the shipping weights of its motorcycles-that is, as the bikes were rolled into their shipping crates. In that state, they have no engine oil, no engine coolant (valid only on liquid-cooled models, of course) and no battery installed.
That makes a little more sense, but it still doesn’t quite wash. If you take the manufacturer’s claimed weight of most bikes, add in the weight of a battery (about 10 pounds), approximately 4 quarts of oil (around 8 pounds) and a couple of quarts of coolant (maybe
another 4 pounds) when applicable, you still don’t account for the 25-to40-pound weight discrepancies that are so common. Chances are that we ’ll find Jimmy Hoffa ’s corpse or discover life in another galaxy before we learn how bike-makers come up with their weight claims.
Inertial guidance
I have a question about flywheel weight. People I’ve talked to say that heavier flywheels allow a bike to go faster, since it won’t have wheelspin and will accelerate faster. But it seems to me that with heavier flywheels, the engine would tend to bog down so that when you open the throttle, the rpm builds more slowly, which is why the wheel doesn’t spin as easily-kind of like being in too high of a gear. I’d like to know the real deal with flywheels. Do the Pros in Supercross, motocross and Superbike racing use heavier flywheels? Ryan Parsons
Cooper City, Florida
There ’s an element of truth to what those “people ” have told you, but there ’s much more to flywheels than that. A simple way to think about flywheels is that they steal some of the energy generated by each combustion stroke, then return that energy as the crankshaft rotates on to the next power stroke. The energy stored in the flywheels after each combustion gives the crankshaft some of the inertia it needs to overcome the effects of compression, valve-spring tension and other sources of engine friction, especially at lower rpm. The greater the flywheel inertia, the more power is stolen on each power stroke and then returned before the next one.
Another way of thinking about flywheels is that they absorb some power on acceleration and give it back on deceleration. Most of today’s high-performance streetbikes (and their racing counterparts) have very> small amounts of flywheel inertia because traction usually isn’t a problem, and the engineers hope to extract as much acceleration as is reasonable from the engine. On bikes that are less performance-intensive, such as cruisers and tourers, the designers tend to use more flywheel inertia to smooth out the engine and make it less sensitive to changes in throttle opening.
Traction is not always optimum, of course, especially off-road, where additional flywheel mass can be beneficial. In conditions where wheelspin can be a major factor, having more flywheel inertia can help the rear wheel maintain better traction by preventing the tire from spinning so easily. It may cause the engine to rev more slowly, but the improved traction allows the motorcycle to accelerate more quickly and predictably, and that ’s what counts.
As a rule, the fewer cylinders an engine has, the more flywheel inertia it needs. On a Single, the crankshaft has to rotate 720 degrees between power strokes, so it requires considerable flywheel inertia to keep the crank turning between those big bangs. A Four ’s crank, however, only has to rotate 180 degrees between firings, and a Six’s just 120 degrees, so they require proportionately less flywheel effect.
You may have noticed that I’ve used terms such as flywheel “mass,” “inertia” and “effect” instead of “weight.” That’s because, in itself, a crankshaft’s weight is not the critical element; what’s important is where that weight is located relative to the axis of the crank’s main-bearing journals. The farther from that axis the weight is located, the more inertia it provides. If you had two cranks of identical weight, one with its mass evenly distributed and the other with its mass concentrated around the outer circumference, the latter crank would have much more flywheel inertia than the former.
When developing an engine, the designers try to determine the ideal amount offlywheel for the bike ’s intended use. Street riders rarely change or alter flywheels unless installing a stroker crank, but for racing, adding or subtracting flywheel effect can be a useful tuning technique. I honestly don’t know how many of the top Pros ride bikes with altered flywheels, and their tuners probably wouldn ’t tell me if I asked.
Recall Roster
NHTSA Recall No. 02V003 Buell X1, S3, M2&M2L Model year: 2002 Number of units involved: 387 Dates of Manufacture: September to November, 2001 Problem: These models were built with front shock mounts that crack and could break, allowing the suspension to drop unexpectedly. This could cause the rider to lose control of the motorcycle. Remedy: Dealers will replace the broken shock mount. Owners who do not receive the free remedy within a reasonable time should contact Buell at 414/342-4680.
