37
BA053101-en
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2.10
Calculation Basis
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2.10.1
Road Load
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Equation
m
a
v
3,6
P
v
v
3,6
P
v
v
3,6
P
F
ref
Roll
2
ref
Flex
3
ref
2
Air
x
⋅
+
⋅
+
⋅
⋅
+
⋅
⋅
=
with
•
v
ref
Reference speed for resistance performance values - normally 90 km/h.
•
v
Driving speed
•
P
air
Air resistance performance [kW]
•
P
flex
Flex performance [kW]
•
P
Roll
Roll resistance performance [kW]
•
a.m
Vehicle mass
Air resistance power P-Air [kW]
The air resistance power is proportional to the surface of
the vehicle front and air resistance coefficient c
w
.
v
)
v
(v
A
c
0,5
P
2
0
Front
W
Air
⋅
+
⋅
⋅
⋅
⋅
=
r
with e.g.
•
Air density
r
(rho)
=
1.1 kg/m
3
•
air resistance coefficient
c
W
=
0.38
•
Front surface
(VEH-width x Height)
A
Front
=
1.7 m · 1.47m = 2.5 m
2
•
Driving speed
v
=
90 km/h
= 25 m/s
•
Head wind speed
v
0
=
0 m/s
results:
•
P
air
= 0,5 · 1,1 · 0,38 · 2,5 · 25
2
· 25
=
8.164 kW at 90 km/h
Flex power P-Flex [kW]
Flex power or resistance is defined as the power loss
which occurs due to the flexing of the tire on the road
surface and/or roller.
v
g
m
P
w
Flex
⋅
⋅
⋅
=
m
Generally speaking the flex power does not have a relevant influence on the test results due to the
minimal flex resistance coefficient.
