rec.autos.simulators

If we are in a steady-state corner at or near the limit of lateral
friction, and you apply the brakes, it creates pretty heavy understeer
in our simulation.  The car is set up with no ABS, and front/rear
brake bias such that the fronts and rears will lock up at the same
brake input in straight-line braking.  Almost everyone I've talked to
thinks that the car should oversteer (completely lose its rear) in
this situation.  I can see two major new torques acting on the body
once the brakes are applied:

1)  The weight-shift forward due to braking would cause an oversteer
torque because the front tires have a greater load and therefore
greater lateral force than the rears.

2)  Since the outside-front tire has the greatest load, the
longitudinal (braking) force of the tires will cause an understeer
torque (this one is not as intuitive, but draw a diagram of a vehicle
cornering, and consider the braking force/torque of all tires on the
body, and the loads on each).

In our simulator, the torque of (2) is greater than that of (1), and
therefore the car understeers in this situation.  I am wondering if
any of you out there that have run into this problem in your own
simulators.  Or maybe someone knows what a real car would do.

Any advice or input here would be greatly appreciated!  Great
newsgroup by the way...I have just recently discovered it :)

-Dave Gierok

PS: If I increase the rear brake-bias, I will get oversteer, but
people that I've asked swear that it should oversteer regardless.

Also, another thing could be if you have so much understeer in the chassis
because of spring settings or bad suspension geometry that it swamps any
other effect.

The other things to check are your combined tire slip functions.  Another
good test is to set the car up with unequal ride heights (left/right or
front/rear) and make sure there aren't any "unexpected" forces due to math
errors in camber and geometry...
-Dave

If the car is rear wheel drive:
Depending on what you have modeled, the motoring torques from the
engine (if the clutch is still engaged) will also slow down the rear
tires. This would act similarly to braking and make the brake
bias calculations harder to get correct.

If it is a race car, you can heavily load up the front tires
with braking force and depending on how you implement
"friction circle" types of effects in your tire model
you can lose the side force capability as you approach
locking up the front tires.

Depending on how you model the effects of that
heavy normal force, the fronts could also lose sideforce
capability from that effect to the point where they still have
less than the rears and the result is final understeer.

Depending on how you model the profile of lateral force
changing with normal force, you can also change the
sideforce capability of the fronts through this effect.
(They might have been just under their sideforce peak
before braking but after loading them could perhaps
be farther under the peak if you move it farther out
with heavier normal force.)

- Matt

from ample experience <g> ...an average car in most cases in such a
situation would begin to oversteer when you apply the brake lightly or
moderately.

If you fully slam the brakes and quickly enough, it will usually understeer
(if you don't slam them fast enough, its body will go in rapid motion
through the movement that accompanies the weight transfers and announces the
oversteer that would happen if you'd not slammed the brakes but only applied
them normally).

An anomaly from this behaviour might in my experience occur in a real car if
the front outside tire doesn't have enough grip relative to the rear tires,
like not having enough weight on the (especially outside) front wheel(s) or
too much weight on the (especially inside) rear wheel(s) (heavy cargo in
trunk?) especially if the road surface is slippery.

An exception could also occur if the outside front tire suspends all the
transferred weight instead of increasing the friction at the contact patch
between asphalt and tire, like if the tire were underinflated.

Too much braking force at the rear wheels could have a similar effect, as
that would reduce the weight transfers.

Hope this helps - apologies for not being able to make more qualified and
knowledgeable contributions.

Achim

As far as mid-corner braking without locking causing oversteer, I
would think that you would be shifting more weight to the front
wheels, increasing the relative traction at that end of the car and
increasing the traction available for steering (plus the decrease in
speed would allow the car to take the turn with a smaller radius using
the same amount of wheel lock).  Probably off again there, too, but
that's what I get when I attempt to visualize it.

Jason

        Certainly braking can cause understeer sometimes.  It depends
on the usual things, weight distribution, brake bias, setup, cornering
load,   I've experienced it in NASCAR2002 and in real front wheel
drive cars.  I think it's related to simply overloading the front
tires.

        Whether or not the understeer you see is realistic, I haven't
a clue.

in the cars I've driven, when driving around a corner near the limit, simply
reducing the throttle a bit infallibly made the rear end come around. I've
used this often in real cars to help steering, and it works in N2002 and GPL
as well.

The cars behaviour of course also depends on factors I mentioned in my other
message, or the car's setup - brake balance, weight distribution, which axle
is driven, diff settings etc.

But I'll leave the theoretical discussion to those who know more about it
than I do :-)

Achim

The brakes are modeled by applying a friction torque to the wheels.
And you're right that both left/right brake torques are equal.  But
with the wheels turned, even if the braking FRICTION is the same on
the front Left/Right tires, their torques on the CAR are not equal
because the outside front tire will have a greater lever-arm.

Also, there is a lot of weight transfer to the outside wheels (maybe
more than there should be), and therefore it looks like the front
inside tire is locking up with very little braking.  The rear inside
tire does not lock up because this car is RWD and set up with a
limitted-slip-differential.

The front springs are SLIGHTLY stiffer than the rear (no sway-bars
yet) and the car tends to be fairly neutral, but leans towards
understeer.

I'll check that out...thanks for the advice

The brakes are modeled by applying a friction torque to the wheels.
And you're right that both left/right brake torques are equal.  But
with the wheels turned, even if the braking FRICTION is the same on
the front Left/Right tires, their torques on the CAR are not equal
because the outside front tire will have a greater lever-arm.

Also, there is a lot of weight transfer to the outside wheels (maybe
more than there should be), and therefore it looks like the front
inside tire is locking up with very little braking.  The rear inside
tire does not lock up because this car is RWD and set up with a
limitted-slip-differential.

The front springs are SLIGHTLY stiffer than the rear (no sway-bars
yet) and the car tends to be fairly neutral, but leans towards
understeer.

I'll check that out...thanks for the advice.