Racecar Brake Upgrade:

Ingredients are:

  • Volvo 740 front hubs to receive rotor hats
  • 2 x DBA069 Mustang rotors 330mm x 28mm (left and right hand)
  • 2 x AP Racing 6-piston calipers CP5570
  • 17mm thick competition disc pads
  • 2 x machined alloy caliper mounting bars from Brunton Engineering
  • 2 x 5/8" ID Girling master cylinders ands reservoirs
  • clutch master cylinder with integral fluid reservoir
  • Balance bar and adjuster cable
  • Workshop facilities and a master mechanic
  • Removal of split circuit brake lines and proportioning valve
  • Modifications to Volvo pedal box

(click images for larger view)

Volvo brake upgrade calculations:

  • Car plus driver = 1250kg
  • Static weight distribution: 55% front = 690kg, 45% rear = 560kg
  • Braking at 0.9g with vehicle centre of gravity 0.5m above road, produces a rotational moment about the tyre contact patch of 0.9 x 1250 x 0.5 = 560kg.m
  • For Volvo wheelbase of 2.77m this creates a weight transfer of 560/2.77 = 205kg, that is +205kg at front axle, -205kg at rear axle.
  • Front axle load under 0.9g stop = 690 (static) + 205 = 895kg or 447.5kg/wheel
    Rear axle load under 0.9g stop = 560 (static) - 205 = 355kg or 177.5kg/wheel
  • Tyre radius (Yokohama A032R, 245x45/17) = 0.32m (approx. 12.8")
  • Front brake torque required = wheel load x 0.9g x tyre radius,
    = 447.5 x 0.9 x 0.32 = 129kg.m (approx. 930 ft.lb)
  • Competition disc pad coefficient of friction = 0.45 approx.
    Rotor O/D = 330mm Approx. radius to centre of pad = 0.155m, therefore
  • Clamping force to generate brake torque = 129/(0.155 x 0.45) = 1,850kg (=4,070lb)
  • Front calipers: CP5570 6-piston: total piston area 50.1cm2
Click here for a link to calipers (then select Application: Road, Type: 6 Piston, Family: CP5570)
  • Area of 3 pistons (per side) = 25.05cm2 = 3.883 sq.inch
  • Therefore hydraulic pressure = 1,850/25.05 = 73.85 kg/cm2 (1,048psi)
  • Master cylinder 5/8" ID, area 1.98 cm2, so rod force for 73.85kg/cm2 = 73.85 x 1.98 = 145kg

For rear, similar calculations to those above give:

  • Rear brake torque required: 177.5kg wheel load x 0.9g x 0.32m tyre dia.= 51kg.m
  • Clamping force req'd = 51/(mean disc radius 0.125m x coeff.of friction 0.45) = 905kg (1,991 lb)
  • Rear calipers: standard ATE 2-piston each 1.49" dia. = 11.25 cm2 per side
  • Hydraulic pressure req'd = 905/11.25 = 80.4 kg/cm2 (1,140psi)
    Master cylinder 5/8" dia. area = 1.98 cm2
  • Rod force for 80.4kg/cm2 = 80.4 x 1.98 = 159kg (350 lb)
  • Brake pedal: Volvo standard pedal ratio = 4:1
    Modified to approx. 4.5:1
  • Total rod load on twin master cylinders = 146kg (F) plus 159kg (R) = 305kg

So for pedal ratio of 4.5:1 the foot pedal force for 0.9g stop is = 305/4.5 = 68kg or 150lb.

Note that rod force for the two master cylinders are similar (146kg, 159kg) which means that the bias balance bar will be near centred. Depending on your F/R caliper areas, you may need to select a different master cylinder diameter to achieve approximately equal rod forces.

I have found that in the Volvo racecar, a pedal force of 150lb is quite OK and in keeping for this application. For a road application, a pedal force of 100lb (45kg) will be more suitable. This can be achieved with an aftermarket pedal box, like Tilton with a pedal ratio of 6.2:1

SPECIAL NOTE: the calculations above are provided for the interest of motorsport enthusiasts only. The brake upgrade described above is not legal for road use in Australia. No liability expressed or implied is accepted by Dick Prince for any use or misuse of the calculations set out above.

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