Service
SERVICE
PAUL DEAN
Herky-jerky Harley
Q My 2003 Harley-Davidson XL 1200 Sportster, which currently shows 39,000 miles, has had a problem since around 15,000 miles. When I let out the clutch and roll on the throttle, the bike moves forward, then lunges ahead in a jerky fashion. It does this in first gear only. It feels like I’m squeezing the clutch in and out, but I am not. I had the dealer look at the clutch, primary chain and tranny gears, but they found nothing out of specification. The dealer also called the Harley factory, but they had no idea, either. This behavior gets interesting when I’m turning corners in traffic in first gear. Can you shed some light on the problem? Alan Metzke Dwight, Illinois
A I think the problem is in the clutch, regardless of what your dealer has told you. Sportsters from 1991 to 2003 have a special device in the middle of the clutch pack called a “spring plate,” which is a large diaphragm-type washer sandwiched between two steel plates that are riveted together. Its purpose, when working properly, is to smooth clutch action by acting as kind of a shock absorber that helps dampen out chatter and grabbiness during initial engagement. If the spring weakens and/or the rivets loosen, the clutch can behave just as you describe, causing the bike to lunge as though you were quickly and repeatedly disengaging and re-engaging the clutch.
Actually, you’re pretty lucky, since the clutch has been acting this way for close to 25,000 miles. It’s not uncommon for the rivets to break and the spring plate to come apart, resulting in a big mess and an inoperable clutch. To prevent such an occurrence on your Sportster, you should have the entire clutch stack, including the spring plate, obviously, replaced as soon as possible.
Flat and happy
Ql’m curious about the difference in vibration-suppression potentials between 90-degree V-Twins and flat-Twins (à la BMW). Any chance you could explain? Chris Craig Posted on www.cycleworld.com
Certainly, and a good place to start is with Kevin Cameron’s lucid explanation of basic engine balance in his “Candid Cameron” segment in last month’s Service column. Using a vertical single-cylinder engine as an example, Kevin described one of the problems inherent in relying on crankshaft counterweights to offset the reciprocating imbalance of a piston shuttling back-and-forth in a cylinder. The counterweights do a reasonable job twice each revolution, largely offsetting the abrupt deceleration of the piston at TDC and BDC; but in between, and most forcefully when they are at 90 and 270 degrees after TDC, they create an imbalance of their own, as there is no significant piston deceleration/ acceleration to offset. So in effect, the counterweights simply trade the engine’s natural up-and-down vibration for a back-and-forth one.
Therein lies the beauty of 90-degree V-Twins. By placing a second cylinder 90 degrees from the first one, there now is another piston imbalance to offset. For simplicity, let’s refer to one of those cylinders as the vertical one and the other as the horizontal one, with both sharing a common crankpin. Just as the vertical cylinder’s piston hits TDC, the counterweights are at 90 degrees ATDC for the horizontal cylinder, but they are at BDC for the vertical one, thus canceling the vertical piston’s deceleration at TDC. Exactly 180 degrees later, when the vertical cylinder’s piston is at BDC, the counterweights are at 90 degrees BTDC for the horizontal cylinder but at TDC for the vertical one, once again canceling that cylinder’s rapid piston deceleration. And the vertical cylinder performs exactly the same balancing service for the horizontal one.
This might sound like a perfect system, but it is not. For one thing, balancing an engine’s reciprocating force with a rotating force is a compromise. Because of the angularity between the connecting rod and its crankshaft throw, the rates of piston deceleration at TDC and BDC are not the same. The forces created by a counterweight, however, are consistent at any given rpm throughout its entire 360-degree rotation, so it cannot provide ideal balance at both TDC and BDC.
Most V-Twins are also subject to another force, called a “rocking couple.” This produces a slight vibration caused by the conu| necting rods being located sideyj by-side on the same crankshaft throw. Though that offset is small 1 (the width of one con-rod), it causes the engine to try to twist in one direction or another as each piston separately reaches TDC or BDC. \Harley-Davidson’s ohv V-Twins are an exception, as they use a “knife-andfork” con-rod design that places both rods on the same centerline. They are 45-degree V-Twins, however, which do not have the same natural balance as do 90-degree Vees.
Opposed-Twins such as BMW’s famed Boxers also have a rocking couple, but those engines possess perfect inherent balance. Instead of placing both connecting rods on the same throw, those engines orient the throws 180 degrees from each other so that both pistons are always doing the same thing but in opposite directions. Both reach TDC at precisely the same instant, just as they reach BDC simultaneously, thereby canceling one deceleration with another equal and opposite one. But because of the considerable offset between cylinders (the width of a connecting rod, plus the thick crankshaft web that connects the two opposed throws), the engine wants to twist in one direction when both pistons reach TDC, then twist in the opposite direction when
they reach BDC. Again, that’s a rocking couple, and it is very noticeable on BMWs at lower rpm.
