ScottCommutes

I have a question that I haven't found the answer to that you guys should easily be able to answer.

How long should the "working length" of a straddle cable be?

It seems to me that if the straddle cable is long, most of the brake cable pulling force is simply directed against the pivots. If the cable is too short, I feel like it will pull the brake pads in a better direction, but it will also obviously hit the tire. Does any of this make a difference?

Mr. 66

Different lengths for different products.

grumpus

The way I set it is so it's normal to a line from the pivot to the straddle anchor points when the blocks hit the rim. With low profile cantilevers you often have to extend the blocks a fair way for the angles to work out. With Mafac/frog-leg style cantis a longer straddle has less or sometimes no negative effect.

cyccommute 

Rather than retype it, I'll offer this Sheldon Brown article on cantilever brakes. I don't agree with everything he says...I think he is way off on lever pull...but his advice is pretty good. I have 3 bikes with cantilevers and all of them are set up with fairly short straddle cables...about 45°. I use the free floating straddle cables rather than Shimano's stupid link wire. I also use Paul Moon Units for the hanger. The Moon Unit is wider and makes for a more responsive brake

riverdrifter

Fantastic looking bike!

FBinNY 

A bit of geometry might help.

Any cable is most effective when pull an arm at 90 degrees, the same as you would be.

So, you're looking for a length that achieve that when the shoes contact the rim. There's a decent bit of wiggle room here, and overall I tend to prefer cables slightly shorter, but use that as a starting place.

You may want to be a bit brave and buy a spare cable and experiment, starting with it clearly too long for a while, then work down until it's too short, before deciding on your final choice.

mpetry912 

as FBinNY says above, a 90 degree angle is a good place to start.

if you need more brake authority or are dealing with something that limits cable pull (like a rear brake on a small frame) then adjust accordingly.

/markp

ScottCommutes

OK - thanks. Sheldon Brown clearly shows that shortening the working length of the straddle cable increases the mechanical advantage of the brakes. However, other things, such as the levers and the brakes themselves, also provide mechanical advantage. You certainly want mechanical advantage, but not to the point of reducing actual brake movement beyond reasonable limits. Changing the working length of the straddle cable is an easy way to adjust the mechanical advantage of the entire system.

Thus, the answer to my original question is to ride, test, and adjust. To increase pad travel, you can increase the length of the straddle cable. To get more brake pressure at the expense of travel, shorten the straddle cable.

andrewclaus

And check the quick release. If it's too short, you won't be able to unhook it.

Aardwolf

I'm new to centre pulls, so I just copied the stradle cable length on the brakes from France.

It's maybe a bit short but it works fine, apparently I have a lot of mechanical advantage.
I'm also using Weinmann levers with QR that is just enough to get 28mm tyres out with a bit of pulling.

79pmooney

If your brake is one with a clamp on one side and a fitted ball/disc on the other, get an extra long straddle cable. (A lot of us use derailleur cable.) Set it up long. Ride it. Shorten a little. Ride some more. Notice the change in power and feel. Keep doing this until you've gone to far, get a new cable, set it to that length you liked and done!

There's no right or wrong here, as long as you have the stopping power. The feel is so much a matter of personal preference that I love tha adjustable straddle brakes where I can customize the length to just what I want. (For me, usually longer than the off-the-shelf with Mafac RACERs. Other calipers vary a lot.)

FBinNY 

It's just about textbook. The designers intended for you to disconnect (unhook) the free side to open the brake and clear wide tires.

BTW, a safety tip. Instead of bending the main wire back, as you did, either leave it straight and trim short enough to clear, or bend in under the hanger and out to the front. If you look behind the hanger, you'll probably find a slot so the cable can stay close and trap the yoke, so it cannot disconnect.

ScottCommutes

Sheldon Brown also provides a mathematical formula that gives the mechanical advantage at the straddle cable. The formula is 1/sin(yoke angle). Yoke angle is the angle between one side of the straddle cable and the horizontal.

A common position might be a yoke angle of 45 degrees, where the two sides of the straddle meet at a 90 degree angle. This provides a mechanical advantage of about 1.4x.

A long straddle cable could limit this mechanical advantage to about 1.1x. Going lower is essentially impossible without an infinitely long straddle cable. Even then, you are still at 1.0x.

The interesting thing is that shortening the straddle cable has a greater effect. If you can shorten your straddle cable down to a yoke angle of 30 degrees (120 degree angle where the two sides of it meet), you have increased your mechanical advantage to 2.0x (double).

Also, the cable angle you see in pictures doesn't mean diddly. What matters is the cable angle when the lever is pulled and the brakes start to work. This changes as the pads wear. Compensating with the barrel adjuster keeps the pads close, but it doesn't preserve your mechanical advantage - your straddle cable effectively becomes longer. Everyone loses the mechanical advantage of a short cable every time they wear down the pads, and there is no easy adjustment for this.

Foolish me. I figured this was an easy question. Turns out there is a lot to it.

Edit: Also worth noting that the straddle length is personal - no one else could possibly set it for you. You need your body weight, your fingers, and your tires on the bike to actually test how the bike stops at a particular setting.

Duragrouch

The length and angle is a compromise, and about more than just the trade between force and travel. Let's use the Mafac Racer brakes above as an example. If you have a very short cable (assuming when the pads contact the rims), pulling up gives a very high straddle cable tension, but not necessarily harder braking, becaused what also matters is the angle of that cable to a line from the arm end to the arm pivot; With a very short cable, it's not pulling at 90 degrees to that line, and 90 degrees is the best leverage angle on the arm. However, shortening the straddle cable to get more force, may have more positive effect, than the negative effect of the pull angle on the arm. This could all be trigged out if you desire, by sketching it all out, with the brake arms at a position where the pads contact the rim, and start with the straddle cable angle and its tension for a given vertical pull, then see how that subsequent pull angle on the arm calculates out. Then look at extremes, shorter and longer straddle cable. Or, you could experiment on the bike.