- Pautan angkup brek radial menjadi pilihan ramai pada hari ini.
- Namun, pautan axial yang ‘normal’ masih lagi boleh dijumpai.
- Apakah kelebihannya memaut angkup secara radial?
To rehash, we’ve seen the basic principles of the brake system in Part 1, about how the pressure from your fingers is transferred into fluid pressure, resulting in braking force.
Next, we took a look at how the ABS system works in Part 2, in order to allow for maximum braking force while avoiding the wheels from locking up.
Continuing the series, we also checked out tips on how to choose the correct brake pads for your bike in Part 3.
Therefore, let’s now examine the methods of mounting the front brake caliper to the motorcycle, or more specifically, about the radial mounting arrangement that’s all the rage these days. Yes, so popular they are that even lowlife thieves couldn’t resist them.
One, the traditional type is the “axial” master cylinder, found on almost all motorcycles with front hydraulic brakes from small to large.
In this arrangement, the master cylinder’s bore is perpendicular to the lever’s travel, seemingly elongated to the left side viewed from the top. A protrusion on the end of the lever pushes the plunger in cylinder, forcing brake fluid into the calipers.
The second and more recent type is the radial master cylinder. The master cylinder’s bore is parallel to the brake lever’s movement, looking more directly fore-and-aft when viewed from the top.
What’s the difference between the two? Other than a different type of piston movement, the radial master cylinder provides a better feel at the lever for what the brake is doing. The radial master cylinder is more rigid as there are fewer moving parts.
With the advent of radially-mounted brake calipers, this format has gone on to be called the “normal mounting.” The caliper is mounted to “bosses” cast into a fork’s bottom tube, with bolts that run parallel to the wheel axle.
There isn’t anything wrong with this type of mounting. However, custom brackets need to be fabricated should one want to install a larger brake disc. Other than that, there may exist a little torsional flex for lateral movement, although it’s hardly perceptible when ridden on the streets.
In this setup, the caliper is still mounted to bosses cast into the lower fork legs. However, the bosses are cast to allow bolts to be fitted directly from the rear, instead of from the side, and perpendicular to the wheel axle.
Being mounted this way means there is virtually no deflection because the braking forces are on the same plane as the brake disc’s rotational forces.
What are the benefits of radially-mount calipers?
First and foremost, being mounted from the rear means all the rider needs to do is install spacers and longer bolts when switching to a larger disc, instead of having to custom make a mounting bracket.
There is a myth surrounding radially-mounted calipers as having more braking power. Want us to be honest?
As we highlighted in Part 1, “brake power” is defined by the amplification of brake fluid pressure, in relation to the size of the master pump’s piston and caliper pistons’ sizes.
What radial mounting does though, with the lack of deflection, provides for a crisper feel when braking, i.e. better braking feedback, and in turn allowing for better modulation of the front brake.
Where it’s usually more important for the track than the streets.
Brake pads are often overlooked for the majority of riders, “Hey as long as it stops, no problem.” Or we just ride replace the pads when the mechanic tells us that it’s time do so. It’s almost like we have no choice in it, except when it comes to paying, “Brape? (How much?)”
Perhaps, it doesn’t have to be so with a little more understanding, then we could dictate on what we actually need, instead of being led by the nose.
Before we go further, be aware that there are two current friction ratings for brake pads, GH and HH.
Both those letters signify the brake pad’s coefficient of friction (CoF); the first being the CoF at normal working temperature, and the second at an extreme temperature of 650 oF (343 oC). Therefore, the ratings indicate how much friction is there at certain operating temperatures. The G-rating offers between 0.45 and 0.55, while the H-rating’s CoF is from 0.55 and up. These ratings are usually stamped on the outside of the pad’s back plate, although there are manufacturers who don’t do so but specify it on the brake pads’ packing, instead.
Having the right type of pad material determines how it feels when the brakes are applied and how it stops a bike.
Organic pads may sound like they’re something grown without chemicals, it just means they don’t contain metal. Instead, they’re a blend of rubber, glass, carbon, aramid, Kevlar (the real contents differ among manufacturers), bonded by a resin.
Organic pads are popular among riders due to their progressive braking feel, which doesn’t “bite” aggressively. Additionally, it’s softer and doesn’t score brake discs. However, they typically wear out faster due to being soft.
The pad material, usually copper particles, is fused to the backing plate under extreme pressure and temperature (the process is called “sintering”) to form a friction material that’s wear resistant. They can handle a wide range of conditions, hence well-suited for any type of riding including trackdays.
Sintered pads offer a stable CoF whether cold or hot, and they bite instantly. Apart from that, they are resistant to fade, perform well in the rain or mud, and usually last longer.
However, the do wear brake discs quickly. Most, if not all, motorcycles use the harder stainless steel brake discs these days. However, if you encounter deep grooves or “blueing” on your discs, you may have to consider replacing your brake pads for those of less aggressive material.
Manufacturers infuse organic pads with some metal material to increase the bite, durability and fade-resistance, while still maintaining the progressive feel and low disc-wear of the organic brake pads. This may be a good compromise for most riders.
They are made by fusing high-strength ceramic fibres and ductile non-ferrous (non-iron) metal filaments at extreme pressures and heat. The metal filaments provide the initial bite while the ceramic compound provides high temperature resistance to avoid brake fade.
