Cycle World Service

CYCLE WORLD SERVICE

Fairing+Honda= weave

I own a 1982 Honda CB900F. After purchasing it, I put on a Shoei FM-3 handlebar mount fairing. When I took the bike for a high-speed run, it developed a very bad shake at around 90-100 mph. I tried tucking in and changing feet and body positions, but to no avail. The handlebars shook so violently, I couldn’t go any faster without fear of being pitched off.

I took off the fairing afterwards, and the problem was solved. I can’t believe Shoei would sell or design a fairing without first making sure it was aerodynamic and would be safe at high speeds (at least up to 150 mph). If anything, a small, sporty fairing should make a bike perform better at high speeds. Your thoughts on this would be helpful, interesting, and appreciated.

Bill Bradley

Utica, Mich.

The problem isn’t simply with the fairing, but the installation of this particular fairing on a motorcycle that doesn’t have much of a stability margin. Recent work by aerodynamicist Kevin Cooper (the man who designed the slippery streamliners for Can-Am) indicates that most weave problems aren’t the result of aerodynamic effects, but more likely result from the increased weight and inertia of the front end. Cooper also points out a fairing could play a subtle role in causing weave by changing the amount of drag on the motorcycle and shifting the center of pressure upwards. This would change the front to rear weight distribution at speed compared to an unfaired bike, and that could worsen weave problems in a motorcycle with marginal stability.

While we can't be certain that Cooper’s conclusions apply to the Shoei fairing mounted on a 900F, it seems possible. In our road test of the 900F in May, 1981, we noted that our test 900F was one of the most weave prone motorcycles we had recently been on, with the bike snaking its way through bumpy high speed corners. The addition of the Shoei fairing, either because of the added weight on the front forks or the changed weight distribution at speed, may simply reduce the bike’s stability margin so the weave occurs at lower speeds and with greater amplitude. The solution? Removing the fairing is the easiest one. A frame-mount fairing may offer wind protection without the weave, but you could only find this out by trying it on your 900F Honda substantially changed the 900F chassis when they developed the 11 OOF, and the 1100 doesn't have the same handling problems. Unfortunately, updating the 900F to 110ÓF specs would be prohibitively expensive.

Catalyst damage?

In the discussion of California emissions in the April, 1983, Round-Up column, you say, “eliminating

petcocks with reserve positions, and adding fuel guages instead, may save the catalysts some trauma from running on reserve.” What is the connection?

You go on to caution against backfires. Is running out on the main tank and switching to reserve causing backfiring? I have never experienced such a condition due to the fact there is no fuel available to collect in the exhaust plumbing and then explode.

Please explain the connection between catalytic converters and reserve position petcocks.

Terry Herr

South Haven, Mich.

Running out of gas isn’t like shutting off a light switch; the fuel levels in the carburetor float bowls drop slowly, and the air/fuel mixture leans out slowly. Eventually it leans out so far that the mixture can’t be ignited by a spark plug, and the engine stumbles with a lean misfire. We've all felt this as the power loss and engine stumble that occurs as a motorcycle runs out of gas. Unburned gasoline is being dumped into the exhaust system, and this continues until the float bowls are dry, or until the reserve position is switched on the petcock and the normal float levels are restored, or until the motorcycle coasts to a stop with a dead engine.

This lean mixture wouldn’t be ignitable by anything in a normal exhaust system, so running out of gas doesn’t usually cause backfires (which are just the explosion of air-fuel mixture inside the exhaust.) However, a catalyst is designed to burn normally unburnable trace amounts of hvdrocarbons, and it causes the lean mixture to burn.

Even this lean mixture releases far more heat than the normal combustion byproducts the catalyst was designed to deal with, and, depending on which emissions expert you ask, this heat can significantly damage the catalyst. An engineer at Yamaha said that the short duration of the lean misfire condition while running out of gas meant it wasn't much of a danger to the catalyst, while an engineer at Honda said that his testing had indicated it could cause major catalyst damage. We could speculate that this may be the result of different catalytic converter designs, but the only thing we can say for certain is that at least one company is very concerned about the running out of gas causing damage to the catalyst. And that may mean the demise of the reserve position on the petcock.

