Service
SERVICE
Not-so-big bang
PAUL DEAN
Q I have a question about Yamaha's new Ri "big bang" engine. It still has four cylinders, just like the Ri did last year, and it even still has 998cc of displacement, so why would the 2009 model be any faster than the 2008? Does the newer bike's "crosspiane" crankshaft make that much difference in engine -performance?
Charles Irvin Dubuque, Iowa
A Actually, the ’09 Rl isn’t faster than the ’08, at least not in pure straight-line acceleration and top speed. The difference brought about by the Rl’s crankshaft design is not in the amount of power the engine produces but instead is in the way it produces it. A traditional inline-Four’s crankshaft is flat, with its outside pair of throws located 180 degrees from the inner pair, giving it evenly spaced firing intervals ( 180-180-180-180). And its single-plane construction requires all four pistons to come to a complete stop every 180 degrees, at both TDC and BDC, creating a considerable inertial loss. On the Rl’s crossplane crank, however, the throws are spaced at 90-degree intervals, resulting in a 270-180-90-180 firing sequence. This unevenness helps rear-tire traction by giving the tire one larger interval during each set of four firing pulses to better maintain grip; it also reduces inertial losses by requiring only two pistons to come to a stop during any TDC or BDC event. The end result is a marginally more efficient engine that provides improved off-corner acceleration as a result of better rear-tire grip under power. So, while the 0-to-60 times and quarter-mile numbers of the 2009 Rl might not be appreciably different than those of the ’08, the newer bike’s lap times on a roadrace course-as well as its point-to-point E.T.s on a twisty backroad-are likely to be better in the hands of a capable rider.
By the way, though the term “big bang” is often used by some people to describe the 2009 Rl’s engine, that is not a correct representation. The term dates back to the two-stroke GP racebikes of the late 1980s and early 1990s when four-cylinder two-stroke racebikes would fire two cylinders simultaneously, hence the “big bang” moniker. I believe it started with inline-Fours, such as the TZ750 Yamaha, later versions of which fired its cylinders in pairs at 180-degree intervals instead of separately every 90 degrees. The concept ultimately evolved to V-Fours that fired in pairs at anywhere between 67and 90-degree intervals, depending upon their Vee spread. This meant there was between 293 and 270 degrees of freewheeling, respectively, before the next firing sequence to help the rear tire maintain grip off of corners. Since no two cylinders fire simultaneously on the 2009 Rl, calling it a bigbang engine is incorrect.
Furthermore, the crossplane crankshaft is far from a new concept. If I’m not mistaken, all current automotive V-Eight engines have crossplane cranks, which date back to the 1920s when they were first used by Cadillac. The name derives from the fact that the throws are positioned 90 degrees apart, putting them in two “planes” that “cross” one another. In effect, the Rl ’s crankshaft is simply half of a V-Eight crank.
More “plane” truths
Q
Okay, guys, help me out here.
I’ve been having an argument with a co-worker who claims that because of its shape, a motorcycle tire is less likely to hydroplane than a car tire, but I disagree. A car tire generally has more ability to move standing water than most motorcycle tires, and a car tire has a contact patch that is some four times greater than what a motorcycle tire has. Am I correct or should I admit defeat?
Tony Bartholomew San Francisco, California
A
Don't throw yourself off the Gold en Gate Bridge, Tony, but you lose this one: A motorcycle tire is less likely to hydroplane than a car tire.
Think of this in the context of surfing, which is another form of hydroplaning but on a board instead of on tires. Do you think a surfer has a better chance of staying on top of the water if he rides the surfboard on its normal wide, flat surface or turns it up on its narrow, curved side? The answer is obvious: The surfer stays afloat quite easily on the wide surface but would tend to sink into the wa ter on the narrow one.
When riding/driving over water, the more you spread out the load, the greater the likelihood of hydroplaning. Car tires spread the load on four wide contact patches, each of which is the full width of the tire. Motorcycle tires, however, have just two contact patches, and each is only a fraction of the tire’s overall width. What’s more, because bike tires have a rounded profile, the pressure on the contact area is not equally distributed across the patch as on a car tire; instead, the center of the patch carries a disproportionate amount of the weight of that end of the bike.
What this all means is that a motorcycle tire is more able to “cut” through standing water than a car tire, thanks to its contact patches being narrower and fewer than those of a car tire relative to vehicle weight. Car tires do offer more area for water-channeling grooves that can raise the threshold of hydroplaning, but their large, flat contact patches still make them more susceptible to skating on standing water. The consequences of hydroplaning tend to be much more dire on a motorcycle than in a car, but the chances of it occurring are not as great.
Mentally challenged XT
Q I have an `86 Yamaha XT600 that recently was both bored and stroked 6mm. I know that the ignition needs to be retarded, but all the books I’ve read dance around the subject. And while I can’t find an adjustable ECU, I can move the pickup coil. There must be an equation or rule of thumb for timing motors like mine. Can you help?
