Quick release hubs are held into the frame or fork with a 5mm quick release skewer which goes through the hub.
There are two key aspects to a thru-axle hub:
- Thru-axle hubs have a large diameter axle which threads into the frame or fork.
- The hubs endcaps rest on a specially designed shelf inside the frame or fork. This makes installing the wheel easier.
Thru-bolts look very similar to thru-axles at first glance, however a thru-bolt does NOT thread into the frame or fork. Instead, it relies on its own included nut, and the hubs endcaps do NOT rest inside the frame or fork. Thru-bolts are uncommon on modern bikes and have generally been replaced with thru-axles.
Thru-bolt front hubs tend to have a 9mm shaft diameter. Thru-bolt rear hubs tend to have either a 10mm or 12mm shaft diameter.
Here’s an example of a front thru-bolt with its nut: