Car Differential - spider gear

[fog37]

Hello,
I need a quick clarification on how the differential works. I understand its purpose and how it operates but I am not sure how, when a car is turning, the spider gear starts spinning to provide the outer wheel with a higher angular velocity? Is it simply because the outer wheel starts to naturally rotate faster than the inner wheel since the ground demands it rotate faster?

When the car goes straight, both wheels spin and the spider gear rotates without spinning...

Thanks!

 

[anorlunda]

This is a case where a picture is worth a thousand words.

 

[fog37]

Thank you!

From what I see, the orange spider gear starts spinning, when the car is in a curve, simply because one of the shafts starts rotating faster...

 

[jack action]

The reason the outside wheel goes faster is because of the torque difference between the inside and outside wheels. This torque difference creates a net torque between both wheels, which means one will accelerate with respect to the other one.

When you enter a curve, the outside wheel tends to slide to «catch up», which creates a friction force braking the wheel, thus counteracting the forward torque coming out of the wheel. For the inside wheel it is the opposite: The wheel tends to slide backward, which creates a friction force accelerating the wheel, thus adding to the forward torque coming out of the wheel.

This is why when you have one wheel on asphalt and the other one on ice, the maximum friction force is different for both wheels and once the wheel on ice has reached its maximum resistance, the torque of that wheel stops increasing. But the wheel on asphalt can still increase its torque. The result is that the wheel on ice starts accelerating (along with the driveshaft). If this starts early enough, the wheel on asphalt won't even begin accelerating and thus its speed will remains at zero.

 

[Tom.G]

Consider this.
If one axle is held stationary, say one tire is on the ground and the other is raised on a jack so that it can spin freely.
When the Ring Gear starts turning, it carries the Spider Gears around with it... but the Spider Gears are engaged to the fixed axle, therefore they have to spin on their own axes. Now consider the free axle, it is also coupled to the Spider Gears. When the Spider Gears turn, the free axle is forced to turn.

When the vehicle is turning a corner, the inside tire, being held to a lower speed than the outside tire, forces the Spider Gears to rotate around it... an analog, or partial, version of the stationary - free situation in the first paragraph.

(resultant rule-of-thumb: whichever tire has the least traction is the one that will spin; and the torque supplied to the other tire is only the sliding torque of the spinning one! Not much help when one tire is on pavement and the other on mud or ice.)