Dont hammer a rim... even a steel one can deform and then it is a bin job but an alloy is pretty easy to damage. Frankly with a wheel stuck on there are a couple of less dodgy options (and if it is for changing a flat then pump it up first... they are rarely so flat they wont hold air for a bit)

a) Put the wheel nuts in loose (enough to stop wheel falling off completely) and drive around over some bumps. This is less practical now the cheapskate manufacturers have stopped using studs and nuts and gone for the bolts which means you can no longer change the wheel in the dark. This will normally loosen the wheel and allow it to come off when you jack the car. No highspeed manouvering is suggested :) b) Take it to a tyre centre and enlist their help... I had an alloy fail to come off the iron centre once (they do corrode nicely together)... in the end car was on the ramp in the tyre place a couple of guys holding a large heavy wooden plank against the inside of the wheel and another with a 10lb sledge hammering the plank (rather than direct on the wheel or tyre to avoid damage)... yes, it did come off eventually but I certainly wouldnt expect it to come off by kicking from underneath and it didnt from method (a) which has worked on others.

Don't hammer a rim... even a steel one can deform and then it is a bin job but an alloy is pretty easy to damage.

Frankly with a wheel stuck on there are a couple of less dodgy options (and if it is for changing a flat then pump it up first - they are rarely so flat they won't hold air for a bit)

1. Put the wheel nuts in loose (enough to stop wheel falling off completely) and drive around over some bumps. This is less practical now the cheapskate manufacturers have stopped using studs and nuts and gone for the bolts which means you can no longer change the wheel in the dark. This will normally loosen the wheel and allow it to come off when you jack the car. No high-speed manouvering is suggested :)

2. Take it to a tyre centre and enlist their help... I had an alloy fail to come off the iron centre once (they do corrode nicely together)... in the end car was on the ramp in the tyre place a couple of guys holding a large heavy wooden plank against the inside of the wheel and another with a 10lb sledge hammering the plank (rather than direct on the wheel or tyre to avoid damage)... yes, it did come off eventually but I certainly wouldn't expect it to come off by kicking from underneath and the first method didn't help, although it has worked on others.