The above comments about magnetic force means your moving extra weight isn't strictly true...

In 540 on a steel floor, pushing the robot along by hand is no different to pushing it along without the magnets in place... Just make sure you position them on the centre line of your wheels and you'll be fine...

Also remember if a magnets rated to hold 50kg.... Thats probably true... If its stuck directly to a sheet of steel.... In a robot it isn't...

540's magnet is 50x50x25 neodymium... 5mm g/c + 1mm of titanium =6mm away from the floor.... The magnet was advertised as having 180kg of pull... After the above mentioned gap it probably gives around 20-30kg of down force...

Ceri - those dewalts blew up without the magnet in place.