At what slope does a car door no longer hold itself open?

[alykabob02]

my boss has given me a nearly impossible task: find out at what slope will a car door no longer hold itself open? in other words, he wants to know how steep a driveway can be before the car door closes on somebody's leg and injures them
(i work for a civil engineering company that consults a town full of complete wackos)
i realize that this will vary from car to car depending on the make, model, and condition of the mechanical parts involved
too bad my boss doesn't have a clue
PLEASE HELP ME!

[Neutrino]

this varies so much from car to car that there is no answer. You can try experimenting with different cars. Or call the auto company and maybe they know how much presure will the door "holder" whitstand and the door weight and dimenssions. so you can make the caculations and parhaps find an average for most cars.

[ivymike1031]

There really are quite a lot of variables that come into play here. I used to design door hardware (including the detent straps), and I know that there isn't a single, simple answer that I can give you that will be of particular use. I also didn't do that particular job for very long or very well (by my own estimate), so someone with more experience in the matter may be able to provide more help. I pm'ed you a link that you may find helpful.

Where are you, in Davis, CA?

[FYRHWK1]

good with people? go find a steep driveway and ask the people if you could pull in with a few cars, tell them what you're trying to do and as long as it isnt full they'll probably laugh their ass off at you, but i cant picture em sayin no. you can get the cars from a rental place, tell the boss this is the only real way to find out since every car door setup is different. spring pressure, door weight and length, mounting point, the actual mechanics of the door, they all play a big part.

Also what kind of cars do you need to test for? stuff like kias have super light doors with stiff springs that could probably stay held open on a 45* grade, although a 2 door camaro would shut on a speedbump.

[911GT2]

Experimentation would be easier than anything else.
There's too many variables.

And FYRHWK1, what's up?

[Jay!]

Just say 21º, and defy him to prove you wrong.

LOL, hey, my sister's in Davis! (but then again, she's a wacko, too... :silly2: )

[v10_viper]

I'd say your boss is kind of a dumbass for asking such a question, no offense to him though. All car doors are different, some are different pressure to close and some are really heavy. Just try to tell him that it's too much of a feat to do. Or just tell him heavier doors tend to close at less of a degree because of the force of gravity.

[ivymike1031]

Originally posted by jay@af
LOL, hey, my sister's in Davis! (but then again, she's a wacko, too... :silly2: )

Well, I used to live there myself...

[FYRHWK1]

Originally posted by 911GT2
Experimentation would be easier than anything else.
There's too many variables.

And FYRHWK1, what's up?

same old sh*t, different day. trying to pass the time at work though i really should be doing homework, i just can't stand calculus, one of these days i'll find a way to design something without using math

[alykabob02]

no, it's actually a formerly-rural-township-now-developing-into-a-yuppie-suburb-with-many-McMansions in NJ
and my boss was asking cuz he needed to try to gather info so the township's board could try to decide out whether to grant a varience to a developer who put in driveways that are steeper than specified in the township ordinance
so it was for a reasonable persuit

[romabimmer]

Once you have these parameters you can figure it out:

The force of the door hinge indents, those things that hold the door in place and the friction force in the hinges.

The mass of the door and the center of mass (center of gravity in this case) of the door.

The position of the hinge(s) in reference to the CG.

The problem is how do you find all this stuff! The mass of door is easy, but the hinge force and friction and the CG requires you have the design specifications or you need to measure it by experimentation.

You can give your boss a basic/generic formula, but not actual results without knowing the specifics.

Now, since the indents and friction force is what holds the door open against gravity you need to find at what angle does the force of gravity overcome that 'hinge force.'

You need to create a free-body diagram where you show all the forces acting on the doors. Put the forces keeping the door in place on one side and the force of gravity on the other side.

Force of Gravity on Door = (Cosine of the angle the door makes level ground) * (mass of door in kg * 9.81)

9.81 is the acceleration of gravity or 1g. The angle of the door is the same as the angle of the incline.

Force on the hinge (torque in this case) = (Force of Gravity on the Door) * (The distance from the CG to the hinges)


If the (Force on the Hinges) is greater than (The Force of the Hinge detent and friction) then the door will close.

Since technically for any specific door the angle is the only variable, you can solve the equations above for the 'Cosine of the Angle'

[911GT2]

Originally posted by FYRHWK1


same old sh*t, different day. trying to pass the time at work though i really should be doing homework, i just can't stand calculus, one of these days i'll find a way to design something without using math

What calc are you taking??

[FYRHWK1]

school calls it MA1012 Calculus IA, its just the entry level first year stuff, i've a deep rooted hatred of math, at least the teacher doesnt make me remember every damn formula myself.

[rsxer45]

Originally posted by romabimmer
Once you have these parameters you can figure it out:

The force of the door hinge indents, those things that hold the door in place and the friction force in the hinges.

The mass of the door and the center of mass (center of gravity in this case) of the door.

The position of the hinge(s) in reference to the CG.

The problem is how do you find all this stuff! The mass of door is easy, but the hinge force and friction and the CG requires you have the design specifications or you need to measure it by experimentation.

You can give your boss a basic/generic formula, but not actual results without knowing the specifics.

Now, since the indents and friction force is what holds the door open against gravity you need to find at what angle does the force of gravity overcome that 'hinge force.'

You need to create a free-body diagram where you show all the forces acting on the doors. Put the forces keeping the door in place on one side and the force of gravity on the other side.

Force of Gravity on Door = (Cosine of the angle the door makes level ground) * (mass of door in kg * 9.81)

9.81 is the acceleration of gravity or 1g. The angle of the door is the same as the angle of the incline.

Force on the hinge (torque in this case) = (Force of Gravity on the Door) * (The distance from the CG to the hinges)


If the (Force on the Hinges) is greater than (The Force of the Hinge detent and friction) then the door will close.

Since technically for any specific door the angle is the only variable, you can solve the equations above for the 'Cosine of the Angle'

I have a minor correction. The force of gravity on the car is actually the weight*the SIN (of the angle of the incline). If you draw similar triangles and relate the angles and the force of gravity directed along the path of the incline you will see that this is true. The cos will give you the force of gravity pointing downward perpendicular to the incline. Also, don't most doors have a little rubber bump on the door frame that compresses after the door reaches a certain angle, and gives some extra force in closing the door? In that case wouldn't the force be non-constant?

[romabimmer]

Originally posted by rsxer45


I have a minor correction. The force of gravity on the car is actually the weight*the SIN (of the angle of the incline). If you draw similar triangles and relate the angles and the force of gravity directed along the path of the incline you will see that this is true. The cos will give you the force of gravity pointing downward perpendicular to the incline. Also, don't most doors have a little rubber bump on the door frame that compresses after the door reaches a certain angle, and gives some extra force in closing the door? In that case wouldn't the force be non-constant?

Yes true it is Sin! Thanks for correcting that.