Service
SERVICE
Dream husbandry
PAUL DEAN
Q I recently calculated the gas mileage on my Suzuki SV650 and Suzuki DR650 (my husband thinks I need two motorcycles). The SV got 50 mpg and the DR got 58 mpg. This provoked a dinner-table question: Does the DR get better mileage because it is lighter, or because it has only one cylinder? Assuming equal weight and optimum shifting, would the DR still get better mileage? My husband says the one-cylinder setup is more efficient because it has less surface area for cylinder volume, so it loses less heat energy. But if the two-cylinder engine had a side-by-side arrangement, wouldn’t it have less surface area for cylinder volume? And does wind drag make a big difference? Any insight would be appreciated. Sarah McCabe
Anchorage, Alaska
A You’re on the right track, Sarah, but on the wrong train. Your husband is correct when he says that a twocylinder engine has a greater surfacearea-to-volume ratio than a Single of the same displacement, but he is not referring to the external surfaces of the cylinder castings; he instead is talking about the internal surfaces that are directly exposed to the heat of combustion. This is determined by adding up the surface area of the cylinder walls, the tops of the pistons and the domes of the combustion chambers. I won’t bore you with the formulae for calculating this area, but suffice it to say that the more cylinders an engine has for any given displacement, the more surface area it also has. In your case, the SV has about 2 percent more surface area than the DR. And just as the surfacearea-to-volume ratio for a Twin is greater than that of a Single of equal dis-
placement, it is even more so for a Triple, more yet for a Four and so on.
Your husband is also right about heat loss. An internal-combustion engine converts the potential energy in fuel into heat energy that drives the piston downward, but some of that energy is lost through the heating of the surfaces exposed to combustion. So, the greater the surface area, the more energy that is lost, which generally translates into reduced fuel mileage.
Many other factors also affect disparities in fuel mileage between two bikes of the same displacement but with a different cylinder count. Motorcycles have comparatively large frontal areas with high coefficients of drag, which is why many cars with weight-to-power ratios inferior to those of comparable motorcycles can reach higher top speeds. Naturally, then, differences in drag between two bikes of equal displacement can affeet their fuel mileage. So can differences in power output, overall gearing, carburetor/injection settings and the measures the manufacturer had to take to make any given engine meet emissions requirements.
By the way, it sounds like you have quite a thoughtful husband up there in Anchorage. Our other female readers must be green with envy about now.
Carb com-pear-o
Ql’ve noticed that on older Honda XLs, the shape of the carb venturi area was an oval, like an egg standing on its small end. As the carb slide was raised just off idle, the venturi area was very narrow, and as the slide progressed upward, the area got wider as well as taller. More modern carbs have a pretty straight-sided slide area. What effect does the bottom shape of the slide have on fuel mixing? Lee “bin Rydin” Riddle Brookings, Oregon
A Those pear-shaped carburetor bores had more to do with air volume than with fuel mixture. For years, an ongoing problem with carburetors on four-stroke motorcycles was throttle response: Opening the throttle quickly at low rpm usually caused the engine to either hesitate or stall altogether due to a sudden drop in intake vacuum.
Automobiles had overcome this problem decades earlier through the use of accelerator pumps, but / most motorcycle carbs-which, by ( design, needed to be much simpler than those used on cars-had not yet adopted (jr¿7/\ such devices.
One solution was to use smaller carbs than were otherwise ideal; a smaller venturi size meant less of a drop in intake vacuum when the throttle was quickly opened, reducing the likelihood of the engine stumbling and stalling. But it also meant less peak power because smaller carbs obviously cannot move as much air as larger ones.
Pear-shaped throttle bores seemed like a way to have the best of both worlds. Placing the narrow part of the bore at the very bottom of the slide’s travel helped the carb behave much like a smaller one at small throttle openings; this, in turn, allowed the upper part of the bore to be perhaps even bigger than normal, thereby offering as much or more performance as a larger carb once the throttle was turned past halfway. Pearshaped bores also helped some bikes, usually off-roaders, have more gradual engine response for better throttle control in tight, technical situations.
Some single-cylinder bikes even had two carbs-one small carb for lowerrpm operation and a bigger one that kicked in at larger throttle openings. But accelerator pumps and constantvelocity carbs eventually became the norm; and now fuel-injection, which does not rely on that intake vacuum signal to deliver the correct mixture, is quickly replacing carburetors.
Duck-ing disaster
QI recently was in an accident with my 2007 1098 Ducati when a car cut me off. Luckily, it happened in town and I was not going fast. I locked up the front brakes
while trying to avoid the car and hit the white stripe they apply to the road surface for what I assume is a bicycle lane, since it is only about 12 inches from the curb. Now I plan on either doing a trackday or joining a club, possibly even enrolling in a Superbike School. Would any of these experiences actually have helped, even though my accident was a “bang-bang” occurrence (meaning not much time from when it started to when it finished)?
