01 August, 2012

Breaking it Down: Grand Cru Cantilever Brake


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.

5 comments:

Tony Lockhart... said...

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.

Calvert said...

Doesn't a longer yoke give you greater modulating control of your braking as on a descent?

VeloOrange said...

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.

tjan said...

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.

Anonymous said...

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.