Differential underdrive

[Brandon505]

Does anyone know the purpose of differentials under-driving the drive-shaft? it says in Halderman 4th edition textbook that the drive-shaft turns as much as 3 times as fast as the wheels

[Dave B.]

First, the wheels don’t drive the drive shaft, it works the other way around. And yes, a drive shaft can easily turn 3 or more times as fast as the wheel(s) it is turning. That’s what the gears in the differential do.

Let’s look at a 1960s car with a 4-speed transmission. Inside the transmission, most of those had a 1st gear ratio of about 2.5 to 1. That means that the engine had to turn over 2.5 times for every turn of the transmission’s output shaft (which connects directly to the drive shaft). 2nd and 3rd gears may have had ratios of – say – 1.7 and 1.4 to 1. This let the car accelerate smoothly. 4th gear was usually 1:1, which means that in high (4th) gear, the output shaft (and thus the drive shaft) turns exactly the same number of revolutions as the engine.

Now we encounter a problem. Most cars and trucks are much too heavy for the engine to drive the rear wheels at a 1 to 1 ratio. So, the differential – which is there to split the drive shaft’s power right and left to the rear wheels – is fitted with a set of reduction gears that allows the engine to move the vehicle in an efficient manner. A light passenger car may have a gear set of – say – 2.7 to 1. A truck that pulls very heavy loads might have a gear set with a ratio as high as 5.5 to 1.

In the old hotrod songs, you’d hear about 4.11 gears (in the differential). That represents a ratio of 4.11 turns of the driveshaft to one turn of the rear wheels. A common gear set used to create that ratio would have 35 teeth on the ring gear and 9 teeth on the pinion.

Making a wild generalization, going to a higher numerical ratio (such as going to a 4.11 ratio from a 3.00) will make the car accelerate faster, but will limit its top speed and also reduce fuel mileage.

All of this was a lot easier to see and understand before front-wheel-drive enclosed all of the separate operating units into a single housing!


Dave B.

[Black Lotus]

The engine needs to be kept in an RPM range that is efficient for it, at the vehicle speed range that is desired.
So, normally the engine likes to be spun much faster than the vehicles' road wheels.
Also, when there is a gear reduction, the torque is multiplied as well.
So having the final gear reduction at the last part of the drive train, it keeps most of the drive train --gear box, driveshaft from being subjected to the wear and tear of the increase torque.
As an example, if you were to under drive the engine by 3 to 1 before the gearbox, the gearbox would need to be 3 times stronger than if it were to take the power at 1 to 1 off the end of the crank, or clutch.