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

[aerohead]

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.

[freebeard]

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

[aerohead]

Originally Posted by freebeard

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

[Isaac Zachary]

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

[aerohead]

Yep! Sometimes, low-tech wins the day.

[Isaac Zachary]

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.

[freebeard]

Scale wind tunnel on a roof rack?

[insert picture of NASA bubble-top Ford pickup]

[aerohead]

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.