Frank M. Mateja III Posted on www.cycleworld.com
A This isn’t a typical Service question, but it’s well worth answering anyway. Frankly, I can’t tell you whether or not your participation in a riding school or a trackday would have allowed you to avoid that mishap. Sometimes, even when you know precisely how to do whatever it takes to prevent an accident, circumstances make it impossible for you to actually do it. There are limits to how quickly a person can react and how well a motorcycle can follow its rider’s commands. Nevertheless, the knowledge that riding schools in particular can offer is priceless in the greater scheme of things. In a controlled environment, you learn more than you ever were likely to learn on your own about what you and the motorcycle can do. You then can apply that knowledge in your everyday riding so that when confronted with a critical situation, you are far more prone to react quickly and instinctively with the right moves, thereby increasing your chances of avoiding the accident. There are no guarantees, of course, but formal training helps put the odds more in your favor while making you a better rider in virtually every way.
Go with the flow
Q Simple question: Why is it recommended to start and warm up an engine before changing the oil? Obviously, the bike had been run and heated up before it was put away, and all that warm, dirty oil got a chance to run back into the pan after shutting the engine off. Wouldn’t restarting it before changing the oil just recirculate dirty oil, potentially leaving contaminants up in the engine that were already down in the pan? Ralph Okunewitch Long Island, New York
A Yes, the dirty oil already was down in the pan, but the idea of warming the engine before draining it is to get as much of the dirty oil out of the pan as is reasonably possible. That seems most likely to happen with warm oil, which flows much more readily than cold oil. Besides, that same oil has circulated through the engine thousands of times before the oil change, getting a wee bit dirtier with each trip; one more isn’t going to make any meaningful difference.
Not quite to the Max
QI have a 1998 Yamaha V-Max with California emissions equipment. I inadvertently merged the right-rear piston with that cylinder’s valves by failing to tighten the rear timing chain after a rebuild. At least it made for quite the conversation piece.
Thanks to the wonders of eBay, I managed to get a new head, complete with all the valves and cams, but it’s not a California head. What will be the outcome if I use the stock California emission cams in the front and nonCalifornia emissions cams in the rear? What should I do to end up with the best performance? Ken Rose
Heber Springs, Arkansas
A If you want the best performance, you should install non-California cams in both heads. Using the California cams in the rear head and the 49-state units in the front won’t cause any installation or tuning problems, and the engine will probably gain a horsepower or two > compared to its output before you amalgamated the right-rear piston and its valves. To meet CARB’s (California Air Resources Board’s) more-stringent emissions standards, the Cali cams have slightly milder timing that results in marginally reduced power levels. You would be using the 49-state cams in just one head, however, so the degree of power gain would be cut in half. Plus, your California Max also has slightly different carburetors. So the end result is likely to be a power gain so small that a sensitive dyno would detect it but the one in the seat of your pants would not.
Alternative thinking
Q I recently got to thinking about alternative front-end designs and that one of their most-hyped advantages is their anti-dive characteristics. Sounds great, but I can’t figure out why this would be. It seems to me that whether using a fork or something from BMW, Yamaha’s GTS 1000, Bimota’s Tesi, etc., when the brakes are engaged, the weight of the bike will be transferred forward, and that no matter what is holding up the front end, it is going to be compressed. Do these alternative front-suspension designs have extreme geometry or do they simply allow much stiffer spring settings? What am I missing? Chris Jankowski
Kettering, Ohio
A What you are missing is the way in which the braking forces are transmitted into the suspension. On bikes with leading-link or swingarm front ends, the calipers attach to the arm in a way that causes the torque reaction of braking to try to pivot the arm downward, or counterclockwise if viewed from the left, which tries to raise the front of the bike.
The transfer of weight onto the front end, meanwhile, is trying to make the arm pivot in the opposite direction, upward or clockwise, which wants to lower the front of the bike. By using carefully calculated geometry to determine the location and length of the arm and the placement of the caliper, those two forces can be largely canceled, eliminating virtually all front-end dive during braking.
BMW’s Telelever system, which debuted in 1994 on the RI 100RS, doesn’t rely on brake-torque reaction to offset front-end dive. It eliminates the steering head in favor of a wide “swingarm” between the chassis and the bottom tripleclamp, with a single shock mounted between the arm and a structure that links the top triple-clamp to the chassis. This allows the front wheel to move up-anddown freely but provides resistance when braking forces transfer weight forward. Telelever doesn’t entirely eliminate frontend dive when braking but does reduce it almost to the point of insignificance. □
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 ivww.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.
