I'm looking at wheel rims, now I'd like to go for mavic 4 or 2cd's but what I am finding is that some are 650c and some are 700c. Can some one explain in simple terms what the difference is ?
The 700c/650c name is just that, a name, it is not a measurement. BITD it used to be a guide to the approximate diameter of a wheel and tyre combo, but no longer works as a system.
Look out for ETRTO numbers.They look like this: **-***. This is the width of the inside of the rim section followed by the diameter of the bead seat, where the tyre bead sits in the rim. This is generally about 12mm smaller than the outside diameter of the rim. This numbering is used on both the rims and the tyres. Match the second, larger number for a tyre/rim fit.
The 700c rims are 622mm in diameter at bead seat, usually marked **-622. This is the normal size adult roadbike size.
The 650c rims are 571mm at the bead seat. Much rarer, usually found on the front of Lo-pros, front and back on some Tri bikes, and on small adult bikes.
To work out which you need for your bike, either measure the existing wheels, or the axle to brake distance on the frame. If you add the rim radius to the likely brake drop, that will give you a rough distance from axle to brake hole, that you can measure off the frame: (622/2)+45mm=356mm for 700c, (571/2)+45=331mm for 650C.
On tyres, again, the ETRTO number will help. Look for something like 23-622 or 22-571. The first number is the width of the tyre on a standard rim, the second is the rim diameter which it will fit. All tyres should be marked with this number.
All the best,
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