Mark Albright Posted on America Online
A Not a whole lot, I’m afraid. Riders who make bore-and-stroke modifications usually set the timing according to recommendations provided by the manufacturers of the affected components. In the absence of such information, they generally resort to a process of trial-and-error to find the best setting. Sometimes, good information can be obtained from other riders who have performed the same mods.
My suggestion is to retain the stock timing and ride the bike to determine if the engine pings or exhibits any other symptoms of ignition timing that is too far advanced for the conditions. If no problems occur, leave the timing alone; but if symptoms do show themselves, reduce the timing in two-degree increments until they go away. I know that repeatedly moving the pickup coil can be a pain in the butt, but it may be the only way to give your XT the timing it wants.
The elastic wheelbase
Q Wheelbase is often referred to very precisely and as an important factor in handling, yet chain adjustment allows for a large variation in wheelbase. What’s up with that?
Jeff Haasch Jackson Hole, Wyoming
A Mighty fine question, Jeff. And the answer is that for all but the most skilled and hypersensitive riders on the street, the comparatively small amount of possible wheelbase variation is neither detectable nor significant. But good, experienced roadracers can tell the difference, which is why they usually monitor their bike’s wheelbase and will install a new chain when the existing one allows the axles to get too far apart. Racers and tuners also regularly fiddle with chain length when changing gearing to arrive at the desired wheelbase.
When we publish a wheelbase figure for our test bikes, we measure that dimension as the bike is delivered to us. The bikes and/or their chains usually are brand-new, so the wheelbases then are their shortest. We fully understand that axles get farther apart as chains wear; but unless we ride a bike long enough to wear out its chain (a process that, with proper chain maintenance, could take tens of thousands of miles and many months), we cannot accurately determine what the maximum reasonable dimension might be. Given that wheelbase variation likely is of very little consequence to 99 percent of all street riders, we provide the as-delivered dimension.
HP by the numbers
Q I read with interest your recent QuickRide on the Kawasaki Voyager 1700. I have an ’09 Vulcan Classic LT 1700. Kawasaki claims the same torque figure for the LT as the Voyager but does not publish a horsepower figure. I was curious to know what that number is, so I applied your formula for computing horsepower (hp = torque x rpm 5252) to the 108 foot-pounds of torque the LT supposedly delivers at 2250 rpm. I got 46.3 hp, which I know can’t be right. What did I do wrong?
Gene Brungardt Posted on America Online
A You assumed that peak horsepower arrives at the same rpm as peak torque, but that is almost never the case. With practically all motorcycle engines, peak power is produced at higher rpm, close to redline, while peak torque is reached at considerably lower rpm. Consequently, you cannot calculate peak hp using peak torque information.
Many people make the same mistake by presuming that an engine produces the most power at the point where it > makes the most torque. But that's not true. In actuality, an engine does not make horsepower; it makes torque. Horsepower is 1 8th-century inventor James Watt's method of measuring the rate of work an engine can perform over a period of time, and its amount varies according to the torque generated at any given rpm. In other words, it's simple math. So, applying the same formula (hp = torque x rpm ± 5252) you used, the most horsepower will be achieved when the multiplication of torque and rpm produces the largest num ber, which is then divided by the constant of 5252.
Let’s suppose that an engine makes 80 foot-pounds of peak torque at 5000 rpm; > that calculates to 76.2 hp. Then let’s assume the torque drops to 75 ft.-lb. at 5500 rpm; that works out to 78.5 hp.
Why more hp with less torque? Because percentage-wise, the increase in rpm ( 10%) was greater than the drop in torque (6.25%), which resulted in a larger number that was then divided by 5252.
As the engine continues to rev, horsepower output will escalate until the percentage of drop in torque is greater than the percentage of increase in rpm.
A good real-world example is the Yoshimura-modified Suzuki GSX-R1000 seen elsewhere in this issue. It made 78.2 ft.-lb. of torque at 10,400 rpm, which calculates to 154.8 hp. But its peak horsepower, 176.1, occurred at 12,700 rpm, where the engine was only making 72.8 ft.-lb. of torque. The torque output at 12,700 was 6.9 percent lower than at 10,400, but the rpm was 22-percent higher, resulting in more horsepower. Beyond 12,700 revs, the drop in torque was greater than the rise in rpm, so the engine then started making less horsepower.
So, how much horsepower does your Vulcan Classic LT make? I asked a few people at Kawasaki but none of them were either willing, or able, to give me an answer. So if you really need to know, I guess a trip to a local dynamometer is in order.
Got a mechanical or technical problem with your beloved ride? Can’t seem to 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 include enough information to permit a reasonable diagnosis. And please understand that due to the enormous volume of inquiries we receive, we cannot guarantee a reply to every question.
