The 2. 25-inch diameter alloy drums of the Kreitler Alloy 2. 25 Rollers provide the greatest resistance of any Kreitler Roller unit, simply because they have the smallest diameter. In fact, these 2. 25-inch rollers have approximately 90 percent more resistance than the largest 4. 5-inch rollers, meaning you'll test your mettle when riding outdoors simply isn't possible, due to inclement weather or frigid winter conditions. Like the Alloy 3. 0 Rollers, these are built to roll in an ultra-smooth and quiet fashion with their ABEC 5 bearings and alloy end caps. They're compatible with Kreitler's Killer Headwind Fan and Flywheel (both sold separately), unlike the cheaper Challenger Rollers. Since they have alloy end caps, you'll find they're heavier, subsequently carrying greater momentum than cheaper offerings made of polycarbonate.