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February 11, 2013 13 Comments
Bicycle wheels need to be straight so rim brakes can be effectively adjusted and the ride smooth and steady. Crooked wheels are barely amusing at low speed and create dangerous wobbles at high. After straightness, however, is the issue of centering. In order for wheels to easily interchange among frames and forks the bicycle world has agreed they should be centered.
Centered means the rim and tire are equidistant from the axle ends. No matter if your wheel is front or rear, internal gears or disk brake, MTB or road, it's meant to be centered. The exception is a very bent frame for which, occasionally, wheels are adapted. No reason to worry about centering if you have a problem without another solution. Let's hope that's rare.
Wheels are centered as they are built. There are a number of ways to do this but a dishing tool (wheel centering is known as "dish") is the overwhelming choice. Another method is to flip the wheel in a truing stand. The rim will run the same distance from an indicator, which ever way it faces if it is centered. You can also use a bike. Flip the wheel around in a fork (front) or frame (rear). If it sits the same way at the brakes, then it is centered. The fork or frame DO NOT need to be straight. Flipping the wheel around ignores frame or fork straightness.
Since some hubs are off center (rear, brake, etc.) a centered wheel may have spokes at different side angles. This, of course, affects spoke length and tension. Just as a mast on a ship supported by two ropes. If one is anchored close to the mast base, it will be much tighter than one with a good bracing angle. This unequal spoke tension is commonplace and rims are designed to handle it.
Many believe unequal side to side tension made wheels less stiff and weaker. While that's true in principle, the limits are far wider than once thought. Good evidence on this can be found in Damon Rinard's great wheel stiffness database, now archived on the priceless Sheldon Brown site. You'll notice that rim deflection with a 26lb (115N) side load is super tiny. Rear wheels seem just as stiff from the right as from the left. How could that be?
When you push the rear rim to the right, the left spokes oppose your motion. They have the best angle so the rim is well braced. Pushing the rim to the left calls on right side spokes. They're not well positioned and the rim ought to be less well supported . In fact, no. Small motion of the rim makes right spokes with their steep angle increase tension quickly. These spokes are already tighter. The combination of start tension and rate of increase makes the rim feel well defended against the side push. Total strength, in case of a major crash, may be less. But for small to medium large loads, the kind we see all the time, the wheel is laterally rigid in spite of the awkward asymmetry of the spoking.
The lesson is, don't fear wheel asymmetry. Learn more about it. With Campagnolo and Shimano 11 speed cassettes, we're experiencing the most asymmetric wheels of all time. These makers are robbing wheels of strength but they're getting away with it. Notice a big grass roots back lash? None. The strength they're robbing is apparently a surplus we didn't strictly need.
The dishing tool has two secrets. One, because it is used on both sides of the wheel, it does not have to be straight. What? Whenever you are willing to flip your measuring stick around a symmetrical structure, you will always get the right answer. In antiquity, builders needed to know such shortcuts. Precision measuring tools were a rarity. Plumb bobs (gravity doesn't lie), stretched string is straight, and boards that are flipped over when judging symmetry tell the truth. I've always wanted a dishing tool made of twisted up burl wood. What a hoot that would be. Dishing tool straightness is cosmetic only (not to knock cosmetics).
The dish tool's second secret is the amazing way it magnifies the wheel error so it's easy to see. When you place a dish tool on one side of a wheel, there needs to be three points of contact. The long bar will touch the rim in twice at 180º and the center post must touch the axle end surface (that meets the frame). Once that is arranged, the tool is lifted off and placed on the wheel's other side. If the wheel is perfect, you'll have the same three points of contact. If not, there's a gap at the axle or at the rim.
If the gap is at the axle, lower the center post to touch. Now three points are contacting. Lift the tool to the wheel's first side. The gap is now at the rim. Hold the bar against the rim at one end so the entire gap is only visible under the second foot. This gap is 4X the centerline error of the wheel. If the gap is 1mm, for example, most builders will endeavor to make it less. In fact, most of us prefer gaps that are barely detectable. A good way to detect a microscopic gap is by lightly tapping the top of the tool bar over the rim. If there's a tiny gap, you'll hear a faint noise. If silence, then no gap.
Remember that this 1mm is 4X the true error. So the wheel's theoretical centerline is 1/4mm from perfection. Now that's precision you don't expect in any other bicycle mechanism besides bearings. Certainly not in any structure. Bars, forks, cranks, frames, are all crude by comparison. In fact, well trued and centered wheels are gauges when it comes to judging fork and frame accuracy. This is why digital dish tools are so uncommon.
So how did that gap come to be 4X the error? Well, let go of the end of the tool bar that is touching the rim. Now the gap is equally distributed under each of two feet. 1mm under one is now 1/2mm under each. If you reduce that error by 1/4mm (by turning nipples), the error disappears because the other side of the wheel is also moving when you adjust the rim. I hope that's clear. But if not, no worries. Just remember the multiplier.
It is wise for a wheel whose trueness is unknown to you, to apply the dish tool twice, at 90º. Wheel centering is subject to rim straightness. Would wheel centering be better if the tool made 360º rim contact? If wheels were randomly untrue, it might be handy. Here's such a tool:
My previous favorite was the trusty Var 143, principally because it's center post slides easily and I can tighten the set screw with one hand while my other hand holds the wheel.
What a delight when brother Jon showed me his design for an artisan wood dish tool last year. Last week, six of these beauties arrived from Montana. This batch are gorgeous. The bar is a bit lighter, still made of aged, laminated white oak. The center post is now purpleheart with a myrtle knob. As before, a clever lamination is employed as a trigger to release the post, like Brett Flemming's EVT tool, Jon's is one hand adjustable.
Once you use a tool like this, nothing else compares. No wonder these are in short supply. One handed convenience, lightweight and durability, artisan beauty and karma; this is an essential part of any eclectic wheel building shop.
Please watch a short movie where I demonstrate dishing tool use.
You can see for yourself how easy it is; especially with a Whitefish Wood tool.
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