650b or 700c: tyre diameter is more than the wheel name
650b is not simply “700c, but more comfortable.” The rim is smaller; the fitted tyre determines how much smaller the complete wheel is. Your frame then has to be approved for the resulting setup.
Use the bead-seat numbers
650b commonly uses a 584 mm bead-seat diameter; 700c uses 622 mm. Schwalbe’s size explanation helps decode ETRTO markings. The rim diameters differ by 38 mm, or 19 mm in radius. Do not confuse 650b with other “650” standards.
The tyre can change the comparison
In a simplified geometric example, a 622 mm rim plus 32 mm tyre height on both sides gives 686 mm outer diameter. A 584 mm rim plus 51 mm height also gives 686 mm. Real tyre height is not guaranteed to equal its nominal width, and loaded radius changes further. These numbers illustrate why the complete wheel matters; they are not an approved frame conversion. Equal outer diameter would still say nothing about lateral tyre clearance.
Get permission for the complete combination
For a GRARO or another Chinese frameset, obtain the maker’s wheel-size and tyre approval for the exact generation. Check both lateral/radial clearance and the effects on bottom-bracket height and pedal clearance. Matching axle spacing does not settle wheel-size approval.
Choose the approved combination that serves your terrain and has obtainable tyres. If approval is missing, keep the intended wheel size rather than treating a forum build as permission. A smaller wheel is a design choice, not a shortcut around a frame’s tyre limit.
Use measured tyre width on the intended rim for the clearance check. If both combinations are approved, plan repeatable wheel swaps separately; matching outer diameter does not align rotors or cassettes.
Sources
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