Service
SERVICE
PAUL DEAN
The sound of music
Q I've asked a lot of people this question since I first started riding 11 years ago, but I've never gotten an answer that made sense to me. Why do engines with different cylinder numbers (Single, Twin, Four, etc.) and arrangements (vee, parallel, opposed) sound so different? If you start with a 500cc Single and add an identical cylinder to make a 1000cc parallel-Twin, the two don't sound even remotely alike. If you twist that parallel-Twin into a V-Twin, it sounds different yet. What's even more weird is that a 45-degree V-Twin like a Harley sounds different than a 90-degree V-Twin like a Ducati. If you add one more cylinder to a parallel-Twin, you have a Triple that sounds like nothing else. Add another to make a Four and the exhaust sound changes yet again. Even when everything inside these engines is exactly the same, they all have different sounds. Why is this?
Darryl `McCormick Saginaw, Michigan
A What you hear emanating from the exhaust systems of all those different engines is, in effect, music. The source of the music is the final stage of the rapid burning of fuel-air mixture in the combustion chamber, and the sound that exits the muffler is affected by the shape, volume and silencing capabilities of the exhaust system. You could liken this to the sound of a single note coming from a trumpet or other wind instrument. If you change the volume or internal characteristics of the exhaust sys tem-which is analogous to a trumpet player pushing a different valve on the trumpet-the sound also changes.
Compression and exhaust-valve tim ing play a role in exhaust sound, as well. Increased compression is equiva lent to the trumpet player blowing harder, and different cam timings open the exhaust valve earlier or later in the combustion process, the end result be ing differences in exhaust sound.
When you add cylinders or arrange two or more of them in different con-
figurations, the changes in sound are more dramatic. This occurs because some sound waves complement one another while other frequencies conflict with one another. The complementary waves can produce a sound that is unique to that combination, and the conflicting waves can either partially or entirely cancel each other. As in music, certain frequencies and tones work together to produce a sound that is nothing like its component parts; conversely, some of that sound can be absent of certain fre quencies and tones that were produced at the source but have canceled each other along the way.
Thus, the uneven firing of a V-Twin has a different effect on certain frequen cies than would be the case with an even-firing 360-degree parallel-Twin, just as the 240-degree firing sequence of a Triple has different complementary! conflicting frequencies than an inline Four with a 180-degree sequence. It's all music, though, an internal-combustion symphony that gives each type of engine its own unmistakable character.
To mix or not to mix?
Q I’m confused. I have a couple of riding friends who are pretty savvy when it comes to motorcycles, and they tell me you should never mix radial and bias-ply tires on the same bike.
My dealer, however, says it’s okay as long as I use the same brand of tires on both wheels. So, what’s the deal? Is either one of these claims correct or are both wrong?
Dale Newsome Winston-Salem, North Carolina
A Although there is some truth to both of those explanations, neither is entirely accurate. Years ago, back when radial motorcycle tires first hit the market, tiremakers strongly advised riders not to mix radial and bias tires. Radiais were new and offered significantly different handling characteristics than bias rubber, and neither the bike manufacturers nor the tire companies yet had enough modeland tire-specific data to predict how a combination of the two would affect the handling of any given motorcycle. So the best policy was to discourage mixing radiais with bias tires.
Since then, radial technology has evolved dramatically, and the manufacturers have had years to develop a better understanding of the effects of mixing tire types. As a result, numerous production bikes, including two new models tested in this issue-the Harley-Davidson Rocker and the Yamaha Star Raider S-come from the factory equipped with a radial on the rear and a bias-ply tire on the front.
This is not to say it’s okay to put any radial and any bias tire on any motorcycle; that could have results running the gamut from awesome to awful. The bike and tire companies have spent millions of dollars and, probably thousands of man-hours determining the feasibility of mixing tire types. So the question is, do you want to open your own R&D department and become the primary test rider? Because unless you follow the recommendations of your bike’s manufacturer or of the tire companies, that’s exactly what you will be doing.
