Service

SERVICE

PAUL DEAN

The dynamics of aero

Q In Cycle World and some other magazines, I occasionally see an article or an item commenting on the poor aerodynamic qualities of motorcycles compared to cars, but I don’t recall ever reading anything that puts those attributes into meaningful perspective. The manufacturers often claim they have designed their latest models in wind tunnels, but to the naked eye, there doesn’t seem to be a major difference between those new bikes and the previous ones. Are aerodynamics really a factor in a motorcycle’s performance or is it just another reason for ad agencies to spout hyperbole? Carl A. Sandifer Grand Prairie, Texas

A Aerodynamics certainly do have an effect on performance, but the bike has to be traveling well up into the triple-digit range for that effect to have much significance. At legal speeds, aerodynamic drag may cost a mile-pergallon or so in fuel mileage, depending upon the motorcycle in question, and the bike no doubt could go a couple of mph faster at any given throttle setting if it were less aerodynamically “dirty.” But the differences in top speed can be huge.

Here’s a true story that offers a wonderful example of those differences. In the mid-1980s, a gentleman (try as I might, I can’t remember his name) who competed in sanctioned speed trials on dry-lake beds decided to convert a wingtip fuel tank from a Lockheed F80

jet fighter into a top-speed four-wheeler powered by a motorcycle engine. After scouring the local salvage yards, he ended up with an engine from a wrecked 1983 VF750F, Honda’s first Interceptor, and simply dropped it into his mini-streamliner. In magazine tests of that period, the ’83 Interceptor reached measured top speeds in the 136to 138mph range; but at a meet at El Mirage Dry Lake sanctioned by the renowned Southern California Timing Association, this aerodynamic little VF750F-powered car went a whopping 177 mph.

Sure, but how stock was the engine, you ask? This fellow knew very little about motorcycles, so aside from changing the oil, he performed no maintenance whatsoever on the 750cc V-Four. He didn’t

adjust the valves, didn’t check compression, didn’t even remove the sparkplugs, and God knows what exhaust system he kluged together. So as the result of vastly improved aerodynamics (and the higher final gearing it allowed), a bone-stock engine in less-than-optimum condition was able to go 40 mph faster than it could in perfect tune in a brand-new motorcycle. That’s how important aerodynamics can be.

Fuel's gold

Q I have been thinking about getting a Power Commander to try to squeeze some extra fuel mileage out of my bike, but I wonder if it’s worth it.

I have a 2004 Yamaha YZF-R6 with a Two Brothers pipe, and right now I’m getting about 30 mpg. Do you think I’ll be able to get any better mileage if I install a PC? Jason Moore

Posted on www.cycleworld.com

A Probably not. Getting better mileage would undoubtedly involve leaning out the fuel mixture, and on modem stock motorcycles, the mixtures already are very lean to allow the engines to meet federal emissions standards. Leaning your R6’s mixture

any farther would run the risk of caus-

ing some type of engine damage-holed pistons, seized pistons, burned valves, fun stuff like that. Even if the outcome were not so dire, a leaner mixture would likely have a noticeably adverse effect on performance.

Although a Power Commander allows you to make the mixture on fuelinjected engines either richer or leaner at any or all rpm and throttle openings, it was developed primarily as a means of adding fuel. In some instances, the extra fuel was deemed necessary to compensate for an engine’s performance modifications; in others, it was to improve power and throttle response by adjusting the emissions-lean mixtures of stock engines; but in any case, mileage was never a significant issue.

Actually, some bikes come stock with marginally rich full-throttle mixtures at or near peak power rpm. Manufacturers occasionally do this for two reasons: 1) The leanness at other rpm and throttle openings tends to generate high engine operating temperatures, and

the engineers don’t want an already-hot engine to overheat when it’s working its hardest; and 2) the Driving Cycle used as a standard for EPA emissions certification does not require most middleweight-and-larger motorcycle engines ever to reach full throttle and peak rpm. The manufacturers therefore often use full-throttle mixtures that are a bit on the rich side to help keep engine temps down while maximum power is being generated. The point here is that you may be able to lean out your R6’s fullthrottle mixture by a few percentage points, but you’ll then be in a Catch-22 situation: If your goal is to improve fuel mileage, you won’t succeed if you spend much time with the twistgrip wide-open; and if you never turn the throttle wide-open, you’ll never benefit from the leaner mixture.

