Motorcycle

Motorcycle agility and rotational mass

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Kevin Cameron has been writing about motorcycles for nearly 50 years, initiallyCycle MagazineAnd since 1992 Cycle WorldI am writing in. (Robert Martin /)

Motorcycle agility depends on three main considerations:

  1. Its total weight
  2. Its steering geometry including the wheelbase
  3. The resistance of its rotational mass to changes in direction.

Weight is basic. The heavier an object, the more effort you need to apply to move it in some way.

Motorcycle steering depends on the lateral movement of the steered front wheels. When turning the bicycle once with the counter steer, steer the wheels from below and tip over in the desired direction.We steer the front wheels and as it moves to one side, its lateral movement steers the rear wheels through the lever arm of Wheelbase.. therefore, The longer the wheelbase, the less steering effect.Each inch of lateral movement of the front wheels.

Dirt track motorcycles have a very short wheelbase, only about 54 inches. Their front and rear wheels are essentially as close to the engine as the function allows and provide maximum rotational response.

A shorter wheelbase will bring the wheels closer to the engine and make the steering more responsive. (Jeff Allen /)

On the other hand, stability is an important quality, as evidenced by the retired couple gliding west past Ship Rock on a large touring rig. This and the need for room and comfort in the cabin require a long wheelbase up to 65 inches.

Understanding rakes and trails

The two numbers that represent the motorcycle steering itself are Rake and trail.. The rake is the angle at which the steering axis tilts backwards from the vertical, and the trail is the distance at which the center of the front tire footprint tracks the projected intersection of the steering axis and the pavement. Typical values ​​for rakes are:

  • Road race: 23.5 to 25.5 degrees
  • Sport bike: close to 24 degrees
  • Old bike: 27-28 degrees
  • Touring bike: 30 degrees

Older motorcycles often have less chassis rigidity, which makes it difficult to stabilize using the smaller rake angles of the faster steering that is common today. However, from the 1950s to the 1970s, it was very common for teams to saw the steering head, adjust it to a different rake angle, and weld it back in place to solve the problem. You get the answer by trying something.

Gyroscope effect and steering

Now we will be difficult steering front wheel. Perform a classic experiment with a bicycle wheel. Remove the front wheel from the bicycle, hold the end of the axle and rotate it to the assistant. When you try to steer the spinning wheel, it resists. The resistance of this gyroscope is proportional to the weight of the rotating object, and each bit of the rotating mass is also proportional to the square of the distance from the axis of rotation and the square of the speed of rotation.

Note the strong effect of distance from the axis of rotation. For wheels, the farthest part from the axis is the tire, followed by the rim. Much smaller distance from the shaft is the material of the brake disc, which is usually 11 to 12.5 inches in diameter.

When a young rider I knew switched from a 250cc machine to a 750, he said, “I have to do my best to do this from turn 2 to turn 3” after the first practice. I reported. -Change to the left. The larger bike was 35% heavier than the 250 he was accustomed to, and had two brake discs on the front wheels instead of one, and front tires on a larger section.

There are many examples of competitive riders who have bent the handlebars with a sincere effort to steer rather than escape from the pavement. When the next corner is in a hurry to you and the bike rolls slowly, you give it everything you have.

This affects all motorcyclists, not just racers. Many have described the dramatic difference in steering ease between the 1975-1982 vintage 500-pound abdominal air-cooled liter bike and the 600 supersport bikes of the late 1990s. The horsepower of the two bikes may be the same, but the 600 may be 100 pounds lighter. The difference in weight helps, but the real big thing about the 600’s faster steering was the reduction in weight and diameter of the spinning front wheels. Wheel sizes have been reduced from 19 inches to 17 inches, with 16-inch wheels in the meantime for several years. The tires have changed from tube type to tubeless, and the wheels themselves have been significantly reduced in weight. The early 7-pound brake discs (which are a great base for floor lamps!) Were replaced by lightweight, thin discs of the 90’s. It was able to handle higher operating temperatures due to the distortion-free expansion of the floating mount. approved.

