Racecar Engineering – September 2019

SEPTEMBER 2019 http://www.racecar-engineering.com 51

rearand52.3percentfront.Thefrontwheels

staticallycarry 140 lbs,orabout4.7percentof

theweight,morethantherears.If thecarwas

setupsoit lockedthefrontwheelsjustbefore

therearsinstraight-linelimitbrakingatthe

startoftherace,it willnowlocktherearsfirst.

Ordinarilywewantthefrontstolockfirst,

atleastfora pavementcar.Thatway,thecar

doesn’ttrytoswapendswhenweapproach

lock-up.Ondirt,though,weoftenusethe

brakestogetthecarrotatinginyawonentry.

Thebrakebiasneededtomakethefronts

justbarelylockfirstis nota constant.It varies

dependingonthecoefficientoffriction

betweenthetyresandtheracetracksurface.

It alsovariesdependingonwhetherweare

corneringorrunningalonga straight.

Thishappensbecausethedynamicloading

ofthewheelsvariesasthevehicleundergoes

accelerations.Whendesigningoradjusting

non-ABSbrakingsystems,weprimarily

concernourselveswiththerearwardx-axis

accelerationsthatwearetryingtoproduce

whenweapplythebrakes.However,weshould

notethaty-axis(lateral)andz-axis(normalto

theroadsurface)alsoaffectwheelloadings.

TECHNOLOGY – THE CONSULTANT

Photos: NASCAR

I runa NASCARpavementLate
Model.Youalwayshearracers
talkingaboutchangingthe
brakebiasduringa race,so
howdoesthathelpthecarlaterina race?
Also,whatwouldbea goodpercentfrom
fronttoreartostartoutat?Howdoesit
affectcornerentry?Andhowdodifferent
percentsaffectsettingupthecar?

THECONSULTANT
Thesimplestreasonforadjusting
brakebiaswhilerunningis to
correctforchangeinweight
distributionduetofuelburn-off.
Buttherecanbeotherreasonsaswell.
A gallonofgasolineweighsaboutseven
pounds.Twentygallons,then,is about 140 lbs
and 15 gallonsis justover 100 lbs.Suppose
thatwehavea carthatweighs3100lbatthe
startofa race,andthatthecarhas 50 percent
rearinthatcondition: 1550 lbsfrontandrear.
Supposethatthefuelloadis carried 20 percent
ofthewheelbaselengthbackoftherearaxle.
Supposethenthatweburnoff 100 lbsofthat
fuel.Whatis therearpercentagenow?
That 100 lbsoffuelwasadding 120 lbsto
therearwheelloadingandtaking 20 lbsoffof
thefront.Whenit’sgone,wehavea 3000lbcar
with 1570 lbsonthefrontwheelsand 1430 lbs
ontherearwheels.Wenowhave47.7percent

To calculate what the brake force distribution

should be to produce shortest straight-line

braking with directional stability, we need to

know the loads at the front and rear wheels in

straight-line limit braking on a level surface. For

this, we will need: the static front/rear weight

distribution; the overall c.g. height; wheelbase;

and the rearward acceleration we think the car

can attain under favourable conditions.

Brake clause

The percentage of the car’s weight that

transfers from the rear to the front is the c.g.

height as a percentage of the wheelbase,

times design rearward acceleration in gs. For

example, a pavement Late Model stock car

might have a c.g. height about 15 per cent of

the wheelbase and slicks that allow it to brake

at about 1.2g. That would mean that 15 per

cent of the car’s weight transfers forward per g,

and 18 per cent transfers at 1.2g.

If the front end has 50 per cent of the weight

statically, it then would have 68 per cent

dynamically. If the coefficient of friction were a

constant, all four wheels would lock together

at 1.2g if the front wheels do 68 per cent of

A NASCAR Late Model in action. When fuel is used up throughout a race the weight distribution will change and the brake bias will need to be adjusted to account for this

Ordinarily we want the fronts to lock first, that way it

doesn’t try to swap ends when we approach lock-up

Biased thinking

Why achieving the perfect brake balance on a NASCAR Late

Model stock car involves weighing up a wide range of variables

By MARK ORTIZ