Oh really? Why don’t you explain to us all how AWD affects polar moment of inertia?
Most performance cars with AWD have a torque split that’s rear biased. As for almost all AWD cars understeering at the limit, how would you know that? Does a 911 with AWD understeer at the limit? Or a BMW 3-Series? Or even an X5?
Yup. Simple physics.
Polar moment of inertia describes the effect imposed by how weight is distributed along a body's longitudinal axis. In the case of a car chassis, it goes beyond simple expressions of weight distribution (50/50, 60/40 etc).
Imagine, if you will two dumb bells of the same mass. One has a really thick shank and small ends. The other has a thin shank and big ends. The dumb bell with the thin shank and big ends will have a higher polar moment of inertia because it will require more turning force (torque) to initiate a change in direction to overcome the inertia of its masses.
So how does this apply to an AWD car? The extra weight at the ends of the car (not the very end, but at the axle which is pretty far from the car's centre of mass or balancing point) to accommodate another transaxle increases the longitudinal distribution of weight. So the turning forces needed to change direction are proportionally higher -- the exact opposite of that which occurs in a mid engine car with a low polar moment of inertia.
But wait, there's more...
As a vehicle turns, it is subjected to centrifugal force. Many think this force acts on the whole car but actually it acts according to where the various masses lie along the chassis. So more weight, the greater centrifugal force acting on that part of the car. Hence more slip.
Your assertion that AWD cars have rear biased torque slip is incorrect. Audi, Porsche, BMW, and MB (except for the A and B classes) do. Toyota, Nissan, Volvo and many others do not. Subaru has a fixed 50/50 torque split in their (now gone) manual transmission cars and a variable 60/40 split in their CVT equipped models. Some AWD systems are very front biased, only sending torque back to the rear when the front wheels slip as one approaches the limits of tire adhesion.
Don't ask me about North American AWD cars... I don't track/buy them and they are all over the map.
Now let's take an extreme example... the Porsche 911. AWD versions of this car understeer far more than RWD versions. This is because the drive forces being transmitted through the rear wheels, combined with the high slip angles of the rear wheels (which don't turn, or at least not very much) can easily exceed available traction. Physics wields a heavy hand and centrifugal forces acting on that heavier rear end, cause the car to oversteer until the Porsche Active Stability Management (PASM) system kicks in.
In early 911 days, when the chassis had a really weird distribution of weight, you could spin the car in a very gentle turn by abruptly lifting off the throttle. Bringing the engine and transaxle closer to the centre of the chassis reduced drop-throttle oversteer. The introduction of AWD to the line placed about 120 lbs or so over the front axle and, surprise, surprise, the car when from displaying terminal oversteer to light/moderate understeer at its handling limit.
Since that time, the introduction of PASM has improved handling a lot by applying asymmetric breaking to keep under steer and oversteer in check. But PASM cannot change the laws of physics, just mask them until the point where the limits of Tire adhesion are reached. At that point, Sir Issac Newton takes over full control and the car behaves according to its masses, the forces being exerted on it and various laws of motion.
Also, keep in mind that passive restraint systems are mostly designed around a front end collision. So manufacturers *purposely* dial in at lest a hint of understeer into the suspension. Doing so appeals to most of the intuitive responses of drivers in a panic situation.
Not many people would want to oversteer, lose control, and slide sideways into a telephone pole or oncoming car... but losing control, understeering, and hitting these objects head-on, with air bags, seals belt tensioner and several feet of crumple zone makes such accidents "survivable" in many cases.
If you've been following my other posts, you might remember me saying that manufacturers use stats for maximum advantage. Weight distribution is no different.
BMW's Ultimate Driving Machine claim of the 80s and 90s was based on their claim that the 50/50 weight distribution of their cars gives them an advantage. This has *some* merit, but doesn't influence things as much as where, and how much, mass lies along the longitudinal axis of the chassis.
Porsche bragged about their 911's "low polar moment of inertia" in ads of the era. The didn't mention that the weight was concentrated mostly in the back, making the 911s of that era about as stable as a thrown hammer when the limit of adhesion of the rear tires were reached.
The same thing can happen in a FWD car when it all goes wrong. I've seen those cars spin in all manner of ways as the driver struggles to take control back from Sir Issac. It's all a matter of inertia, though manufacturers generally dial in so much understeer into the chassis of inexpensive FWD cars to make them relatively stable even in the hands of an idiot.
Sorry for the long reply but I find physics rather fascinating and can drone on a bit. I have also raced in the past and authored a few technical books on cars (don't ask titles, I'm not selling and finished that business many years ago).
Hope this helps explain the importance of mass distribution on handling.
