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Vertebrae, the most advanced cable housing in the world

Rim brakes vs disc brakes: pros and cons of cable and hydraulic brake calipers.

Aug 9, 2009 by Dr. Leslie Dean Brown

Let’s face it: anyone who rides both on and offroad knows that even current road bikes have less-than-ideal brakes. With the advent of 8 and 9″ rotors, braided lines and quad piston calipers for downhill MTBs, “lame” is probably a better word. Road bikes are faster than their cross-country MTB counterparts, so they should really be equipped with more powerful brakes. In this article, this logic is questioned in depth.

Before we get any further, most conservative road cyclists either argue that road bikes don’t need more power or that it wouldn’t be a good idea with such skinny tyres. Here at vertebrae, we think that these arguments are rather weak – MTBs are often ridden on uneven terrain with slippery surfaces yet most riders manage to stop their bikes extremely well.

• Powerful brakes result in less hand fatigue during long descents
• Improved modulation gives you more control of your bike not less
• The ability to brake later increases average speeds.

You might think that having more powerful brakes require heavier-built calipers. And you’d be absolutely correct. But obviously keeping the weight of components to a minimum is paramount for competition bikes, therefore it’s not very practical to design and manufacture super-chunky massive road racing calipers. Nor can we do much about the brake levers themselves which are already inherently flex-free. (incidentally, we think that the lack of practical ways to improve current road-going brakes is a likely reason that riders have until now been content with such wimpy stoppers and argued for them in the past)

So here’s the point of this article. We thought we’d discuss why there is traditionally such a big difference in performance between hydraulic disc brakes and conventional rim brakes. Remember that in actual effect, a 700c road rim is a 26″ diameter rotor! How can we then harness the power of a rim brake without adding any extra weight? Not surprisingly, much of the difference is down to the brake lines themselves…

DISC BRAKES:
Firstly, let’s discuss disc brakes. Disc brakes used on bicycles are in fact at a major disadvantage since their outer diameter is 3-5 times less than a wheel rim’s brake track. Straight away that means they loose 3-5x the power of a set of rim brakes (all else being equal). There are two ways in which they make up for this rather significant deficit:

1) Brake fluid (either DOT or mineral oil) is essentially ‘incompressible’, at least while it remains below its boiling point. When the brake operates, the hydraulic brake line is under pressure; power is robbed by the housing as it expands along its length. This is the principal reason that braided steel lines are used to upgrade hydraulic brakes in all racing disciplines. Even so, brake fluid absorbs water, which can then boil at a lower temperature (leading to spongy brakes).
2) The body of a disc brake caliper is more compact than that of a rim brake caliper, stouter means stiffer, hence there is less overall energy loss as the brake pads are squeezed together. Further, metal rotors, which are essentially solid, can resist the compressive forces of the opposing brake pads and pistons. Put another way, the brake tracks of a hollow metal or carbon wheel rim are more prone to deformation by similar sideways forces. This in turn means that a pair of disc brake pads can sit mere fractions of a millimetre away from a rotor whereas rim brakes lose that advantage…

That is basically it. There is no “number 3” on the list. Although hydraulic systems work with less inherent friction than their cable-operated counterparts, this additional cable friction is very easily overcome [a quick test is to see just how little effort it takes to pull a cable without the cable pinch bolt done up]. Furthermore, it appears that organic and sintered metal disc brake pads used in conjunction with steel disc rotors have a lower coefficient of friction than rim or v-brake pads used with alloy or carbon rims (their operating temperature is another story).

In other words, there is no magic “secret” to a disc brake’s power. They take the same force applied by your hands (the finger effort) and transfer that energy into slowing you down by friction. The initial effort is basically converted (or multiplied) by the leverage ratio of the actual brake lever compared to it’s pivot point (just like any conventional brake lever) and then again by the ratio of the area of the master cylinder to that of the piston(s).

Let’s not forget that bicycle and motorcycle disc brakes are unassisted. Cars on the other hand which utilise disc brakes implement the engine’s vacuum to cleverly assist the action of your foot (without which it requires much more effort to stop the vehicle). So even cars equipped with disc brakes on all 4 wheels still need a brake [vacuum] booster to improve their performance to an acceptable level.

