Obtained a mystery frame the other day, and was wondering if there is a 'rule of thumb' measurement from the dropouts to the brake mounting holes on the fork or seatstays to determine correct wheel size?
Dropout to brake mount will give you a good idea of the intended wheel size, but often more than one wheel size will work, depending on the drop of the brake calipers you fit.
In a nutshell:
(dropout centre to brake bolt hole dimension) equals (rim radius plus brake reach)
You can use the tyre bead seat diameter to estimate the effective rim radius - that's 630mm diameter (315mm radius) for 27", 622mm (311mm) for 700C, 584mm (292mm) for 650B, 571mm (285mm) for 650C, 559mm (280mm) for 26". Most modern road brakes have a maximum reach of about 47-49mm, but longer models are available.
Most road bikes built in the last 20 years use 700c wheels and either 47mm or 57mm reach brakes.
Once you've measured, try a wheel in the frame. If there's clearance for the tyres you want to run, and the rim is within range of the brake hole, you're good to go.
Frames built for 650C can often be used with 26" (559mm) rims. Frames built for 27" can usually be used with 700C. Frames built for 700C can be used with 650B if the brake calipers are replaced with long-reach models.
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