# Opening a car door at a high speed

Tech2025
What would happen if a car is driving down a highway at 80Mph and one of the doors is fully opened. Will it make a big change and make it slow down or start turning?

Outhouse
It will increase drag, it has the possibility to turn the vehicle. It has the possibility to slow the vehicle down But not much in either case.

To many unknown variables for an exact answer.

sophiecentaur
Gold Member
That's a good question. I guess it would depend on what the contact with the road was like. If there wasn't good traction, I'd expect the car to turn but it could depend on something subtle like the air flow over and under the door. I think there would be a big risk of the door causing lift and tipping the car over (a third possibility).
Looks like a good project for Mythbusters.

Tech2025
Staff Emeritus
Slow down yes. Even opening a window creates more drag.

Turning, I'm not sure. It depends on many factors.

sophiecentaur
Outhouse
I think there would be a big risk of the door causing lift and tipping the car over (a third possibility).

There is no risk at all for that, none. If magically that was possible, a person could not have the strength to hold the door open with that much force.

A door open will not cause lift, and doors have different shapes that will have different effects.

80 is not high speed, and not fast enough to overcome the weight of the vehicle. You would have to attach a small airplane wing to a car and then use it as a flap is used to cause lift.

Outhouse
Slow down yes.

Depends.

If cruise control is on, the car will not slow down.

Gold Member
2022 Award
What would happen if a car is driving down a highway at 80Mph and one of the doors is fully opened. Will it make a big change and make it slow down or start turning?
What I'd like to know is how are you going to GET it open and how will you KEEP it open? 80Mph wind against a car door is going to be a LOT of force. I doubt one person could do it, particularly given the odd angle at which you'd have to exert the force. If you tried to walk in an 80mph wind, holding a car door directly against the wind, I'm sure you could not stay upright.

Homework Helper
Gold Member
2022 Award
What I'd like to know is how are you going to GET it open and how will you KEEP it open? 80Mph wind against a car door is going to be a LOT of force. I doubt one person could do it, particularly given the odd angle at which you'd have to exert the force. If you tried to walk in an 80mph wind, holding a car door directly against the wind, I'm sure you could not stay upright.

Actually, having just spend a week in the Scottish mountains, I can say that it's difficult enough to walk against a 60mph wind without the handicap of a car door.

Outhouse
What would happen if a car is driving down a highway at 80Mph and one of the doors is fully opened

Assuming the door was mechanically opened fully, a slight pull on the steering wheel to correct its path would be felt, with a slight increase in throttle to maintain speed.

CWatters
Gold Member
What I'd like to know is how are you going to GET it open and how will you KEEP it open?
There is a lot of special effects expertise that could be brought in here.When they want to slip a car into a roll, they fit a ram underneath which lifts the side of the car up. The same sort of force would be needed and I'm sure it would be simple bread and butter stuff. But you are right; an arm and shoulder wouldn't do the job.
A door open will not cause lift, and doors have different shapes that will have different effects.
I bow to your superior knowledge there.

Tech2025
Yes, the door would theoretically be held open mechanically.

Mentor
Time for a calculation. The front doors on my truck have roughly 10 square feet of area, and open about 50 degrees. The projected area of one door fully open is 10 ft^2 X sin(50 deg) = 7.7 ft^2. Assuming Cd = 1, then the drag force = 0.5 X rho X V^2 X area = 0.5 X 0.075 lb/ft^3 / 32.2 ft/sec^2 X 117^2 ft^2/sec^2 X 7.7 ft^2 = 120 lbs force. Double that for both doors fully open.

The power to overcome that drag is 120 lbs X 117 ft/sec / 550 ft-lbs/sec/hp = 26 hp.

Asymptotic and sophiecentaur
rootone
It would definitely increase drag, so would slow the car unless the driver puts their foot down.
It could also produce a turning momentum which might not do much on a normal road but would be disastrous if the road had ice on it,

Gold Member
Not a door on a car, but the Falcon F16 with air brakes being looked at.
The mechanic is opening them by hand in the video for a check post or pre flight.
Same principle as a car door - induced drag for landing, or in flight maneuvers.
At around 150 knots about 1200 pounds extra drag, with the force depending upon speed of the aircraft.
So, in principle, the car door question does have a real life application!