Drag force is calculated by building an approximate aerodynamic profile of the car from a co-efficient of drag and a cross-sectional area. Drag force is relative to the product of those two values times the square of velocity, so you want to keep both as low as is reasonable.
in the following discussion, length is the size of your car in the direction of forward motion and width the size of the car at 90 degrees to forward. Height isn’t directly visible in CDDA because the game has a top-down 2D view.
Co-efficient of drag increases when:
- there is a ram at the front of the car
- there isn’t a quarterpanels at the front of the car (large increase)
- there aren’t any quarterpanels before any windshields (large increase)
- there aren’t any windshields before any passengers (large increase)
- any passengers aren’t inside the vehicle (large increase)
- there are any turrets or floodlights (moderate increase)
- there are any wind turbines (large increase)
- the back of the car isn’t a trunk door or quarterpanel
- the car isn’t at least twice as long as it is wide
Cross-sectional is determined by the vehicle’s height and width. Width is dependent on the vehicle’s width in tiles, but there’s not a 1:1 correspondence.
Height depends on vehicle design:
- have no quarter panels, doors, full boards, or aisles gives a very low height
- quarter panels increase the height a little
- doors increase the height a little more
- full boards increase the height even more
- aisles increase the height even more
- solar panels, turrets, floodlights, and turbines all increase height a bit
only the worse of quarter panels, doors, or full boards count.
The final drag force coefficient is displayed in the vehicle interaction menu. It generally has a large effect on how fast your vehicle can go, though very heavy vehicles with treads or road rollers tend to have their speed limited more by rolling resistance than drag.
Opening a door causes passengers to not be inside anymore (usually), which increases drag.