Car Parts and Accessories

Passing on power to Auto Car Wheels

On the last stage of car journey to the road wheels, power from the car engine passes through the final drive.

This assembly has three jobs to do: it gears down the speed of the propeller shaft to the speed required by the car wheels; it allows the inner car wheel on the powered axle to turn more slowly than the outer car wheel when a car rounds a bend; and, except when the car engine is mounted across the car, it turns the drive through a right angle to drive the wheels.

The flywheel of a medium-sized car runs at speeds up to 6000 rpm, and that of a sports car at up to 7500 rpm. Such car speeds have to be reduced greatly before power is transmitted to the road wheels, which even at 110 km/h turn at only 750-1150 rpm, according to their size.

In top gear, the speed-reduction ratio provided by the final car drive ranges between about 4.5:1 and 3:1—that is, taking the 3:1 ratio, the propeller shaft revolves three times for every single revolution of the wheels. With conventional cars, this reduction takes place entirely in the back- axle assembly. In cars with all-indirect gearboxes, part of the reduction is done in the gearbox. Cars with 4-wheel drive have a reduction for both axles.

The reduction is achieved by an assembly called the crown car wheel and pinion. A pinion on the propeller shaft drives a larger car gear, the crown wheel, which is mounted in the centre of the axle car housing.

Speed reduction depends on the numbers of teeth used in the crown car wheel and pinion. If, for instance, the pinion has 10 teeth and the crown wheel 40, the propeller shaft turns four times to turn the crown car wheel and the road wheels once. This gives a 4: 1 speed reduction.

Rotating with the crown car wheel is the differential, a gear system which allows for differences in speed between the powered wheels when rounding a bend. The crown car wheel and pinion also turn the drive line through 90°. This is possible because they are bevel gears, meshing with their axes at right angles to each other.

There are three types of bevel car gears — straight, spiral and hypoid-spiral. Straight bevel gears have teeth cut in line with the axes of the shafts, and spiral bevel gears have curved teeth. Hypoid-spiral bevel car gears also have curved teeth, but the axes of the wheels are not in the same plane. This means that the centre of the car pinion can be set below the centre of the crown wheel, giving a lower propeller shaft. Consequently, the tunnel in the floor which houses the propeller shaft can be made lower, or can even be eliminated.

How the differential allows Auto Car Wheel speeds to differ

When the car rounds a bend, the inner wheels trace a shorter path than the outer ones. If both driving car wheels were fixed rigidly- to one axle driven by the crown wheel, they would have to run at the same speed, causing the inner wheel to skid.

This is avoided by dividing the car axle into two half-shafts. Each half-shaft is driven independently by the differential, so that when the inner car wheel slows, the outer car wheel speeds up, and the crown wheel turns at the average speed of the wheels. If the inner car wheel turns at 50 rpm and the outer car wheel at 100, the crown wheel will run at 75.

Limited-slip differentials to counter Auto Car Wheel-spin

The disadvantage of a differential is that it will allow a car wheel which cannot grip a slippery surface to spin at twice the speed of the crown car wheel, leaving the opposite wheel motionless.

This is because the differential always applies equal torque (turning effort) to each road car wheel. If one wheel is spinning, doing no work, the other car wheel will not do any work either.

This problem is overcome in some high- powered cars by a limited-slip differential, using self-locking or frictional devices.

In one common type of limited-slip differential, clutch cones are fitted between the differential bevel car gears and the cage in which they are housed. Springs between the car gears press the conical surfaces into contact, creating a frictional resistance to any difference between the speeds of the gear and the cage.

The resistance is not enough to stop the differential action when the car is following a curved path; but it builds up when the driving torque applied to the differential is increased—when using full throttle in a low car gear, for example.

Driving torque tends to force the differential bevel car gears apart, so adding to the load on the cones and increasing their resistance to speed differences between the half-shafts. This prevents wheel-spin.

Possibly related posts: (automatically generated)
Auto Car Wheels Transmission/Final Drives

• Auto Engine Parts Guide, Transmission Operation
• Automobile Transmission Axles Care
• Car Parts Guide, the Auto Clutch
• Breath a Fresh Truck
• More about Auto clutch: transmit the torque, or turning force, from the engine to the transmission
• What type Gearbox do you have on your car? Transmission and Auto Manual Gearboxes Guide
• Caring for Tires Auto maintenance part 3
• Selecting the Auto Part Gears in Different Designs
• Car Pushing Starting/ Car Clutching it
• Complete Auto and Truck Repair Cornerstone Automotive