The Challenger models feature aluminum drums and polycarbonate end caps. Polycarbonate end caps are built to last, but provide a more economical option when compared to the all alloy models. As the diameter decreases, the resistance increases. 3.0-Inch has 40-Percent more resistance than 4.5-Inch while 2.25-Inch has 90-Percent more resistance than 4.5-Inch.