at home I have a york fitness bar and york weight plates...the bar is 5kg I believe...I stack 4x10kg plates and 2x5kg plates on there for a total of 50kg. Now at the gym the bar is 20kg, but it is quite long and thicker. When I add just two 5kg plates, it feels heavier than the ones I have at home, why is this? surely the fact that it is thicker requiring stronger grip doesn't make that much of a difference, if I add 2 10kg plates, it gets really hard, yet at home it is easy...why is it like that?
Are you certain about the unloaded bar weights for your home and your gym? Look up the specs on your bar and the bar at the gym. Grip also makes a difference, but should not be that profound.
It doesn't look like York puts up specs on most of their bars (which is bad practice IMO) but you can probably send them an e-mail. Don't know what brand or model the gym bar is, but 20kg is pretty standard. Are you sure the plates at home are weighted properly? That's a pretty strange phenomenon. 55 kg should definitely feel heavier than 40 kg.