Efficiency of RWD vs. AWD at high speed

[Braymond141]

Still trying to find out what kind of drag equation that would apply to all the components of an AWD car.

 

[Greg Freeman]

Still trying to find out what kind of drag equation that would apply to all the components of an AWD car.

I seriously doubt you'll find something simple that gives you an answer that will give you something usable for the drag on the components. It will depend on the size/shape/design of what's in the car, lubrication, etc. I challenge anyone to prove me wrong because I want to know how it was figured out myself.

To be honest getting a very accurate answer will just trick you to make you think you know something to an extreme amount of precision about the car, but you really don't. Those 17% to 25% figures that you quoted and I'm surprised that you didn't mention when you first asked the question, and I also have no idea where they came from, are probably the best answer you will get.

Why?

-You don't know how accurate your vehicle drag coefficient is
-You don't know how accurate your speedometer is
-You probably won't be accurately measuring windspeed in any real world case
-You won't accurately know the air density (which will affect not only drag but the motor's ability to develop power)
-You won't know anything about rolling resistance
-You still don't have an accurate number for crankshaft horsepower
-The accuracy of the dyno is also something unknown (but you could probably get an accurate number for that because hopefully the guys that own the dyno keep it reasonably calibrated)


I'm not trying to be a killjoy or anything, but people in controlled environments where a lot more is known about the situation are happy to get an answer within 5% most of the time, with much simpler experiments. All the additional precision in the world for one aspect of the problem isn't going to make the whole thing more certain.

 

[Ulysees]

What about equations guys? The OP was asking for them. Frictional torque in drivetrain:

$$T_f = k_f S \omega^g$$

$$S$$ = area of contact, therefore longer drivetrain (ie larger area of contact) produces more friction at a given angular velocity $$\omega$$.

Angular velocity:

$$\omega = k_t v$$,

$$k_t$$ = transmission factor
$$v$$ = speed of the car

Hence

$$T_f = k_f S (k_t^g) (v^g)$$

If the engines and air drags are identical, the car with the smallest $$T_f$$ over the race wins.

If the transmission ratios were identical (meaning $$k_t$$ factors identical), the winner would always be the car with the smallest surface $$S$$, ie the one with shortest drivetrain (the RWD).

But transmission ratios are different so the RWD wins at high speed (its transmission factor $$k_t$$ is low, and also its frictional surface S is less due to the shorter drivetrain).

At low speed the other wins because drivetrain friction becomes less relevant, it is small compared to the engine torque. So at such speed it's the $$k_t$$ factors of the engine-torque transmission that make a difference, and the AWD happens to transmit more torque to the wheels.