Disc brake dynamics - The actual difference between mechanical disc brake and hydraulic disc brake

Disc brake dynamics - The actual difference between mechanical disc brake and hydraulic disc brake

Regardless how much engineers try to convince us, that modern high end mechanical disc brakes, controlled by high-end braking cables and high-end cable housings, can be real match to a hydraulic disc brake, the actual picture is quite different. Mechanical brakes suffer from cable stretching, regardless of everything. Yes, yes, I know the blah-blah-blah-blah that high-end cable in high-end cable housing do not suffer that much as low-end cables in low-end housings… but at the end of the day, the high-end cable also stretches. Even this combo has so called slack, which appears and accumulates during long term use. This is the reason for the brake to be inefficient, in most cases. Another issue is that almost all mechanical disk brakes have only one moving piston, which moves only one brake pad. In this case the static one must be set up properly a tiny hair away from braking disk, in order to make it work at all. This is the reason for 50% of inefficiency, and the braking cable stretching is the other reason that causes inefficiency.

Part 1 Mechanical disc brakes

Let’s look both reasons separately, but remember to take both into account, and keep in mind that inefficiency of one adds to the inefficiency of the other, giving total result of higher inefficiency then each one separately.

1. Inefficiency of the mechanical disc brake caused by one moving brake pad

Inefficiency caused by one moving braking pad, is half of the reason why mechanical disc brakes are not good at stopping. In this case the moving brake pad must be set to 1mm away from the brake disk and the static one must be set a tiny hair away from the brake disc – barely enough to make sure that the static brake pad does not touch the disc.  After we made our setup properly, let us pull the brake and see what happens. The moving brake pad moves in, makes contact with the disc, and slightly pushes the disc against the static braking pad. The disk eventually contacts both braking pads, but what actually happens? In this case, the disc has some elasticity and pushes against the moving brake pad, which means the rider must pull the brake lever even harder, to force the brake to overcome the resistance. This leads to uneven wear of both brake pads. Moving brake pad wears out faster than static, because it creates more contact and more friction, than static one. On average uneven wear is 38,7% faster on moving brake pad than on static one. This lack of balance in forces reduces the braking efficiency, because the disc actually wants to bend away from static brake pad. This creates the lack of balance, because you need to assure good push from both brake pads, even from both sides to make a good efficient braking. The strive of the braking disk to bend away from the static brake pad actually is what creates the lack of balance.

2. Inefficiency of the mechanical disk brake, caused by braking cable stretching over long-term use.

Braking cable stretching over long-term use is inevitable, because the system is based on tension. When the rider pulls the brake lever, the majority of the force goes into the moving brake pad, while some force is lost over one insignificant stretch of the braking cable. This stretch is what makes the loss of braking force. So as we noted we have two reasons to loose efficiency, so in order to stop, our rider needs to pull the lever harder. This increases the moment of short-term stretch and the loss of brake force. The result is hard pull, to achieve good stop. Some time this can be very bad, for example you ride the trail in a park, you have blind turn, and a mother with child appears out of nowhere. Fast stopping in situation like this is critical. Keep in mind that a full-force, pull will stop the bike in time, but it will stress the braking cable further more, and stretch it further more. Now the next moment comes. The brake lever is released. The whole system returns as it was at least in theory. Practically, the braking cable do not fully recover. A tiny amount of stretch remains. During intensive stopping whole season, this can lead to notable long-term stretch, because each time the braking cable stretches, and recovers incompletely. These tiny stretches which remain, are the key here. One or two of them bear no meaning. Large number of them, accumulate after whole season of intense braking, and can lead to notable stretches, creating bigger amount of slack. To fix this issue you need to re-tension your cables. Keep in mind this effect will not disappear. It will become heavier in time. Remember abused braking cable can tolerate less abuse than brand new, in full health. In time the cable needs replacing, if the cable is ignored, and ran like this it will snap. This is very good time to replace the braking cable housings, while you are at it. At least your brakes will work smoothly when you need them, assuming the brake pads are adjusted properly.

