Next car?

[Irvrobinson]

AWD is beneficial for many things, but not necessarily dry handling because of a thing called "polar moment of inertia". This, along with poor torque split, is why almost all AWD cars understeer quite a bit at the limit.

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?

 

[Irvrobinson]

Looks like you're not entirely wrong on account of A3,
from https://www.autozeitung.de/audi-a3-sportback-bmw-118i-kompaktklasse-test-171993.html#fahrdynamik

On the handling course, the Audi is very agile. Change of direction is easy for him, the skilfully tuned ESP holds discreetly in the background. Who presses the ESP button, raises the control threshold of the system and can look forward to well-dosed load change reactions, which - skilfully provoked - the Audi A3 Sportback 1.8 TFSI even easier around the corner. However, the picture tarnishes a comparatively imprecise steering and strong body movements, which are at the expense of accuracy. Even the somewhat doughy brake pedal feeling bothers something, although there is nothing to complain about the braking power.

I know a lot about the A3, since I'm judging them based on my S3, which is just a duded-up A3. The 2016 S3 has precise steering, good braking feel and power, 40/60 F/R AWD, and it still understeers a lot on a curvy road. I should have test-driven it on curvy roads, but I was so impressed around town that I fell for it and bought one. Just talking about the understeer is making that M2 Competition sound better.

 

[2channel lover]

I think it's fine for minivans.

I once had a 63 mile (each way) driveway to parking lot commute, and bought a 4 cyl Toyota Tercel...FWD with a manual trans. Got me thru the '93 and '95 nor'easter...our RWD bimmer wasn't trust worthy to get us to the train station during the snow storm.

That thing served me well...when we moved down south I decided to keep it....lo and behold...my mechanic bought from us with 130k+ miles for his kid in HS.

Of course, that's totally different animal from a performance car.

 

[GrimSurfer]

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.

 

[Irvrobinson]

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 far, so good.

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.

For one thing, you're confusing moment of inertia with polar moment of inertia. They're different. Second, you're focusing on the effects of the weight of the AWD drivetrain additions, while not focusing on the effects from even power distribution on four tire patches rather than two. The front wheels applying drive torque have an effect that overwhelms the minor weight difference.

But wait, there's more...

I hope so.

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.

I said performance AWD cars have rear-biased torque distribution. Toyotas, Nissans, and Volvos often, but not always, have transverse-engine layouts anyway, and as I mentioned, they suck.

Don't ask me about North American AWD cars... I don't track/buy them and they are all over the map.

There's aren't very many interesting coupes or sedans with AWD anyway (except Cadillacs), and domestic SUVs don't count.

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.

First of all, you've mixed up your acronyms. PASM is an electrically adjustable hydraulic shock absorber system. You're referring to yaw control, which Porsche calls PSM, or Porsche Stability Management. It does work through the antilock braking system by applying braking on each corner independently when it detects speed differences between the wheels. The main point you've missed is that AWD multiplies the tire patch power application area by 1.75-2.0 (depending on the F/R section widths) and divides the power among the four contact patches. On a 911 this has much more effect than a ~5% increase in weight distribution on the front wheels. (Not all of the AWD mechanism is in the front axle. There is a center differential, among other things.)

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.

You're making that up. BMW's often do have a 50/50 weight distribution, but that's not the basis of their marketing message.

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.

Again, you've mixed up polar moment of inertia with moment of inertia. And old 911s had skinny rear tires and no PSM. Modern RWD 911s don't spin, unless you're an idiot. And the latest 911 generations have been pushing engines a bit forward in the chassis to improve weight distribution.

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.

Not in my experience. They just understeer. Modern FWD cars with yaw control won't spin unless you turn it off.

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.

I think you need a refresher course.

 

[GrimSurfer]

So far, so good.



For one thing, you're confusing moment of inertia with polar moment of inertia. They're different. Second, you're focusing on the effects of the weight of the AWD drivetrain additions, while not focusing on the effects from even power distribution on four tire patches rather than two. The front wheels applying drive torque have an effect that overwhelms the minor weight difference.



I hope so.



