汽车专业英语_课件第3章.pptVIP

　　But it does not work. You tell him you have a better idea. You move the existing weight, and now it works! Why? 　　Remember that all of the force is transmitted through the driven wheels, so the moved weight increased the traction where it was needed. Weight distribution is very important, since you can increase traction just by moving existing weight from one part of the car to the other. 　　Have you ever heard that front wheel drive cars are better in snow and ice than rear wheel drive vehicles? Front wheel drive cars aren’t heavier. But the engine is very heavy and is located above the front wheels. This helps traction in front wheel drive cars because the weight is right above the driven wheels. 　　So, in summary, traction is important for transmitting the forces from the wheels to the road. If any of your wheels are spinning rather than rolling, you probably need more traction. Traction can increased by adding a non-slip material around the wheels (like a tire) or by moving weight over the driven wheels. But, remember, it is also important to have efficient wheels, which are usually thin and lightweight. 　　Axle 　　The axle is the component on which the wheel turns. It must be strong, straight and polished to reduce friction losses. Is a smaller diameter axle better? Why do “Hot Wheels” toy cars have such tiny axles and to so fast and far? The larger the diameter of the axle the farther away from the center they contact the bearings. Like a book dragging on the table all the way out at the corners, a large axle will be harder to turn. 　　Axle Bearings 　　When you have two things rubbing against each other and you want them to move freely, friction slows things down and wastes energy. For example, try sliding a coin and an eraser across the table. The reason the coin slides much more easily is there is less friction between the coin and the table than there is between the eraser and the table. 　　One case where friction is very undersirable is in the wheel axl