Times used to be simpler: quick-release axles with five-millimetre axle diameters were the standard on both road bikes and MTBs. Front wheel hubs were 100 millimetres wide everywhere, rear wheel hubs measured 130 millimetres on the racer and 135 millimetres on the MTB. However, a lot has happened since then: suspension travel has increased on MTBs, disc brakes can now be found on many racers, and instead of eight gears on the rear wheel, the norm has shifted to twelve. In short: The demands on technology grew and the good old quick-release could no longer keep up. One thing is clear: thru-axles (TA) allow a stiffer connection between the wheel and frame or fork due to their larger diameter and the closed dropout in which they are inserted, which was possible with quick-releases (QR). Most of the axles we use on modern bicycles are not thru-axles in the original sense of the word, but screw-in axles, as they are threaded. But because we always have to put them through the wheel for assembly, the term has become widely accepted. Some thru-axles are mounted with tools, some have a screw lever that is either fixed or can be folded down and secured under slight tension. With the introduction of thru axles, however, the number of axle variants has risen sharply. And There has also been a lot of changes in hub dimensions in recent years. So that you keep track and know which axles and hubs you can combine, we are providing you with an overview of the common dimensions of hubs and axles on mountain bikes, road bikes and gravel bikes.