It's all about mu

[Skyllz]

In short... the OP explained thew theory behind why I love driving in the rain, the car feels so alive and responsive despite the much lower overall grip.

[wparsons]

Originally Posted by Suberman

There's no doubt the BRZ can handle. It's after it stops handling so well that the issues arise.

On dry road the car grips well enough but on wet roads, and as this whole topic got started, and snow or ice it reveals an Achilles heel.

It's not a handling issue, it's a driving issue.

The whole argument got shifted to grip levels and eventually this thread because you keep blaming the chassis for the oversteer on low grip surfaces but that's simply not the case. The oversteer you're complaining about is from asking the rear tires to provide more combined (lateral + forward acceleration) grip than they can. This will ALWAYS lead to oversteer with a RWD car or understeer with a FWD car. You don't need complicated physics to know that the wheels getting the most power are going to be the first to break loose.

I have never had mine oversteer off power, and even with trail braking or a big sudden lift of throttle mid corner they don't get upset or spin wildly. They'll rotate a couple degrees, but it's not like an Elise, MR2, S2000, etc that'll bite your head off for lifting at the wrong time.

I'm going to make a video for you to show how easy it is to get these cars to understeer off power and the kick them into a ton of oversteer just by giving them gas at the wrong time in a turn, and then take the same turn driving properly and show that you can actually take the turn faster without any drama. And even better, I'll shoot another video doing the exact same thing in a B5 S4, which is known to be an understeering pig when pushed.

Now that @Dave-ROR is watching this thread, maybe he'll throw in some videos of the difference between a slight (desirable) rotation at turn in and a wild drift?

[brianc]

love the tags. epic.

[drewbot]

Originally Posted by wparsons

Best part, I'm not an engineer and don't work at a power plant at all. I figure he either tried to look me up online and failed, or was trying to make a lame homer simpson joke.

Whenever I see your username, I immediately think of this engineering company:

[7thgear]

Originally Posted by Suberman

These boards are a democracy. You can accept or reject whatever you wish.

privately owned boards are most definitely not a democracy.

what kind of lawyer are you anyway?

[Suberman]

Now we all agree that tire rubber on road mu is a constant for practical purposes (the variables are vertical load and horizontal force, which when divided produce the same number (that would be the constant) referred to as the coefficient of friction) we understand why the friction circle is a useful concept.

Each contact patch has its own friction circle and the vehicle as a unit has a friction circle resulting from the sum of the four friction circles.

Each contact patch has its own friction circle. Neither the shape nor the size of the contact patch is relevant because the vehicle weight does not vary.

Each friction circle has a maximum size, exceed the forces described by the friction circle for any contact patch and that contact patch will slide, reducing the size of the friction circle rapidly, relatively speaking.

A vehicle with track and wheelbase does not need steerable wheels to corner.

To turn a bicycle requires that the handle bars be at an angle to the direction of the bicycle opposite to the desired direction of the turn.

A car with steerable wheels always initiates a turn by understeering.

A car that uses driving wheels to steer will initially corner better than a car that uses one axle to drive and the other to steer but will reach the limits of cornering sooner.

There's no free lunch where mu is concerned. Use mu to drive the car and it isn't available to corner.

Therefore, a car set up to oversteer will be slower than a car set up to understeer. This is why a stiffer front bar on the BRZ will make it lap faster up to the point the drive wheels can no longer add forward thrust. Look at any touring car racer from the 60's and 70's when all were rwd and note that they all lift the inside front wheel in a corner. Note that most, if not all fwd race cars converted from road cars lift the inside rear wheel instead.

The human body isn't very good at detecting g. That's why astronauts get queasy. Maximum g isn't a useful idea for analyzing car handling. Actual g cannot be measured conveniently. You need an accelerometer or two or three which measure g in one dimension, each, as fitted to a car. Most cars have one to detect yaw in conjunction with a steering angle sensor to input to the stability control. Most important, maximum g tells you nothing useful about lap times. The driver is always trying to corner with less g than the car could generate, that's what separates the quick from the deadly.

