Winfried

Hello,

While it seems obvious that the cable moves one arm towards the rim when pulling the brake lever, I can't figure out how the other arm moves on V-brakes or dual-pivot sidepull brakes.

Some articles I read says that it's thanks to the housing, but how does the brake lever push the housing against the brake arm so as to move the pad towards the rim?

Thank you.

"The cable housing connects to one arm, and the inner cable runs across the top of the tire to the opposite arm. When the brake is applied, the housing pushes on one cantilever while the inner cable pulls the other." (Source)

Andrew R Stewart 

Simple levers and action/reaction stuff. The inner cable is trying to shorten between the tops of the arms (because some of the inner cable is being pulled into the lever at the bars). Depending on which arm has less friction in it's pivot an arm will move the pad towards the rim and at some point the other arm will also move. The ideal is both arms moving pretty much at the same time. But if one pad stops the arm movement first then the other arm's pad will continue to move till it's pad contacts the rim too.

Some linear brakes allow the cable casing noodle stop and the inner cable bolt to be swapped to their other sides, the cable can enter the brake from either side. I wouldn't get hung up on whether it's the casing pulling one arm or the inner pulling the other arm because it's both happening at the same time. Andy

FastJake

It may help to think of it as pulling against the spring tension in each arm rather than the arm itself. If you have a bike with V-brakes you can easily disconnect each spring individually and see what happens.

dsbrantjr

The brake lever does the same thing at the other end of the housing, pushes on the housing while pulling on the inner cable. If it didn't push the housing it would not be able to pull the inner, the whole thing would move.

cyccommute 

You have to think about it as a whole. The inner cable doesn’t work independently from the outer cable. They work together. Disconnect the noodle from the brake and pull on the lever. You can get a little movement on the brake arm that the inner cable is anchored to but you can’t pull enough cable slack to get the brake pad to the rim. Think of it as the inner cable is anchored on the right hand side of the brake in your picture while the cable housing is anchored on the left side of the brake. You are pulling those anchor points towards each other by shortening the inner cable.

The spring on the brake doesn’t really get involved in the braking. The springs on a mechanical brake only serve to return the brake to the open position.

70sSanO

They work because the housing operates one arm and the cable operates the other arm. Even though the cable and housing are fixed lengths, neither are attached to a immovable object.

So when you pull the lever both the cable and housing are operating the brake arms together. As Andy stated earlier the resistance of each arm determines if one moves first (easier/faster). In the best case they move together at about the same rate.

Regardless of rate when one hits the rim and stops the other will keep moving. Think about it, if the arm with the cable attached stops first there is no more cable to pull; the brake can’t move anymore. What moves the other arm is the cable housing.

John

70sSanO

An easy way to see this is to put a block of wood on the non-cable attach side between the brake pad and the rim. You are making one side immovable. Pull the brake lever. The cable will move the arm towards the rim and the other arm will not move. Reverse it and the housing will move the arm and the cable side will just sit there.

John

fietsbob

Reading about 'the lever' is a good foundation https://en.wikipedia.org/wiki/Lever

Miele Man

See:

Bowden cable

https://en.wikipedia.org/wiki/Bowden...ble%20housing.

Cheers

Winfried

Thanks everyone for the infos.

"As the cable pulls against the housing the arms are drawn together." (Source).

So, it looks this is how the magic happens: 1) the housing is incompressible, 2) pulling the lever pulls the inner cable which pushes against the housing which reduces the radius of curvature of the housing, and since the contact point in the lever doesn't change, 3) the housing pushes on the brake arm at the other end.

In other words, the "housing" arm of the caliper would not move were the cable + housing laid in a straight line between the lever and the brake arm?


(Source)

cyccommute 

Mostly right. Regular brake cable housing is slightly compressible so it has just a little give. It’s not a lot but it’s there. There’s enough compressibility that it makes lousy housing for indexed shifting.

Winfried

But then, brake housings and shifter housings are two different beasts.

https://bike.bikegremlin.com/8548/ca...ing-standards/

70sSanO

Interesting comment. I have never thought about an absolutely straight housing because it doesn’t exist in the bicycle world. It would be possible to mount a caliper to a piece of wood and run 12” of housing with a cable on a straight line to a lever on the same piece of wood. You would have to put a block to simulate the rim. I believe it would work since there is a greater amount of lever pull than just the cable side.

You should look at inline/interrupter brakes. They function using the opposite principle. They push the housing to operate The brake. But the result is the same.

John

Edit Added: It is interesting stuff. I haven’t given it a whole lot of thought other than it works when I’m merrily rolling along at 25+ mph... lol!

Winfried

It's actually a very elegant solution: Very simple, does the job, and has been doing so for over a century.

cyccommute 

Didn’t used to be. Then someone came up with a better way to shift that required more accurate movement of the inner cable.