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In my players update method I calc the new world matrix then I update a boudingsphere by:

MyBoundingSphere = MyBoundingSphere.Transform(World);

However when the the model's World has moved vertically down MyBoundingSphere moves vertically up?! (i.e. MyBoundingSphere.Center.Y increases instead of decreases);

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Please post some more code so we can have a more detailed look. – r2d2rigo Oct 9 '11 at 19:28
Is this 'world' transform a real world transform, or is it some sort of 'delta' transform? If it's an actual world transform, it would cause your bounding sphere to fly away at high speed. – Blecki Oct 10 '11 at 2:28

During a BoundingSphere.Transform(Matrix m) operation, the bounding sphere moves in the same direction as m's Translation vector tells it to. Your problem must be elsewhere.

Have you checked that the sphere starts out in the right place? (before the downward movement of the model).

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