The spacer ring is an element of the bicycle steering mechanism, with the help of which the height of the position of the handlebars relative to the bicycle frame is adjusted.
How to choose a lantern piece?
If your frame does not satisfy you with the height of the handlebar, then do not rush to change it to another or cut the fork stem. Everything can be solved with a simple lantern piece. It will reliably hold the desired height, despite the loads that the route will give you. And you can set the height of the handlebars to within three millimeters, saving yourself from unnecessary expenses and diseases that would certainly follow a wrong fit on a bicycle.
If every gram is very critical to you, and above all — reliability, then you need to go for carbon spacer rings.
Spacer rings are available in 3mm, 5mm and 10mm heights.