A guest post by Casey Fittz
Cantilever brakes have stood the test
time of time due in a large part to their simplicity. They provide
ample tire and fender clearance, and once properly, set up require
little maintenance. Unfortunately, getting them properly set up can
be challenging. Our cantilever brakes, the Grand Cru MK2 and
the Grand Cru Zeste brakes, are designed with extra features to make
their setup easier. While this blog post is intended to provide a basic
understanding of how to set our brakes up specifically, the theories apply to most other cantilever brakes.
First off, some important things to
know:
Mechanical advantage: This is the ratio
of the “force out”, at the pads, to the “force in”, at the
brake lever. An important thing to remember is that energy is
conserved. Mechanical advantage can also be looked at as a trade off
between force and distance. A higher mechanical advantage corresponds
to a larger output of force at the brake pads, but means a smaller
distance traveled by the brake pads. Energy is going to be lost from
your braking power due to housing flex, cable flex, cable friction,
and plenty of other things. This is another great thing about the
simplicity of cantilever brakes. There should be an extremely small
amount of flex in the brake arms, therefore less energy lost. The
same thing certainly cannot be said for a lot of long reach caliper
brakes. It is possible to have too large of a mechanical advantage,
because the larger your mechanical advantage is, the smaller the
range of motion of the your pads will be. This would mean needing to
have your pads very close to your rim and not being able to release
your brakes. The primary adjustment of mechanical advantage on
cantilever brakes is the straddle cable yoke height. For our brakes,
the lower the yoke height the higher the mechanical advantage.
Cantilever Brake Types: There are two
main variety’s of cantilever brakes on the market today, wide
profile and low profile. Wide profile cantilever arms stick out from
the frame, often close to horizontally, while low profile arms point
toward the sky. Wide profile cantilevers have a simple setup and
fairly consistent mechanical advantage. Low profile cantilever brakes
are useful for pannier or heel clearance on small frames and provide
access to a larger mechanical advantage. However they are generally
harder to setup well. We make both types, the Grand Cru CantileverMK2's are our wide profile model, and the Grand CruZeste are our low profile model.
Properly set up, brakes should feel a
bit squishy. This generally means that you have enough mechanical
advantage that you're compressing the brake pads. This isn't a
perfect indicator because if you have a lot of brake housing that is
going to make your braking feel squishy as the housing compresses.
When setting up brakes in the bike stand, the temptation is to set
them up so they feel stiff. This intuitively seems like it would
correspond to the most braking power because it makes the brakes seem
more responsive. However, remember that a low mechanical advantage
means more pad travel. This feeling of responsiveness is coming from
the higher pad amount pad travel, and actually means that your
mechanical advantage is too low.
There is a handy interactive calculator made by Benno Belhumeur that allows you to adjust different
cantilever brake parameters and see how they effect mechanical
advantage. For those of you who enjoy things like that.
The above picture shows mechanical
advantage vs. yoke height curve for our brakes. (Yoke height is
measured from the brake studs). Note that this is the mechanical
advantage of the brakes only, it ignores the MA of the brake lever.
As you can see, the MK2's will give just about the same mechanical
advantage no matter where you the put the yoke. A 100mm yoke height
is about as low as your going to be able to get. The Zeste brakes
have a much larger accessible range of Mechanical advantage. This is
part of what makes them tricky to properly set up. What we generally
suggest is to set the yoke height at about 6 inches(150mm) from the
brake stud as a starting place. Then adjust the yoke height based on
feel, remembering that a lower yoke height corresponds to a larger
mechanical advantage. The other problem with the mechanical advantage
varying so much based on yoke height is that yoke height is going to
increase as the pads wear. For this reason we have a barrel adjuster
at the end of our straddle cables. This means that you can easily
tighten your straddle cable as your pads wear.
On our brakes it is also possible to
adjust the brake pad post, bringing the brake pad closer in or
further away from the cantilever arm. For the low profile brakes this
adjustment will make a small difference in mechanical advantage. On
wide profile brakes the mechanical advantage can substantially
decrease as you move the pad away from the cantilever arm. For this
reason we suggest backing the post all the way out so the pad is as
close to the cantilever arm as possible on wide profile brakes.
So you can see how a simple design like the cantilever brake can have a multitude of ways in which it can be set up and why proper set up can make a huge difference in the performance of the brake.
So you can see how a simple design like the cantilever brake can have a multitude of ways in which it can be set up and why proper set up can make a huge difference in the performance of the brake.
6 comments:
Very interesting post! I know there are a number of seasoned riders who frequent this blog, so I'll put a question out there about wider profile canti brakes.
I've got some of the generic Tektro canti brakes with the wider profile. I have noticed that when lowering the yoke (in order to gain mechanical advantage), I tend to lose the ability to easily remove the straddle wire. I'm guessing this has to do with a shorter straddle wire (since it's moving closer to the canti studs). With that issue in mind:
**is there a way to gain mechanical advantage on wide profile brakes while having easy removal of the straddle wire?
**is there a sequence used to properly adjust, or set up, your canti brakes? I generally tend to set the orientation of the pads, followed by the height of the yoke, and then I finish off by fine tuning with the straddle wire. Perhaps there is a better way to do it.
Doesn't a longer yoke give you greater modulating control of your braking as on a descent?
Tony,
There is an inverse relationship between pad travel and mechanical advantage. The higher the mechanical advantage the lower the distance traveled by the pads. The problem your running into with not being able to release the brake is probably caused in part by needing to have your pads very close to your rim in order to compensate for the lowered travel range of the pads. The Tektro aero brake levers have a pin to release some cable and allow you to easily release your brakes. The Dia Compe 204 non-aero levers also have a similar quick release.
Compressing the brake pads? That is some mechanical advantage! Good post. If you are interested in brakes and cantis in particular, I'd suggest tracking down Jan Heine's articles on brake, a good addition to this post.
Are the two lines in the diagram reversed? I expect the high-profile cantilever to have flatter curve, but the diagram (as of 2012/08/02) has a flatter curve for the low-profile cantilever.
No, Anonymous the graphs are not swapped. The leverage on the wide profile look more powerful, but it isn't the only thing in play. What the wide profile brakes have going for them is a near constant mechanical advantage as the lever is pulled. (the right hand part of the graph is near level) This gives good pad clearance, and excellent modulation, but the fact that the straddle cable "arms" are nearly parallel to the actuating cable for practical arrangements limits the mechanical advantage unless the lever arms are made ridiculously long.
The secret of the compact cantilever brake is the force multiplication that happens due to the high angles between the straddle cable runs and the actuating cable. The force is multiplied by the secant (1/cosine) of this angle. Of course the travel is reduced by the same factor...no free lunch.
Levers and cams are well known ways to trade travel for force, but the catenary (what the straddle cable is, in engineering terms) is a neglected alternative. This is what gives the compact cantilever brakes such high mechanical advantage.
Unfortunately, the mechanical advantage droops off as the brake actuates, pretty much opposite of what most would want*, but in practice it seems to be livable, and many brake levers and hand kinematics have the opposite effect, which mitigates this to some degree. Also grip control becomes less precise the harder we squeeze, so in practice these brakes tend to rate excellent in terms of modulation.
*A gently rising mechanical advantage is seen as desirable because it allows both powerful braking, and good pad clearance. Too much of this supposed good thing leads to brakes that are "grabby" and hard to modulate.
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