On opposed-Fours, though, such as Honda’s Gold Wing motor, the potential
rocking couple of one opposed cylinder pair is offset by the other pair, which always is trying to create a couple in the opposite direction. The end result is silky smoothness at virtually all engine rpm.
Feedback Loop
Q There’s another way to use tools like the Motion Pro Adjustable Torque Wrench Adaptor (“Tool Time,” January ’08 Service) that doesn’t require tables or torque recalculation: Just keep the extension at 90 degrees to the torque-wrench handle. My college-boy engineering mechanics class taught me to balance torques and forces at each end of the extension to keep it in equilibrium. At 90 degrees, your hand pushing on the end of the torque wrench causes torque to be applied to the head of the torque wrench, which is resisted by an equal and opposite torque at the end of the extension where it meets the torque wrench. And that torque is resisted by an equal and opposite torque at the fastener end of the extension, which is exerted by the fastener being tightened. Since the torque wrench measures torque at the head where it attaches to the extension, that’s the same torque that is applied to the fastener through the extension.
All bets are off, though, if the angle between the extension and the torque wrench is not 90 degrees. When that happens, the length of the moment arm changes, and you’re back to tables and calculations. Scot Marburger Dublin, California
A Good tip, Scot, but that very same information is clearly explained in the documentation that comes with the Motion Pro Adaptor. I chose not to mention the 90-degree factor in that “Tool Time” segment for the same reason I didn’t explain the formulae for calculating effective torque values when using the tool at other angles and lengths: I had insufficient space for detailed information that would have been relevant only to people who had an Adaptor-and hence, the instructions that come with it-in their possession. But in retrospect, I can see that such information would have been useful for anyone, including you, who has either fabricated or purchased a similar tool. So, thank you very much for the feedback.
Performing a Service
QIn the February issue, you printed a Service letter titled “Compression expression” that claimed to ex-
plain compression ratios. What you stated in your response was just a ratio of two volumes. I can not disprove this information in relation to design compression ratio, but I can say it is almost completely useless unless used from a design standpoint to figure design compression ratio. Your actual compression ratio would be much more useful and is measured as Ratio = Absolute Discharge Pressure/Absolute Suction Pressure. For example, if you had a cylinder that was able to build up 160 psi gauge max cylinder pressure, you would add 14.7 psi (atmospheric) to arrive at 174.7 psi absolute. Divide that by 14.7 psi atmospheric and you get 11.88:1 actual compression ratio for that cylinder. Your design compression ratio would more easily be obtained through literature and specifications. You could then compare it to your actual compression ratio and determine your cylinder efficiency.
Nate Caron Thompson, Connecticut
AI appreciate the input, Nate; it provides me with an excellent opportunity to explain the purpose of Service and the limitations a forum such as this presents.
When preparing my replies, my objective is not to compose an engineering thesis or write an SAE paper on the subject under discussion; neither am I offering specific instruction to anyone attempting to design a new motorcycle component or system. I’m simply trying to help Cycle World readers who are not engineers or experienced technicians better understand the basic principles that govern the whys and hows of motorcycle behavior-the operative word being “basic.” Virtually every subject discussed here is one that, if explained to its fullest, would fill every page of this magazine. Indeed, entire books have been written on practically all of the topics covered in Service. But all I have available here are a few column-inches in which to provide information that, at best, barely scratches the surface of the matter at hand. On occasion, I have erred or failed to hit the target squarely; I’ve never claimed to be infallible. But I like to think that I have helped far more people than I have confused. Whether or not I have succeeded is something only our readers can decide.
By the way, the method of measuring compression ratios that I described in that response-comparing TDC and BDC volumes-is the same one used by engine builders everywhere.
Counter-intuitive
QFve been trying to understand the fundamentals of countersteering. I’ve read and heard a few theories but still am not quite convinced. We all know it works, but I can’t seem to figure out why. Could you please shed some light on the matter? Marcio Matandos Säo Paulo, Brazil
A This is a subject that seems to confuse a lot of people, so let’s try to keep it as simple as possible. When you are driving your car down the road and turn the steering wheel to the left, what happens? Yes, the car turns to the left, but it leans to the right. Inertia wants the car to continue going straight, but the friction of the tires against the road forces it to go left; the chassis, meanwhile, reacts by leaning the other way, to the right. The harder you turn the wheel to the left, the more that inertia makes the car lean to the right.
