It's all about mu
 

G isn't useful for understanding handling balance. Technically, g isn't even applicable as it is the acceleration of Earth's gravity which works only in a vertical plane. Certainly, when figuring out what's happening at each end of a car g is a pretty useless concept.

It's all about mu. Specifically static mu. Which is surprisingly constant for these purposes. Accident investigators successfully use assumed mu as well as experimentally measured mu.

The key to understanding how handling works is to understand mu.

Mu is the same for all four contact patches. In theory, all four contact patches develop the same frictional force for a given vertical loading. Size of the contact patch is not significant.

How a vehicle uses that total force determines how it handles.

Slip angle results from mu. The distortion of the tire exhibited as slip angle is the reason static mu is the only relevant factor unless and until the car is overdriven and slides. Kinetic mu is markedly lower than static mu, and, kinetic mu has nothing to do with slip angle. Only while the contact patch grips the road statically does the tire produce slip angle forces. Once the grip is lost, slip angles decrease as does total traction and cornering grip.

This is why oversteering cars are slower, they reach kinetic mu earlier than cars which understeer.

Technically, a car stops oversteering when the tires begin to slide. A sliding car also cannot understeer, by definition. It doesn't matter which end of the car slides, as soon as at least one axle slides then the car is technically skidding and going slower than it could if it were to return to static mu conditions.

Because mu is a constant and identical for all four tires (one reason you must never mix tires) all four tires have the potential to develop the same grip. Axle weight distribution isn't as important as people think because total force is proportional to vertical load. Total grip varies but so does the loading the grip is required to deal with. This is why lighter people don't slip and fall any more readily than heavier people. Were mu not a constant the world would be a weird place.

The more driving force you put through the drive wheel contact patches the less grip is available for cornering. The more powerful a car is the more it must be set up to understeer to be fast.

Mu on snow and ice is generally low. In fact it makes a big difference which it is. Driving on snow does not engage the mu between the rubber and the snow it engages internal mu in the snow layers. Mu on ice does engage the mu of the tread rubber. Tread design is critical for both surfaces but entirely different.

A car that is balanced on snow or ice will be well balanced on wet or dry pavement. A car that handles well on dry or wet pavement may not be so good on ice or snow.

Why?

The answer lies in the mu.

Now you know.

µ*

Hm... Thanks?

I'm having a hard time understanding what you're trying to convey. You're saying lateral acceleration using "G" as a base of measurement is absurd, instead we should be measuring the friction of the tire?

And how does tire friction (size of contact patch apparently irrelevant) magically tell us how well a car handles?

By definition I thought understeer was simply when the car doesn't turn or yaw in the direction of the turn and simply pushes toward the outside of said turn, whether the front or rear tires are under, at, or beyond optimal slip angle is completely irrelevant. For instance, a typical FWD car plowing into a turn, front tires sliding well past slip angle, is THE definition of understeer.

Also if axle loading isn't important, then by your logic a pickup truck should handle just as well as a Mazda RX8 because it has the same tire on all four axles....

Op for president of Canada.

I think he's trying to say that he got the wrong tires on the wrong car for his winter driving surroundings.

If a problem can be solved by money, then it's not a problem.

And this all brings me to one conclusion: bullshit baffles brains.

Just when I thought you couldn't be dumber...

There is far too many things fundamentally wrong with what you're saying, so here we go (again).

You clearly didn't take any physics classes I see... G is a widely accepted unit of measurement used to measure acceleration forces. The only relation to gravity is that 1G is the same acceleration force applied to an object by gravity. It's the same thing as using BAR to measure air pressure, 1bar is atmospheric pressure.

You are the ONLY person I've ever seen try to argue that G isn't applicable to anything except vertical acceleration. That shouldn't surprise me though, you're the only person trying to argue most things you argue.

source: http://wordnetweb.princeton.edu/perl/webwn?s=g-force

This is quite possibly the worst over simplification I've ever seen in my life. You're assuming that weight doesn't shift around in a corner (it does), that the tires stay flat at all times (they don't) and that heat held by each tire doesn't affect mu (it does).

No, wrong again. A car that is understeering broke traction at the front before the rear, a car that is oversteering broke traction at the rear before the front. Both could have broken traction at the exact same point in the turn.

Technically, you're wrong again, by definition. Understeer (by the real definition) is when the yaw angle of a car is less than it should be for the given steering input, if you want to talk slip angles it is when the front wheels are at a higher slip angle than the rears. Oversteer is when the yaw angle of the car is greater than it should be for a given steering input, relating to slip angles it's when the rear tires are at a greater slip angle than the fronts. It's actually impossible to be under or oversteering without one end of the car sliding.

