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Old 07-20-10, 02:15 PM   #1
wile e. coyote
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weak rear cantis on women's frame

Yes I did my searching elsewhere, just doing a sanity check here.
I've got a 'girls' Giant Iguana with Suntour XCM cantis. Though it's old, it saw little use. The front canti's stop perfectly fine despite potential hardening of the shoe rubber. Ran new cable, cleaned the rim and shoes, etc, I cannot get the rear brake tight.

It uses the typical pulley system, having a dropped top tube, so I'm wondering if the problem is inherent to this system? I'm thinking I should run a solid chunk of cable and convert to linear pull.
I'm giving this to a friend and doubtlessly she will use the rear brakes despite the rock solid fronts. (I could just run the right lever to the front...heh)

The x-press shifters are funky, I kind of like them.

-ned
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Old 07-20-10, 03:31 PM   #2
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try eagle stop salmon break pads. That fixed it for me.
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Old 07-20-10, 06:01 PM   #3
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yes, I am aware of kool stop eagle claws. It was a question of normal force, not coefficient of friction...
I thought I would get some interesting replies about cable stretch and routing and stuff. waahh why cant I ever get good answers here?
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Old 07-20-10, 06:29 PM   #4
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See Sheldon Brown's website entry on cantilever brakes. Yes indeed, with small frames (e.g. typically those ridden by women, but not exclusively of course), it can be difficult to position a rear cantilever brake cable yoke high enough to provide optimum squeezing force without it butting up against the cable housing stop. (I'm not familiar with the geometry of your particular bike.) We had this problem on our first tandem and were never able to "square the circle" as it were. Front canti was great, rear sucked. We gave up and converted to dual-pivot side-pull caliper brakes. (The frame clearances were too tight anyway to allow the large tires and fenders that are the whole point of cantilever brakes.)

I would think linear-pull brakes would solve the problem since they don't draw a cable yoke up toward a stop and run out of travel. Sorry in advance if my answer is not sufficiently good and interesting to meet your standards.
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Old 07-20-10, 08:56 PM   #5
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tnx yeah I think between the pulley assembly and the longer cable run, there is just too much stretch. The front is pulled directly off of the stem, it's drilled so there's no headset adapter in the mix, super rigid. Maybe I should drill through the seatpost, I jest but it would take the pulley out.
I think I'll try some other canti's and pads while I'm at it. The Suntour XCM's are bolted, no stud like traditional cantis.

Sir Sheldon is often my first source of advice. My notebook is full of transcription from his cheat sheets.
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Old 07-21-10, 12:55 AM   #6
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Originally Posted by wile e. coyote View Post
..I think between the pulley assembly and the longer cable run, there is just too much stretch. ...Maybe I should drill through the seatpost, I jest but it would take the pulley out.
Rear brakes are often more finicky due to the longer cabling run, but it's not as much stretch as it's flexing and compression of the housing that's the villain. To get things as good as possible you really need to take care with the routing.
I'm not so sure about switching to v-brakes would be of much help, they too rely on the brake cable entering pretty much in line with the brake arm. If you come at them from below you have to run a sizeable loop in the cable to get to the noodle, and those loops are killers for brake efficiency. A pulley otoh is reasonably solid.
Maybe one could use one of those flexible noodles to get a decent cable run with a perpendicular approach to a v-brake?
I worked on a mixte frame once where I moved the brake from the seatstay bridge to the "top tube" stay bridge, and whas then able to run the youke cable on either side of the seat tube. Wasn't ideal, but it sure beat the original setup.
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Old 07-21-10, 01:12 AM   #7
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I offer suggestion.. more than fiddling with whats there.. new stuff ..
Best Brake for the purpose : Magura Hydrostop
the only Hydraulic rim brake , by being Hydraulic it works well no matter what the hose length, or path. No rust filled U bend to collect water in the housing possible.
Straight bar levers , Closed system [no expansion tank like discs needed]
HS 33 work really well
lots of bikes in Germany and Europe have used them for many years.

My Koga Miyata Trekking bike came with them on It, Its a bike for carrying stuff and going around the globe if desired.

they Do Mount on cantilever frame bosses .

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Old 07-21-10, 01:18 AM   #8
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one advantage a V-brake has is the brake cable noodle coming off the side instead of from the center.
This makes it less awkward to route the cable around the seat post.

The key to problem solving is eliminating all variables and getting down to the real problem.

Is the weakness coming from the brake itself or the cable running up to the brake?
Do the cantis move freely on the bosses?
Are the springs giving too much or too little resistance?
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Old 07-21-10, 12:42 PM   #9
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Originally Posted by wile e. coyote View Post
yes, I am aware of kool stop eagle claws. It was a question of normal force, not coefficient of friction...
I thought I would get some interesting replies about cable stretch and routing and stuff. waahh why cant I ever get good answers here?
Why do you care if friction is obtained using normal forces or coef of friction? Increasing either will increase the friction force. Also, the salmon pads seem to be stiffer than the black ones so there isn't additional compliance in the pads to add to the squishiness caused by the cable housing.
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