Truth is, if you were to use a PC to improve your engine’s performanceand performance seems to be one of your criteria, since you already have swapped the stock exhaust for a freerbreathing aftermarket replacement-the mileage might worsen. Most aftermarket pipes exaggerate any lean spots in the rpm range, but those usually can be

remedied on fuel-injected bikes through enrichment with a Power Commander.

In the end, though, your question is best answered with simple math, not fuel-injection modifications. If you pay the good Internet price of around $275 for the PC, plus approximately $50 of shop/dyno time to get it custom-tuned to your lean-jetting requirements, you’d have invested about $325 to make the modification. Were it successful, you could, at best, expect only about a 2-mpg improvement in mileage. If premiumfuel prices remain anywhere near their current $3.30 a gallon, it would take you approximately 45,000 miles to recoup your investment.

A better solution? Go easier on the throttle and you’ll improve the mileage much more dramatically-and for free.

Radials aren't just tires

QCan you explain what makes

radial-mount brakes so special? To my old-fashioned eye, all calipers are concentric with the disc circumference, regardless of the method of mounting. Is the supposed “advantage” simply morerigid mounting? The torque against the

fork leg (around the axle) caused by braking forces seems the same either way. In which case, what’s so special?

I’m a born-again biker from Britland, and this issue makes me think of when I previously rode 25 years ago. Back then, many people, including some motorcycle journalists, thought that increasing the preload on shock absorbers actually stiffened the springs.

Hugo Rose

Colchester, England

A Radial-mount calipers offer a couple of advantages, one of which is, as you jokingly suggested, increased rigidity. The traditional method of mounting attaches the caliper at its mid-point and its trailing edge; this leaves the leading edge unsupported, giving that end enough leverage to slightly misalign the caliper with the rotor during hard braking. Radial mounting, however, attaches the caliper at both the leading and trailing edges, effectively eliminating the possibility for such misalignment. As a result, some manufacturers have been able to reduce the diameter of the front rotors and eliminate the need for

FeedbackLoop

Qln to the a question August trom issue, reader you responded Andy Dewey (“A glaring problem”), who was trying to remedy the instrument glare in his Yamaha Star Stratoliner windshield when riding at night. If you could, please pass along the information that the meter light can be dimmed. Matter of tact, the meter backlight, LCD readout and needle backlight all can be adjusted independently to any one of six levels of brightness. The adjustment is done through the meter’s Select and Reset buttons on the handlebars. The full procedure is explained in the owner’s manual. It’s a pretty neat feature that was designed specifically tor this situation. Mike Ulrich

Yamaha Motor Company Testing Division Cypress, California

different-sized pistons by adopting the radial-mount system. Additional rigidity could be accomplished in other ways, of course, such as using exotic materials and more-elaborate manufacturing processes, but the radial method is just as effective and much more cost-efficient.

A I was not aware of that dimming capability, Mike, and I'm sorry to report that I even made the very same

mistake in our September-issue road test of the Stratoliner (“King Kong vs. Godzilla”), despite having perused the owner’s manual; apparently, I somehow overlooked that section. For that, I apologize to Mr. Dewey, Yamaha and all Stratoliner owners who may have been misinformed by my response. After receiving your letter, I adjusted the instrument brightness on our test Stratoliner, which is still in our fleet, and found that the lowest level of illumination very effectively eliminates the glare in the windshield during night riding.

On the other hand, the dimmer obviously has no effect on the daytime glare created by all the chromed brightwork surrounding the instruments and on the fork’s top tripleclamp. The glare in the shield during the day is not as distracting as it is at night with the instruments at full brightness, but it’s still an annoyance that slightly diminishes the pleasure of riding what is otherwise a very enjoyable motorcycle.

Another potential advantage of radial mounts is to allow a switch to largerdiameter rotors without the need to replace the complete mounting system. In the worst case, a new set of calipers (with its pistons on a radius matching that of the new rotors) and longer bolts are all that

would be needed. In fact, if the increase in rotor diameter were small enough, it might even be possible to retain the existing calipers and simply move them outward with spacers between caliper and mount. These options may explain the engineers’ decision to orient the bolts on radial mounts perpendicular to the front-wheel’s axle rather than parallel to it.

