Vertical acceleration due to car going over speed bump

[Complete]

Homework Statement

Find the vertical acceleration of a car going over a speed bump a foot wide and 4 inches tall.

Homework Equations

Acceleration = dv/dt

The Attempt at a Solution

http://imgur.com/55t2WCQI am getting an answer that is around 25*g
I know this is way too much.
neglecting the shocks on the car, what assumptions am I messing up that would allow me to do this problem correctly? This isn't homework, rather a small part of a larger project where I need the forces to do failure analysis.

 

[SammyS]

Homework Statement

Find the vertical acceleration of a car going over a speed bump a foot wide and 4 inches tall.

Homework Equations

Acceleration = dv/dt

The Attempt at a Solution

http://imgur.com/55t2WCQ


I am getting an answer that is around 25*g
I know this is way too much.
neglecting the shocks on the car, what assumptions am I messing up that would allow me to do this problem correctly? This isn't homework, rather a small part of a larger project where I need the forces to do failure analysis.

Was that 20 mph speed given ?

Was anything else given, which you have not included here?

 

[Complete]

Nothing was given other than the thing I am designing should be able to withstand the force of going over a speed bump. I originally had it at 30mph but my number was way higher so I lowered it hoping that would be my problem. I looked up online the higher end dimensions of a speed bump and got a smaller width of 1 foot and a height of no more than 4 inches. Thanks for the reply.

 

[CWatters]

That's quite a severe speed bump compared to those in here..

http://riverridge.bizland.com/Site/Speed hump data/DESIGN STANDARS FOR SPEED HUMP.pdf

Try calculating the radius from the chord & height. I got 6.5"

 

[Complete]

Yes, but speed bumps and speed humps are different. Also I was assuming I should use the bump where the force would be greatest. maybe I was going too high with 4'' but http://www.innoplast.com/sbs6.html was closer to what I was imagining going over.

 

[Complete]

r = (4² + ¼*12²)/2*4 =6.5
Yes, but I see naught where you are going with this?

 

[CWatters]

Not sure what to say. Your figures are in the right ball park. Others get similar .. eg 22g mentioned here.

https://www.physicsforums.com/showthread.php?t=647187

Don't forget the tyres absorb a lot.

 

[CWatters]

r = (4² + ¼*12²)/2*4 =6.5
Yes, but I see naught where you are going with this?

I was going to work out the acceleration another way but the small radius caught my eye. If curved as per the original drawing it would have very steep sides - perhaps not as bad as a vertical kerb but still.

 

[CWatters]

Article here suggests humps are typically designed to reduce speeds to 15mph where as bumps target around 5mph.

http://www.dmgov.org/Departments/Engineering/PDF/FAQ9_Speed_Bumps_and_Humps.pdf

Speed bumps, on the other hand, have a more abrupt design. They consist of a portion of raised pavement, but because of their abruptness their use is very restricted. In fact, most speed bumps are found in parking lots and or along private roadways. Their height is typically between three and six inches, and they are usually only one to three feet long. Speed bumps produce substantial driver discomfort, damage to the vehicle suspension, and/or loss of control if encountered at too high a speed. This is one reason speed bumps are not used on public roadways. In general, vehicles must slow to about five miles per hour or less for a speed bump (compared to 15 mph for a speed hump).

 

[Complete]

Thanks. This has been helpful.

 

[Complete]

Fixed it and I got 5.4g as my maximum acceleration vertical. Thanks again.

 

[haruspex]

Fixed it and I got 5.4g as my maximum acceleration vertical.

Sure, but acceleration of what? The tyres, certainly, but a bit less for the wheel as a whole and much less for the vehicle as a whole.
Also, passenger discomfort is not just a matter of peak acceleration. It's a compound oscillator problem. With a short bump, a low speed can produce a much larger amplitude rise in the vehicle than a higher speed.