Bicycle Mechanics - brake housing for derailleur cable?
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03-31-12, 05:39 PM
Another one of those questions: :o
I know one should definitely not use derailleur housing for brake cables, but is there any reason not to use brake cable housing for derailleur cables?
I'm talking Jagwire Hyper quality housing.
I had enough left over, and it means matching cables and saving $. Yes? No?
03-31-12, 05:48 PM
Derailleur cable housing needs to be as compressionless as possible for indexing, and most of it has linear windings rather than spiral windings to reduce the compression.
Also, the inner brake cables are generally larger in diameter than inner derailleur cables and would likely get stuck or drag within the derailleur cable housing, and derailleur cables being smaller in diameter could move around too much within brake cable housing for accurate indexing. Jagwire makes both brake cable and derailleur cable sets.
EDIT: Also, because the two cable housings are different outer diameters, the end ferrules from the brake cables housing may not fit properly into the shifters or rear derailleur.
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.
03-31-12, 06:17 PM
Thanks, guys. Thoughtful and informative, as always. (genuflect icon goes here)
This is for friction shifting with Suntour barcons and a wide touring rear end. I've done a brief shake down, and it seems to be good. Still, before I commit the the bar tape I'll see if my LBS has a length of derailleur housing to match. Might as well do it right.
If it's friction shifting, you won't have any problem at all, go ahead and tape the bars and enjoy the bike.
03-31-12, 07:57 PM
Oh, great. Thanks, yet again.
I'm ready to finally wrap and ride.
is there any reason not to use brake cable housing for derailleur cables?
It's not accurate enough for indexed systems >7spd. Might even give you trouble on that 5mm spacing, depending on the cable pull of the system you're using.
This is for friction shifting
In that case, not a problem.
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