Join Date: Apr 2009
Location: New Rochelle, NY
Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
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There's a difference in how brake and index housing flexes.
Despite the irony of the name, top end brake housing is more compressionless than index housing. But when flexed the section at the inside of the curve can't compress, but the area to the outside is free to expand (imaging a flexed slinky to see what I'm describing). That means that imaginary line down the middle lengthens, and will change the trim of a derailleur as the housing flexes.
With index housing, the compression at the inside of a curve equals the elongation at the outside, so the neutral axis (the imaginary line that doesn't change) is right down the middle and so flexing won't change trim.
That's the logic for long spiral index housing. As to the reality, there really isn't really that much housing movement during normal riding, and I'm not sure that cable flex is an issue either way. The Jag brake housing you're using is relatively stiff and will probably serve fine for index housing, though you'll need to remember to keep the bars fairly straight when you adjust trim.
Go ahead and give it a shot, but don't cut the inner too close or crimp the tip too tight in case it turns out not to be OK after all.
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