Compressionless brake outer

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gloomyandy
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Compressionless brake outer

Postby gloomyandy » 5 Dec 2017, 10:36am

Thinking of replacing the cables on my mechanical disc brake setup. What cable outer (full length) would folks recommend. I think it currently has the OEM SRAM ones installed which are spiral bound, I'd like to try something with a linear binding (like a gear outer). I have used Jagwire on another bike and would probably use them again, but happy to give something else a go. So what would you recommend. Levers are SRAM, calipers are TRP Spyres.

hamster
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Re: Compressionless brake outer

Postby hamster » 5 Dec 2017, 10:41am

I always like the Transfil Black Snake ones, failing that Campagnolo outers.

PH
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Re: Compressionless brake outer

Postby PH » 5 Dec 2017, 1:46pm

Using the Gusset ones, haven't used any others cables or any other disc brakes so can't compare, but these do the job and were decent value. Used on TRP Spyke with flat bar levers.
http://www.charliethebikemonger.com/gus ... 6786-p.asp

PeterBL
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Re: Compressionless brake outer

Postby PeterBL » 5 Dec 2017, 1:49pm

I recently mounted this set, it is quite good: https://www.ribblecycles.co.uk/trp-disc ... sion-less/

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horizon
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Re: Compressionless brake outer

Postby horizon » 5 Dec 2017, 1:51pm

Can anyone just clarify for an ignoramus like me why suddenly it's OK to use linear cable outers on brakes?
The experience of travel is something that you have to pay for but can never buy. Ho Ri Zon Chinese philosopher

mattsccm
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Re: Compressionless brake outer

Postby mattsccm » 5 Dec 2017, 1:53pm

Built for the job I suspect.

PH
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Re: Compressionless brake outer

Postby PH » 5 Dec 2017, 2:00pm

horizon wrote:Can anyone just clarify for an ignoramus like me why suddenly it's OK to use linear cable outers on brakes?

Yes, built for the job. The reason you wouldn't use a gear outer is the compression would break the outer, those made for brakes are designed to withstand that.

Stevek76
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Re: Compressionless brake outer

Postby Stevek76 » 5 Dec 2017, 2:11pm

They don't just have a linear wire layer but a second strengthening layer as well (eg kevlar) to stop the inner rupturing the outer.

Obviously using a regular gear cable is still very much a bad idea.

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horizon
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Re: Compressionless brake outer

Postby horizon » 5 Dec 2017, 2:41pm

Thanks everyone for that!
The experience of travel is something that you have to pay for but can never buy. Ho Ri Zon Chinese philosopher

Samuel D
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Re: Compressionless brake outer

Postby Samuel D » 5 Dec 2017, 2:50pm

How much compression can there be in a couple of feet of regular housing made of tightly-coiled square-section wire? I cannot see where the compression is coming from that would be reduced with lengthwise wire (if anything, the converse). For this reason, I wonder if the performance benefits of compressionless housing are actually coming from somewhere else, e.g. reduced friction.

PH
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Re: Compressionless brake outer

Postby PH » 5 Dec 2017, 3:25pm

Samuel D wrote:How much compression can there be in a couple of feet of regular housing made of tightly-coiled square-section wire? I cannot see where the compression is coming from that would be reduced with lengthwise wire (if anything, the converse). For this reason, I wonder if the performance benefits of compressionless housing are actually coming from somewhere else, e.g. reduced friction.

Don't know the numbers, but if you were daft enough :oops: to use brake outer on indexed gears you'd find they didn't index very well... The outers on disc brakes are usually full length and the pads on disc brakes don't move very far to start with, I doubt it would need much compression to make a difference.

Samuel D
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Re: Compressionless brake outer

Postby Samuel D » 5 Dec 2017, 4:05pm

PH wrote:Don't know the numbers, but if you were daft enough :oops: to use brake outer on indexed gears you'd find they didn't index very well...

I’d be amazed if brake cable housing wasn’t stiffer in compression than gear cable housing, at least for modest total angles of bending. But gear cable housing is designed to reduce length changes when the cable bends, as happens with handlebar-mounted controls when the bicycle is steered. So it does work better for indexed gears (or friction shifting with handlebar-mounted controls).

PH wrote:The outers on disc brakes are usually full length and the pads on disc brakes don't move very far to start with, I doubt it would need much compression to make a difference.

The full length is harmful for compression but it shouldn’t matter how far the pads move (only the cable).

Not saying this to argue but to try to get to the bottom of why compressionless housing works – if indeed it does! Has anyone tested the compression of various housing types?

Brucey
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Re: Compressionless brake outer

Postby Brucey » 5 Dec 2017, 4:48pm

Samuel D wrote:How much compression can there be in a couple of feet of regular housing made of tightly-coiled square-section wire? I cannot see where the compression is coming from that would be reduced with lengthwise wire (if anything, the converse). For this reason, I wonder if the performance benefits of compressionless housing are actually coming from somewhere else, e.g. reduced friction.


You only need look at failed spiral wound cable to see what happens; where the cable bends the coils are open on one side and only touch one another (badly) on the other side. The usual effect of this is that the cable housing adopts (where possible) a single radius rather than a compound radius when the inner cable is under tension. There is a length change associated with this change of shape.

However the cable housing ends and fittings usually constrain the housing some way, so that there is a mixture of shear and bending on the cable housing spiral. As soon as the inner cable is under tension there is a proper shear and bending load on the spiral and this causes the coil turns to start sliding over one another. Again there is a length change.

Some cheap indexed systems are put together on bikes using spiral wound housing, presumably because it is cheaper. The shifting performance invariably improves when proper index housing is used instead.

The effects of cable sticking friction are distributed differently when the housing compresses vs when it doesn't. This may alter the way the cable feels, even if the coefficient of sliding friction is the same. Note also that the work done to get the brake on increases every time you introduce more flex into the system.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

PT1029
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Re: Compressionless brake outer

Postby PT1029 » 5 Dec 2017, 8:25pm

I don't have numbers. But a XT lever + rear BB7 disc brake on the back of my tandem was spongy enough it either rubbed or was too slack (disc was new/straight, and cable was exposed runs along the frame tubes).
I replaced the spiral outer with linear brake outer (Jagwire) and the problem was solved, so it works.
I have seen gear linear outer used (for a short time only...) on brakes, not a pretty sight!

Roadster
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Re: Compressionless brake outer

Postby Roadster » 6 Dec 2017, 2:29am

The coils of a spirally wound outer are perhaps not as tightly packed as they appear. Even though the gaps are only a few microns, they'd all add up over the length of a cable.
I also wonder whether the diameter of a spirally wound outer expands under pressure, reducing the force applied to the end stops.