I'll agree on the first part, but the second part is flat out wrong again. In your magical world that would mean a car like a Ferrari 458 would have to be an understeering nightmare to put power down, but it's not even close. That's why to PROPERLY drive fast you have to balance braking/throttle inputs with steering inputs. If you just floor it mid corner while at maximum cornering force any RWD car is going to spin, regardless of how much it naturally understeers off power.

You really can't wrap your head around the difference between a chassis that oversteers off power and power oversteer, can you? I'll make it VERY simple for you.

If you enter a corner coasting and the car wants to spin that's power off oversteer, if it wants to push that's power off understeer.

If the car is neutral off power it's VERY easy to predict if it will under or over steer under too much power. Is it RWD (or rear biased AWD), it'll oversteer. Is it FWD (or front biased AWD), it'll understeer.

It doesn't matter how well the chassis is set up, if you can't drive properly it will ALWAYS slide under power.

Remember when you tried to bring up the friction circle? If you actually understood them you would also understand that the key to maximizing grip is that you're always balancing lateral and forward grip. Entering a corner you're hard on the brakes while going straight, then as you start to feed in steering you're backing off the brakes until you're at the apex, at which point you start to release the steering angle and feed in power. You can't be at full braking and full steering angle or full power and full steering angle, there simply isn't enough grip available for that unless you're driving WAY below the limits.

And the single biggest flaw with what you're saying, mu is actually a really useless number to bring up when talking about chassis balance since it is, like you said, constant on all four tires. A different mu will change the ultimate grip provided, but it's not going to change the chassis balance at the limit at all.

I hadn't checked this site since I put my car away for the winter, my first day back...

And all I see is you braying about the same thing as you were in November. Either you're complete rubbish at explaining yourself to an entire forum full of people, or nobody cares what you have to say. Teacher or troll, you've lost your audience.

Time to get a new schtick or move on.

Actually, the grip tires have with pavement is not entirely Newtonian.

Asphalt is not smooth for the same reason that rubber makes a good tire. The rubber forms to the pits of the asphalt aggregate. In physics, this is called conjoined surfaces. It's the reason why a larger contact patch can allow for more traction.

If the physics of a road tire was as Suberman says, we'd all be running on skinny little tires because it classic Newtonian physics, pressure and mu dictate the amount of force transferred normal to the force (of gravity in the car example). However, drag cars, and most race cars that put the majority of their rear tires run a staggered (wider rear than front tire)

It's also the major reason why lighter cars corner harder than heavier cars, why roll bars work etc. etc. etc.

What Suberman said above is purely academic, dumbed down "Magic physics world" BS, that doesn't really explain anything about how our cars work and doesn't prove why an understeering car would actually be better (even though it isn't).


Suberman.... if you are really desparate for so much more understeer, why don't you just go out and put the front tire pressure up to 40-45psi, and drop the rears to 28psi or something to achieve maybe a 30-31psi operating temp?

I've blocked Suberman so that I don't have to read his BS in other threads... but I just feel the need to make sure Canadians specifically aren't reading his non-sensical tirade against the 86's, and spewing of miss-information on a forum dedicated to cars.

Moderators: Please change thread title to "It's All About Me" LOL

Hi Everyone,

This is my first post here so I figure I should introduce myself. My name is Sean. Last year I finished my masters in mechanical engineering studying vehicle dynamics and tire modelling. I have attached my thesis for the curious reader and to hopefully have somebody actually read the damn thing since I spent 2 years developing it! I have been participating in grassroots racing for nearly 10 years driving a wide variety of cars including a Hyundai accent, Toyota mr2 Spyder, Toyota Celica GT4, Mazda Rx-7 and now the BRZ.

I would certainly not say I am an expert, but I do have some insight on this topic and after reading several posts from suberman I would like to clarify some of the false information.

First I would like to make something abundantly clear

THE COEFFICIENT OF FRICTION IS NOT CONSTANT UNDER ANY CIRCUMSTANCES!

The effective coefficient of friction for a given tire is a function of not only temperature, pressure, age, and road conditions, but most importantly normal load. That's right the coefficient of friction is dependent on the normal load

MuFn = Ft
Where, Mu f(T,P,t,Fn)

Where Mu = coefficient of friction, T = temperature, P = Pressure, t = time, Fn = Normal load, and Ft = Tire lateral force

I have attached a typical plot of lateral tire force response as a function of slip angle. Each line represents a different normal load. starting at a normal load of 200 Newtons and incrementing by 200 Newtons up to 1000 Newtons. (The maximum normal load is relatively low because this is a 13" FSAE tire)

Several things can be determined from this plot.

