Racecar Engineering – September 2019

52 http://www.racecar-engineering.com SEPTEMBER 2019

Dirtcarscanhavehugeamountsoflateral

bias.Rulespermitting,theyoftenhavenoright

frontbrakeatall.Whentherulesrequirea right

frontbrake,theyoftenhavea driver-controlled

proportioningvalvefortherightfront.

Judgetread

Tounderstandtheeffectoffront/rearbrake

biaswhentrailbraking,it’snecessarytoget

togripswiththetractioncircleprinciple:the

tyrecanmakeaboutthesameamountofforce

inanydirection.If it’satthelimitofadhesion

incornering,it willbreaklooseif youaskit

tomakebrakingforceatthesametime.If it’s

atthelimitinbraking,youcannotgetany

corneringforcefromit.Youcangetsomeof

each,butwhenyouaskformoreofone,you

can’tgetasmuchoftheother.

Therefore,morefrontbrakeaddsundersteer,

ormakesthecartighter,whenbrakingand

turningatthesametime.Morerearbrake

addsoversteer,orloosensorfreesthecar.

CONTACT

Mark Ortiz Automotive is a chassis

consultancy service primarily serving oval

track and road racers. Here Mark answers

your chassis set-up and handling queries.

If you have a question for him, please don’t

hesitate to get in touch:

E: [email protected]

T: +1 704-933-8876

A: Mark Ortiz

155 Wankel Drive, Kannapolis

NC 28083-8200, USA

To understandtheeffectof frontto rear brakebias when a race driver is

trailbraking it is necessary to getto gripswiththe traction circle principle

TECHNOLOGY – THE CONSULTANT

the braking. Since the coefficient of friction
diminishes some as load increases, the fronts
would probably lock just a little before the
rears. So we want a brake bias somewhere in
the range of 68/32 to 70/30.
The retardation force at the contact patch
depends on: the ratio between tyre radius and
brake pad acting radius (roughly the radius to
the middle of the swept surface); the pad to
rotor coefficient of friction; the caliper piston
area (more area gives more force); and the
hydraulic pressure fed to the caliper.
The hydraulic pressure fed to the caliper
depends on: the master cylinder piston area
(less area gives more pressure); and the force
on the master cylinder pushrod.
The force on the pushrod depends on: force
on the pedal; pedal pad to balance bar motion
ratio; and apportionment of balance bar force
between the two master cylinders.
A pavement Late Model has similar tyre
sizes front to rear, although there are small
variations, mainly to achieve stagger. Brake
rotors (discs) are generally as large as the
wheels will allow, front and rear. Calipers are
generally of similar design front and rear, but
with bigger pistons in the fronts. The most
common combination is 1.750in in front and
1.375in in back. This means that the diameter
ratio is 14/8 to 11/8, or 14/11, and the area
ratio is the square of that, or 196/121. The front
caliper piston area percentage is then 196 /
(196 + 121) = 61.8 per cent. The brake bias
would be about 61.8/38.2. The rears would lock
first, if the line pressures are identical.

Front loading
We can give the front a bigger share of the
line pressure by giving it a smaller share of
the master cylinder area. To give the front
70 per cent of the stopping power when the
balance bar is centered and pushrod forces are
equal, the rear master cylinder needs to have
(70/61.8) x 50 per cent = 56.6 per cent of the
piston area and the front one 43.4 per cent. The
front master cylinder area needs to be about
43.4/56.6 = .767 times as great, so the diameter
ratio needs to be the square root of that, or
about .876. That’s just about exactly 7/8, so we
would be close if we use a .875in front master
cylinder and a 1.000in the rear.
We can then adjust from there using the
balance bar. Typically this will only give us
about a seven percentage point adjustment
range – from about 73/27 to 67/33. It is possible
to build ones with broader range, but size
and rigidity become issues, so commercially
available ones are fine tuning devices, and we
need to get close using the rest of the system.

So far we’ve been assuming a symmetrical

car, braking in a straight line. However, with

an oval track car, things get considerably

more complicated. We often will not have the

same rear percentage on both sides of the

car statically, due to wedge adjustment. We

often will not have equal longitudinal load

transfer on the left and right in braking, due

to asymmetrical springing and suspension

geometry. We usually will have more than 50

per cent static left weight. We will usually have

tyre stagger at both ends of the car.

Often, the driver is going to trail brake: the

car will be turning while braking. Depending

on the track, the driver may be braking for

the first third of the turn. The car may never

be running entirely straight. The turns will

probably be banked, and in many cases there

will be some banking on the straights.

On asphalt, our concern may be to keep

the car from spinning while the driver is trail

braking. On dirt, we may be using the brakes to

persuade the car to rotate in yaw.

The engine will also be a factor. Engine

braking may effectively add rear brake. Also,

the driver may be a left foot braker who uses

the throttle to partially overcome the rear

brakes, in a controlled manner.

The racecar will also usually have a side-to-

side brake bias, as well as a front to rear bias.

Even if the right and left brakes are identical,

tyre stagger and tyre pressure difference will

create unequal retardation on the right and left

sides of the car. This is the main effect that front

tyre stagger has on the racecar. But it doesn’t

affect the car much except in braking.

On dirt tracks a bias to the rear brakes is often useful on turn-in to help rotate the car. There can also be plenty of lateral bias