Collision shape and Model problem

I used BEPUphysics for my XNA 4.0 game , the problem is that I load my model.fbx from into the game then add it to the space of physics engine , but when I change the scaling matrix of my Model then collision did not work , if I keep model scale the same then my model response to collision and every thing look good !

some says to me that your collision shape (it's often box ) that bound your model have to be changed also if the Model changed ? is this true ? (If yes !!! what is this ????)

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The physics calculations are performed on collision shapes because they are simpler and therefore faster for collision calculations then the polygon mesh. Since the collision is only calculated based on the collision shapes scaling the vertices will not effect the collision. So you have to resize the collision shape, too. – danijar Jan 11 '13 at 18:21
thank you sharethis, So why when scale my model the collision not working ?? if you say that I have to resize the collision , the problem is when I making my model bigger no when resize it ?? ex : Model.transform== matrix.scale(1f); // work Model.transform = matrix.scale(5f) or (2f) or whatever any number bigger than 1f it's not working ?? how to solve that problem in my Engine . I just need steps. – Mohammad Ahmed Jan 11 '13 at 19:20
Somehow you have to find a fitting bounding box (collision shape). Depending on your mesh, a cube or a sphere or a cylinder or whatever will fit better. But anyway you have to find its dimensions depending on the mesh size. Just redo the dimensions calculation each time the mesh scale changes. By the way, please avoid redundant question marks and format code using ` and `. – danijar Jan 11 '13 at 19:37
thank sharethis , the last think is that how " redo the dimensions calculation each time the mesh scale changes " how to do this ?? the problem is that I don't know how resize the dimension of my collision box according to mesh changes !! this is exactly my problem sharethis :( – Mohammad Ahmed Jan 12 '13 at 16:55
Sorry but that depends on your physics engine which I never heard of. You should look for a documentation. – danijar Jan 12 '13 at 17:06