Bob, This is gonna be like the three bears. One cable too short, one too long, and one just right.
I think your arguement is good to illustrate what is going on, but not exactly correct either. If my back of the envelope analysis is right, it makes little difference. Back to your arguement, If you have two cars, with an infinitely long V, it is easy to see that your pull force in the middle is split evenly and both cars get half of your pull. Kind of nice. And the force in each cable is also half of the pull. Now make that really short, to the point of being Car -----------|-----------Car. We pull in the middle as before. What is certain is that the sum of forces in the vertical direction must be zero. So, if I pull with some force upwards (or downwards), the sum of the two up (or down) forces on the cars must also be equal to the pull. So there is no difference from the first case. However, the force on the cable goes to infinity in this extreme case. So we have not changed the vertical pull with this arrangement, but we have added a strong horizontal pull. And the centerpull brake is sensitive to this as well (push on the side of an arm endpoint and the pad will touch!). So, there is a force multiplying effect. I will also claim, that the geometry of the arm is more sensitive to vertical pull (long piece of metal runs along the top relative to the portion of the arm from the pivot to the pad) than horizontal pull (roughly equal length of metal from the pivot point up to the elbow, compared to the pivot point to the pad). So, for practical straddle cable lengths, I bet it does not make a big difference.
And this supports my experimental results, when I have shortenned and lengthenned the straddle cable it has not made a big difference.
Also, my arguement that each lever of the centerpull brake is less sensitive to lateral than vertical pull is probably not applicable to a cantileaver brake, I would have to think about that one a bit (I see a new thread).
Dan Kasha Watching the grass grow in rainy SLC Utah
Bob and Steve wrote:
>With a longer straddle cable, you get LESS brake power.
> You are applying brakes inwardly, not upwardly,
Just the opposite... with a longer straddle cable you get more braking power (i.e., more mechanical advantage) but longer lever travel per given movement of the brake pad.
You are right, the pads are indeed moving in. However, the end of the caliper arm is moving up, and that's what matters since that is where the cable is attached. Shortening the cable brings the direction of pull more in line with the arm, thus it takes a harder pull to get the lever to move up (but also a shorter pull... the lever will operate the brake faster but with less power).
>Similarly, if you have a rope stretched between two cars in =
neutral, and you pull on the rope, they will move toward each other. =
But, when you have pulled them together and the rope is pulled far from =
the cars forming a "V", they will not so readily continue to move toward =
each other, momentum notwithstanding.
Not a very good analogy, since the cars are rolling along a straight line axis. Brake arms are not, they are rotating around a pivot. Assume your cars are facing each other and the cables you're tugging with are attached to the front bumpers... now before you tug the cables, drive a giant stake thru the trunk of each car into the ground, then see what happens when you stand between the cars and pull on the cables... not a thing. But stand at a right angle to the cars and the front of both cars will move towards you, pivoting on their respective stakes).
Bob Hovey Columbus, GA
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