Aluminum

* Spacers to adjust the chainline of your freewheel or fixed gear cog.* Set of 5pcs - 1mm, 1.5mm, 2mm, 2.5mm, 3mm* Inside diameter 1.385" / 35.18mm - made to fit on standard 1.37" threaded hubs for fixed cog or freewheel. ** THESE ARE NOT MADE FOR CASSETTE OR FREEHUB - THEY FIT STANDARD HUBS ONLY **The picture above shows a flip-flop hub. It has fixed gear threading on one side, and standard freewheel threading on the other. These are the proper hub styles for use with our spacers. Use of spacers is not to be taken lightly with freewheel and fixed gear cog applications. Every millimeter of spacer used is one less millimeter of engaged threads between the hub, and freewheel or cog. Inadequate thread engagement can cause significant thread damage, especially if you mash it hard. But sometimes you need to adjust your chainline and no other simple fix exists. * Use of cog/freewheel spacers is high-level bicycle mechanics. Buyer accepts all responsibility for making drivetrain modifications. FREE SHIPPING USA SELLER - DELIVERED IN DAYS, NOT WEEKS