• Understanding the Problem
• Designing brake systems to cope
• Pad-Disc Interface
• Friction Materials
• Brake Fluids
• Brake cooling
• Retarders

DUTY CONCERN
• City running (250-350°C) Lining and rotor wear
• Alpine Descent (600-700°C) Fade, wear, vaporisation, pedal travel
• Autobahn Stops Disc cracking, hot judder
• Fade test (e.g. AMS ) Fade, pedal travel

• Critical effect of temperature on brake wear
• Front Spoiler can severely reduce pad life
• Significant effect on service intervals and cost of ownership
600
500
400
300
200
100
0
Temperature (°C)

• Grossglockner
– Industry Standard
– 10 miles
– 10 % gradient
– 600-700°C Discs
– 150-200°C Fluid
800
700
600
500
400
300
200
100
0
Alpine Temperatures (°C)
Driving Soak
Front
Rear
Front Fluid
0 5 10 15 20 25 30 35 40 45
Time (mins)

• Up to 300 kw initial heat input
• 600-700°C Differential
• Disc cracking
• Hot judder (Blue spotting)

80
70
60
50
40
30
20
10
0
COLD
HOT
40 m

- Designing brake systems to cope
• Avoid excessive duty on any brake
• Optimising brake distribution

• Commercial Vehicles
– Predominance and load sensing valves
– Coupling force control to spread duty levels with advent of brake by wire
• Cars and light Commercials
– Front brakes have high duty
– Need to maximise rear braking duty

10
9
8
7
6
5
4
3
2
1
0
Net Effective Pressure
Thresholds
Front Rear
High rear thresholds (e.g. arising from integral PCRV in RWC)
- resulting in excessive front duty
800
700
600
Front
Large differential between
Front and rear axles
500
400
300
Rear
200
100
Front Fluid
0
00:00 05:46 11:31 17:17 23:02 28:48 34:34 40:19

(ZCrit = Decel at point of rear lock)
0.25
0.2
0.15
0.1
0.05
0
0
Typical PCRV Non ABS characteristic
PCRV
0.2
0.4
0.6
0.8
1
0.25
0.2
0.15
0.1
0.05
0
0
EBD
0.2
Same system with EBD
0.4
0.6
0.8
x axis = deceleration, y axis = proportion of rear braking
0.25
0.2
EBD failure gives rear lock @ 0.4 g
EBD Failure Zcrit 0.4 g
0.15
0.1
0.05
0
0 0.2
0.4
0.6
0.8
1
0.25
0.2
0.15
0.1
0.05
0
0
System revised to give rear lock @ 0.7 g EBD Fail
EBD Failure Zcrit 0.7 g
Resulting 30% Reduction in rear braking at 0.2g
0.2
0.4
0.6
0.8
1
1

• Minimising brake fluid temperature – pad under-layer
• Less heat into caliper gives increased lining degradation
• Heat needs to go somewhere

Devised by Eric Thoms of Scania

• Flow from tap = Brake energy in
• Height of Water = Rotor temp
• Size of hole = cooling capability
• Plan area = rotor heat capacity

• WIDE BUCKET
• Large Heat Capacity
• Good Cooling
• Low Temperatures
E.g. Aluminum MMC Discs

• WIDE BUCKET
• Large Heat Capacity
• Good Cooling
• Low Temperatures
E.g. Aluminum MMC Discs
• TALL BUCKET
• Small Heat Capacity
• Moderate Cooling
• High Temperatures
• E.g. Thin solid steel or cast iron disc with plasma ceramic coating running at up to 1100
°C

• Benchmarking against current vehicles
• Single stop temperature rise calculations
• Fade and Alpine descent prediction
• Sizing by packaging constraints

id od
• Thickness t
• Vent factor
• Diameter OD
• Diameter ID
• Calc area
• Calc vol.
• Disc mass m
2.16
cm Density
0.7
7.873
g/cc
Spec. Heat S 550 J/kg/ °C
26.2
cm Veh mass M 1910 kg
16.1
cm Brake balance 80% front
336 cm 2 Max speed 230 km/h
507 cc
3994 g
= 63.9 m/sec

• K.E. = ½mV 2 Total energy kinetic energy = 3898110J
• Energy per front brake = 1559244J (80% shared)
• Calculate temperature rise T: Energy = mS d
T
• Therefore temperature rise, T = 710°C
Rotor Type
Drum
Solid Disc
Vented Disc
SSTR ( °C)
350-400
550
600-650
If the above SSTR values are exceeded then in-service problems could occur

• Indication for single stop on the autobahn
• Reasonable quick indicator
• Empirical evidence for SSTR figure
• Shortcomings
– No allowance for the brake cooling
– Not for an AMS test or alpine descent
– Should not be relied upon for new designs

• Uses increased computing power availability from PC’s and workstations
• Predictions can be much more involved
• Possible to calculate predicted temperatures for a fade test and Alpine descent
• Better judgements at the concept stage.