Pipe dreams
I want to install a different exhaust system on my 2001 Yamaha YZF-R6 and would like to know which one works the best. Getting a good sound is important, but what I want most is a boost in performance all through the rev range. Fd like something that would give my R6 a little more zip at cruising rpm, but I don’t want to kill any top-end power. These systems aren’t cheap, so I’d like to know which one will do the best job before I buy it. Aaron Silberman
Posted on America Online
I’ve lost count of the number of readers who have asked me which aftermarket exhaust system is best for their particular motorcycle. Unfortunately, I have to give the vast majority of them the same answer: I don’t know. Unless I or my colleagues here at the magazine have tested the exhaust system in question on the make, model and year of motorcycle in question, we have no hard data to tell us how well that particular combination performs. And we won’t reiterate the pipe manufacturer’s claims or pass on unsubstantiated reports gleaned from a friend of a friend of a riding buddy.
What’s more, I won’t give you an appraisal of any exhaust system based on that same system s performance on another motorcycle. If for example, we got excellent test results with an XYZ exhaust system on a 2000 Yamaha Rl, that does not mean an XYZ system of the same type will work just as well on a 2001 R6, a 2002 Kawasaki ZX-6R, a 1999 GSX-R750 or any other make and model. Although a lot of bikes in any category share the same basic type of engine design, there are too many critical tuning differences between them to guarantee that an exhaust system s effect on any one of them translates directly to its effect on the others. I’ve seen instances in which a particular aftermarket exhaust slightly improved performance on one manufacturer’s 600, but did very little on that same company’s 1000 and actually killed a little performance on its 750. So, as much as I’d like to help you, Aaron, I have no credible way of doing so.
The big squeeze
I have noted that most motorcycle manufacturers specify the use of regular gasoline in their engines, even though the compression ratios are high enough to warrant premium. I have a 1990 Honda Transalp and a 2000 Kawasaki Concours, and I put premium in both, even though the manuals specify 87 octane. What gives?
Joel Widman Posted on www.cycleworld.com
A common misperception is that high-octane gasoline contains more potential energy than low-octane gasoline, and that as such, the use of high octane in high-performance engines produces more power. That ’s not true. The primary difference between highand low-octane fuel is that high-octane can withstand a higher degree of compression without self-igniting. If compression causes the fuel mixture to ignite before the sparkplug has a chance to do so, that creates a condition called “pre-ignition,” which causes a reduction in performance and possible engine damage.
When the fuel mixture is ignited-either by the sparkplug or by compression-it burns in a “flame front” that moves rapidly away from the point of ignition. When the mixture is ignited by both pre-ignition and the sparkplug, two flame fronts are created that work against one another. In a best-case scenario, the conflicting flame fronts cause a loss of combustion efficiency and power. In a worst-case scenario, the flame fronts collide with such force that the impact is heard as a pinging noise called “detonation.” If allowed to continue, detonation can cause severe damage to the exhaust valves, piston crowns and piston rings.
None of this has anything to do with the energy in the gasoline. If an engine does not suffer pre-ignition or detonation on regular fuel (a matter that the manufacturers determine during their development of the engine), regardless of its compression ratio, there is no need for it to burn high-octane gasoline. It will perform no better on the high-priced fuel than it will on the cheaper stuff.
Mass confusion
According to the stories I’ve been reading about the new YZF-R1, Yamaha apparently raised the engine about 18mm in the frame. Cycle World used the term “mass centralization” as the rationale behind this change. Could you please elaborate? I’ve always thought a low center of gravity was the holy grail of motorcycle design. How did Yamaha manage to change the laws of physics and make the handling quicker by raising the center of gravity? Baxter Wood
Posted on America Online
Yamaha’s engineers didn’t change the laws of physics with the new Rl; they merely abided by them. Remember that demonstration in high-school physics class that showed you can swing a broom back-and-forth more easily and quickly if you hold it close to the bristles rather than at the very end of the handle? The broom has a greater “polar moment” when the pivot point is at the end of the handle; grabbing it closer to the bristles moves the pivot point nearer the greatest mass, reducing the polar moment. It doesn’t matter if you move the mass closer to the pivot point or the pivot point closer to the mass; both have the same effect, which is to centralize the mass.