Rotate the crank backwards

Show the riders what they can do and they want more. Carbon fiber or forged mug wheels in today’s races are still lighter than the cast mugs that were first available in late 1973. Yamaha noticed what was called a “gyro cancel” when it adopted the Jack Shaft Drive for its early two-stroke road race fours. Using the jackshaft from the center of the crank to the clutch on the right will reverse the rotation of the engine.

This sounds strange, but a small diameter (less than 100mm) steel crankshaft that rotates “backward” at 10,000 rpm can cancel much of the gyro effect of the two wheels (about 2 feet in diameter). The latter is because it rotates at 1,400 rpm. (Or about 100mph). The wheels are six times the diameter of the crank, but only rotate at one-seventh the speed. The weight of the crank is about the same as one of the wheels on the bike, so a rough first estimate would look to reverse the rotation of the engine. You can cancel half of the bike’s “gyro resistance” to roll operations, as opposed to the weight of the wheel. It’s worth having when you’re in a hurry.

The Ducati Panigale V4 model has a crankshaft that rotates in the opposite direction. The 2022 SP2 model also features carbon fiber wheels. Ducati claims to be the most agile Panigale ever.

The Ducati Panigale V4 model has a crankshaft that rotates in the opposite direction. The 2022 SP2 model also features carbon fiber wheels. Ducati claims to be the most agile Panigale ever. (Ducati /)

Since 1981, Yamaha has adopted contra-rotating propeller crankshafts on 500GP bikes. This had the effect of neutralizing the crank gyro effect. Honda switched from a 3-cylinder to a 4-cylinder in 1984, but initially one crankshaft was spinning in the same direction as the wheel.

Other rotating parts?

What about other rotating parts such as clutches and gearboxes? The primary gear means slower revs, and the dependence of the gyro effect on the square revs significantly reduces the contribution of these components. Yes, there are some, but it’s about a quarter pound of that of a crankshaft.

Honda added an idler in 1987 to reverse the rotation of the crankshaft. NSR500GP bike..rider Mick Doohan Later, he commented, “It helped me a bit, but it didn’t solve all the problems.”

Yamaha’s original when the MotoGP era began in 2002 M1 The engine introduced reverse engine rotation. Over the years, others have adopted it. Today, its use is universal in class. With a top speed of 220 mph, the wheels of these bicycles spin at over 3,000 rpm and the engine spins at 18,000 rpm. It’s a lot of gyros! To steer them, the rider needs a lot of help in the form of lighter rotating parts and a backward rotating engine.

In races in the late 1980s, switching to carbon carbon discs significantly reduced the mass of the brake discs (carbon density is about 1.5, which is, of course, similar to coal density, but steel density is 7.8. is). After doing a lot of experimentation with GP’s 16-inch wheels and World Superbike 16.5, the tire company decided to focus on 17-inch wheels instead to make it easier to apply race discoveries to production tires. Did.

Responsiveness / stability trade-off

Part of the Academic Motorcycle Stability Community, As the wheel diameter shrinks, it becomes unstable and the ugly head backs off... And surely it did. The 500GP bike I saw on the 16-inch front in the 1981 season wasn’t the cup for every rider. After all the obstacles, it vibrated threateningly.

This is the case for passive systems. The faster the control responds, the more it responds to disturbances other than the operator’s commands. Conversely, the more stable the vehicle, the stronger it resists all types of interference, including operator commands.

Some people are convinced that a gyro effect is needed to steer a two-wheeled vehicle, but experiments are being conducted. The test front end was built with two wheels. One touched the road and the other rotated in the opposite direction on the same axis, canceling the gyro effect. Experimenters have discovered that this bike can still be ridden normally.

What does this have to do with everyday motorcycles? Many riders are happy to get tired of it at the right speed, with the right amount of control, so it’s hard to argue. On the other hand, despite our best plans, emergencies occur. We would be pleased if our motorcycles react quickly and easily when they do so.

https://www.cycleworld.com/story/blogs/ask-kevin/motorcycle-agility-and-rotating-masses/ Motorcycle agility and rotational mass

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