understeer-oversteer

[Lawrence C Leung]

Both types of cars unload the rear suspension under braking. Thus they
act the same. 

It's under powering out that there is a notable difference. Applying
tractive (pushing or pulling) force to a tire limits its lateral (side to
side) grip. 

The more tractive force applied (either braking or accelerating) the less
lateral grip.

Got that?

Now, when entering a turn, both cars should be braking with the same
amount of tractive effort (assuming the same mass), so they should act
about the same, ignoring the fine tuning nuances of anti-dive, spring
rates, damper rates, static and dynamic weight distribution, etc. Let us
assume the cars are set up the same. Since there are no dynamic
differences to either car under braking and entering a turn, they should
act the same. 

So, if one enters a turn too quickly with any (AWD, FWD, RWD, 0WD - cart)
kind of street car (cars with aerodynamic aids are different), there is a
whole lot of turning force required (lateral grip) in order to get the
car to turn and rotate (yaw) into the intended direction. Grip is
proportional to the downward force on the wheels, and if that force is
insufficient, the car's front end won't be able to turn, and UNDERSTEER
occurs, IF the driver does nothing to attempt to correct for the
situation. 

SInce most drivers instinctively brake (yes, too late!), they transfer
weight to the front wheels, adding lateral grip to the front wheels. The
front of the car begins to move towards the center of the turn AND the
car begins to rotate, like a top, about it's "spin" axis, it begins to
yaw. This also turns the rear wheels into the turn creating the slip
angle needed to get the rear of the car to start going towards the center
of the turn. At the same time, the front of the car is operating with
less slip angle than the rears (the tires are more closely pointed in the
direction where they are going) giving them more grip than the rears.
Finally, due to the weight transfer, the rear of the car gets lighter, so
even less weight, hence lateral grip is availible at the rear, AND the
car is now rotating (beginning to spin). If the driver continues to brake
or lift throttle in this situation, the rear will break loose and begin
to spin out, a condition called "lift throttle OVERSTEER". All cars will
experience this too different degrees depending upon how hard it's
pushed. This is the situation where the driver AND passengers are scared,
b/c if you've ended up in this situation, you've probably got no room to
go (hence the need to brake and turn!) and the car wants to spin right
into the obstacle you're trying to avoid. Again, this applies to all
cars, F,R or AWD.
 
Now, powering out. Here's the real difference. Remember, tractive effort
reduces lateral grip. 

So, for FWD, applying power reduces lateral grip at the drive wheels,
hence, less lateral grip at the front of the car (compared to the neutral
rears), and understeer, more steering input is needed in order to
maintain the same rate of turn. This is in-spite of the front wheels
pointing and driving into the turn to create yaw. There is still less yaw
developed than un-driven tires would generate for the same steering
angle. (the term is slip angle, a whole 'nother thread ). Worse yet,
acceleration transfers weight off of the front driving wheels, reducing
the tires overall effectiveness for both accelerating and turning.  The
rears just follow the front wheels path.

For RWD, applying power reduces lateral grip at the drive wheels, hence,
less lateral grip at the rear of  the car compared to the front wheels,
which are causing the car to rotate or yaw, into the turn. Because of the
difference of grip, and the rotation caused by the yaw, less steering
input is needed in order to maintain the same rate of turn called
over-steer. WAY too much power to the rears will overcome their ability
to provide tractive effort, and will also overwhelm any lateral control
of the rear tires. This is called a "power slide". Now, weight transfer
is a the same as for FWD, however, although there is less steering grip
availible, there is more traction at the rear wheels availble, reducing
the tendency to powerslide without EXTREME use of power. For cars
designed for the street, most RWD cars will still understeer exiting a
turn under full power, due to weight transfer off of the front tires,
however. Things such as camber angles are also used to aid in keeping
street cars on the understeer side of the equation, as it is generally
considered more "safe" for the average driver (see "lift throttle
oversteer" above).

Note, no mention of AWD such as "quattro" (MAC) yet. Under braking, same
things apply. Under power out, note tractive effort is distributed to
both axles. Hence, the fronts are not as loaded as FWD, so some lateral
grip is availible to create yaw (reducing Understeer) and the rear wheels
are carrying less tractive effort too, reducing "power slides". Overall,
total behavior under power tends to favor understeer in AWD cars anyway,
just significantly reduced. 

HTH,

LL - NY


On Sat, 18 Nov 00 22:14:51 -0400 Steven Addy <steveaddy at mindspring.com>
writes:
>Do I have this correct?  - I'll try to get the overs and unders 
>straight 
>as I appear to be getting dyslexia in my old age.   
>
>When entering a curve too rapidly, a typical driver hits the brakes 
>and 
>this action causes the vehicle load to be directed toward the front 
>outside wheel. A car with understeer  also tends to shift its load 
>toward 
>the front outside wheel when entering a curve. For safety reasons,  
>therefore, front wheel driven cars are specifically designed to 
>understeer as the load shift caused by emergency braking also 
>increases 
>traction on a driven wheel. 
>
>When entering the same curve, a vehicle with oversteer tends to shift 
>its 
>load toward the rear. Braking, however, causes the vehicle load to 
>shift 
>toward the front and this decreases traction at the rear wheels where 
>
>it's needed most. In a car with oversteer, therefore,  braking in a 
>curve 
>will increase the liklihood of a spin and it must be driven as if on 
>ice 
>or gravel. 
>
>Steve A.    
>
>