Service

SERVICE

Paul Dean

Rubber match

I’ve noticed that when your magazine compares motorcycles and puts them all on the same tires, you either raise or lower the bikes to compensate for differences in tire height. I am curious if the manufacturers do this, since the same bike sometimes has different brands of tires installed at the factory. Ken Moore Concord, California

Actually, Ken, we don't always make chassis adjustments as a result of putting all the bikes in a comparison on the same tires; we only do so when any of the tires we install are significantly different in height than the ones we take off We try to replace the original-equipment tires with ones of the same width and aspect ratio, but for various reasons, that’s not always possible, practical or in the best interests of equality for all the bikes in the test. So, if the differences in tire height are substantial—and the vast majority of the time, they are not-we will adjust the ride-height accordingly to compensate.

The bike manufacturers, however, do not make any such compensating adjustments when equipping their new motorcycles with different tires. Those who use more than one brand of OE tire do so only after extensive testing; and in most cases, they have either worked closely with the tire companies to provide tires

built to specifications tailored for each particular model, or have chosen tires that are so close in critical dimensions that no chassis adjustments are necessary> before the bike rolls off the assembly line.

Low-side lament

I low-sided my 2000 Yamaha R1 about 3 weeks ago, and, thankfully, the frame sliders pretty much saved the bike and my jacket saved my skin. But in riding the bike after the accident, I’ve noticed that the left clip-on is closer to me than the right one is when the wheel is straight. I checked the stops, and I took the fork tubes out of the triple-clamps and reset them, but it’s still kind of crooked. A friend said the closest thing he could imagine is that the lower triple-clamp is slightly out of whack from the crash. Is there any sure way to tell if the tripleclamp is the problem before I spend $300, only to find out my steering is still off? The front axle is slightly bent, but would this cause the clip-ons to be off?

R. Keesler Okinawa, Japan

Did you ever consider that maybe it’s the clip-on that’s bent? Some part of a handlebar usually hits the ground during a low-side, and that impact can easily generate enough force to bend the bar. And in any event, do replace the front axle before going any farther; it’s not an expensive part, and as long as it’s bent, the front wheel will be out of alignment in one plane or another.

Determining whether or not the triple-clamp is bent is a fairly easy operation. Remove the front wheel and both fork legs, then, one by one, slide both legs back up through the tripleclamp. If you feel any significant resistance when sliding the legs into place, either the triple-clamp or the affected leg is bent. To check for a bent leg, rotate each one 360 degrees in the tripleclamp, and if it hangs up at any point, the leg is bent.

Checking for a bent clip-on handlebar can be a lot trickier. I suggest that while the fork legs are removed, strip both clip-ons of their switchgear and other components, then butt the two together back-to-back for comparison purposes. They are mirror-images of one another, so if you align them accurately, you should be able to detect some difference in the angle of the two bar sections. Based on what you have told me, it’s my guess that the handlebar is the culprit, not the triple-clamp. But please, the axle...

Pop quiz

I have a 2003 Suzuki SV650 that I’ve equipped with an M4 carbon slip-on and a K&N air filter. When blipping the throttle at lights, as we occasionally do, the engine seems to backfire through the airbox.

It isn’t an actual backfire but a “pop” of air escaping, like the engine doesn’t want to rev and then does so after the pop. This also occurs when I turn the ignition off. It doesn’t happen very often but occurs enough to pursue a cause.

Jeremy McDermott Freehold, New Jersey

Technically, the condition you describe is in fact a backfire, so named because it is the sound of the engine “firing back” through the intake tract. When an.engine pops or belches fire out the exhaust, that is called an “afterfire.”

To meet prevailing emissions standards, motorcycle engines are forced to run with very lean fuel mixtures, especially in the lower rpm ranges and at smaller throttle openings. If the mixture is too lean, the engine will tend to backfire when the throttle is opened quickly at > or near idle. The manufacturers spend enormous amounts of R&D time achieving just the right balance between all of an engine’s tuning elements to get it to meet emissions standards while still performing up to expectations. But altering any of those tuning characteristics can upset that balance enough to negatively affect at least some aspects of the engine s behavior.

When you changed your bike ’s air filter and exhaust, you increased its ability to move air through the engine, but you did nothing to the SV’s fuel-injection system to compensate for that additional air. This has leaned out the mixture enough to cause an occasional backfire when the throttle is snapped open at idle. And the limited range of the SV’s fuel-injection map does not allow it to adequately compensate for that leanness. In all likelihood, the mixture is excessively lean throughout most of the rpm range, but the off-idle backfiring is a more obvious symptom than those that occur at other rpm and throttle positions.

