Service
SERVICE
Paul Dean
Gimme five!
Why does a big V-Twin with massive torque need a close-ratio six-speed gearbox? I own a 2004 Suzuki SV1000, and just the other day had to do a triple downshift to get by a truck when the road narrowed from two lanes to one. The SV’s big V-Twin engine lets me spend a lot of time in the 3000-5000-rpm range (redline is at 11,000), but when all-out acceleration is needed, I still need to click down two, three or even four gears. Maybe six speeds make sense on a high-revving 600 with a peaky powerband, but on a big motor with a torque curve as flat as my dining room table, five’s enough! Can’t five gears be made to cover the same ratio range without making the shifting clunky or less reliable? Avi Halpern San Diego, California
Problem is, the SV’s torque curve really isn’t as flat as your dining room table. If it were, you wouldn’t have to downshift as far or as often to make a fast pass. The curve is flatter than most produced bv high-performance inline-Fours, and the power delivery is very linear, without significant dips or spikes, but it doesn’t peak down in the 3000 to 5000-rpm range; it peaks at 7200 rpm, where it makes 30 percent more torque than it does at 3000.
Suzuki and other manufacturers put six-speed boxes in their high-performance V-Twins to improve the maximum rates of acceleration. More gears and closer ratios mean less of an rpm drop
between gear changes. That, in turn, gives the rider the ability to keep the engine spinning closer to its peak power rpm, whether on a dragstrip, a roadrace track or a twisty backroad. The trade-off is that under some other circumstances, including the one you describe, the rider has to execute more downshifts to get the revs up into that peak-power range.
What actually propels a motorcycle is torque at the rear wheel, which is engine torque multiplied by the overall gear ratio in use at any given time. Lower gears have higher numerical overall ratios, which means they provide a greater mul-
tiplication of engine torque at the rear wheel. So regardless of the number of transmission speeds, a downshift or two or three will always provide better acceleration when you ’re just cruising at lower rpm in top gear.
Since the SV1000 and its sportier sibling, the SV1000S, are considered highperformance bikes, Suzuki chose to make this compromise. But all of the company’s V-Twin cruisers, which are not considered performance models and have lower-rpm engines with even flatter torque curves than the SV’s, are equipped with five-speed transmissions.
Oranges and apples
I have a question regarding the Honda VTX1800F tested in your July issue and the VTX1800N in the “7 For Sturgis” comparison last December. In top-gear roll-ons, the F was 1.3 seconds faster from 40-60 and 60-80 mph, even though, according to your figures, it weighs about 23 pounds more and its torque was rated at 96.7 foot-pounds compared to 107 for the N. Why? Kevin Bailey
Posted on America Online
Unfortunately, the performance numbers for the bikes in the ‘‘7 For Sturgis” test are not directly comparable to those we normally publish. Out of necessity, we conducted that test entirely in and around Sturgis under much different conditions than those we usually encounter. Obviously, we used a different dynamometer in Sturgis, and that alone invalidates close comparison between the horsepower and torque numbers generated there and those produced on our dyno. What ’s more, Sturgis is at 3500 feet, whereas our performance numbers usually are generated down at sea level. The dyno in Sturgis used an altitude-correction factor to adjust the power and torque figures to sea level-a common feature on most dynos— but the numbers for the roll-on and eighth-mile tests, also conducted at 3500 feet, were not adjusted. Plus, we weighed the N-model on different scales than the calibrated ones we use here in Newport Beach. All this might explain why the N had slower roll-on times than the F, despite producing more torque on the dyno. And don’t forget that the 1800F we tested in July was stock, while the 1800N in Sturgis was fitted with a Cobra exhaust system, further muddying the waters when trying to compare the two.
All these factors clearly support a basic tenet about performance: Comparing numbers derived on different dynamometers by different bikes under entirely different circumstances invariably provides more questions than answers.