Lonesome lobe
Q In a brochure for a little-known Chinese-made motorcycle, I read that its one-cylinder four-stroke engine has a “single-lobe camshaft.” How on Earth can that be possible? If it is true, what are the advantages, if any?
Carnell Woodley
Posted on www.cycleworld.com
A Yes, it is possible, but that
Chinese motorcycle is not the first to employ such a design; some overhead-valve engines dating back many decades used single-lobe cams, and so do a few current manufacturers of smaller general-purpose and powerequipment engines.
How single-lobe-cam engines work is very simple. The followers for the intake and exhaust valves are positioned in line with that single lobe, and they are located precisely where they need to be to open their respective valves at the proper time. So the single lobe does double duty, controlling the opening and closing of both valves.
As far as I am aware, there are just two advantages to such a design. One is reduced manufacturing costs; a one-lobe cam is cheaper and easier to make than a two-lobe cam. The other potential advantage is that a cam with a single lobe can allow the valvetrain to
be marginally narrower.
There are disadvantages, as well. Having just one lobe to work with hampers the ability to have separate intake and exhaust valve lift and timing. The lift can be increased or decreased slightly through the use of followers with different ratios, but the basic timing remains the same for both valves. What’s more, since one lobe has to operate two valves, that lobe is potentially subjected to twice the rate of wear.
V vs. I
Q Is there a difference in the torque output of a V-Four as opposed to an inline-Four of the same displacement? Just curious. Roy Trauber
Posted on www.cycleworld.com
Ain theory, no. If all other tuning factors are equal (bore, stroke, compression, valve size, valve timing, intake and exhaust specs, etc.), the two should produce the same torque and horsepower. In lowtraction situations, the V-Four may prove to be slightly more tractable, since its uneven firing sequence could give the rear tire a better opportunity to hook up between combustion events. But on a dyno, the two should be comparable, if not identical.
Feedback Loop
Q I Motorcycle am a longtime Mechanics Cycle World Institute reader in Phoenix, and a student Arizona. at the In the
December issue, you answered a question regarding progressively wound fork springs (“Which way is up?”). I do not feel you were incorrect in your answer, but in my suspension and chassis class, my instructor told us that the reason the OEMs want the soft end (closely wound coils) of the springs toward the bottom is because they are completely immersed in oil there. When the spring goes into coil-bind and the coils make contact with one another, the oil dampens the sound, keeping the overall noise output within the ever-constricting EPA regulations. I hope this helps clear up the ever-present fog when it comes to suspension tuning. Dustin Baenen Posted on www.cycleworld.com
A Thank you for the input, Dustin, but I’m afraid your instructor’s explanation doesn’t make sense. There is no part of the EPA’s sound-measuring procedure (or that of any other country’s comparable agency) that would ever cause the fork springs to reach the point of coil bind. That would require the bike to hit a large bump or hole during the testing process, or for the test rider to grab a big handful of front brake, but neither is the case. The procedures vary from one country to another but generally involve simply accelerating the motorcycle from one rpm to another in a specified gear and measuring the sound output from a specified distance.
Recall Roster
NHTSA Recall No. 07V535000 Harley-Davidson Dyna FXD, FXDB, FXDC, FXDL, FXDWG Model year: 2008 Number of units involved: 538 Problem: On certain California-only motorcycles, the catalytic muffler protective heat shield was not installed. This condition could allow the rider and/or passenger to have direct contact with the muffler and cause injury. If contact occurs while riding, it could cause loss of control, possibly leading to injury or death.
Remedy: Dealers will install the heat shields on the mufflers.
Owners not receiving this free remedy should contact Harley-Davidson at 414/343-4056.
NHTSA Recall No. 07V510000 Victory Vision Model year: 2008 Number of units involved: 326 Problem: On certain motorcycles, the voltage regulator/rectifier assembly may permit an overcharging condition, which, in conjunction with a loose battery connection, could cause a stalling condition. An unexpected loss of engine power could cause a loss of control of the motorcycle, increasing the risk of a crash.
Remedy: Dealers will replace the voltage regulator/rectifier and inspect both battery cables for tightness.
Owners who do not receive this free remedy may contact Victory at 763/417-8650.
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.