To drive effectively you ignore g and rely on your eyes. This is why you can "drive" GT 5. In fact, your eyes are so important and your inner ear (or "seat of the pants" if you prefer ) so unimportant that drivers can match their lap times achieved on a real circuit by "driving" on a computerized display of that circuit. GT 5 has come a long way in duplicating the physics. Clearly you don't drive GT 5 by the seat of your pants.

Humans can drive because they evolved to hunt with projectiles. We are masters at assessing relative motion. That's what you are doing when you drive. Watch where you're going.

[s2d4]

@Dave-ROR

[maloney2]

Trying to correct the errors in all these posts reminds me of grading student papers in vehicle dynamics in grad school. You wanna give them marks because at least they're thinking and trying to apply what they know, but at the same time the conclusions drawn from the facts they do know are so far off you don't know where to begin.

[wbradley]

Somebody please write a disclaimer for Suberman's posts so no kids go out and get themselves killed or injured after reading them.

[wparsons]

Keep digging, you're so far over your head it isn't even funny.

Originally Posted by Suberman

Now we all agree that tire rubber on road mu is a constant for practical purposes (the variables are vertical load and horizontal force, which when divided produce the same number (that would be the constant) referred to as the coefficient of friction) we understand why the friction circle is a useful concept.

Each contact patch has its own friction circle and the vehicle as a unit has a friction circle resulting from the sum of the four friction circles.

Each contact patch has its own friction circle. Neither the shape nor the size of the contact patch is relevant because the vehicle weight does not vary.

Each friction circle has a maximum size, exceed the forces described by the friction circle for any contact patch and that contact patch will slide, reducing the size of the friction circle rapidly, relatively speaking.

Congratulations, you've said nothing useful in that whole block of text.

What's actually useful about the friction circle is balancing inputs (steering, throttle and brakes), not trying to analyze the chassis dynamics. The point of analyzing the friction circle is to see that you're using as much grip at all times, be it for braking, cornering or accelerating. If you're relying on a friction circle to set up the chassis balance you're barking up the wrong tree.

Originally Posted by Suberman

A vehicle with track and wheelbase does not need steerable wheels to corner.

Every wheeled vehicle by definition has a track width and wheelbase (forgetting a segway or unicycle), or are you talking about a track as in a tank tread? If so, why on earth are you bringing up skid steer vehicles in this discussion?

Originally Posted by Suberman

To turn a bicycle requires that the handle bars be at an angle to the direction of the bicycle opposite to the desired direction of the turn.

Are you claiming that on a bicycle you need to turn the bars to the left to make the bike turn right? Simply astounding.

Originally Posted by Suberman

A car with steerable wheels always initiates a turn by understeering.

I really don't think you know what understeer is at all. I'm starting to think that you believe understeer is anytime the wheels are pointed into the turn and oversteer is anytime the wheels are pointed out of the turn, which is simply 100% wrong.

Originally Posted by Suberman

A car that uses driving wheels to steer will initially corner better than a car that uses one axle to drive and the other to steer but will reach the limits of cornering sooner.

Again, what? Are you trying to claim that FWD cars will always have better turn in? You really need to go knocking on the doors of every supercar manufacturer to tell them how wrong they have things.

I hate to burst your bubble, but off power (like at corner entry) which wheels are driven has no real relevance at all (ignoring very mild engine braking).

Originally Posted by Suberman

There's no free lunch where mu is concerned. Use mu to drive the car and it isn't available to corner.

Mu is not a force, it is simply a coefficient. Ignoring the complexity of tires and weight transfer, mu is the same whether the car stopped or cornering at the limit of adhesion. Mu != grip, it simply is the potential for a surface to provide grip. Even in the most basic equation mu is only a factor in the force, not the resultant force.

Originally Posted by Suberman

Therefore, a car set up to oversteer will be slower than a car set up to understeer. This is why a stiffer front bar on the BRZ will make it lap faster up to the point the drive wheels can no longer add forward thrust. Look at any touring car racer from the 60's and 70's when all were rwd and note that they all lift the inside front wheel in a corner. Note that most, if not all fwd race cars converted from road cars lift the inside rear wheel instead.