The solution is to have the injection system remapped with one of the several electronic devices on the market designed for that purpose. The most common are the Dynojet Power Commander, the Yoshimura EMS and the Techlusion TFi. Not only will the backfiring disappear once this modification is properly completed, the engine will perform better throughout the entire rpm range.

Exhaustive research

Why do two-stroke engines use expansion chambers while four-stroke engines do not? Jason Dunham Marion, North Carolina

It~ a matter of timing-exhaust timing, to be precise. On a two-stroke, there are no actu al exhaust "valves ",~ the exhaust port is merely a big hole in the cylinder that is opened and closed by the piston. As a result, two-strokes have symmetrical exhaust tim ing, which means that the exhaust port opens and closes when the piston is at the very same loca~'ion in the cylinder The port opens a certain number of degrees Before Bottom Dead Center when the piston is on its way down, and it closes at exactly the same number of degrees After Bottom Dead Center when the piston is on its way up. But a four-stroke engine has real exhaust valves whose opening and closing are dictated by the camshaft’s exhaust lobes, which can be oriented to make those events take place anywhere the engine designer pleases. So, four-strokes can have—and in fact do have—asymmetrical exhaust-valve timing.

On a typical four-stroke, the exhaust valve might open when the piston is at about 40 degrees Before BDC on its way down, remain open throughout the piston’s entire upward stroke from BDC to TDC, and not close until the piston is moving downward again at around 15 degrees After TDC. That’s a total open time of235 degrees (40 plus 180 plus 15). On a typical two-stroke, however, the piston first uncovers the exhaust port when it is somewhere around 80 degrees before BDC, and closes it at 80 degrees after BDC-an open total of just 160 degrees.

Therein lies the problem. With such a short window for exhaust extraction, combined with the inability to stage the exhaust opening and closing events independently-and let ’s not forget that a twostroke must produce twice as many complete combustion cycles as a fourstroke in the same number of crankshaft revolutions-a two-stroke needs help.

It gets that help with an expansion chamber, which takes advantage of the natural sonic waves that travel through the exhaust tract. During the first part of the exhaust cycle, the design of the expansion chamber creates a negative pressure wave at the port that helps extract the burned gases from the combustion chamber while also helping to replace them with fresh air-fuel mixture; and during the latter part of the exhaust cycle, the negative wave is replaced by a positive wave that prevents some of the incoming air-fuel mixture from being swept out the exhaust and wasted. Entire books have been written about expansion-chamber principles and design, so I won’t even attempt an explanation here, other than to say that the dimensions of the chamber-head-pipe length and diameter, diverging and converging cone angles, center-section length and diameter, stinger length and diameter-all play a role in helping overcome the disadvantages of symmetrical, short-duration exhaust timing.

On a four-stroke, these same factors are not as much of a problem, because the engineers can greatly reduce their effects with camshaft profiles. Still, a fourstroke exhaust does utilize some of the same principles as employed on expansion chambers, but not necessarily to the same degree or with the same configuration as a two-stroke. Stepped head pipes, collector junctions, expansion boxes, megaphones, baffle styles and sizes, etc., all are tuning elements that can help a four-stroke produce optimum power, a broad powerband or, in some instances, even both.

Slippin' and slidin'

Here’s an anecdote in reference to your “Feedback Loop” letter in the January issue regarding the mold release on new tires. A few years back, I sent my Red Wing oxfords in for new soles, a service apparently performed only at the factory. I got the shoes back, put them on in the bedroom, walked to the top of the stairs and promptly went horizontal, landing about four steps south of where I started. Long story short, the silicon mold release had not been removed from the soles, and the result was ice skates with no blades. I wrote the company about the slippery soles, and on my first attempt I got a “cover our ass” reply. I wrote back and told them it wasn’t my intention to sue Red Wing, I just wanted them to be aware of what could be a devastating liability problem. This time, the president wrote back, apologized and explained the mold-release problem-and assured me that the word had gone out that all soles had to be cleaned prior to being shipped. This just crossed my mind when I read your letter about tires. Jim Baxter National Motorists Association (NMA) Waunakee, Wisconsin

Do I see another NMA in your future, the National Mold-release Association? Thanks for the amusing story. □