Springs 60mm wide are a durable highway spring designed to withstand the rigors high speed can inflict on suspension.
Spring twisting can occur due to a combination of load centre of gravity, weight, cornering and shock loading caused by badly maintained roads.
45mm wide springs are suitable for metropolitian travel while 60mm springs are most suited to high speed travel under heavier loads.
Example, persoanlly I would not build a car trailer with the intention of interstate travel using 45mm wide springs.
