January 29, 2011 1 Comment
[Note: this is #10 of a series of 20]
A craft can be fun owing to charming materials, a satisfying process, elegant tools, and a worthy product. Wheel building has all of these. And among tools, none is quite so effective and clever as a dishing tool. What could be as simple as the gentle arch with central indicator of this classic tool?
As many of you know, the arch locates like a diameter, its ends resting on opposite sides of the rim.
The central indicator hovers over the hub, reaching down to just touch it. So adjusted, the tool can be lifted off and placed on the other side of the wheel. The wheel is perfectly centered if the tool fits it in the same way on both sides.
Be careful that the tool doesn't inadvertently touch a spoke. It's only meant to touch the rim and the hub dropout face.
The wheel is out of center if either one of the arch ends doesn't touch the rim.
Or if the center indicator doesn't reach the hub.
Anyone with tool and wheel in hand can see the asymmetry and deduce how to change the relative positions of the hub or rim by tightening or loosening spokes.
(1) The utter simplicity of the display. It takes no training, no dials or scales, no accurate features. The arch could be made of crooked driftwood. The center indicator could be a tongue depressor pivoting on a thumbtack. No problem, because the tool moves from one side to the other. The readout is comparative, so the tool needs no marks, measures, or symmetry to tell its story.
(2) The actual centerline error of a wheel is magnified, offering precision to any user. The tool can be fitted so an error shows as a gap either at the rim or at the hub. It's your choice. A gap at the rim (hold one tool end against the rim so the other end lifts off) shows a 4-fold magnification of the wheel's centerline error. For example, if the rim centerline is 1mm off, the tool gap at the rim will be 4mm.
A tool so easy to use and with such powerful accuracy is a potent weapon. And wheels can be built to its precision because we have 32 separate points of adjustment and can put the rim anywhere we want. The result is awesomely centered wheels for nearly every rider. Since bicycles are single track, balanced vehicles, this centeredness is crucial. Without centered wheels, our bikes wouldn't track straight, too challenging for many riders.
Just remember this 4X exaggeration of the wheel's centerline error. No use obsessing over insignificant errors. I find wheels perform wonderfully if centered within 0.25mm. That would be a 1mm gap for the tool at the rim. Easy to see and manage. And 0.25mm is 1/4 of one percent of a front hub's width. That's accuracy no fork or frambuilder could maintain. The average wheel is FAR straighter than its frame.
In fact, experienced framebuilders use wheels as gauges to check their work. And average wheels are more than accurate enough to judge the highest end frames. I'm sorry to say that we wheel builders never use frames to check our wheels. Accuracy doesn't flow in both directions.
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