Thread: Automotive Differentials View Single Post

 Quote by slideways Basically yes. Aside from wear, the open diff plus computer controlled brakes is theoretically a better system even though many of the car people shudder at the idea. However, here's why it's good. The open diff always splits the torque. That means at any point in time we can send half the available torque to either wheel. Most of the time our concern is that we have more torque than one wheel can handle thus that wheel slips and the other wheel is thus limited to the low torque value of the slipping wheel. With a wheel brake we can increase the the torque an axle handles by adding the brake torque to the traction "torque" (of course that traction goes through a few steps before becoming a torque in the axle). With the brake torque and the traction torque added to 1/2 the engine torque we now can deliver the other 1/2 of the engine torque to the other wheel. Even better we can do this when the wheel speeds aren't supposed to be the same. We can now effectively control torque and speed of both wheels. The speed is controlled because, theoretically, the speed of the engine (diff housing) is the average of the two wheels and, thanks to our traction brake system neither is slipping. We control torque to each wheel individually via two inputs. The first is the engine throttle. That determines the max torque to each wheel. The second is the brake application. Brake application determines the reduction in torque to either wheel. Thus torque to say the outside wheel is 1/2 engine torque while torque to the inner wheel is 1/2 engine-brake load. This also has a critical advantage in that you don't have the sudden spike in torque like you do with a LSD as it starts to break free. With a LSD we already said while cornering the slower wheel gets more torque. Unfortunately that also happens to be the unloaded wheel when you are cornering hard. If, due to cornering, the inside tire unloads to the point where it starts to slip (ie the torque is greater than the traction available) it will speed up. Well if you look at the formula for torque to the wheels you can see that if the inside tire speeds up then the clutch torque can drop to zero. That results in a sudden INCREASE in torque to the outside tire. Well that tire was already doing it's best to keep the rear end in line as you were turning. No it's like you were on the limit and VTEC just kicked in yo. You exceed the grip of the outside tire and the car spins. Not cool. With off road things might be different. A true locker or LSD sometimes might be better than a brake based system (again ignoring the wear and brake heating issues). I said with the open diff you can only send 50% of the torque to any wheel. It may be there is a time when, say climbing a rock, where one wheel has PLENTY of traction but you need more than 50% of the engine torque to actually get the vehicle to move. I'm not an off roader so I'm just speculating.
You are correct.

Off road, the brake based systems tend to at least require programming changes to allow some spin, to "get going".

What happens off road for example on systems that have poor programming, is that the truck will just sit there shuddering. Essentially, as soon as some torque is applied, the system senses slippage and brakes the spinning tire....and if none of the tires has enough traction on its own to make the rig MOVE, well, the poor things sit there shivering and shuddering as each tire tries to rotate, goes faster than another tire, and is promptly braked.

By allowing some initial spin/rotation to occur, it also allows the other tires to try to pitch in on the propulsion thing, so some momentum can be gained.

This is also why, from a propulsion stand point, the front lockers can outperform the rear lockers.

Essentially, the locker really only helps if one side slips, by allowing the OTHER side to keep pushing you up the hill, etc.

The tires that slip the MOST though are the front tires, which are more lightly loaded, and tend to have less wheel travel to stay on the terrain.

So, lockers help the axle which slips the most, more than they help the axle that slips the least....and if it doesn't slip at all, the locker is not a factor.

Of course, its a lot harder to STEER a locked front end.

:D