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I have installed on my bike brand new cantilever brakes (Shimano CX-70).

On the rear, the braking is justas efficient as I would have expected it to be.

But on the front, it feels quite weak. Front braking is usually more efficient that rear but in my case it is not. Actually, stopping by using only the front brake would take very long. It is just powerful enough to maintain a low speed in downhills or increase the braking power of the rear (I get a little more power when squeezing the levers from the drops position).

But obviously the rear braking can only be as much efficient as the rear braking can get, with the limitation of all the weight and traction concentrated on the front that should make it the most efficient brake.

Obviously I checked the installation of my brake:

  1. The pads are in perfect condition
  2. They get in contact with the rim about 1-2 mm from the very top of the rim (and the beginning of the tire)
  3. they have a < 2 mm clearance between rim and pad and that distance is the same on both sides
  4. I installed properly a cable stop (http://www.aspirevelotech.com/images/Paul%20Components/041-FunkyMonkeyFrontCableHanger-Black.jpg)
  5. the cable condition is very good, as is the housing and it is properly lubricated

What else could it be?

For the record, the rims are also in very good condition and I do know that the brakes are not exactly the same model on the front and the rear and yes I have purchased the right versions and yes the front is installed in the front and the rear on the rear.

I mean, I could understand that the brakes are crap, but in that case a similar model would not be working properly on the rear. So even if the whole mechanism were just exactly operating the same as in the rear, I would get at least as much and probably even way more efficiency on the front.

I just don't understand.

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If they appear to be interchangeable, you might try swapping the front and rear pads, to eliminate the pads as the difference. –  Daniel R Hicks Apr 3 '13 at 10:50
    
You can also experiment with shortening the straddle cable. You can jury-rig a shorter one with some cable and clamps from a hardware store, then buy an adjustable cable if that seems to help. –  Daniel R Hicks Apr 3 '13 at 10:54
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2 Answers 2

up vote 3 down vote accepted

I have had similar problems in the past, and I would group them in these "categories":

  1. Your rims are different. One of them has a smoother surface, or a different alloy, or a different quality alloy, and the actual friction is lower in one of the rims. I have this problem currently with one of my bikes;
  2. One pair of brake pads is different. It can be from a different batch, or different model, or different brand, or different material, etc.;
  3. If your pads are the (very common) asymmetric type, THEY SHOULD ALWAYS HAVE THE LONGER SIDE POINTING BACKWARDS! Everytime you brake, the pads tend to rotate "outwards", pulled by the moving rim, and that's why it's always best to assemble the pads with the longer side pointing backwards;
  4. One or both of your brake bosses (and overall bike structure) flex when you brake. Specially on the front, where the "rotating outwards" can combine with the (carbon?) fork "opening" while braking. This is very successfully solved with a (somewhat infamous) "brake booster";
  5. There are some possible geometric tunings, but this specific model doesn't seem to allow for tuning the pad position or the straddle cable length. If that were the case, attaching the pads away from the brake arms (thus widening the angle between the arms) and increasing the angle of the straddle cable gives the best mechanical advantage;
  6. The pads have not burned in yet. Going for a tough ride, with long descents and possibly some dirt, could allow the braking power to emerge;
  7. The braking surfaces keep being contamined by some non-cycling-related influence in your house: car exhaust near the front wheel, use of some spray product near the bike, oil coming from some wierd place, etc.

I hope this helps!

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To list it all: 1/ the rims are the same model and are quite new. 2/ The pads "felt" the same when installing them although the "nail-test" is not worth a lot 3/ The pads are almost symmetric and the documentation was very specific on their direction of installation 4/ That might be an idea, although I don't suppose I can test it anyhow else that by buying one of those... 5/ thanks for the info but N/A for me 6/ that is what I keep hoping for, I have not reached the 100 km step yet... 7/ I will look out for that but I don't see what it could be... –  Nic Apr 3 '13 at 6:17
    
About 6/, I think mileage per se is not a good indicator for pads burning in. What I would do is to find a long descent, and give the brake two or three good squeezes while coming at a high speed, until almost stopping. Sometimes it's better to squeeze and release repetitively instead of a long squeeze. Sometimes this will work when dry, but in my experience a mildly wet braking surface works better (I wonder if this is why drilling aluminum works best with water, or if it has more to do with the braking pad itself). Anyway, good luck, and give us some feedback if possible! –  heltonbiker Apr 3 '13 at 13:46
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The "efficiency" (mechanical advantage) of a brake system is determined by its geometry (brake arms and brake levers) and generally can't be changed.

The effectiveness of the brakes can vary, though, based on general maintenance issues (lubed cables, etc), the brake pad material, and the alignment of the pads.

You say you "replaced" the brakes. What were the old brakes you replaced -- cantilevers? If so, did they have the same geometry? Did you keep the same brake levers or install new ones?

In general, if the front and rear have the same geometry and the same components they should have the same effectiveness (up to the point of rear wheel skid).

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Do note that a part of the "mechanical advantage" equation is the "straddle cable" that connects the two sides of the canti. The longer it is, the less force is applied to the brake pads. –  Daniel R Hicks Apr 2 '13 at 23:49
    
The original brakes were V-Brakes. I replaced them because I changed the levers because I changed bar geometry. –  Nic Apr 3 '13 at 6:10
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