Motorcycles have a pivot point, called the “roll center,” which is the longitudinal (front-to-rear) axis along which the bike rotates when leaning into a turn. That axis is not located where the tires make contact with the road; it’s considerably higher, somewhere closer to the middle of the motorcycle/rider package, depending upon the physical attributes of the bike and rider. When the bike banks over to, say, the left, the part of it that is above the roll center tilts over to the left while the part below it swings out to the right. As a result, any mass that is located below the roll center has essentially the same inertial effect-or polar moment-as an equal mass located above it. Thus, the designers try to lower anything that is above the roll center to get it closer to that axis, and raise any significant mass located below the roll center for the same reason.
This principle also applies in the transverse (side-to-side) plane. The closer the outermost masses are tucked in toward the roll center, the easier the bike will bank over into a turn. That’s why, years ago, designers of four-cylinder sportbike engines began moving the components that once were located on the ends of the crankshaft (alternators, starter mechanisms, etc.) somewhere else, usually behind the cylinders where they wouldn’t make the engine as wide, benefiting both cornering clearance and polar moment.
Send ’em packing
I have a Vance & Hines exhaust system that needs to be repacked. Is there a special exhaust-system material I should use or should I just do it with some regular fiberglass packing (like the kind used by boat makers and auto-body shops)?
dskettles
Posted on America Online
There is indeed a special packing material intended just for this task. It is fiberglass, but it ’s purpose-built with a special weave that helps it stay together under the extreme heat and high-pressure shock waves it must endure in a silencer. Regular fiberglass packing tends to wad up very’ quickly and then come apart in chunks that soon get blown out the exhaust. Most muffler manufacturers use some form of this specialized packing, so it doesn 't necessarily have to come from Vance & Hines.
Brake dancing
What is trail-braking? I read about it quite often in magazines, including yours, but I’m not sure if it is rear braking, front braking or what. I do understand that it has something to do with braking into a corner, but what is the technique and is it something I can use on the street, like when I exit off a freeway onto a circular exit ramp? Mike O’Connell
Posted on www.cycleworld.com
Trail-braking is indeed a technique used when entering a corner. But it might be easier to comprehend if you first understand how motorcycles used to enter corners in the bad old days. Back then—when tires were skinny and hard, and chassis were flexi-flyers-the accepted go-fast technique for entering a corner was to scrub off as much speed as possible by braking hard while the bike was still perfectly upright, then letting off the brake altogether just as the bike was flicked over into the corner. If done any other way, the tires were very; likely to lose grip and the chassis deform enough that the rider could easily lose control. But as tires began to be made much wider and considerably stickier, and as even streetbike chassis became capable of formerly GP-caliber cornering forces, riders found they could continue front-wheel braking as the bike was banked over into a turn if they eased off the lever just enough to prevent the tire from losing grip-trail-braking, in other words. That meant they could start braking later, bank over into the turn at a higher speed and still manage to scrub off any excess cornering speed by trail-braking as they approached the apex. As the tires and chassis continued to improve, riders found they could trail-brake deeper and deeper into the turns.
Whether or not this is an acceptable technique for street riding is really up to the individual. Unlike being on a racetrack, a rider on the street can't always be sure of the road conditions entering a corner; besides, not all riders have the skill and feel to effectively trail-brake without dumping themselves on their heads. Like so many other aspects of motorcycle riding, trail-braking is an excellent technique, but only if done properly. If you think y oud like to learn how to trail-brake, I suggest you do so under controlled conditions, such as during a track day on a roadrace course, and sneak up on the technique gradually rather than attacking it all at once. □
Got a mechanical or technical problem with your beloved ride? Can’t seem to find reasonable solutions in your area? Maybe we can help. If you think we can, either: 1) Mail your 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 and click on the Feedback button. Please, always include your name, city and state of residence. Don’t write a 10page essay, but do include enough information about the problem to permit a rational diagnosis. And please understand that due to the enormous volume of inquiries we receive, we can’t guarantee a reply to every question.
