Pressure Drop Follow Up

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The one time I go short format on a blog and all my contact points just blow up with follow-ups about yesterday's blog

First, this phenomenon is generally linear along the depth axis (the deeper the rim section, the more resistant it is to compression) and material (carbon exhibits this behavior generally less than alloy). 

Second, like Rod Stewart sang, the first cut is the deepest. The first time you cause and adjust for this wheel behavior is the most it's going to happen. When you put a tire on after the ajustment, it's not such a big effect. A useful guess is that the lingering effect is about half of the initial effect.

2.5th, you should see what happens to tension as we bed the spokes. We don an initial spoke line correction as part of lacing, and then do a bit as we bring the tension up. Once we get them nearly up to tension, they get their big bedding in. That can drop tension by 50%. How we do that is a trade secret. We're allowed to have those. 

Third, the difference in spoke tension drop between a tire at, say 80 psi and one at 110 isn't that big at all. You don't need to adjust your wheels because you like high or low pressure, nor do you have to change your tire pressure because we did this step with a 100 psi tire as a guide. 

Fourth, your rim's sidewalls bulge just a tiny bit when your tire is fully inflated. We've studied this extensively on alloys, and our pressure recommendations are informed by this. The general behavior is that the brake tracks (or the equivalent place on a disc rim) will open by a small amount (~.15mm) up to a point, after which it really starts to open up more. We want to keep you the hell away from that point. Danger be there. I'd be willing to guess that the common guesses on the relative lineup of how this happens on rims we use are wrong. Again, carbons generally do this less than alloys. We don't measure this spread on every wheel that goes out, but being aware of it sure does inform what we look at in the "with the tire on and inflated" part of the build process. 

Believe it or not, there's a good fair bit of IQ and a really broad range of experience brought to bear on all of our processes. Nobody, and no thing, is perfect, but we really really (really) do have your best interests at heart. Almost painfully so. That's something we can never adequately express through our pithy yet eloquent blogs. Every the whole "professionals built the Titanic and amateurs built Noah's Ark!" line of logic comes up, we die a little bit inside. A f-ing LOT goes into trying to put perfect in a box every freaking time. 


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  • Brian on

    This post makes me love you guys a little bit more. <3


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