# Forces on an object being run over?

1. May 8, 2004

### andy_padgett

I'm currently trying to complete a Finite Element Analysis of a design i have made at uni.
Put simply I have designed a new 'Stinger' type device for the police to use to stop pursuits with.
I am trying to calculate the force that will be placed on my design when placed in the road and run over by cars etc.. - Obviously it must be strong enough to withstand these forces.
The profile of the design is triangular, 35mm high and 75mm wide, with pretty much a 45 degree angle.
I have tried to work out the force using basic impact equations, but obviously the force will not be hitting the object head on e.g. only the bottom 35mm of the tyre will hit it, the profile of the object will have some effect, etc..
Is there any way to make this more accurate?

With my calculations the impact force is about 2000N at a speed of 60mph.
This does not take into account the weight of the car, should i add 1/4 of this to the impact force (to account for the 4 wheels) to get the final force so i can complete the FEA?

Any help or pointers would be very appreciated!

Many thanks,

Andy

2. May 9, 2004

### arildno

Are you using formulae for direct impacts, or do you also consider the generation of spin as a result of the impact?
(Your example is a bit too technical that I can follow you completely)

3. May 9, 2004

### andy_padgett

At an impact force of 60mph. With the width of the SVAS being only 75mm (0.075 metres) the impact force will only be in effect for;

(1 miles = 1.609344 kilometre, therefore 60mph = 96.5km/ph or 96500m/h or 26.81m/s)

Width / Impact speed = 0.003 seconds (3 milliseconds).

The Stinger will want to be accelerated at 26.81 / 0.003 = 8936.7ms^2
If one modular part of the SVAS weighs 200 grams then the force would be;

Force = Mass x Acceleration
F = 0.2kg x 8936.7ms^2
= 1787.34 N

Hope that makes it a bit clearer.

4. May 9, 2004

### arildno

Well, first of all, sorry for inspiring "false hopes" about my ability to help you out.

The aspects of collisions that I know about, limit themselves to the calculations of momentum and spin change (i.e. when the collision period is set to zero)

I'm unfamiliar with impact analysis that estimates the length of the impact period, and the real magnitude of the forces acting.

Hopefully, others at PF are more knowledgeable than I am (I'd suggest posting in the engineering forum, unless you've alrady done so).

5. May 9, 2004

### Gokul43201

Staff Emeritus
I guess this means that the stinger gets carried away by the vehicle. Should that be happening ? In any case. for a complete analysis, you will also want to add (i) friction acting along the bottom surface of the stinger, (ii) the weight of the vehicle ( this could be a fully loaded truck ! ) acting down from the tip of the stinger, and (iii) the normal reaction of the road surface - which is not one simple number.

To balance torques, the normal reaction from the road will have to be ,
(i) 2 different numbers, if there are only 2 points of contact with the road, OR
(ii) a linearly varying function of horizontal position, if the entire bottom surface is in contact with the road.

I'm not exactly sure what the stinger looks like. If it is really just a triangular prism, then the latter case will be true. But then I'm not sure it will be pointy enough to actually blow out tires - coz the apex angle is just 90 deg, right ?

PS : I read somwhere that the British have invented some new superstrong mesh wiring thingy that will make the stinger obsolete. You might want to check that out too.

Last edited: May 9, 2004
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