9

This confuses me. The point of a differential is to make it so the wheels don't slip when one needs to be turning faster than the other, such as during turns.

So if the rear wheels are directly connected by a solid axle, doesn't that force them to turn together? How is turning handled?

19

FWD cars dont have a rear axle, the wheels are mounted independently, there may be some situations where there is a tube there, but it will not have an axle shaft.

  • 2
    To expand on this, @Aerovistae, you may be getting confused after seeing cars with things like Twist Beam Suspension which can look like a solid axle from behind. – Dan Feb 1 '13 at 10:11
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    I wouldn't go so far as to say FWD cars don't have a rear axle. They could certainly have a rear axle--but the rear wheels would ride on independent bearings, so you're right about the key point. From wikipedia: "An axle is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to its surroundings, with the wheels rotating around the axle." – mac Feb 1 '13 at 15:21
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    @mac "Stub Axle" tends to be the phrase, there. – Dan Feb 1 '13 at 15:27
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The wheels just ride independently on bearings, they aren't connected by an axle like in a RWD vehicle.

I think you misunderstand the true purpose of a differential. A ring and pinion is necessary to take the rotation of the driveshaft and change it's rotational axis. That is, the driveshaft is spinning about an axis from front to back of the vehicle and something needs to convert this rotational force perpendicularly to turn wheels.

True that differentials (non-spools) allow one wheel to "slip" but that is a design to handle that scenario rather than the true intent of the differential.

I hope this makes sense.

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    I actually disagree a little with this - a differential is there to allow the wheels to turn at different speeds and there are plenty of examples of vehicles with a 'solid' drive axle. – Dan Feb 1 '13 at 13:57
  • I'm with @Dan. There are plenty of mechanisms that "change the rotational axis" without allowing a rotational speed differential (notice the similar terminology--no coincidence) between the two output shafts. The purpose of an automotive differential is to allow such a speed differential. – mac Feb 1 '13 at 15:18
  • It's not enouugh for them to turn at different speeds, they also need to both deliver power. Something that sprag clutches or "detroit lockers" don't do. – mckenzm Sep 5 '16 at 23:51
2

It seems to me there's some confusion about terms here.

An axle is just a bit of ironmongery holding the wheels in place. The wheels can rotate independently of the axle: there's a bearing between wheel and axle.

Here's a Ford model T front axle:

Ford T front axle

The two wheels are connected, and suspension is provided by leaf springs between the axle and the chassis. This has some drawbacks:

  • the axle is heavy, so unsprung weight is high. This means a less comfortable ride.
  • movement of one wheel is transmitted to the other. This reduces the freedom each wheel has to react to changes in the pavement. A pothole on one side of the road will be 'felt' by both wheels. Again, reducing ride quality.

To solve these issues, modern cars usually have independent suspension: there is no physical connection between the wheels. Both wheels are attached to the car body instead. The term 'axle' is still used to indicate the two wheels that are in line across the car.

When the wheels are driven, there is a physical connection: the drive shafts. Then you'll also see a differential in the middle, to allow the wheels to run at different speeds when you take a turn.

Here's a Ford model T rear axle:

Ford T rear axle

In this case, the axle is a tube. The drive shafts run inside the tube.

In independent suspension, there's normally no tube so the drive shafts are exposed.

(images borrowed from this site)

1

On FWD cars the rear wheels do not transmit the power, they are free to rotate freely

0

The purpose of the differential is to allow two axles to turn at different speeds while delivering power to them.

If wheels are not used to deliver power to the road then they can just be allowed to spin independently. No differential is needed.

So a front wheel drive vehicle has a differential at the front. A rear wheel drive vehicle has one at the rear. A four whel drive vehicle has differentials at both the front band back and often a center diff too.

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