# Why does the Spider gear (differential) start to rotate about its axis when a car takes a turn?

1. Oct 3, 2014

### rambharath94

Whenever the car goes about a differential comes into action.This action generally starts with the spider gear starts to rotate about its axis,this causes increase in speed in one wheel and decrease in other.I want to how this spider gear starts to rotate about its axis what causes it to rotate ???

2. Oct 4, 2014

### Damo ET

If you consider both rear wheels going round an apex of a corner, the inner wheel being closer to the apex than the outer wheel travels less distance through the corner. Imagine the corner is a portion of circumference of a circle, the closer you are to the centre point of the circle, the smaller the circumference will be, and as there is a fixed distance between the two rear wheels, one is travelling further to get around the corner than the other in the same time period. This difference in distance results in differing axle (spider gear) motion from one side to the other.

Damo

3. Oct 5, 2014

### rambharath94

Thank you Damo,
There must be some force acting on the side gear ( slowing it down) which starts the rotation of the spider gear! I'm searching for that force which causes a load to act on the inner wheels thr by slowing it down (this inturn slows down the inner side gear) then the spider gear starts to rotate abt it this causes the other side gear (outer) to rotate fast helping the outer wheels to cover a large distance.

4. Oct 5, 2014

### rcgldr

update - lost an edit, this post is fixed now.

During the transition into a corner, while the tires accelerate to their steady state speed in a turn, the inner tire is slowed down by an "backwards" (opposing) torque, while the outer tire is sped up by a "forwards" torque. The torques feed back through the spider gears, and the side gears accelerate during the transition until everything reaches some steady state.

Last edited: Oct 5, 2014
5. Oct 5, 2014

### rambharath94

thank you rcgldr,
y should the inner tires experience more opposing torque?,can u elucidate more on this, please also provide the nesscary terms e.g. name of the force causing opposing torque. if u can draw a diagram.

6. Oct 5, 2014

### jack action

oing into a turn:
• the inner wheel wants to go faster than it needs to follow the path;
• the outer wheel want to go slower than it needs to follow the path;
Hence:
• the inner wheel tends to spin;
• the outer wheel tends to skid;
Those two extra (opposing) friction forces, create the small torque that makes the spider gears accelerate. Once both wheels reach their correct rpm to follow their respective paths, the small torque disappear, the axles and spider gears stop accelerating and thus their new acquired rpm become constant. And the torque is back to being the same on both wheels.

There are nice animations on HOWSTUFFWORKS to help visualize a working differential.

7. Oct 5, 2014

### rcgldr

Somehow I lost an edit, but jack action's post explains what happens. During the transition, the inner tire is slowed down by an opposing "backwards" torque, while the outer tire is sped up by a "forwards" torque.

8. Oct 7, 2014

### Ranger Mike

see Automotive Differential Dec 31,2011 by bugatti
specifically post #14 from Tea Jay..old engineering film

9. Nov 29, 2014

Thank you Jack Action you explained very well.i found your view helpful about spinning of spider gear.

10. Nov 29, 2014

### rambharath94

Thank u ppl ,

I watched the videos but im still ambiguous ab wat causes the backward torque

11. May 21, 2015

### Soham De

12. Sep 15, 2015

### rohitjinnu16

i saw the animations the motion to the rear wheels is provided by the spider gears only,so only one input.may be i think an epicyclic gear train needs 2 inputs to give a positive drive.what about the other input.please explain if i am wrong in any way.

13. Sep 15, 2015

### jack action

There is a resistive torque (due to friction) from the inner wheel and another one from the outer wheel and an input torque from the driveshaft; So you have 3 torques working together.

If you remove the driveshaft torque (car in neutral), then you still have the inner and outer wheels' torques. It is the difference between those two torques that will dictate if the spider gears turn or not (whether there is a driveshaft torque or not).

14. Sep 15, 2015

### rohitjinnu16

yeah i got it han q