How can i calculate the drag coeeficient with the height and width?
 

Good evening,

I would like to know how can i calculate the drag coefficient of a 1990 toyota corolla with the height and width measurements?
Thanks

I don't think you can. Cd is a dimensionless number.
en.wikipedia.org/wiki/Dimensionless_numbers_in_fluid_mechanics
At most you could estimate A, for area (~78-85%).

I can't find the source now, but I remember .32 as the cD on the car.
Although the car doesn't look sleek, the car was well designed for low drag.

By comparison, have a look at the VW Box ....I mean Fox :p
Despite the car looking like a box, it had a cD of .31!
This is better than a car such as this Firebird at .33 !

"User name checks out." :)

Here's mine:

https://ecomodder.com/forum/member-f...8-100-1154.jpg

(I'm coming down on the selling price)



(...I'd even throw in the roof rack)

Force = velocity^2 x cross sectional area x coefficient of drag x the density of the air.

To find the coefficient of drag you need to know:

• How dense is the air where you're at (usually 1.225 kg/m^3)
  
• The cross sectional area. That's not just height and width. You have to take a picture of the front of the car from far away and use a bunch of geometry to figure out the actual area.
  
• The mass or weight of the entire vehicle, including yourself and fuel.
  
• The rolling resistance, although with enough testing you can kind of figure out both your rolling resistance and coefficient of drag.
  
• And a way to measure speed and time to figure out deceleration.

Calculate your cross sectional area in meters squared.

Speed up pass a certain speed (i.e. 60mph) on a known flat piece of pavement and throw it in neutral. When the car slows down to that speed (60mph) start the timer. When it has dropped a small amount (i.e. 55mph, 5mph less) stop the stop watch. The more accurate your measuring instruments and techniques are the better the results will be. Several repeated tests are best. Try both direccions on the same road and avearage out the results in case the road isn't perfectly flat.

Now take your speed drop and turn that into meters per second per second (m/s^2). For an example, from 60 to 55mph is about 2.235 m/s. If it takes 1.5 seconds that about 3.353 m/s^2. Now take your weight and turn that into kilograms and multiply that by your m/s^2 to get force in newtons. Subtract any force you believe or know rolling resistance takes up. You could push a pressure operated scale against the car and see how much force it takes to push the car in neutral and convert the number into newtons. Or do a lot of tests at different speeds with different guesses until you find a rolling resistance number that doesn't change much at different speeds.

Take the average speed in m/s (between 55mph and 60mph is 25.7) and square it (multiply it by itself.). Multiply that by your cross sectional area and the density of your air in you area in kilograms per cubed meter (usually 1.225kg/m^3 for an average number) and then divide that into half.
Now take your force in newtons and divide that into the number you just figured out above.

You should now have your coefficient of drag.

I almost bought a Dasher because of you freebeard. Seller had already started parting it out by the time I decided to contact him though.

Pardon me, but I think I made a mistake. Take the force in Newtons and multiply that by two. Then take the other number from the velocity squared multiplied by the density of air and the crossectional area, but don't divide into two. Then divide the newtons doubled into that number to get coefficient of drag.

M_a_t_t -- [If I might ask] what was he asking?

Else one could put the car on a tow rope with a GoPro strapped to a spring scale, and measure it directly.

So lets say it takes 3 seconds to go fron 60 mph to 55 mph then what would i do?

Multiply 3 seconds (s) times 2.235 meters per second (m/s) to get 6.705 meters per second per second (m/s^2). Multiply that by kilograms mass of your vehicle (don't forget to include your own mass).

The resulting answer will be force in newtons (N) at around 25.7m/s.

There is no accurate way without putting the car in a professional wind tunnel.

Coast-down testing is notoriously inaccurate.

Total drag is CdA. In other words, the coefficient of drag X frontal area. The height and width would be used for the Area. The coefficient of drag has to be measured in a coast down test, or a wind tunnel.

Nobody likes my spring-on-a-tow-rope idea. :(

Sounds good to me!

I forgot that DIY tests (spring scale tow or coast down) only work if wind is basically zero.

To be honest, I assumed that people were ignoring it because it doesn't measure aerodynamic drag.

I think the idea is to measure rolling resistance. Cost down tests measure rolling resistance + aerodynamic drag. So the hard part is usually separating the two.

Once you've subtracted the rolling resistance, what's left?

[five minute delta]

Ok freebeard you made me comment on this:

I first thought that this spring scale was an excellent method until I thought about it some more and came up with many issues with repeatable results and myriads of conflicting things that would show up as increased or decreased scale readings. Kinda like torque does not relate to PSI holding values in a screw.

I accept the criticism.

It was a thought experiment.

Which lead to further thoughts about averaging the data. What if the spring were pulsed with a solenoid to give a fluctuating reading, said fluctuations greater than the noise?

Corolla
 

* The 1988 Corolla GT-S was Cd 0.33, @ around 18.302-sq-ft frontal area.
* The next-gen Corolla was jointly produced by Toyota- General Motors, as the Corolla / Geo PRIZM, I believe, at the New United Motor Manufacturing, Inc. ( NUMMI ) factory in Fremont, California, where Tesla Motors presently resides.
This car, in 1992, had grown to 20.8 sq-ft frontal area, at Cd 0.33.

spring on a rope
 

In the past, 'shrouding' trailers have been used to isolate R-R from overall drag.
Inside the trailer, the test vehicle is completely shrouded and impervious to aerodynamic forces, other than wheel windage.
The car is anchored to the tow vehicle via a tow-bar, which incorporates a linear strain-gauge load cell, designed to allow calculation of the rolling-resistance force.
Locked in place until up to test velocity, the tow-bar mechanism is unlocked for the duration of the test, then re-locked once data acquisition is completed.
Afterwards, this data can be used in conjunction with SAE-approved coastdown results, to help isolate strictly aerodynamic forces.
Amateurs could use a 'fish'-scale of appropriate range for lower resolution measurements.

I'll take that as confirmation. In this case the tow vehicle would need the Templ shroud.

shroud
 

The test vehicle is inside the shrouding trailer ( it's airtight, with scrubbing seals dragging the road for the entire perimeter of the trailer), a box of dead air.
The tow-bar is connected to the inside front of the trailer, pulling the test vehicle, which is only touching the load cell and the tire/road interface.

That's a lot easier than building a road in a vacuum.

easier
 

Yep! Sometimes, low-tech wins the day.;)

I wonder if there's a way of constructing an enclosed trailer, but that's actually open in the front and back, for testing air drag.

Scale wind tunnel on a roof rack?

[insert picture of NASA bubble-top Ford pickup]

for testing air drag
 

In past times, cars have been tested on top of flatbed railroad cars, pushed from behind by the locomotive, so as not to upset the airflow coming at the car.
Drag force measuring equipment was below the 'floor' of the flatbed, leaving only the vehicle exposed to the flow.
If we take a 30% test section blockage-ratio as the absolute maximum allowable, for zero-yaw flow conditions, a trailer which surrounded a car, with an open throat large enough to respect the 30% factor, would be so enormous that one couldn't use it on public roadways.
Also, rigging up test equipment sensitive enough to measure forces would, be in the way of the airstream ( a sting ), and road vibration conditions might knock the equipment out of calibration before you could capture any useful data.
If you had a hill top, with very strong reliable near-constant velocity winds, you could build a tunnel with a tail-vane, large enough, on top of a circular rail track-lazy-susan, which would slew it's inlet into the wind at all times. Measuring equipment would be under the floor.