It's a smoothing issue mate, try turning smoothing off in the render settings.   You can also change the smoothign angle on your goalpost object.  Select it and go to the properties for that object, you should see an angle there somewhere, if you reduce that number it will deform less.  I don't know the optimal angle for the goalposts, but 30 suits most of the models I've used it on.

John.

Fugazi (without the aid of a safety net)

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