Our top-of-the-line PD4 gets another frame upgrade, this time in the form of 148mm boost spacing. This allows us to use a wider rear hub axle, which means we can increase the width of the hub flanges, resulting in a stronger, stiffer rear wheel. Add in the 12mm thru axles front and rear, sealed bearings throughout and the PD4 is our most bullet-proof (pardon the pun) model in the range.