Hi,

I'm building rather simple tank chasis simulation, with elements like engine and transmission. Quite a few car physics tutorials where helpfull but it seams that most of them avoid the subject of how differential should properly work. What I mean is this - assumption that drive shaft rotates with the same speed as driving wheels. From what I understand, when car starts to turn and drive wheels starts to turn with a different speed, differential will "cancel" that difference and driving shaft will be spinning just as you drive forward. What I'm trying to understand is how this "canceling" actually happens. in a sense that as rotation of wheels can be "slowed down" by other forces, which can be different on different side. How do I propagate this difference in rotation back into driving shaft? This can't be a simple sum.

In case of tanks it get's interesting. I was looking at this artcile: http://www.gizmology.net/tracked.htm

and if I want to model Double Differential Steering I would need to somehow move torque between separate tracks, calculate propulsion and friction for tank itself, calculate torque effecting tracks and then calculate new angular velocity of driving shaft from those effects.

The differential is a drive dear containing a pinion gear set.

Here is a pic of the differential gear from the GM 6T70 transmission:



The large gear is the drive gear. The inner set of pinion gears are what is important. If you look closely at the pic, the two smaller pinion gears are directly connected to the differential assembly via a shaft. The drive shafts are connected through the assembly and splined to the larger two pinion gears. The large ring gear is the main drive gear. What this does is allow the drive shafts to turn independently of the drive gear. When the vehicle is moving forward, the pinion gears are stationary and the whole pinion assembly is rotating in sync with the differential. When the vehicle is turning, the outer wheel needs to turn slower than the inner wheel, just as a point on the edge of a spinning disk is spinning slower than the point near the center. At this point, the pinion gears start to turn, allowing for a difference in drive shaft speeds.

I work for a company that remanufactures transmissions and have worked on the GM 6T70 exclusively for the past year, BTW.

How you plan on translating this into code, I do not know. I'm really not sure why you would need this level of physical modeling.

I don't really want to simulate a complete physical interaction of the differential itself. More to understand how torque from wheels is passed back through differential to driveshaft.

Let's say that driveshaft turns at 30rad/s, vehicle rolls straight so both wheels spin at 30rad/s * differential ratio. let's assume that differential ratio is 1. When vehicle turns right wheel spins at 20rad/s and left one at 40rad/s.

But what if you start to brake both wheels while turning? Let's say that their new velocity is 15 rad/s and 35 rad/s, how do I figure out at which speed driveshaft is suppose to be turning now? is it really just (RightWheelTorque + LeftWheelTorque) / 2, that would be too easy...

First off, the torque is passed from the drive shaft TO the wheels, not the other way around. [edit] Misread your post.

What happens is that the inner wheel is still turning forward, while the outer wheel is actually going in reverse. The result is a net speed difference between the wheels. I don't know the math behind this, however.