Wheelspin in an open differential = zero torque?

[hari00968]

Hello.

An open differential works by the principle of distributing torque equally to both wheels, no matter what. And I think I understand how that works, so no problem with that.

Consider the case of one of the wheels on ice, and therefore having a very low traction limit. Almost every book I've read says that in this case, because the torques are equal on both wheels, and since the torque on the wheel lying on ice is almost zero, the torque on the other wheel is also zero. This makes sense from the point of view of torques being equal, but my question is, how CAN the total torque transmitted be zero? I mean, where's all the engine's power going then? You're still flooring the throttle, and it's still burning fuel, crankshaft turning, everything, where is the power going?

 

[SteamKing]

Where most of it goes anyway: out the tailpipe.

 

[Averagesupernova]

You are not burning the fuel that you think you are. Obviously you are burning some fuel but generally throttle position compared with speed of the crank gives a pretty good indication of the power that the engine is making. I think you will find that your foot isn't into it as far as you think. Oh yes, almost forgot, you are doing a fair amount of heating of the tire.

 

[256bits]

Put the tranmission in neutral and press the accelerator. You could ask the same question. Out the tailpipe as Steamking said ie just waisting fuel.

In the ice scenario, the energy goes to heating the tire, the wheel bearing, the spinning dfferential, the transmission, and the moving engine components.

 

[Lsos]

The torque on ice is not zero, but close to zero. You will find that when you're on ice the engine doesn't have much load on it, as it was mentioned almost like it was in neutral. If you give it too much gas the engine will simply go towards redline. If you keep the wheels spinning at a constant speed, you'll see that you're not giving much gas at all.

 

[Ranger Mike]

how CAN the total torque transmitted be zero? I mean, where's all the engine's power going then? You're still flooring the throttle, and it's still burning fuel, crankshaft turning, everything, where is the power going?
Torque transmitted is not ZERO. Only when the transmission is in neutral, do we have near zero torque transmitted. It is near Zero because a minimal amount is transmitted as the engine pushes to one side of the motor mounts under acceleration of the internal components...crankshaft , flywheel etc..
When on ice, there is significant torque applied through the drive train to the rear wheels. Because of the self canceling action of the differential, it " feels " like there is zero torque applied but if in doubt, jack up the vehicle and with the transmission in neutral, try to turn the drive shaft or half shaft by hand...there is a lot of torque required to turn the gears and both wheels.

 

[brewnog]

how CAN the total torque transmitted be zero? I mean, where's all the engine's power going then? You're still flooring the throttle, and it's still burning fuel, crankshaft turning, everything, where is the power going?
Torque transmitted is not ZERO. Only when the transmission is in neutral, do we have near zero torque transmitted. It is near Zero because a minimal amount is transmitted as the engine pushes to one side of the motor mounts under acceleration of the internal components...crankshaft , flywheel etc..
When on ice, there is significant torque applied through the drive train to the rear wheels. Because of the self canceling action of the differential, it " feels " like there is zero torque applied but if in doubt, jack up the vehicle and with the transmission in neutral, try to turn the drive shaft or half shaft by hand...there is a lot of torque required to turn the gears and both wheels.

...But it's nothing like the amount of torque applied when a wheel actually has traction.

 

[spanky489]

torque should be constant in your situation and even in normal driving conditions, only thing that changes is the torsion in your transmission elements.

 

[brewnog]

Why would torque be constant under normal driving conditions? That makes no sense to me.

 

[spanky489]

sorry i misinterpreted the word torque because in my language it means the same thing as T= F*r, which is independent of the number of revolutions but in america i guess you guys use torque as the power of moment as well, which is P=T*w

 

[brewnog]

I don't think it's a language misinterpretation Spanky. Torque is the same concept in both of your examples.

 

[olivermsun]

Keep in mind that, while your one wheels is spinning into oblivion (and probably digging a small groove in the ice) the other three wheels are probably stuck in ruts.

Tiny bit of torque on the spinning wheel = enough to keep it spinning vs. friction on the ice
Tiny bit of torque on the other wheel = not enough to move your car out of its ruts.

Plus all the other losses mentioned before.

 

[hari00968]

I didn't realize 'other' losses would be this significant. While these losses contribute to the load, I'm assuming it's still a very small load compared to the normal load on a wheel when a car is coasting along at a decent speed. So I'm guessing flooring the throttle is a bad idea in such a situation? Because if you get the engine to produce all its power it's just going to make whatever is free, spin really really fast right?