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Plastic transmission cable outer won't last forever, here's how to replace it

Gear cable outers are essential for accurate shifting. They can degrade because of corrosion or they can wear and split from friction against the frame or damage from UV light. Here’s how to replace them.

Our guide below shows you what we believe is the best method to replace a defective piece of gear outer. We've included a list of the tools and materials that you will need to complete the job and in some cases where you can buy them. If there are others that you prefer then feel free to let everybody know in the comments.

Tools & Materials

Sharp cutter wth crimpers

Cable cutters

Outer cable

Gear cable outers are a vital part of your bike's indexing system. They carry the inner cables and provide a smoothly-running environment that makes shifting gears easy. They need to be in good condition, with no kinks or wear spots, and to have smooth, large-radius curves wherever they turn corners.

Cable outers are made up of wire strands running along the length of the cable, held together with a hard plastic outer shell. The wire makes the outer virtually incompressible and also keeps the distance between the ends constant when the ends move. That's essential for accurate indexing.

If your cable outers are too short the friction between outer and cable increases, making for poorer shifting. If in doubt, outers that are a little too long are better than ones that are too short.

The importance of good cable cutters can't be overstated. A clean cut is essential for good shifting. You want a square end with no wires sticking out so that it's easier to fit a ferrule and the outer consistently resists compression.

You also want ferrules that fit tightly around the outer. If they slip on and off without any assistance then they're too loose. 

The big tip is to look at your existing cable runs, and see if you've got too much outer. You might be OK, but often new road bikes have lots of cable from the shifters to the stops on or around the head tube. If you're not sure, not bothered or the length is perfect as it is, then just trim the new run the same length as the one it's replacing.

How to fit cable outer 02

1 The original outer.  We've removed this run of outer from the rear derailleur of our bike. The length is okay, and the shifting is good. That said we think we can lose a bit to close up the loop and create a run that's a bit less likely to be snagged – this cyclo-cross/gravel bike gets ridden in all manner of stick infested places. Measure the run to be replaced (or simply hold it, straightened out, next to the new bit to mark the spot.

Sharp cutter wth crimpers

2 Get the right cutters. Some cable cutters are only designed to cut inner cables; their jaws won't go round the complete circumference of the outer housing. These ones have the size jaws you're looking for. If you use undersized cutters you'll make a mess of the outer and end up with worse shifting, not better. 

 

How to fit cable outer 03

3 Check twice, cut once. Before you make a cut measure the new run. Mark the cable without cutting it and offer the run up to the frame to see if it really is going to be right. If it is, make the cut. With sharp cutters a firm, quick squeeze will cleave through the cable in one clean cut, with no ragged plastic or pokey bits of wire to interfere with ferrule fit. 

 

How to fit cable outer 01

4 The freshly cut end. The plastic will have been cut to show an even and smoothly trimmed circle of support wires. These will press directly on the inner face of the cable ferrule, which is why the outer cable doesn't compress under shifting load.

Cutting the outer often squashes the PTFE liner, making it difficult to get the cable in. To open it, either  run a spare inner cable up the outer, from the end you haven't just cut, or pick the closed tube open with a needle. 

 

How to fit cable outer 04

5 Fit the ferrules. The inner cable often won't easily pass through the ferrule and into the cable outer when you fit them together, and inner cables that have already been trimmed can fray if snagged. To avoid all this, slide the first ferrule on to the inner wire, then the new run of outer housing, then the second ferrule (as above). With all the elements threaded on to the inner cable, press the ferrules on to the ends of the housing.

 

Grease inner cable (2).jpg

6 Grease the cables. Don't forget to add a little bit of grease to the inner cable in the zone where the outer housing will sit. It helps seal the new run of outer from water ingress and ward off rust. 

 

Slide new outer on

7 Fit the outer. Slide the completed new run along the inner cable.

 

How to fit cable outer 05

8 Seat the lead ferrule into its stop on the chainstay. Make sure that it's fully home. If it's not all the way in, then it could move when you pull the gear cable, throwing out the shifting adjustment.

 

How to fit cable outer 06

9 Thread the inner cable into the rear derailleur via the hole in the barrel adjuster at the rear of the derailleur. Then locate the other end of the new outer run into the barrel adjuster. Make sure it's well home. 

 

How to fit cable outer 07

10 The finished run. It should be a neat and graceful curve. As an easily remembered rule, outer cable curves on the final run to the rear derailleur should be about the same radius as the skin between an outstretched forefinger and thumb, not tighter or wider. ​

​>> Read more: The full archive of road.cc maintenance articles

 

2 comments

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TypeVertigo [429 posts] 3 years ago
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Good tip on threading ferrules first through the shift cable before pushing the lot into the housing. I frayed some shift cables cutting them too early.

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ktache [1695 posts] 6 days ago
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Managed to pick up a titanium cabel outer fid/marlin spike as an add on to an online order.  It fits the cable outer inner bit perfectly and makes it very round.  Lovely little object.

I have attempted to search for it, but cannot come up with the page.