Rev counting

I went up three teeth on the rear sprocket of my ’98 Suzuki Bandit 600, even after I read

all the tech about picking up low-end grunt versus top-end speed, taking a hit on gas mileage, more vibration, etc. But overall, it’s been a worthy trade-off. My questions are: 1) Can I expect a reduction in the long-term reliability of the bike, since the motor is spinning about 500 rpm faster at normal highway speeds? 2) Should I be performing certain maintenance (oil changes, valve adjustments) more frequently, given the extra engine revs? Greg Sullivan L ¿c % White Plains, New York

A Were it my bike, I might check the valve clearances 1000 to 1500 miles more often, but otherwise, I wouldn't do anything different in my maintenance program. More valve openings and closings per mile are like ly to acceler ate changes in valve clearance,

but the rest of the engine, including the oil, won’t be significantly affected by the extra/higher revs.

In fact, what shortens the service life of engine oil more rapidly than anything else are short trips at lower rpm that never allow the oil to reach its full operating temperature. Stop-and-go riding also is tough on oil, whereas highway riding at legal speeds puts the least demands on an engine and its lubricant.

Just one or a two-fer?

QWhy do I see some Suzuki Hayabusas that have been modified with a single exhaust system instead of a dual system? I always thought that a dual exhaust would improve performance. I am thinking of getting a new exhaust for my ’Busa, so is there something about dual-muffler systems I need to know?

Louie Almanze

Lackland AFB, Texas

A On a stock bike, the exhaust has to meet the EPA’s sound-emissions standards, and the most practical way to accomplish that feat while still allowing the engine to make competitive power is with exhaust-system volume; the bigger the internal capacity of the system, the easier it is to limit exhaust noise to a legal level without creating so much backpressure that the engine can’t breathe at higher rpm. On the big-inch, high-powered Hayabusa, the factory felt that the best method of obtaining that volume was either with one enormous muffler-like the howitzer on the Yamaha Road Star War nor-or two more-rationally sized ones. For several reasons, including cornering clearance, right-left balance and styling, Suzuki chose the latter.

TECH TIPS

Do you frequently have to add air to your bike’s tubeless tires? If so, the loss may not be due to a puncture; it could be the result of the way in which the tire was mounted. If you or the shop that mounts your tires don’t pay attention to the following details during the mounting process, tiny amounts of air can possibly escape past the bead, the valve stem or the valve core, causing a slow but consistent pressure drop.

I After dismounting the old tire, carelully clean the entire bead segments of the rim. If necessary, use a solvent or a Scotchbrite pad to remove any deposits that could prevent the tire's bead from forming an airtight seal. 2. Occasionally, the bead on a new tire will contain small, thin ribbons of rubber that are leftovers of the manufacturing process. Most tires these days don't come out of the mold this way, but if you end up with one that has, remove the ex cess rubber with a sharp razor blade be fore mounting, being very careful not to nick the actual bead. 3. Install a new valve stem, They're cheap, easy to replace and prone to leak or crack as they age. 4. Before mounting the tire, lube both of its beads to ease installation. There are numerous tire-mounting lubricants on the market, but soapy water also works well. 5. When installing the new tire, try not to distort the bead any more than is neces sary. This is best accomplished by ensur ing that the part of the bead that already has been levered into place always is pushed all the way down into the deepest part of the rim, called the "well. 6. When installing the core in the new valve stem, don't tighten it as though you were torquing the lug nuts on a monster truck; just snug it down. Overt~ghtenting can distort the valve and cause it to leak. The recommended torque (though very few people will-or could--ever actually measure it) is only 1 .5 to 5 inch-pounds. 7. Always install a valve cap. Metal caps are preferable, especially for racing and track days, but a plastic cap is better than no cap at all.

Recall Roster

NHTSA Recall No. 06V201 000 Harley-Davidson Sportster 1 200R, 883R Model years: 2004-06 Number of units involved: 9506 Problem: On certain motorcycles, the front-brake master cylinder assembly was built with inadequate lubrication on the secondary cup. This could cause the sliding resistance of the piston to be excessive, which in turn could cause the return of the piston to be slow upon release of the brake. If the piston does not return completely, the rear brake light could remain illuminated, which could increase the chances of a rear end collision. Remedy: Dealers will rebuild the master cylinder. Owners not receiving this free remedy should contact Harley Davidson at 414/343-4056. .~ ..~ .~

Remove noise from the equation, how ever, and the whole volume thing goes out the window. The vast majority of aftermarket mufflers are designed to improve peak power with very little regard for EPA noise regulations, so internal volume is not an issue. Mufflers of this type usually are either straight-through in design or have minimal baffling, only providing as much back-pressure as their builders deemed necessary for optimum perfor mance. This means they can be much smaller than stock while also offering very little internal restriction. As a result, they are too loud to be legal but efficient enough that only one is usually necessary to get the job done.

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 949/6310651; 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 10page 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.