Besides that, the metal filaments carry away heat quickly to reduce rotor wear and deformation (disc warp).
Everytime we squeeze and press down on the brake levers, the brake system causes the motorcycle to slow down. it works day in, day out, throughout the bike’s lifespan. But have you ever thought about how it actually works?
While the traditional cable-operated drum brakes are available on a certain number of bikes, they are being phased out for the hydraulic brake system, more commonly called disc brake system.
The brake system converts kinetic energy (contained in a moving object) to thermal energy (heat) by using friction. Brakes have evolved over time and some brake systems could actually slow a bike quicker than the latter could accelerate. In a recent overseas test on the BMW S 1000 XR, the bike accelerated from 0 to 160 km/h in 6.1 seconds, covering 151 metres. That fast! But it slowed from 160 to 0 km/h in 5 seconds in less than 100 metres.
The basic working principles of the hydraulic disc brake system is easy to understand. When you press the brake lever, the master cylinder pump pushes the brake fluid through the brake hoses to the calipers. In turn, this pressure pushes on the caliper’s pistons which have brake pads attached to them. The pads are compressed on to the brake disc.
Let’s look at the parts of a hydraulic brake system:
More commonly called the “brake pump” or (“bulek pom” by your typical Chinese mechanic at the kedai motor), it converts mechanical force (when you press the lever) into hydraulic pressure. The brake lever pushes on a piston that presses on the brake fluid. The force with which you pull the lever is the leverage ratio and the size of the master cylinder piston determine the amount of pressure is subjected through the system, sometimes exceeding 1,000 kPa.
Hoses transmit pressure from the master cylinder to the calipers. They are typically multilayered, with a Teflon inner lining surrounded by braided nylon, or Kevlar, or stainless-steel reinforcing layer, and finally wrapped in a protective sheathing.
Contrary to popular belief, stainless steel-braided hoses DO NOT stronger braking. They provide a more consistent braking feel as they don’t expand like rubber hoses do when subjected to eyeball-popping hard braking.
Rubber hoses lose their strength over time, thus need to be replaced every four years.
This is also inexplicably called the bulek pom in the workshops. The real job of pumping braking fluid is handled by the master cylinder.
Anyway, it’s at the caliper where the hydraulic pressure is multiplied. This is because the pressure from the master cylinder is exerted uniformly on the much larger area of the caliper pistons. An adult male’s hand grip typically exerts only between 0.4 to 0.6 kPa, thus that has to be increased to more than 1,000 kPa.
The disc transfers the brake pads’ resistance to the tyre contact patch. Brake discs are usually made of stainless steel with variable amounts of iron. Modern discs are also drilled to assist in cooling, besides shedding water and debris.
Another incorrectly named item, usually called minyak brek (brake oil). It has nothing to do with oils. The misconception probably arose from being referred to industrial hydraulic fluids that are petroleum based.
The brake fluid is the medium which transmits force from the brake lever to the brake pads. The brake fluid isn’t as simple as one may be inclined to think. Other than being non-compressible to effectively transfer the pressure, it needs to have low viscosity to work with ABS components, has good lubricity for the master cylinder and caliper piston seals, offer corrosion resistance, and importantly has high boiling point.
There are four grades of brake fluids. Glycol-based ones are DOT 3, 4, and 5.1, hence are mixable. DOT 5 is silicone-based and can’t be mixed any other type.
The glycol-based fluids are hydrophilic, meaning they suck in and absorb moisture from the air. Mixed with water, the brake fluid’s boiling point is lowered, causing brake fade. That’s why brake fluids need to replaced every two years.
DOT 5 on the other hand, is hydrophobic and rejects water. However, after repeated heating and cooling cycles, bad master cylinder and caliper seals, it will also ingest water eventually. However, DOT 5 brake fluid does not pull moisture out of the air own its own, thus have a longer lifespan. Military vehicles usually use DOT 5 since they sit idle for long periods of time.
Additionally, being silicone-based, it’s not caustic leading Harley-Davidson to using this previously.
Does this mean we should all convert to DOT 5? The short answer is “NO.” DOT 5 brake fluids are expensive, has high compressibility and higher viscosity (thicker) and thereby unsuitable for everyday use. Harley-Davdison has since reverted to DOT 4.
Each grade usually denotes the fluid’s boiling point, from the heat resulting from friction, rather than its chemical contents.
The US Department of Transport (DOT) specified each grade’s “dry” and “wet” boiling point. The latter is deemed to be completely free of moisture, while the latter contains 3.7% water, common after a year of regular use.
DOT 3: 205 oC (dry), 140 oC (wet).
DOT 4: 230 oC (dry), 155 oC (wet).
DOT 5: 260 oC (dry), 180 oC (wet).
DOT 5.1: 260 oC (dry), 180 oC (wet).
Observe how much performance drops away between dry and wet. That’s why brake fluid should be replaced every two years. Since the standard was set in the USA, we may need to replace it even earlier due to our climate’s high humidity and constant rain.
Water in the fluid lowers its boiling point, casing the brake lever to feel spongy and reducing braking performance – called “brake fade.”
That’s it for Part 1. We’ll talk about brake pads, caliper mounts, ABS, carbon brakes and so forth next time.
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