Dyno indifference
When testing a bike on a dyno, does tire wear or the level of fuel in the tank have any impact on the results? By that, I mean are the numbers affected if the bike is run on old tires versus new (different tire diameter), or if the gas tank is full (40 to 45 pounds of weight) compared to it being closer to empty? Walter R. Machnicz Beacon Falls, Connecticut
As long as the engine is not standing for fuel, the amount of gas in the tank during a dyno run is insignificant. The tank only has to dispense gas at the same rate at which it is being used, and modern bikes have no trouble doing that, even when low on fuel. The actual weight of the gas also has no effect on a dyno run. A bike ’s overall weight is not a variable on a rear-wheel dyno so tong as the rear tire is sufficiently inflated to prevent it from flexing excessively, which can rob power
Neither does rear tire diameter affect the numbers, because the dyno always “knows ” the tire’s circumference. An electronic tachometer tells the dyno ’s computer how fast the engine is turning, and a sensor on the dyno ’s roller drum tells the computer how fast the drum is spinning; the computer then automatically calculates the ratio between the tire and the drum, even on the fly. If a bike on the dyno had some kind of super-soft rear tire that would go from brand-new to completely worn-out during a single run, the computer would constantly recalculate the tire-todrum ratio during the run and produce accurate results at all engine rpm.
The knock doc is in
I ride a 2003 Suzuki SV1000S with only 3000 miles on it that I bought used about a month ago. I love the bike and take excellent care of it, but lately I have been feeling a sort of hammering that starts near or in the engine and transmits through the handlebars. I only feel it at idle; as soon as I increase engine speed, it disappears. The engine runs strong and there is no smoke of any color coming out of the exhaust. I don’t know if this “knocking” is being caused by something that has come loose or if it is more serious. The previous owner installed a Power Commander, and sometimes I feel some glitches in the fuel-injection, so I thought that it might be causing the sound. I wanted to ask your opinion before taking the bike to a service garage where I will be given a dramatically exaggerated explanation of the problem.
Jaime Camhi Berkeley, California
Based on your final comment, I assume that your Suzuki dealer is not on your Christmas card list. I’ll try to do better. I’m unable to sample the noise myself, however, so I can’t offer a sure-fire diagnosis, but I think that what you are hearing may be emanating from either the clutch or the primary drive-or both.
Like the vast majority of other motorcycles, your SV uses a multi-plate clutch in which the fiber drive plates lock into the outer clutch hub via wide tabs on the plates that fit into matching slots in the hub. The tabs fit closely in the slots but not snugly, and that clearance increases slightly as the tabs wear with use. When any engine is idling, the rpm fluctuates very slightly between each firing pulse; this is especially so with a V-Twin, which fires at uneven intervals. As the rpm fluctuates, the clearance between the tabs and the slots allows the clutch plates to rattle back and forth in the hub, producing an audible “knocking”
sound. When the revs increase, the firing impulses occur in more rapid succession and the clutch plates then do not have sufficient time to rattle back and forth, so the noise stops. If this is the source of the knocking, it’s nothing to worry about.
Recall Roster
NHTSA Recall No. 05V252000 2005 Victory Hammer Number of units involved: 712 Problem: On certain motorcycles, the camshaft chain-drive sprocket located on the crankshaft may have cracked upon installation. If cracks are present, the sprocket may fail during operation, which could cause the engine to lock up and result in loss of operator control, thereby increasing the risk of a crash. Remedy: Dealers will remove the lower right engine cover, inspect the sprocket for cracks and replace either the engine or the necessary components. Owners who do not receive this free remedy should contact Victory at 763/4 1 7-8650.
Then there the primary drive, which uses straight-cut gears on the crankshaft and clutch hub. With straight-cut gears, the mesh of the mating gear teeth is critical. If the mesh is too close, the gears can give off a whining noise; f the mesh is too loose, a loud rattling can result. To combat this, many motorcycles, including the S~ have a narrow gear on the crankshaft, directly be hind the primary drive gear This second gear is spring-loaded in a way that takes up any freeplay that may exist between the two main primary gears so that the primary drive runs more quietly On your S~ one or more of the springs in that narrow gear may have broken or gotten weak, allowing the primary gears to have freeplay that s' causing the knock ingyou hear This is not a major pro blem that will lead to engine damage, but you may want to have someone inspect the primary drive just in case.
Got a mechanical or technical problem with your beloved ride? Can't seem to find work able 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 CW1 Dean@aol.com; or 4) log onto www.cycleworld.com, click on the "Letters to the Editor" button and enter your question. Don't write a 10-page essay, but if you're look ing 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.
