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See the image below for my mental model of a barrel adjuster. It's essentially just a metal tube that gets longer or shorter as you twist. I also understand that making it shorter will remove tension to a cable and making it longer will increase tension by removing slack. I use these all the time to index my gears and adjust breaks.

What I don't understand is how it actually works. As far as I can tell, it's just an independent little dingus that gets longer or shorter while the cable continues to move freely through it.

What am I missing? How is it actually tightening the cable?

enter image description here

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    In your diagram, the adjuster would indeed do nothing at all, as the length of cable between the anchors (the blue walls) would remain the same. What you're missing in this diagram is the cable housing, which keeps the distance between the end of the adjuster and the distant anchor constant. Apr 26, 2022 at 9:59

5 Answers 5

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Crucially, the length of the inner cable is fixed, because it is anchored at each end, both at the lever and at the brake caliper or derailleur. It passes through a housing of a fixed length. The little bit of cable sticking out of the tunnel at the end is the difference between the two lengths.

If we shorten the tunnel though, there is a bit more of the fixed length cable sticking out. This allows the brake arm or derailleur to move further away from the tunnel exit. The brake arm or derailleur is the only part of the system that is free to move on its own.

If we lengthen the tunnel, there is a little less exposed cable length remaining, so the distance from the tunnel exit to the brake arm or derailleur must get shorter. This therefore pulls against the spring of the mechanism, creating 'tension' in the cable between the end of the housing and the fixing point.

The barrel adjuster allows you to shorten or lengthen the tunnel as you wish, which gives more, or less, remaining length of the cable so that you can tune the cable tension.

The little dingus is far from independent.

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    I was discussing your excellent answer with a friend today. Maybe it’s just us, but the significance of the cable housing is actually extremely surprising. I’m pretty sure most people see it as just a protective “skin” for the cable. The fact that it plays suck a significant role in regulating the tension in the cable is pretty darn cool.
    – ebrts
    Apr 29, 2022 at 1:57
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    @ebrts I wouldn't even describe it as a significant role. The cable and housing are a paired system, period. Without the housing, the system just doesn't work. It's all Newton's third law.
    – MaplePanda
    Apr 29, 2022 at 5:19
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    @ebrts: I’d say that the housing is actually more important/difficult/complex. It’s relatively easy to make a strong, flexible cable which doesn’t stretch much. But doing the same with a hollow tube is much harder.
    – Michael
    Apr 29, 2022 at 7:34
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    @Michael Especially since it's far easier to create a small, flexible, thin, light wire that is stable under tension than it is to create a small, thin, light housing that resists compression.
    – DavidW
    Apr 29, 2022 at 14:23
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    @ebrts: you might be interested to find they have a lot of applications outside of bikes as well. en.wikipedia.org/wiki/Bowden_cable Apr 29, 2022 at 18:09
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The cable and the cable housing are anchored at each end.

Inline adjusters or the barrel adjuster on a brake lever or derailleur work the same way. They make the cable housing longer or shorter relative to the cable.

enter image description here

As you unscrew the barrel adjuster you make the cable housing longer. This makes the cable shorter - relative to the cable housing - and tightens the cable

enter image description here

As you screw in the barrel adjuster you make the cable housing shorter. This makes the cable longer - relative to the cable housing - and the cable loosens.

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This is my understanding...

The cable length is fixed. The cable is attached to the shift lever at one end and the derailleur at the other.

The shift lever body is fixed to the bike. The shift lever is movable.

The derailleur attachment point is movable and moves the derailleur in and out.

When the shift lever is activated, it pulls on the cable. The cable doesn't stretch. It gets pulled through the cable housing and moves the derailleur at the other end.

The cable housing is fixed to the bike at both ends, but is flexible.

When the barrel adjuster is rotated anticlockwise it starts to add length to the cable housing.

As the cable housing is fixed at both ends, this extra length makes the cable housing bend a little bit more.

This means the cable has further to go round the loop inside the housing.

As the cable length stays constant, something has to move. The only movable point is the derailleur attachment, and this is pulled towards the shift lever body.

This makes it look like the cable is being "shortened" between the shift lever body and the derailleur.

The diagram shows this effect greatly exaggerated.

It's a bit like a bowstring. As you pull on the bowstring, the bow flexes and the ends of the bow move towards each other.Barrel adjuster diagram

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  • Excellent first answer - Welcome to the site !
    – Criggie
    May 11, 2023 at 21:33
  • Yes the cable housing is flexible and it's not elastic, meaning it cannot be compressed nor stretched and this is by design, which is why the cable housing has a very close relationship with the cable running through it. Because the cable housing is not elastic, it has the ability to increase tension in the cable running through it. And this close relationship is why the jacket can push on one brake cantilever arm as the cable proportionally pulls the other cantilever arm. What does this look like? en.wikipedia.org/wiki/Bicycle_brake#/media/…
    – MindDBike
    Feb 11 at 13:00
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A more extreme example of the principles in play is how cross-levers work for brakes. When they are installed, the cable passes through them untouched. Instead of pulling on the cable, they work by pushing the housing away, in essence making it longer.

Cross levers are different than "suicide levers" or "turkey levers" as those have a little lever arm that weakly pushes the regular brake lever. Cross levers, acting on the housing, work with any other lever, and can deliver 100% the braking force of a regular lever.

Cane Creek cross levers Animated

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Nothing in the cable system is fixed and static over time.

The inner cable will subtly stretch over time, with most of that being a "bedding in" or settling in the first month of installation. It settles down in the middle of its life, and toward the end the strands can individually break which stresses others, increasing the length again.

The outer housing is set into ferrules which live in frame stops. These will seat themselves more completely over time too.

Smaller variation comes from frame-flex, dirt/rust and even temperature changes.

The barrel adjuster lets the rider/mechanic remove these slight changes without having to re-clamp a pinch bolt, which is an inaccurate way of doing anything.

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    Nice info. Also the brakepads wearing off causes the brakes to go slack. I usually start with the barrel adjuster small, and expand it as the pads wear. It's nice not having to repinch the bolt.
    – Robert
    Jun 28, 2023 at 0:12

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