When you are riding your motorcycle down the road and turn the handlebar to the left, the bike, like the car, leans to the right, and does it for essentially the same reason. The bike, however, turns to the right instead of the left like the car.
To understand why, stand with your feet spread as far apart as is comfortable and have someone try to push you over to one side by pressing on your shoulder. Unless you’re very small and/or light, the pusher has to exert a lot of force to even come close to knocking you over. Now stand on just one leg and have him try it; he can push you over with just one finger.
That’s the fundamental difference that allows the same steering action to let the car turn left but make the bike turn right. The car, being a two-track vehicle, has a wider base that lets it tum in the steered direction despite the lean; but the bike, being a single-track vehicle, has such a narrow base that virtually any steering input makes it lean in the opposite direction, and there is nothing to stop it from doing so.
Except steering into the lean. In keeping with our original example, once the bike starts leaning to the right, the only way to prevent it from falling all the way over and hitting the ground is to steer it tn thí» rirthf which either halts or reverses the leaning action. If you steer right just a little bit, the bike stops failing in that direction and the lean angle stabilizes; if you steer right even more, the bike will sit up and-unless you point the wheel straight as you reach the vertical position-start leaning to the left.
I’ve not addressed the physics that control motorcycle steering-gyroscopic precession, centripetal force, etc.-but as I promised earlier, my goal was to keep it simple. I hope my doing so has helped you gain a better understanding of countersteering.
It takes a thief
Q Someone tried to hot-wire my ’99 Kawasaki ZX-7R but failed. Now I need to change the ignition switch, but I can’t remove the two bolts that hold it in place because their heads have been snapped off. My Kawasaki dealer said the bolts have to be forced out, and he couldn’t guarantee that removing them would not damage other parts. I have the replacement ignition switch, new bolts and spacers, but I cannot start the bike and have no way of taking it to the shop. I thought of trying to drill the bolts and taking them out with those special broken-bolt tools, but that would require me to remove the whole front end. Another suggestion was to cut a straight groove in the bolts and use a screwdriver to remove them, but I have no tools small enough to cut the groove. Any suggestions would be appreciated. Hyilil Jung Jacksonville, Florida
A Those bolts are special fasteners with Torx heads designed to break off during installation on the factory’s assembly line at a certain predetermined torque value. This makes defeating the fork lock or compromising the ignition switch more difficult for thieves. But it also makes replacing the switch more challenging for either the owner or a shop mechanic.
Replacement does not, however, require removal of the entire front end. You can gain unrestricted access to the bolts by removing only the fork’s top tripleclamp and unplugging the wires that lead from the switch. You then can easily drill a pilot hole in the middle of each bolt and use any of several types of bolt or stud extractors (E-Z Out, Craftsman Bolt Out or Screw Out, etc.) to remove them. With the triple-clamp removed, you also should be able to use the method you described-cutting a straight groove in both bolts and removing them with a screwdriver. All you would need to cut such a groove is a hammer and a small chisel.
What to wear
QGot any idea why the tires on my 2004 Suzuki Bandit 1200S always wear out first on the left side? The bike has never been down, and it goes straight when I take my hands off the bars. I’ve checked wheel alignment several times and it’s always right on the money. The bike only has 17,000 miles on it, but I’ve replaced the tires twice because they were almost worn out on the left but still had some tread on the right. Sheldon Parker Cuyahoga Falls, Ohio
A Two factors cause tires to wear more rapidly on the left than the right: crowned roads and left-hand corners. To facilitate adequate drainage during rainstorms, most road surfaces are higher in the middle than on the sides.
This allows water to run off onto the shoulders rather than puddling in the travels lanes where it can cause severe hydroplaning. As a result, the contact patches of motorcycle tires spend more time shifted slightly off-center to the left because the road angles to the right. In many cases, the crown is so slight that you might not even notice it when riding along; but if you pull over onto the shoulder, get off the bike, crouch down low and look across the road to the other shoulder, you usually can see that the surface is higher in the middle than on the sides.
What’s more, unless you ride very slowly and conservatively, you probably go around left-hand comers slightly faster than around right-handers, thus increasing the wear on the left side of the tires. You can see farther around most left-hand turns, since any obstructions that limit your view of the road ahead (hillsides, bluffs, buildings, trees, signage) are farther away from you on the left than on the right. And in any given comer, the radius going left is longer than it is going right. The differences in speeds between left and right turns generally are small, but their cumulative effect over several thousand miles contributes to greater tire wear on the left. U
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.