I said performance AWD cars have rear-biased torque distribution. Toyotas, Nissans, and Volvos often, but not always, have transverse-engine layouts anyway, and as I mentioned, they suck.



There's aren't very many interesting coupes or sedans with AWD anyway (except Cadillacs),and domestic SUVs don't count.



First of all, you've mixed up your acronyms. PASM is an electrically adjustable hydraulic shock absorber system. You're referring to yaw control, which Porsche calls PSM, or Porsche Stability Management. It does work through the antilock braking system by applying braking on each corner independently when it detects speed differences between the wheels. The main point you've missed is that AWD multiplies the tire patch power application area by 1.75-2.0 (depending on the F/R section widths) and divides the power among the four contact patches. On a 911 this has much more effect than a ~5% increase in weight distribution on the front wheels. (Not all of the AWD mechanism is in the front axle. There is a center differential, among other things.)



You're making that up. BMW's often do have a 50/50 weight distribution, but that's not the basis of their marketing message.



Again, you've mixed up polar moment of inertia with moment of inertia. And old 911s had skinny rear tires and no PSM. Modern RWD 911s don't spin, unless you're an idiot. And the latest 911 generations have been pushing engines a bit forward in the chassis to improve weight distribution.



Not in my experience. They just understeer. Modern FWD cars with yaw control won't spin unless you turn it off.



I think you need a refresher course.

Re: PSM vs PASM. Ok wrong acronym. I guess I'm just an air cooled guy in a water cooled world!

Re: Traction. None of this changes how a friction circle works. When you're steering and putting power into the same contact patch, lateral load limits will be less. Why? The Tire only has so much traction to give.

Example: Easy to rotate a RWD car while spinning the tires. The traction used for acceleration leaves nothing for rear lateral grip. Basic car control stuff.

Re: Polar moment of inertia, moment of inertia. Splitting hairs.

Re: Lying about BMW's emphasis on 50/50 weight distribution. If you're going to call somebody a liar, aircon, at least do a google search first (BMW + ad + 50/50). Otherwise, you come off looking like an idiot.

BMW is still using this line as the focal point of their advertising, which started in the 80s. How do I know this? Cause I was there, son. I could find a link for that too, but you'd only ignore it.

Re: Old 911s and narrow tires. They didn't weigh 3400-3500 lbs either, Irv.

A 70s 911S was about 2400 lbs. Those tires had about the same size contact patches as a new, heavier watercooled Porsche on wider tires. So this begs the question why newer Porsches have higher lateral grip than older ones. Lots of reasons, but tire compounding is a big part of it.

As you probably know, tire size doesn't equate to contact patch area... unless it's flat. The real issues are vehicle weight and inflation. Club racers and autocrossers fine tune their car's handling through inflation adjustments. Basic stuff, with one or two psi capable of making a bigger change in handling than 5-10 mm of cross ectional width.

Don't take my word for it. Read Carrol Smith's "Prepare to Win", Fred Puhn's "How to Make your Car Handle", Jay Lamm's "All Wheel Drive Handbook" or Skip Barber's "Going Faster!". I could name others on my bookshelf, but start with those. They're really good.

Re: FWD cars with yaw control. Yes, they will spin and in very dramatic fashion. Yaw control systems only work to a point. They do not and cannot overcome the laws of physics. Once the car starts to rota fast enough, yaw control can't stop it. If things worked as you suggested, we'd never see rice rockets slide sideways into telephone poles. Yet, it happens all the time.

It sounds like you've never been on a track, much less competed on one. Maybe you read other books. That's OK too. This stuff is not everyone's thing.

 

[Irvrobinson]

Re: Polar moment of inertia, movement of inertia. Splitting hairs.

I could go on debating with you point by point, but this one statement tells me it's futile.

 

[GrimSurfer]

I could go on debating with you point by point, but this one statement tells me it's futile.

I might enjoy the debate more than you.

Now is a pretty good time to quit. I could tell you how to dial-out the understeer on your S3, but you're smart enough to figure that out on your own now, aren't you?

Just don't buy the M2 and complain about its ride!

