Hello everyone
Firstly, I was thinking about AshG's point regarding welding and I must admit he is right. Think about a drum skin. If you apply a load in the middle, previously it would be transfering the load at the top and bottom - by welding the sides you reduce that load and distribute it around more - you also now have the drum skin affect.
With regard to the stiffness of the plate. The best way I can think of describing it is to get a piece of of strip 1mm thick and 30mm wide. Holding the strip across two benches with the wide side up, press down in the middle and watch it bend. Now rotate 90 degress and try and press down and bend again - it's a hell of a lot stiffer. For the maths a simple example here -http://physics.uwstout.edu/statStr/statics/Beams/bdsne47b.htm
If you look at the equation for a rectangle then the moment of inertia is
3
bd
12
Where b is the 'breadth' and d is the depth. Hence by making the section deeper you stiffen it by a cubed factor.
In the case of the pedal box you could add some form of strengthening rib across the back. However, by adding a narrow rib across the back you willl produce a stress raiser.i.e. a point or line where stress is concentrated. Hence it is better to add either a wide rib (such as a piece of box) or lots of smaller ribs. Flyerncle, your idea will add some strength to the plate, but not a great amount for the weight you will add. For a n interesting play - you could try taping a piece of card across the opening and try addining different forms of strengthening. I personally would look at some form of cross brace in the engine bay, this could be braced above and below to feed the load into the tubes either side.
Side impact is an interesting one. In this case stiff may not be the best. The obvious thought is that you don't want the side of the car to be 'crushed' but there are actually two things to consider - one is energy dissipation the other is penetration. You could make the side impact area extremely stiff, but what happens to all the force when you are hit. If you had a 'solid' chassis and you were hit then all the force would be transferred through the car and into you. 30 or 40G in a very short period of time can cause some serious damage to the human body. Your better off trying to disspate that same energy over time, some some flex and bending in the chassis can be good. Think of modern crumple zones. The guys I use to work with designing racing monocoques used to allow about 20mm of bending over a typical space frame. The idea being that the driver might get a little squashed and bruised - but the severity of his injuries would be less.
That's some food for thought!
|