1) The peak lateral tire grip occurs at a different slip angle depending on normal load
2) The reduction in performance after the tire is fully saturated is more dramatic at higher normal loads
3) The increase in lateral grip with increased normal loads is a situation of diminishing returns. This is most easily seen by looking at the right side of the graph after each tire has saturated, but this is also true when comparing the peak lateral force at each normal load.

The third point is very critical and is the reason roll bar tuning allows for the designer to change the balance of the vehicle between oversteer and understeer.

As the vehicle corners, weight is transferred from the inside tires to the outside tires due to the roll moment generated at the center of gravity. Now if the coefficient of friction were the same than it wouldn't matter how much weight transferred since the sum of the lateral grip between the left and right sides would always be equal.

Since the coefficient of friction diminishes with increased normal load, we can tune the amount of relative weight transfer between the front and rear axle in order to force one end of the vehicle to generate more lateral force and thus generate a yaw moment. This diminishing coefficient of friction is also why a lower center of gravity increases overall grip.

Assume every tire is exactly the same at the same temperature,age, etc.

Also assuming the chassis is a perfectly rigid body, the roll angle must be the same between the front and rear axle of the vehicle. This means that if the roll stiffness of the rear of the vehicle (roll bar plus spring) is higher than the front of the vehicle than more weight transfer will occur on that axle. Because of the diminishing coefficient of friction, the rear axle will generate less force overall and cause the vehicle to yaw into the turn. The opposite is also true, more front axle roll stiffness will cause the vehicle to yaw away from the turn.

This is only one of several factors that affect the handling of the vehicle, but to attribute the handling characteristics to one factor as suberman has in regards to the rear wheel geometry of the vehicle is utter non-sense. The overall vehicle handling is a factor of all of these things and the relative weight of the suspension characteristics, tire characteristics and environmental characteristics on the system response must be evaluated in each handling situation in order to understand how much one of these factors contributes to the purportedly undesired oversteering tendency of the vehicle.

suberman if you feel the vehicle has too high a tendency to oversteer this can easily be remedied by changing the roll stiffness of the vehicle, changing tires, wheel alignment, or rear differential tuning, depending on the situation you find the vehicle oversteering too greatly in.

I can assure you the suspension is not set up from the factory to oversteer inherently as this would result in too many accidents and lawsuits.

The one thing Subaru did to allow a bit of fun with the car is put on those skinny tires so that the car can power over with its chassis twisting 150 ft-lbs of torque.

I hope this post was informative. Here is a link to my thesis for anyone interested.

http://137.207.14.230/vdc/downloads/...loney_masc.pdf

Sean

^^ GREAT information, but don't worry... Suberman will claim you're wrong too.

In the meantime, Suberman should give this video a watch and try to say that this car is understeering, or that it would be faster if it was understeering:

[ame="http://www.youtube.com/watch?v=xgKjp1pq7iI"]Ferrari 458 vs Ferrari 430 - Top Gear - BBC - YouTube[/ame]


You can CLEARLY see that at turn in (except maybe the hammerhead) there's some rotation (mild oversteer), and at every exit there is also mild oversteer. Not drifting with the tail out 20*, but there's at least a couple degrees of rear slip. That's the fast way around a track, and the fast way to set up a car. You can control oversteer without slowing down, the only way to control understeer is to slow down.

To add to what Sean said above, if you want to test out how little traction you'll gain in snowy corners with more understeer, disconnect one of the end links on the rear sway bar. The chassis will be balanced way towards understeer in that state, but it's still going to oversteer VERY easily if you get on the gas like before.

Another thing he fails to realize... he keeps making comments about the torque, but doesn't seem to think about common sense. Take an Audi S4 (his previous favorite sports car), it puts about 230lb/ft to the wheels stock, these cars put around 140lb/ft to the wheels stock. 230/4 = ~58lb/ft per wheel, 140/2 = 70lb/ft per wheel, no kidding it breaks traction easier with 20% more torque going to each drive wheel.

if the earth's movement through space was instantenously stopped (like a car hitting a conrete wall), would half the people fly off while the other half squished into nothingness?

Oversteer scares passengers, understeer scares drivers.

Suberman needs to either improve his driving skills, grow a pair, or start sitting in the passenger seat where he can focus on less challenging tasks like finding the right radio station or getting my sunglasses out of the glove compartment when I need them.