 

[Irvrobinson]

I could tell you how to dial-out the understeer on your S3, but you're smart enough to figure that out on your own now, aren't you?

Just don't buy the M2 and complain about its ride!

No, I'm not smart enough to know how to "dial out" understeer on a car with a 58/42 weight distribution and symmetric wheels and tires, so please enlighten me.

The original M2's ride was okay for me on a test drive. I haven't driven the M2 Competition yet, but given my history I doubt it would be a problem.

 

[GrimSurfer]

No, I'm not smart enough to know how to "dial out" understeer on a car with a 58/42 weight distribution and symmetric wheels and tires, so please enlighten me.

The original M2's ride was okay for me on a test drive. I haven't driven the M2 Competition yet, but given my history I doubt it would be a problem.

Elementary problem, with an easy solution.

I won't tell you directly, since you haven't accepted what I've written to this point.

I will give you a phrase that, with some reading and research, will tell you precisely what you need to address your problem.

Lateral weight transfer.

Feed a man a fish, and you satisfy him for a day. Teach him how to fish, and he'll be satisfied for a lifetime.

 

[Mikado463]

Elementary problem, with an easy solution.

I won't tell you directly, since you haven't accepted what I've written to this point.

I will give you a phrase that, with some reading and research, will tell you precisely what you need to address your problem.

Lateral weight transfer.

Feed a man a fish, and you satisfy him for a day. Teach him how to fish, and he'll be satisfied for a lifetime.

LOL, all of it meaningless if you drive on these God forsaken roads of Pennsyltucky !

 

[GrimSurfer]

LOL, all of it meaningless if you drive on these God forsaken roads of Pennsyltucky !

... and it won't affect the ride comfort of the car. LOL.

But let's not give Irv any more clues. The one I've provided is obvious to a car guy (not somebody who likes cars, but somebody who also knows how they work). I've also given him three book titles, each of which that contain enough info on their own to the answer the riddle.

Trust me. He's better off working this out on his own.

 

[Irvrobinson]

... and it won't affect the ride comfort of the car. LOL.

But let's not give Irv any more clues. The one I've provided is obvious to a car guy (not somebody who likes cars, but somebody who also knows how they work). I've also given him three book titles, each of which that contain enough info on their own to the answer the riddle.

Trust me. He's better off working this out on his own.

So, just for my edification, Yoda, when you say "dial in", do you mean an adjustment of existing parts, or do you mean replacing parts with other parts?

 

[Irvrobinson]

Re: PSM vs PASM. Ok wrong acronym. I guess I'm just an air cooled guy in a water cooled world!

Re: Traction. None of this changes how a friction circle works. When you're steering and putting power into the same contact patch, lateral load limits will be less. Why? The Tire only has so much traction to give.

Example: Easy to rotate a RWD car while spinning the tires. The traction used for acceleration leaves nothing for rear lateral grip. Basic car control stuff.

Re: Polar moment of inertia, moment of inertia. Splitting hairs.

Re: Lying about BMW's emphasis on 50/50 weight distribution. If you're going to call somebody a liar, aircon, at least do a google search first (BMW + ad + 50/50). Otherwise, you come off looking like an idiot.

BMW is still using this line as the focal point of their advertising, which started in the 80s. How do I know this? Cause I was there, son. I could find a link for that too, but you'd only ignore it.

Re: Old 911s and narrow tires. They didn't weigh 3400-3500 lbs either, Irv.

A 70s 911S was about 2400 lbs. Those tires had about the same size contact patches as a new, heavier watercooled Porsche on wider tires. So this begs the question why newer Porsches have higher lateral grip than older ones. Lots of reasons, but tire compounding is a big part of it.

As you probably know, tire size doesn't equate to contact patch area... unless it's flat. The real issues are vehicle weight and inflation. Club racers and autocrossers fine tune their car's handling through inflation adjustments. Basic stuff, with one or two psi capable of making a bigger change in handling than 5-10 mm of cross ectional width.