RIM BRAKES:
Without getting too far off track, now let’s consider rim brakes – dual pivot calipers and v-brakes in particular. A 26″ or 700c rim has a diameter about three times larger than an 8″ disc brake rotor. Like disc brakes, rim brakes also use the pivot locations to amplify the braking force at the lever and again at the caliper. The force is transferred (or better, applied) through the brake cable; tension is exerted on the cable, while the housing itself is under compression.

Unlike the hydraulic fluid used in disc brakes (which is supposed to remain incompressible), the trouble with the cable housing used in most rim brakes is that not many materials can withstand these compressive forces adequately. All cable housing basically distorts under load. So what people typically feel as “cable stretch” is more like the outer housing deforming or buckling under compression.

The traditional solution has been to eliminate all the absolutely necessary cable housing from modern bikes, using the bicycle frame itself as a load-bearing member to support the cable between two “cable stops” and thus stiffen things up a bit. It works wonders, although this problem still persists with the remaining housing.

What generally happens when you use a brake equipped with standard bowden housing? As you begin to squeeze, much of the travel at the brake lever is just to get the pads to touch the rims. Setting this initial gap is critical to the performance of a rim brake (too far away and the total power available is instantly crippled, too close and the brake pads rub when the wheels flex laterally). Continue squeezing harder on the brake lever and the pads will continue to press up against the rim walls with progressively less and less force. Finally towards the end of the brake lever stroke the housing gives way and you don’t get any more braking power at the caliper/wheel as you continue to squeeze harder on the brake lever. This problem is exacerbated with rear brakes that use full length cable housing and tandems with long cable runs because the lever hits the handlebars well before transmitting the full force which your fingers are capable of.

Hence one of the principal disadvantages of all cable-operated brakes (dual-pivot calipers, v-brakes and mechanical disc brakes included) are the brake lines themselves. Reducing flex in the brake caliper arms, pivots and pad holders would substantially improve a brake’s performance, however there is not much point in doing that if there is still close to one metre of cable housing flexing all down its length.

Vertebrae eliminates this primary weakness by utilising a material and design which does not significantly flex under compression. This special material (Al2O3 ceramic) exhibits 32% less deformation than steel and about 4.3 times less than aluminium alloys, while the design of vertebrae provides a much greater surface area to resist those compressive forces in the first place. Both of these traits allow vertebrae cable housing to cope with the loads applied with minimal distortion.

The moral of the story is that with a set of vertebrae installed, your brake levers are never going to touch your handlebars. Because before they even get close, your rear wheel is either already locked up and/or it’s in the air. Expect to be use your brakes more “from the hoods”. The minuscule amount of flex you can now feel is truly due to the brake cable stretching and calipers flexin’. And any other weakness is down to your lever/caliper pull ratio…

Thanks for reading this article and by all means feel free to submit any relevent comments!

-Leslie

2 Responses to “Rim brakes vs disc brakes: pros and cons of cable and hydraulic brake calipers.”

  1. Mirta Says:

    Hello would you mind stating which blog platform you’re working with? I’m planning to start my own blog
    in the near future but I’m having a hard time deciding between BlogEngine/Wordpress/B2evolution and Drupal. The reason I ask is because your design seems different then most blogs and I’m looking for something unique.
    P.S Apologies for getting off-topic but I had to ask!

  2. Dr. Leslie Brown Says:

    Hello,

    Sure, I am using a current version of wordpress with a fairly heavily
    modified murdock-10 template (which is very old but still works ok
    because it’s very simple).
    I can’t speak for your other choices as I haven’t used any of them
    unfortunately.
    My other website (tenerife-training.net) is also done using worpress,
    although it too was very heavily modified…

    I do like using wordpress although I wouldn’t use it for everything.

    For example, I would like to make another completely separate website in
    the science/inspiration field.
    It should have a wikipedia editable section, with a forum or something
    where people can easily discuss interesting topics and easily updatable
    blog style posts on the homepage.
    I am not 100% convinced that wordpress would be the best content
    management system (CMS) to host all that.:-\

    If you just want to host a blog, yes wordpress is pretty good at doing
    that.:-)
    Thanks!

    Leslie

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