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7/28/2019 Automotive Differentials, Release
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Automotive and Racing Differentials
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Overview Why do you need a differential? Typical Types of Differentials
Brief Overview
Advantages and Disadvantages
Differential Applications Porsche ALMS LMP2 Prototype Differential Configuration
Description of Layout
Adjustability
Influence on Vehicle Performance
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Why do we need a differential? Quantifying a Turn
centerr
outerrcenter
rinnerr
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Assumptions
Pure rolling, no wheel spin Constant yaw velocity, No Yaw Accel.
Known Constants
Vehicle Track Width LTW
Lateral Acceleration ALat Vehicle Speed Vs
Tire Radius rtire
Example:LTW = 72 in.
ALat = 2.0 g
Vs = 60 mph
rtire = 14 in.
Determining Corner Radius:
This set of equations quantifies the action
required by the differential to negotiate a
given corner
%1.5100*
6.17
9.0
9.07.166.17
6.173.7
8.128
7.163.7
5.122
3.712/1422
8.1281232360
602
360
5.1221172360
602
360
123
2
12/72120
2
1172
12/72120
2
120
1202.320.2
4667.160222
slip
innerouterdiff
tire
outerouter
tire
innerinner
tiretire
outerouter
innerinner
TWcenterouter
TWcenterinner
center
Lat
scentersLat
RPM
RPMRPMRPM
C
lRPM
C
lRPM
ftrC
ftrl
ftrl
ftL
rr
ftL
rr
ftr
ftA
Vrr
VA
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Artificial Wheel Spin Percentage
0.0%
10.0%
20.0%
30.0%
40.0%
50.0%
60.0%
70.0%
80.0%
0 50 100 150 200 250 300 350 400 450
Corner Radius (ft)
InsideWheelSpin(%)
Artificial Wheel
Spin Percentage
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Options to a Differential Train wheel, the angle of the wheel allows the train tomove laterally and change the rolling radius of the train
wheel
Tire stagger
Utilized in Oval Racing
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Typical Differential Configurations Open Differential
Locked Differential, Spool
Limited Slip Differentials
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Limited Slip DifferentialAdvantages: Manages vehicle traction mechanically
Completely passive system, no user or electrical controller inputrequired
Disadvantages:
Many moving components, more susceptible to failure
Expensive to produce
Applications:
Optional OEM equipment, performance oriented or high endproduction vehicles
Road racing
Types of Limited Slip Differentials:
Salisbury style
Viscous
Torsen
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Salisbury Style Differential Purely mechanical differential which relies on pressure angles to generate a clampingforce on a clutch pack to resist different wheel speeds
Highly adjustable and can be characterized through simple simulation or test
equipment.
Minimal temperature sensitivity
Expensive to manufacture due to all of the components involved, tight tolerances
Small performance and operational window
Components can wear out and the performance of the differential can be adversely
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Viscous Differential Hydro-mechanical differential that relies on the viscous properties ofan oil or grease to generate a resistance to different wheel speeds
Highly influenced by temperature
Semi-adaptive, force is dependent wheel speed difference when
responding to slip. Characterized as a speed sensitive differentialrather than a torque sensing differential.
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Torsen Style Shortened from Torque Sensing Differential.
Relies on gear pressure angles to cause a mechanical binding in a geartrain that resists different wheel speeds.
Non adjustable, and difficult to quantify the performance without expensiveequipment.
Has a large performance window, which enables it to be used in a widerange of applications
Still relies on torque input from the wheels to respond and can only apply aratio of the torque to the non-slipping wheel
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Porsche LMP Differential Configuration Differential Layout
Combination of a Salisbury Differential and a Viscous Differential
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Adjustability Optional friction plate material
Sintered Steel Plates
Sintered Bronze
Carbon Matrix
Clutch pack preload Static clutch pack compression with preload springs
Clutch pack configuration
Number of friction faces
Clutch pack pressure angles
Optional coast and drive pressure angles Optional Viscous Differential Stiffness
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Influence of Optional Friction Materials
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Influence of Clutch Pack Preload
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Clutch Pack Configuration Increasing the number of friction facesincreases the area of the friction surface
Increasing the friction area translates a
larger portion of the differential clamping
load into differential force, giving a tighter
differential
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Optional Viscous Coupling Stiffness
To change the Viscous coupling stiffness the entire unit must be changed or modified.The stiffness of the coupling is adjusted by changing the amount of sheering plates or the
viscosity properties of the oil or grease
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Influence of the Viscous Coupling on Total Differential Force
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Simple Representation:
.deg45
300
300300
45tantan
lbsF
lbslbsF
F
Torque
Torque
Clamp
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Influence of the Differential on Vehicle Dynamics Yaw stability is commonly referred to as vehicle balance, the differential can
have a large influence on the yaw stability of the vehicle.
Understeer Balance Condition, Tight
Nuetral Balance Condition
Oversteer Balance Condition, Loose
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Braking Zone During this phase of the corner the differential will have the leastamount of influence. The rear brakes are determining the speed
and torque of the rear wheels and at the same time the torque
coming from the engine is small as well, engine braking only. In a
symmetric left to right brake bias setup the forces from the brakes
will be equal. The influence of the differential increases progressively as the brake
pressure decreases
The differential influence will increase as the inertia of the engine
components increases
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Braking Zone Forces
RFyF
RFxF
xD
FRONTyD
REARyD
CGM
LFyF
LFxF
RRxF
LRyF
LRxF
RRyF
+Y
+X
DIFFyF
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Resolved Moment
xxX
yyY
DFM
DFM
LFLF
FRONTLFLF
Moment Contribution LF Moment Contribution RF
xxX
yyY
DFM
DFM
RFRF
FRONTRFRF
Moment Contribution LR
xDiffxXyyY
DFFM
DFM
LRLR
FRONTLRLR
Moment Contribution RR
xxX
yyY
DFM
DFM
RRRR
FRONTRRRR
LRLRRRRRRFLFRFRF xyxyxyXyCGMMMMMMMMM
Resolved Moment for a Left Hand Turn:
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Mid-Corner Rolling Phase The middle of the corner is where the yaw velocity is at its highest, and there is
little or no influence from the throttle or brakes, zero longitudinal acceleration
RFxF
xD
FRONTyD
REARyD
CGM
LFxF
RRxF
LRxF
+Y
+X
DIFFyF
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Corner Exit On Throttle Phase
RFxF
xD
FRONTyD
REARyD
CGM
LFxF
RRxF
LRxF
RRyF
+Y
+X
RRDIFF yyFF
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Questions
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Quiz1) What is the not a reason for having a differential in your car?
a) Increase tire life
b) Decrease the turning radius to help the car turn in a
parking lot
c) Accommodate the two different arcs carved by the car
through a turn
d) All of the above
e) None of the above
2) Which differential will respond to wheel speed differences?
a) Salisbury Differential
b) Viscous Differential
c) Open Differential
d) Torsen Differential
e) None of the above
3) Which differential has the largest performance window?
a) Salisbury Differential
b) Torsen Differential
c) Viscous Differential
d) Open Differential
e) None of the above
4) During which phase of the corner does the differential have the
lease amount of influence?
a) Corner Entry
b) Mid Corner
c) Corner Exit
d) The Entire Corner
e) None of the Corner
5) If the driver is complaining about poor acceleration out of the
corner, what adjustment to the differential would you make tohelp fix the drivers problem?
a) Increase the differential force to decrease inside wheel
spin
b) Decrease the differential force to increase the inside
wheel spin
c) Do nothing and tell the driver to quit complaining and
push the gas pedal harder
d) The differential can do nothing to help solve the
handling issue