Don't take my word for it. Read Carrol Smith's "Prepare to Win", Fred Puhn's "How to Make your Car Handle", Jay Lamm's "All Wheel Drive Handbook" or Skip Barber's "Going Faster!". I could name others on my bookshelf, but start with those. They're really good.

Re: FWD cars with yaw control. Yes, they will spin and in very dramatic fashion. Yaw control systems only work to a point. They do not and cannot overcome the laws of physics. Once the car starts to rota fast enough, yaw control can't stop it. If things worked as you suggested, we'd never see rice rockets slide sideways into telephone poles. Yet, it happens all the time.

It sounds like you've never been on a track, much less competed on one. Maybe you read other books. That's OK too. This stuff is not everyone's thing.

Wow, that's quite a patch you made to that post. Ya know, one difference there seems to be between you and I is precision in use of language. You claimed the ultimate driving machine marketing was "based on" that 50/50 weight distribution factor. "Based on" tells me that it is the primary factor, and you couldn't know that (because it wasn't the only factor in their marketing story) unless you were part of the BMW marketing organization. Were you?

Also, are we discussing street driving or track driving? I thought we were discussing street driving. Which is it?

 

[GrimSurfer]

So, just for my edification, Yoda, when you say "dial in", do you mean an adjustment of existing parts, or do you mean replacing parts with other parts?

No clues more, me give you. Figure it out for yourself young Skywalker. Use the force, you must. Or some research you must do.

 

[GrimSurfer]

Wow, that's quite a patch you made to that post. Ya know, one difference there seems to be between you and I is precision in use of language. You claimed the ultimate driving machine marketing was "based on" that 50/50 weight distribution factor. "Based on" tells me that it is the primary factor, and you couldn't know that (because it wasn't the only factor in their marketing story) unless you were part of the BMW marketing organization. Were you?

Also, are we discussing street driving or track driving? I thought we were discussing street driving. Which is it?

I just added a vid. You know, in response to your post saying I make things up which I initially let slide... until I concluded you were actually serious. (It's hard to tell, sometimes you say the silliest things).

Street driving, track driving? Is there a difference driving the Dragon's Tail, swerving to avoid an accident, or auto crossing? Sure, the location and duration of the "events" are different, but the dynamic handling challenges are similar.

The track is a great place to hone skills and learn set-up without endangering lives or breaking laws. So it's a much better place to dial in a suspension than trying to do it on the street (where a poor choice of mods or adjustments can send your car off into the rhubarb or worse).

P.S. The difference between us is that I actually know what I'm talking about. Otherwise, you would known exactly what solution I was referring to when I used the term lateral weight transfer. It's so obvious.

P.P.S. Still not going to tell ya! Not while you're being an ass.

 

[Irvrobinson]

The difference between us is that I actually know what I'm talking about.

The evidence demonstrates otherwise.

 

[BoredSysAdmin]

I enjoy this bickering so much

 

[GrimSurfer]

The evidence demonstrates otherwise.

If you knew what you were talking about, you wouldn't have complained about your vehicles handling. You would have avoided disappointment through a rigorous test drive or resolved your handling issue by now. It's been three years after all.

It's really not that hard to change handling in a car like an S3. The basic elements are there and the foundation is a solid one. The trick is knowing how manufacturers build understeer into their suspensions and what to do to change this without the car turning into an oversteering and directionally unstable (read: dangerous driving) go-cart.

So good luck with that. You know what you're doing and I don't. No need to ask me how to solve your problems. You've got this.

 

[Irvrobinson]

If you knew what you were talking about, you wouldn't have complained about your vehicles handling. You would have avoided disappointment through a rigorous test drive or resolved your handling issue by now. It's been three years after all.

Actually, you're correct about the test drive miss. I didn't intend for it to be a tight-curves car, and it drove so nicely on the roads I did test it on, that after about 45min or so I just said I'll buy one. I didn't realize how compromised it was until last summer, and then on a challenging road in our new location I was appalled. As I suspected by my look underneath with the car lifted, Audi experts I consulted said essentially nothing was adjustable. I made the decision not to do any parts swapping or mods, since the architecture of the car is so compromised. Another test drive miss was how useless the Nav system is.