mfge425 resistancevv

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AUTOMOTIVEAUTOMOTIVE

ENGINEERINGENGINEERING

(MFGE 425)(MFGE 425)

Asst. Prof. Dr. Serhat ERPOLAT

ATILIM UNIVERSTY

2007-Fall



Basic PerformanceBasic Performance

CalculationsCalculations -- 22



RROLLING RESISTANCEOLLING RESISTANCE

W f R r r =

n

r V ba f **+=

The rolling resistance, Rr , is due to deformation of road and

tire and to the dissipation of energy through impact.

Its value mainly depends on vehicle speed, tire inflation

pressure, vertical load on the tire, tire diameter and the road

surface.



Typical variation of the rolling resistance coefficients for

passenger cars with vehicle speed is as shown above. However, it

is quite often that this variation is ignored altogether for simplicity

of analysis.



Effect of tire pressure on tire wear:Effect of tire pressure on tire wear:

Proper tire inflation is a key ingredient in driving safety, long

tire life and low fuel consumption.



W f R

V ba f

W

W K K

p

pK K aa

r r

r

R

R

ref

=

+=

−

−=

**

2121

*

[an approximation of aref for SBR tires is 0.0125bref for SBR tires is neglected; but 1.5 10-5 @high speeds]

[K 1=1.3 & K 2=0.3 for steel belted radial tires]

Rolling resistance approximations:Rolling resistance approximations:

For load surfaces other than concrete or asphalt, use a multiplication

factor:

- Stone paved ~ 1.5 or more -Firm sand ~ 4 or more

- Pot-holed ~ 3 or more -Loose sand ~ 20 or more



ApproximateApproximate coeff coeff . of rolling resistance:. of rolling resistance:



( ) ( )100

GW GRW

x

W Rg ==≅

GRADIENT RESISTANCEGRADIENT RESISTANCE

For a vehicle climbing up a

gradient , the component of its

weight parallel to the road surface

acts as a resistance to its motion.

based on the assumtion that sin θ ≈ tan θ = 1/ x if θ is small enough (i.e.

x > 5)

GR : Grade ratio (1/ x)

G : Grade ratio in percentage



AIR RESISTANCEAIR RESISTANCE

The air resistance is the force exerted by the air that

opposes the motion of a vehicle passing through it.



airtorelativespeedvehicle:

vehicletheof areafrontal:

coeff.dragessdimensionl:

pressuredynamic:

047.02

r

f

D

r f Da

V

A

C

q

V AC R =

f Da AC q R =

( )( )width Maxheight Max. A

f ..80≈

AIR RESISTANCEAIR RESISTANCE



The typical modern automobile achieves a drag coefficient of

between 0.30–0.35. SUVs, with their flatter shapes, typically

achieve a C d of 0.35–0.45.



0.24 – Audi A2 1.2 TDI, 2001 0.27 – Merc Benz C-Class, 2001-2007

0.372 – Ferrari, 1996 0.57 – Hummer H2, 2003

0.7 to 1.1 – F1 Car???



DragDrag vsvs. Lift. Lift

Lift is an aerodynamic force resulting from the pressure difference above and

underneath a car. It decreases the weight of the car in effect. Drag (i.e. air

resistance) and lift are usually mutually exclusive.



Unlike standard cars that produces upward force, F1 cars can produce a

downforce equal to their weight at 125km/h to 130km/h, and at 190km/h the ratio

is roughly 2:1. Therefore, theoretically, F1 cars can drive upside down from

130km/h.



Bugatti Veyron



%2



• Parts rotating at engine speed : crankshaft, flywheel, clutch,etc.

• Parts rotating at propeller shaft speed: transmission shaft, etc.

• Parts rotating at wheel speed : Drive axles, wheels, tires, etc.

INERTIAL RESISTANCEINERTIAL RESISTANCE

While the vehicle is in linear motion, an inertial resistance to motion due to

rotating parts also exists. Rotating masses can be grouped into:



Moment of Inertia ( J ) : A measure of the resistance offered by a body to

angular acceleration.

+=

2

w

T vvi

r

J ma R

eqvi ma R =

veq mm =

2

222

1wv

d t ed pw

r m

ii J i J J ++

+=γ

( )20016,003.1 d t ii+=γ

(An effective linear acceleration resistance)

(γ is the ‘rotary mass factor’)



mfge425 resistancevv

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