If you're so confident on this, find a pro driver nearby to drive your car 100% stock and see what their fastest lap is, then repeat with one rear sway bar link disconnected. I guarantee you they'll be faster with the bar connected. You could do the test blindly, but they'll know by the first corner anyway.

Originally Posted by Suberman

The human body isn't very good at detecting g. That's why astronauts get queasy. Maximum g isn't a useful idea for analyzing car handling. Actual g cannot be measured conveniently. You need an accelerometer or two or three which measure g in one dimension, each, as fitted to a car. Most cars have one to detect yaw in conjunction with a steering angle sensor to input to the stability control.

What relevance does any of that have to grip or lap times?

Originally Posted by Suberman

Most important, maximum g tells you nothing useful about lap times. The driver is always trying to corner with less g than the car could generate, that's what separates the quick from the deadly.

Are you seriously suggesting that the fastest way around a track is driving at less than 100% of the car's potential? If you watch any real racing, the ONLY reason to drive at less than 100% is to preserve tire life. If you want the fastest lap you're right at 100% the whole time.

Originally Posted by Suberman

To drive effectively you ignore g and rely on your eyes. This is why you can "drive" GT 5. In fact, your eyes are so important and your inner ear (or "seat of the pants" if you prefer ) so unimportant that drivers can match their lap times achieved on a real circuit by "driving" on a computerized display of that circuit. GT 5 has come a long way in duplicating the physics. Clearly you don't drive GT 5 by the seat of your pants.

Humans can drive because they evolved to hunt with projectiles. We are masters at assessing relative motion. That's what you are doing when you drive. Watch where you're going.

Again, 100% useless in this conversation. Adding paragraphs of stuff you think sounds smart doesn't make you any less wrong.

Let's keep it to the facts.

1) You think understeer is faster - it isn't
2) You think that adding understeer with a stiffer front sway bar will make the car have more traction in the snow. - it won't

I've already provided lots of examples as to why, and if you read relevant information you'll see that I'm not wrong.

[f0rge]

I can drive in GT5 because I've been playing playstation for years, it has nothing to do with relative motion because the screen isn't moving.

My GF, who has been driving for 10 years, could not complete a lap in GT5 to save her life. Not because she can't drive, because she sucks at video games.

[7thgear]

more mu for me

more mu for you

i'll have a mu

you'll have one too

[wheelhaus]

Originally Posted by wparsons;

Are you claiming that on a bicycle you need to turn the bars to the left to make the bike turn right? Simply astounding.

1) You think understeer is faster - it isn't

Actually the part about the bike is somewhat correct. On a bicycle, the effect is nearly undetectable because you are 90% of the total weight of the machine and shifting your weight has a dramatic effect on the bicycle and its wheels. At high speed its more noticeable. A motorcycle is a better example, where you are only a portion of the total weight and have to actually steer the bike. To steer a motorcycle at speed you actually "counter steer" by pushing the bars opposite the desired direction of turn, and it steers the tires wide and leans the CG inward. The bike will attempt to right itself due to rake and trail steering geometry, so you maintain or increase this pressure to hold a line in a corner. The angle of steering is minute, bar movement is nearly undetectable. On a bicycle, you're moving pretty slowly, and this weight shift happens almost instantly, that you're then steering into the turn to maintain balance.

Understeer may not be faster per se, but it does help stabilize the car. A car that wants to oversteer more readily can cause driver stress. Sometimes a chassis tuned for a bit of understeer is just enough to reduce driver stress and allow him to push harder or drive for longer stints in an endurance race, which may result in faster times.

[7thgear]

yeah the bicycle thing is weird but doesn't really apply to cars as much

you turn left a bit but your body wants to keep going straight so it leans you and the bike to the right, then you turn into the turn and off you go... but it all happens so quickly you don't notice it.

but for a better explanation

http://www.exploratorium.edu/cycling/brakes3.html