Re: Brake Pads, Brake Balance, Understeer versus Oversteer

[Pete Read <pete.read@xxxxxxxxxx>]

This thread started with Dave Hogg's report of his serious track 
incident.  His E36 M3 oversteered (back end came around) down the
Summit Point turn 4, known as the chute, resulting in heavy
damage to his car.


Brake Balance, Understeer/Oversteer
- -----------------------------------
It was suggested that his choice of brake pads (Pagid Carbon
front and PBR Deluxe rear), caused the oversteering condition
by changing the brake bias towards more front end braking power.

Actually the reverse is true -- more front bias causes understeer.
As Gary Lin said, "the less grippy rear pads result in more 
cornering traction at that end".  And note that the understeer 
versus oversteer balance is related to both turning and braking 
at the same time.  This is commonly called trail braking because 
brake pedal pressure is trailed off as the car is turned and
cornering forces increase.

Tires can handle longitudinal (braking/acceleration) and lateral 
(cornering) forces at the same time, but at the expense of each 
other.  As more braking force is applied, less lateral force is 
available, and vice versa.  So as a car is turned into a corner,
cornering force increases and braking force must decrease.  The 
front to rear distribution of brake force affects the cornering
balance.  More rear brake means less rear cornering force and
can result in oversteer.

Think of an extreme example such as a bootleg turn where a car is
spun 180 degrees.  To initiate the spin, a car is quickly turned 
(high cornering force) and then the emergency brake for the
rear wheels is applied.  The front wheels with no braking force
grip much better than the rears with both cornering and braking
force, so the rear end slides out (oversteers).


Front Brakes Always Lock First (front big brake kits are safe)
- --------------------------------------------------------------
I can remember two or three years back when Jim Moran asked if
big front brake kits adversely affected the front/rear brake 
balance or bias.  It got me thinking and I did some reading.

Now it seems obvious, but I hadn't realized that automobiles are 
required by law to have their front brakes lock first for stability 
reasons.  If the rears lock first, the back end can come around 
(oversteer and spin).  Front locking causes temporary loss of 
steering control, during the tire skidding, but the vehicle 
maintains stability with the front end pointed forward.  During
cornering and braking, front bias causes understeer, which is
much more easily managed by the average driver than oversteer.

So big front brake kits do change the brake bias, but in a safe way.
In theory, a car will stop better with the correct front to rear
bias, because each tire will do its fair share of the stopping.  
Larger than stock front brake kits (without ABS to solve the bias 
problem at the limit) should overload the fronts and under utilize 
the rear tires.  They certainly will change the balance towards
more understeer, at least below the limit where ABS kicks in.

As a practical matter, many people install larger front brakes 
to solve the problem of heat related brake fade at track events.  
Front brakes are more highly stressed because, due to weight transfer 
during braking, the front tires have more grip and need to handle 
more of the stopping duties (approximately 70% on most cars).
The greater heat capacity of big front brake kits usually more than 
compensates for the less than perfect brake balance, so stopping power 
is improved.  However ultimate performance requires a matching rear 
brake upgrade.

  Pete Read
  '88 M5
  Arlington, VA

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End of bmw-digest V9 #189
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