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how much cable for rear derailleur

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how much cable for rear derailleur

Old 01-28-15, 08:00 AM
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how much cable for rear derailleur

I've never given much thought to this but what's the correct amount of cable from the rear cable braze to the derailleur?

I looked at the Park website and found two pictures of good and bad, but I've alway been somewhere in between

Park Tool Co. » ParkTool Blog » Shift Housing Length

Would different derailleurs command different lengths? what about friction vs. indexed?
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Old 01-28-15, 08:08 AM
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Shifting cable is shifting cable, usually sold in one length, to be cut down upon installation. Exception is tandem cables, which are longer, and actually needed on some non-tandem bikes, like triathlon bikes, with longer runs than usual.

Cable Housing is a different matter. For downtube shifters, you only need about 12" or less from the rear cable stop to the RD. For shifting at the bars, you'll need about 18"/ea for the front and rear shifters, maybe a bit more if you run bar end shifter housing under the handlebar tape.

And then there are all kinds of special situations, like full run of housing from the shifters, some bikes which run housing internally, etc.

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Old 01-28-15, 08:42 AM
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BITD it was thought that cable housing cut as short as needed was best, perhaps because the housing back then was pretty compressible and lacked the low friction liner of today's housing. These days there's a trend to have one's housing loops a tad longer then the minimal. But I never would suggest a certain length without test fitting, there's just too much variance between bikes housing end points (like an brifter and the dt stop on a tall bike vs a small bike, or where along the frame the stop or port is located). Andy.
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Old 01-28-15, 09:37 AM
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In my opinion the good picture on the Park site is too long. I have the loop big enough that the housing goes into the rear derailleur more or less straight like they show but not as big as their picture.
Maybe I'm doing it wrong as the SRAM manual shows a big loop as well.
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Old 01-28-15, 07:31 PM
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My SRAM RD's all have front entry points for the cable --> no loop.

That loop in the Park picture does looks bigger than required.
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Old 01-28-15, 11:58 PM
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Originally Posted by Slash5
In my opinion the good picture on the Park site is too long. I have the loop big enough that the housing goes into the rear derailleur more or less straight like they show but not as big as their picture.
Maybe I'm doing it wrong as the SRAM manual shows a big loop as well.
Sram RDs, at least the 10 speed ones, definitely need a bigger loop than Shimano (the Shimano that I'm familiar with anyway). It's probably because of the angle the cable goes into the derailleur.
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Old 01-29-15, 02:59 AM
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Originally Posted by Slash5
In my opinion the good picture on the Park site is too long.
+10
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Old 01-29-15, 03:21 AM
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qclabrat, The size of the loop to the RD depends on where the cable enters the RD and where the frame's housing stop is located. There isn't really a set length. I use a long enough length of housing that a flush fit into the RD is accomplished regardless of the angle of the RD when on either bottom or top cog.

Loop size grew as more cogs were added to the rear because the cable moved less for each single cog shift and friction from a too tight of a radius caused imprecise indexed shifting.

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Old 01-29-15, 07:09 AM
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IMO there's no "right" length. Instead focus on getting the housing to meet the end fitting squarely. There's rarely an issue with how it meets the chainstay fitting unless it's way too long, so go by the RD fitting. If too short, the housing will pull toward the front of the bike creating an angle there. If too long it will push toward the back. But when just right the housing will meet the fitting straight, making for a smooth line for the inner wire.
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Old 01-29-15, 09:17 AM
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WRT the "set" length of housing loops. Actually there is a standard length, at least for Shimano rear ders, if all the aspects are within Shimano's specs. The big S has published recommended specs for pretty much any/all elements that influence the rear der housing loop length. Like how far from the hanger mounting hole is the chain stay stop and the angle WRT the chainstay that the hanger's "B" screw stop is at. So if the bike has followed all the little details as Shimano wants one could order the "set" length off a supple sheet and know that as your thousands of bikes were being assembled by the 12 year old girls in Asia (please take kindly to my poor attempt of a joke) will have quite acceptable housing loop lengths.

But in the real world of shop or home installing this detail isn't to be known, followed or assumed that the OEM assembly got it right. Hence I cut the housing length an inch or so too long , test fit and then trim down to what I've learned works well. Smooth and even curves with the ends of the housing entering the stops/der adjusters without any kinking or doglegging. And these days I find a touch longer the ideal is a better side of the fence to fall on then a tad too short. Andy.
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Old 01-29-15, 12:40 PM
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For several years now I've been using the cable and housing kits from Shimano and Campagnolo. The Shimano pre-cut short housing loop is 12 inches and the Campagnolo pre-cut housing loop is 13 inches. I've gotten into the habit of measuring them each time I open a new set and those lengths have been consistent over a period of about 8 years. I assume they both think they've got it right.
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Old 01-29-15, 01:07 PM
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Smooth, lowest friction arc, in order to achieve this , a bit longer is better than too short , particularly as Indexed shifting is fussy about the friction
between the command lever on the bars, and the action at the rear..

the cable routing down the seat-stay may offer a straighter line than one from under the chain-stay ..

so where the braze on stop is on your frame is a variable. preventing a Rote answer. It in the end, Depends..

Last edited by fietsbob; 01-29-15 at 01:12 PM.
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