MXW004 - Brass Rear Axle Caps The Brass Rear Axle Caps simply add more overall weight to the Mini-Z 4x4, which delivers better traction and overall crawling performance over that of the stock 4x4. Adding the brass front steering hubs and axle caps add even more performance because adding weight at the lowest point on the chassis is also going to have a considerable effect on lowering the vehicles center of gravity. The lower the center of gravity, the better the performance.