Quick wheel alignment

When I was still riding chain driven scooters with my customary verve, and stretching chains and wondering how to ensure that the rear wheel was properly aligned, I was given a good tip. Spinning the rear wheel after adjustment and observing the chain sideplateto-sprocket distance on both sides of the sprocket to see whether the chain rode against one side of the other was at least a good test for gross misalignment. Good for filling stations and roadsides when away from home. It also forces you to inspect the rear sprocket regularly.

W.W. Hunt

Dayton, Ohio

Revolutions Per Minute

I understand that the “r” in “rpm” represents revolution, but what is it that revolves in one minute? Is it a 360° rotation of the crankshaft or the single completion of the two or four-stroke cycle of all the pistons involved? I think I've read where rpm readings are taken

from below the engine head and others are taken from the camshaft in an overhead cam engine. Anyhow the tachometer is registering the speed of some moving engine part. The speed ol that engine part is involved but what sole cyclical event takes place that revolves per minute?

W. Doget

Union Lake, Mich

Rpm, when used to describe engine speed, stands for crankshaft revolutions per minute. An engine turning 8000 rpm has its crankshaft revolve 8000 times everv minute, regardless of whether it’s two or four-cycle, or how the actual measurement of engine speed is made.

A tachometer is the instrument that measures engine speed, and different tachometers do this in different ways. Mechanical tachometers are connected to a revolving engine part by a sheathed cable, and display a number that is proportional to the cable rpm. Notice that the cable doesn't have to be turning at crankshaft rpm as long as the tachometer designer knows the relationship between cable rpm and crank rpm; the scale on the tach face and the tach needle movement are adjusted to display the correct crankshaft rpm for a given cable speed. Electronic tachs are basically similar, but they count electrical pulses instead of watching the speed of a revolving cable. Once again, the tach designer has to be aware of the relationship between the electrical pulses and crankshaft rpm; if the tach is counting ever v time a spark plug fires, it will have to be calibrated differently for two and Jourstroke engines.

Interceptor shifting

In our May test of the Honda Interceptor, we complained about the notchiness of the Interceptor’s shif ting. Since then we've discovered at least a partial solution to this problem: lubricate all the joints of the shift linkage. Much of the effort of shifting was overcoming friction in the linkage. A fter the lube job, the shif ting still isn't in the Suzuki 750 class, but it’s much improved.

Another cause of high e ffort shif ting on añy motorcycle is a dragging clutch. Honda models with hydraulically actuated clutches may süßer from this problem if they have any air in their hydraulic lines. The air reduces the travel of the slave piston, and the clutch may not completely disengage. So if your Honda has suddenly become hard to shift, try bleeding the clutch hydraulics.

Faster V65 Magna

I recently purchased a V65 Magna and I'm wondering if lightening the bike and putting a header on it would make it run a faster quarter-mile? Your test V65 did a quarter in 1 1.07 sec. Could I possibly make it do 1 Os?

Dan Kneisler

Provo, Utah

The changes you mention might make the V65 quicker, but not necessarily in a quarter-mile. Our test bike missed a 10 sec. quarter not from a lack of power, but because our rider had to shut off the throttle in second gear to keep it from flipping over backwards on top of him. We can think of two easy ways to make a V65 run a 10 sec. quarter: First, hire Pee Wee Gleason to ride it. Second, install a stickier rear tire, struts in place of the rear shocks, and a wheelie bar. Hiring Gleason is the easier of the two, but anything done to lower the bike and reduce its tendency to wheelie will also make it quicker at a drag strip.

Rough ride in Chicago

I live in Chicago and I ride a GPz750. As you might know, Chicago is renowned for its monster pot holes and swollen and collapsed patches of same in short lousy roads and highways. Would aftermarket items like forks, fork braces, swing arms, and air shocks by manufacturers such as Fox, Marazocchi, Kayaba, or DG Performance help me or hinder me?

Michael Mazanec

Chicago, Illinois

There’s only so much you can do with the standard suspension travel. If you set the bike up stiff enough not to bottom over monster pot holes, it’ll be rough riding over small bumps. If you soften iffor the small bumps, you’ll be jolted by the pot holes. The best solution to this dilemma is more suspension travel, but that’s difficult to achieve without extensive modifications or the purchase of a street/trail bike like an XL600. An alternative approach would be to increase the progressiveness of your suspension by using air shocks or normal shocks equipped with motocross style bump rubbers. That would give you a better ride under normal conditions while resisting bottoming over pot holes. But don't expect a fantastic improvement in ride without increasing the suspension travel.