In part three, we made a pair of suitable gauges and measured rim spoke seat diameter. In part four, we measure the two remaining critical dimensions: the diameter of the hub flange spoke drilling circles and the distance of each flange from the wheel centre line.
Of these, the first is simple both to imagine and to measure. With the notable exception of the original Pedersen hub gear, hub flanges are generally circular and the spoke drillings in them sit evenly spaced on a circle. It is the diameter of this circle that needs determining. Use either calipers or a Vernier gauge to do so as carefully as possible, with the easiest means often being to measure across the internal/external edges of two opposite holes.
Flange offset, the distance by which the flange is distant from the wheel centre line, is easily determined in a symmetrical front hub by measuring across flanges and, since each sits equidistant from the hub centre line, dividing by two. Another method, and the one to use for a non-symmetrical hub such as a derailleur rear, is to measure inwards from the outside face of the hub locknut to the flange centre line.
With the width of the hub Over Locknut Dimension (OLD) known, it is easy to subtract the measurement from 1/2 the OLD (i.e. the hub centre line) to find the flange offset.
For example: OLD = 130mmm (standard for current road rear hubs), ÷ 2 = 65mm; non-drive-side locknut face to flange centre line = 28mm; 65 – 28 = flange offset of 37mm. If you are not sure of the OLD of an older hub, measure it using the calipers.
On the drive side, locknut face to flange centre line is 47mm, so flange offset = 18mm.
On the front hub, OLD is, for the road, 100mm, so a 15mm locknut to flange dimension gives a flange offset of 35mm.
Note all these measurements, collating them per wheel side so that you separate front wheel left and right sides and rear wheel drive and non-drive sides. In practice both sides of a symmetrical front wheel wheel will use the same spoke length, assuming you stick to the same lacing pattern. Which we look at in part five.
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