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Grand Crux Zest brakes

Old 01-14-20, 04:02 PM
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wvridgerider 
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Grand Crux Zest brakes

Anyone use or have used Velo Orange Grand Crux Zeste can't brakes?
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Old 01-14-20, 05:14 PM
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I haven't, but the instructions page listed on their website (near the bottom of this page) shows a really high straddle cable adjustment. They recommend six inches. That seems awfully high for low-profile cantilever brakes like these. I'd probably set those up with a much lower straddle cable if I didn't read those instructions first.
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Old 01-16-20, 10:19 AM
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The high straddle cable is likely because the arms are so long. Low straddle cables was the trick BITD when trying to get maximum leverage out of shorter armed brakes. The trade off, of course, is that high leverage means less travel, squishy feel, and potential close tolerances and rubbing if your rim is slightly out of true. A higher cable means the arms will move in more for each mm of cable pulled from the levers. It also means that the dreaded 'negative force curve' (the harder you pull the levers, the higher the straddle cable goes, and the less leverage you have pushing the pads against the rim) will be less than with a low straddle cable.

I would guess that if you wanted to use a low straddle cable, you could do so with V-brake 'long pull' levers and get acceptable brake performance, but that's just a guess.
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Old 01-16-20, 08:44 PM
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Good point about the long arms. I guess in theory, if your arms are long enough, you could basically have a straddle cable flat across. With every unit of cable pulled from the brake lever, the arms would move together by that same amount (one half on each side), right? That would essentially "convert" their geometry to that of linear pull brakes, right? Where you get one unit of brake arm convergence for one unit of cable pull? In that case, I guess a long pull lever would definitely be the right lever to use.

Am I thinking about that correctly?
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Old 01-17-20, 10:09 AM
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No. Linear pull brakes have a noodle or roller that allows the main brake cable to pull directly sideways, so 1mm of cable travel = 1mm of travel of the arms at the anchor points. This is not true of centre-pull cantilevers.
When the straddle cable is straight across (lowest possible position), the arms move very little for any given movement of the lever. On old cantilever systems I liked to set them up super low like that when possible, but this resulted in very tight rim/brake pad clearance and mushy lever feel.
Think of a clothesline - it is nominally straight across from the back of my house to the garage, but you can move the centre of the clothesline up and down significantly without pulling my house and garage closer together - the tension in the cable is enough that it causes deformation in the system (the line stretches and the sheaves might flex or deform, but no movement at the ends.

Because cables can only act in tension in a straight line, the main brake cable pulling straight up on the middle of a straddle cable straight across can create (theoretically) infinite tension in the straddle cable - the straddle cable will have a 'lateral' component to its movement and a 'vertical' component to its movement, and since there is very little of the vertical direction in a very short straddle cable, the tension needs to go very high to counter the tension in the 100% vertical main brake cable. It's a basic trig equation.

Further,
In general, if a machine (linkage, gear, or lever system) increases force, it decreases movement, and vise versa - if a machine increases movement it decreases force/ just like the low gears on your bike that allow you to ride up steep hills but at the cost of distance travelled per pedal revolution vs. high gears that allow you to cruise at high speed with slower pedal movement, but has the cost of needing more force at the pedals to go up hills. You don't get something for nothing.
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Old 01-17-20, 02:02 PM
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Originally Posted by Wilfred Laurier View Post
Because cables can only act in tension in a straight line, the main brake cable pulling straight up on the middle of a straddle cable straight across can create (theoretically) infinite tension in the straddle cable - the straddle cable will have a 'lateral' component to its movement and a 'vertical' component to its movement, and since there is very little of the vertical direction in a very short straddle cable, the tension needs to go very high to counter the tension in the 100% vertical main brake cable.
Yeah, I was probably trying to make the math simpler than it is. By pulling straight up on the center of the straddle cable, there would be a lateral movement at the cable ends, but I now understand that it wouldn't be a 1:1 relationship.
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