I need some 5 ray rims but the ones I find on the internet are way too expensive, so I decided to "roll my own" by creating a 3D model and having it machined from a piece of aluminum.

I've chosen to go with 20x2.125, or roughly 500mm in diameter and 50mm in width, tires for my rim. I'm looking at the nominal section widths of the standard and for this size tire I see that a size of 21C, 23C, 25C rim is required. Nowhere is mentioned what C stands for tho. I assume it's from crotched but I'd like a word of comfort on that. So basically this means that my rim's inner width (the one labeled 19 here) must be between 21 and 25 millimeters?

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    I cannot believe that having wheels machined from a 20x20x2.125" billet is cheaper than buying mass produced ones. Commented Oct 14, 2018 at 2:08
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    You have to understand that the "inch" values are pure fiction. They supposedly refer to the outer diameter of the tire (not the bead circle diameter), and often are not even that. A 27 inch tire has a larger BCD than a 29 inch tire. Commented Oct 14, 2018 at 2:40
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    @php_nub_qq I think your estimate of metal cost is way off. See here: metalsdepot.com/aluminum-products/6061-aluminum-sheet-plate 2.5" x 1' x 1' 6061 plate is over $400, and that's smaller than you need. Commented Oct 14, 2018 at 3:04
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    As I said, pick a tire you like, then read the ISO numbers off the sidewall and use those to dimension your rims. Commented Oct 14, 2018 at 3:05
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    @Criggie I'm building a mini electromobile. I figured spoke rims and hubs are not strong enough as it will weigh around 250kg with load + I really want it to look nice, and spoke rims are not a particularly good suit for a "car". I'm going buy bearings that are rated for my required weight and build custom flanges to mount rotors for disk brakes. It sure will be a lot of work but hey I signed up for it :D
    – php_nub_qq
    Commented Oct 14, 2018 at 14:20

1 Answer 1


Usually the inside width of the rim should be at least half the tire width and can be almost as wide as the tire itself. So for a 50-622 tire anything between 25 (a bit less would probably still be okay) and 50mm should work.

The problem with wide tires in narrow rims is that the tire will be unstable in corners at low pressure and can break the rim at very high pressures. I’ve ridden 35mm cyclocross tires inflated to less than 2 bar on 13mm rims and wouldn’t recommend it.

  • I will surely follow Daniel Hicks' advice though, to first find tires.
    – php_nub_qq
    Commented Oct 14, 2018 at 14:24

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