• Uses assumed cooling coefficients
(often based on a current vehicle)
• Significant changes in the brake cooling rate will not be reflected in the predictions.
• Currently most effective technique

• Often need to design the biggest rotor possible in the packaging space available
• Method most often used in practice
• Engineer’s preference
• Brake rotors not often oversized
• Decisions made at the concept stage for brake sizing made on optimistic assumptions

• Weight always increases
– (First prototype is always the lightest)
• Subsequent versions often include estate or high performance derivatives
• Customer expectations continually change
– Increasing service intervals
• Changing technology
– e.g. reduced overrun braking on auto boxes
• If the brake size is marginal, then subsequent lining choices are compromised (limited to high mu linings)

• Wheel size tends to define rotor sizing
• Wheel Types
– Steel wheels more restrictive
– Alloy wheels not normally fitted to base-line
– Cold formed fabricated alloy wheels worst

• Clearances
– Wheel to Caliper
– Caliper to disc
• Caliper stiffness defined by
“b” affecting:
– Pedal travel
– Pressure distribution
– Aluminium calipers
• Require greater b dimension b a

• Effect of uneven pressure distribution
• Localised Heating
• 6 stops from 80 km/h
(IBT < 100 °C)

• Large thermal gradients cause differential expansion of the disc brake material
– Thermally unstable disc is likely to induce cracking and both hot and cold judder
– Energy needs to be fed into friction ring evenly
– Stable disc design with minimal coning
– Minimal runout and bolt-up distortion
– Suitable disc material

• Thermal stresses greater than mechanical stresses
• Coning gives rise to
– Increased pedal travel
– Taper wear
– Uneven heat input (risk of forming hot bands)

o i o c o c u r
• Minimise stiffness of top hat and undercut

• Thermal Stresses >> Mechanical
• Maximising Thermal Conductivity
– Graphite Flake Structure
– High Carbon Cast Iron (Free Graphite)
– Silicon,Titanium,Vanadium, Copper,
Molybdenum
– Ferrite and Pearlite different characteristics

• The perfect material doesn’t exist
• Important to understand parameters
• Casting processes vary
• Partnership with the right foundry essential

ACTUAL AIR FLOW ACTUAL AIR FLOW FLOW
• Critical effect on brake temperature and performance
C
L
ACTUAL AIR FLOW
FRONT SPOILER
INTENDED AIR FLOW
Fig 1 Air flow along spoiler

• Complex air flow through wheels
– Steel and alloy wheels cars and 4x4
– Wheel trims
• Wind tunnel tests
– Various techniques
• Timing of cooling tests
– Need to establish cooling before B.I.W. tooling
(Testing first prototype can be too late)

• C.F.D. can be used to predict air flow
– Flow around body
– Flow through ventilated disc
– Wheel design
• CFD can be performed early in program
– At stage in programmed where body in white design can still be influenced

• Service Problems Include:
• Brake fluid reactions with components at elevated temperatures.
– Certain combinations of fluid and brake pipe can suffer adverse reactions at sustained elevated temperatures. Cu/Zn is extracted from the brake pipe and reacts with the fluid.
– Reservoir platiciser reacting with brake fluid at elevated temperatures
– Certain combinations of hose and fluid producing deposits in fluid
• Collapsing Vacuum pipes
Nylon pipes may survive elevated temperatures if carrying positive air pressure, but can collapse due to combination of internal vacuum and temperature.
• Seizing Vacuum non-return valves
Non-return valves can seize shut if subjected to elevated temperatures, especially in the presence of fuel
• ABS sensors melting
Metropolitan Police usage can generate temperatures of up to 800 ° C and melt
ABS sensors, especially if dirt shields are not used.

• Need to perform thorough hot room tests
• Take great care with heat shield deletion

• Need to spread heat energy between axles
• Need to optimise component design and materials

Forced cooling of ventilated discs
• Need for BMW Autosport to improve the brake cooling on M3 motor sport vehicles
• Increasing engine power brought about a marked increase in braking duty
• A solution was to introduce forced cooling into the vents in the front discs.

• Inner race rotates at half rate of outer race
• Supplementary race to force cooling air
• Reduction of 80°C at Nurburgring


Vented Disc
Fan
AIR FLOW
Supplemetary bearing race