Extended Inner Race Bearing Mandrel

Most roller skaters don’t use spacers between bearings in the wheels. To me, this is completely crazy!
A spacer between bearings helps share axial load between the two bearings, limits moment loading on the bearings (since the bearing seats are not particularly stiff in many wheels (Shore 85A polyurethane is common for bearing seats!). It also allows the axle nut to bottom out hard on the bearing inner race, eliminating the need to “adjust” the axle nut preload. It also allow the axle to stretch slightly, increasing friction on the nut face and threads, which limits to potential for the nut backing off.

One of the reasons that roller skaters don’t use spacers if because there is essentially no standardization in bearing spacing, which means you’d have to have a custom, matches set of spacers for each wheel.

Skateboard wheels do appear to have a much more standard bearing spacing however (11mm), and that spacing is a little larger than any roller skate wheel (~7.5mm-9.5mm).
Some bearings sold to skateboarders have extended inner races that are sized to replace the spacer.

This tool is designed to allow me to turn down the extended inner race on these skateboard bearings in order to customize them to a specific set of roller skate wheels.

The mandrel itself is not all that tough to make, the main thing is achieving a really narrow slit for expansion, such that there is still some meat on the expanding cylinder to grip the inner race of the bearing. The slitting saw was in the region of 0.010” thick!