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Disc Brake Kits for Road Bikes: Everything You Need to Know

December 20, 2023

Just a few years ago, hydraulic disc brakes were met with disdain by many road cyclists. Professional riders, in particular, vocally expressed their dissatisfaction with the new braking technology, some even reaching the point of loathing.


By 2024, all race bikes in the WorldTour will come equipped with disc brakes. Nearly all bicycle manufacturers have adopted disc brakes for their products. Even the UAE Airlines cycling team, which only upgraded to disc brakes in 2022, now exclusively uses hydraulic disc brakes.


The shift away from rim brakes is not limited to high-end models; affordable road bikes are also gradually adopting this trend. Some may feature mechanical disc brakes instead of hydraulic ones, and you'll find many road bikes priced below £1000 equipped with disc brakes.


So, what's the deal with disc brakes on road bikes? Why is there so much discussion around them? Do they represent a significant departure in bicycle technology? And when comparing disc brakes to rim brakes, which braking system comes out on top?


What are Road Bike Disc Brakes?


The fundamental difference between traditional rim brakes and disc brakes lies in the location where braking force is applied. As the name suggests, rim brakes directly grip the sides of the wheel rim. Thus, the wheel rim serves both as the main component of the wheel and the structure for supporting the tire and brake mechanism.


In contrast, disc brakes transfer all braking force to a single disc, which is smaller in diameter and can be directly mounted on the hub, much like in everyday cars, motorcycles, or virtually all wheeled vehicles. The brake calipers are still mounted on the frame and fork, meaning they are closer to the hub axis.


Another significant difference is how different types of brakes typically operate. With a few exceptions, rim brakes fall under the category of "cable-actuated brakes," meaning the brake lever is connected to the caliper by a brake cable. When you squeeze the brake lever, the cable is pulled, and the caliper clamps onto the wheel rim.


Disc brakes are typically hydraulic, with the brake cable replaced by a pre-filled brake fluid and sealed brake hose. Squeezing the lever pressurizes the brake fluid, pushing the pistons in the caliper and causing the brake pads to grip both sides of the disc.


Mechanical disc brakes, on the other hand, also use a brake cable to connect the lever and caliper. In this case, the lever and caliper act on a single-sided or double-sided brake pad to effectively engage the disc. Due to the friction and stretching of the brake cable, mechanical disc brakes are slightly less efficient and more affordable than hydraulic ones, commonly found on budget-friendly bikes.


Disc Brakes vs. Rim Brakes: Why Disc Brakes are Superior


Disc brakes offer several key advantages over rim brakes. Firstly, they can generate greater braking force, meaning less force is required on the brake lever for the same deceleration compared to rim brakes. This can be a significant benefit for riders who need to maintain a firm grip on the brake lever during extended descents or for heavyweight riders finding traditional brakes challenging to modulate (a concern shared by touring cyclists and tandem riders).


Another advantage is the ability to adjust braking force by changing the size of the disc. Larger discs provide lighter brake lever feel and improved heat dissipation, while smaller discs suit riders who don't require extra braking power. Admittedly, the braking force of any wheeled vehicle is fundamentally limited by traction. As many have experienced, controlling a bicycle with rim brakes on wet or slippery surfaces is relatively easy, presenting the second advantage: control.


Disc brakes offer greater adjustability compared to rim brakes, allowing riders to easily and precisely find the braking threshold. Controlling the bike up to the point of lockup becomes more manageable.


Disc brakes provide more predictability and linear braking, with consistent performance across different weather conditions. This is especially noticeable when compared to carbon fiber rim brakes, which perform poorly on wet surfaces and exhibit suboptimal heat dissipation in dry weather.


Bicycle manufacturers leverage another advantage of hydraulic disc brakes—braking efficiency remains unaffected regardless of the orientation of the brake hoses. This allows for complex internal cable routing designs and increased integration of handlebar components. The hydraulic hoses and derailleur cables can be routed internally from the brake levers, improving the bike's aerodynamics.


Disc brake wheelsets can also be made lighter than rim brake wheelsets. In the design of rim brakes, considerations must be made for the friction of the brake pads, tire expansion, and the heat generated during braking, while disc brake wheelsets only need to consider tire pressure.


When designing disc brakes, optimizing the aerodynamic performance of the wheelset becomes easier. Additionally, disc brake frames can be built differently from rim brake frames. While the left fork leg and the left rear dropout of the frame need reinforcement for disc brakes, other smaller stressed areas of the frame can be made thinner. Bicycle manufacturers have also learned how to make disc brake bicycles more aerodynamic than rim brake bicycles.


Disc Brakes vs. Rim Brakes: Why Rim Brakes are Better


The primary advantage of rim brakes is their lightweight design. Although the weight difference between disc brake and rim brake kits is not substantial, adding disc rotors can typically increase the weight of a disc brake kit by several hundred grams. However, the latest generation of bike frames and wheelsets can offset this weight difference with enhanced lightweight construction.


Nevertheless, the simplicity of rim brake systems offers several advantages. Parts are usually inexpensive and readily available, and there is high compatibility among various brands across different years. Rim brakes are easy to maintain when needed, even in roadside or remote conditions. They are also straightforward to adjust, and unlike some disc brake setups, the annoying friction and squealing noises between the caliper and rotor are not persistent issues.


The most important and noteworthy point is that the latest disc brake kits are better than ever, particularly in mid to high-end setups, providing ample braking power.


From an aesthetic standpoint, some argue that rim brakes, to a certain extent, are more visually appealing. While this viewpoint was widely accepted when disc brake road bikes first appeared, we can now say that this consideration has become less important. The simplicity brought about by disc brakes and internal cable routing (especially around the handlebars) is sufficient to offset any concerns about the appearance of disc brake rotors.


In conclusion, the choice between disc brakes and rim brakes for road bikes involves weighing the advantages and disadvantages of each system. The cycling community's shift towards disc brakes signifies a significant evolution in technology, offering riders improved performance, control, and adaptability across diverse riding conditions. Ultimately, the decision between disc brakes and rim brakes will depend on individual preferences, riding style, and the specific demands of the cyclist.


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