April 30, 2015 3 Comments
Season's underway and builders everywhere are slinging parts as the weather (in the N Hemisphere) woo's riders outside.
I'm hearing more questions and grumbling about all the over-dished rear wheels landing in our laps. No time to waste with some advice about this situation. First, it's not related to climate change!
We builders can navigate the extreme specs of 10 and 11 speeds as long as a few basic ideas are kept in mind.
(1) A wheel's geometry is determined by the hub and rim maker. The cross section triangle describes your spoke tension balance. This balance is beyond the builder to make significant improvement.
The tension balance is the center-to-flange distance ratio, non-drive side (Y) to drive side (X). Those distances are easily measured and derived:
X = r (cos b)
Y = l (cos a)
Wheels with 10 or 11 speeds and low spoke count are functioning well in the market despite tension ratios often greater than 2:1. Some brands employ off center rims, others use variable left-right spoke count. However, among those with traditional rim and spoke layouts there is still big concern. Pro’s use both and results do not suggest anyone is in trouble.
(2) Avoid over-tightening the drive side simply to have more tension on the opposite. Max tension for the rim should not be exceeded. NDS spokes may appear too loose but they rapidly reach high tension with mere millimeters of lateral rim movement. In practice, lateral stiffness is barely compromised.
(3) Ultra low non-drive tension is a pain. Measure their tension and reduce inequality between pairs. Use an effective thread compound. Which are effective? Bench tests (just spokes and nipples) will confirm your program is good.
(4) As wheel builders, we understand wheel geometry better than others. We need to remind (no whining) those who will listen that extreme dishing is dumb. In time it will be an historic novelty. We endure it today but it serves little purpose. Adding disk brakes and widening the rear triangle improves the situation, so some relief is coming.
(5) Never forget that nothing takes the load off wheels more than tires. The larger and more supple the casing and the lower the pressure, the happier and more effective the wheel. Thankfully, this is a strong, current trend so builders cannot complain too loudly!
Don't hesitate to mount wide tires to narrow rims if the frame permits. Rims with 17mm inside width (like old Mavic E2's) can easily wear 2.1" (53c) tires. The narrowest rims at 13mm inside width easily carry 35c tires. Remember, though, these are for moderate pressure (50-75psi) and must be tested (like all tire fits) by an experienced mechanic and then ridden carefully to learn the handling before going wild.
Send comments. What are you doing this year?
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