CALCS BAS011
CAR PERFORMANCE ESTIMATE

NOTATION

Afrontal area A (ft^2)
Caaerodynamic constant
Cddrag coefficient
Cllift coefficient
Cttransmission constant
DriveFWD F, RWD R, 4WD 4
Mmass (t)
mucoefficient of friction
Ppower (bhp din)
P1a/Mfront wheels aerodynamic load fraction
P1s/Mfront wheels static load fraction
Zcg/WB(vertical centre of gravity)/(wheel base)

APPLICATION

Estimation of car performance when frontal area, drag coefficient, mass, power and weight distribution are known. Some of the uses of the program are as follows:
* Verification that published test performance is realistic
* Performance estimation for cars not tested
* Estimation of power or Cd.A
* For cars with same Ca.A, estimation of performance or power.

OUTPUT

Max speed, and tabulated data of acceleration times & g forces
Graph showing velocity-time & g-time plots

SEE ALSO

other progs & procs

THEORY

references

GUIDANCE

Ca Constant of proportionality for aerodynamic drag. This is based on tested max. speeds of a broad range of road cars and should typically be taken as 0.02177 . For a formula one car a value of 0.0196 was found to be appropriate: the variation of the value compared with that for road cars is not suprising as the two classes differ greatly. The method of deriving this constant can be supplied.
Ct Constant for power, gearing and weight correction. This is based on tested acceleration of a broad range of road cars and should typically be taken as 0.45 .
A frontal area (ft^2). In deriving the values of Ca and Ct, Cd.A values were taken from Autocar magazines dated 25/09/82 and 11/12/82. It can be difficult to obtain values of A for a particular car but if the max. speed and power of a variant are known, the program can be repeatedly RUN varying A until the known max speed is output, noting that it is only necessary to use a unit Cd of 1.0 during this process. Having obtained an effective Cd.A, the program can be used to estimate the effect of varying any of the input parameters for the car. It may be possible to obtain the value of A either from manufacturer's data or by measurement. A possible means of measurement is to photograph the car from the front, sum the area constituents by means of grid lines and scaling the result by means of a reference measure visible on the photo. Note that the visible parts of the tyres must be included in the area summation.
Cd drag coefficient. This may either be obtained from manufacturer's data or Cd.A may be estimated in the manner described for A above.
M mass (tonnes). This must be an operational mass, typically including two adults, 1/2 tank of petrol, water and oil.
P power (bhp din). DIN is recognised as being a strict measure of power and the uniformity this provides is the principal reason for using it.
Drive: F R or 4 (FWD RWD or 4WD). Drive configuration constitutes one of the parameters determining traction and therefore acceleration at low speeds.
mu road/tyre coefficient of friction (traction parameter). For road cars a value of 0.9 is probably reasonable.
P1z/M front wheels static load fraction (traction parameter). Equivalent to front/rear weight distribution.
Zcg/WB (Vertical Centre Of Gravity)/(Wheel Base) Traction parameter. For road cars a value of 0.25 is probably reasonable.
P1a/M front wheels aero. load fraction. Difficult to obtain. Will have negligible effect on road cars, use a value of 0.5
Cl lift coefficient. Difficult to obtain. Will have negligible effect on road cars, use a value of 0.0
The following sites have been found from a search on 'car manufacturers specifications':
www.carfolio.com
www.theinternetautoguide
These sites have weight distribution & aerodynamic data available but there are some omissions. Advice of other data sources would be welcomed.

INPUT FORMAT

DAT011title
CaCtClmuZcg/WBP1a/M
entertabdelimiteddata
CaseCdAMPP1s/MDrivecase title
entertabdelimiteddata
ENDDAT

EXAMPLE

Input

DAT011example
CaCtClmuZcg/WBP1a/M
.02177.450.00.9.250.5
CaseCdAMPP1s/MDrive
1.421.851.283.61FAmbassador1.7
2.32201.4120.6FRover75
ENDDAT

Output

OUTPUTAmbassador1.7
MaxSpeed:97.3 mph
Acceleration:
0toMPHTime(s)g
101.020.447
202.130.348
303.860.227
406.330.163
509.690.121
6014.20.0894
7020.50.0628
8030.10.0389
9049.50.0162
OUTPUTR75
MaxSpeed:122 mph
Acceleration:
0toMPHTime(s)g
101.040.44
202.080.433
303.460.285
405.40.21
507.950.162
6011.20.128
7015.30.101
8020.60.078
9027.60.0578
10037.60.0391
END011