Winter wheel build – part three; rim diameter

Before you get to start your wheelbuild or even to order your spokes, you’ll need to do some measuring. There are three basic dimensions to be determined: the diameter of the hub spoke hole circle (or pitch circle), the distance of the hub flange in question from the wheel centre line and the diameter of the rim at the nipple seats. You’ll also need to decide on the lacing pattern of the spokes, but that can wait for the moment.

First, for no other reason that it involves a gauge that can be made at home, we’ll measure the diameter of the rim at the nipple seats. These are the locations for the spoke nipples; in a modern box-section clincher or tubular rim, the nipples sit down inside eyelets and it is vital for correct spoke length selection that the diameter of the circle on which they sit in the rim be measured accurately.

Why? Because the spokes in the finished wheel must be neither too short nor too long. In the case of the former, the end of the spoke should reach the base of the slot and must sit beyond the shoulder of the nipple that bears against the eyelet. This is because a spoke that is any shorter places tensile force on the nipple below the shoulder; in other words, the pull of the spoke acts through the neck of the nipple.

The nipple will usually shear off at this point as it is not designed to withstand such loading. If the spoke is long enough, its thread pulls on the part of the nipple above the shoulder, compressing the nipple against the eyelet and obviating the risk of shearing under tension.

A little too long is permissible in an eyeletted rim as the excess won’t rub against the rim tape and damage the tube. However, properly too long a spoke and the nipple may reach the end of the thread before full spoke tension is achieved, limiting wheel life and potentially preventing correct truing of the rim.

So, some way of determining how the spoke sits in the rim at correct length for the nipple is what is needed. The answer is simple. Thread a nipple onto the spoke so the spoke end sits flush with the slot and you have the rim end of things. Now you need to know the distance from one side of the rim to the other. For this you need two spokes threaded onto nipples as described. First, however, cut each spoke to exactly 200mm long. Then, use thread locking compound to secure the nipples to the spokes for durability. You have gauges!

Insert your two gauges into eyelets on exactly opposite sides of the rim. Count eyelets to make sure you are not offset by one; 16 for a 32hole rim etc… Now simply measure the distance between the ends of the spokes, add 400mm (the lengths of the two spokes) and you have your rim diameter. In this case (Mavic’s Open Pro) we have 204mm + 400mm = 604mm. Voila!

Note that this is not the external diameter of the rim, nor the bead seat diameter…

Discuss in this forum thread.


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