Slack is something which is good to be aware about and take good action in time to prevent Bigger problems and accidents involving injured people. The other problem is that longer braking cable suffer this more than shorter braking cable. Actually if there is longer cable to begin with, there is more braking cable to stretch, long-term stretches are bigger, efficiency drops further, slack accumulates faster, and to a greater degree. When I mention longer cables I mean considerable difference in length, such is found on every bike – the front brake has MUCH shorter braking cable than the rear brake, this is why the front brake suffer less from the slack effect, and the rear brake suffer more from the slack effect. This is the reason people to be complaining more of their rear brake causing more trouble than the front brake.

Safety tip:

if you are using combination of cheap low-grade braking cables in combination with cheap low-grade cable housings, at the end of the active season, they are worn enough to be considered unreliable and ineffective. They must be replaced at the beginning on the next season. Yes, cheap goods are never good. This is why they are cheap. If you use high quality, high-end braking cables in combination with high quality high-end cable housings, you will need to put them in the beginning of the season, enjoy them whole season, at the end of the season prepare the bike for the winter, and re-tension them at the beginning of the next season. How long they last before replacing? It depends on how intensive you stop but on average they last 3 – 5 seasons before they need to be replaced.

Part 2 - Hydraulic disk brakes - the jewel in the King’s crown.

They react fast

They are reliable

They are easy to use

They are easy to control the amount of brake force you apply to the wheel, with single gentle squeeze of the brake lever with one or two fingers.

They are strong, and powerful

They do not suffer any of the short-term or long-term inefficiencies of their mechanical counterparts.

Welcome, to the world of modern hydraulic disk brakes. These brakes quickly gain market share, replacing basically everything before them. Unlike many of you might guess, these are very simple and very efficient devices, and their simplicity, makes them reliable. This is basically the simplest form of hydraulic device, small piston pushes hydraulic oil, which pushes two larger pistons which push the braking pads to the braking disc. Hydraulic oil does not compress or thins out over time, and if the brake itself is not damaged or leaking hydraulic oil, and properly bleeded to completely remove all air from the system, with time this brake does not deteriorate, there is nothing to deteriorate in it. the only thing you need to do, is to monitor the wear of the brake pads and replace them before wear becomes critical. That all the care they need. There are no braking cables to stretch and cause you headaches, no cable housings, to add to the list of things to do before riding safely. These brakes have two moving pistons and two moving brake pads, which basically means if the brake balance left-to-right is done right, both brake pads will touch the brake disk at the exact same time. Proper setting for these brakes is the disc to be centered exactly in the middle, and each brake pad should be 0.5 millimeter away from the disc. Because of this balance, the inefficiency of one moving and one static brake pad does not exist. This also means both brake pads wear evenly, and the problem of one wearing faster than the other also do not exist. Brake force increases dramatically because the simple rules of hydraulics, say that if a small piston pushes hydraulic oil, which pushes larger piston, the larger piston will move slower than the smaller piston but will apply more force while it moves. In this case braking forces double, because on the other side there is not one but two pistons, connected to the same hydraulic system same hydraulic oil which is pushed by the same small piston, which makes the two larger pistons to move twice slower but with twice the force, they will apply to the brake pads and the disc, unlike the case with one small piston and one bigger. This is how your car brakes work. The same way. If you have these fancy 6 cylinder calipers, per wheel, which means 3 cylinders on one side and three on the other, multiplied by factor of 4 for all 4 wheel of the car, you have quite some power there in your pedal. The same rule apply here. You have quite some power at your finger tips to play with. Like all this is not enough, you have some more power amplification, the brake lever is based on what is called type 2 lever, which means you have very long arm, mounted on one pivit, and near the pivit is the small hydraulic piston. The result is that the closer the piston is to the pivit point, the slower it moves but with greater force. Add this power amplification to the overall power you have to play with. This type if disc brakes not only deserve respect, they demand it, and they will get it from you the painful way. Many people unaware of all this basic physics laws just squeeze hard. The brake locks the wheel, bike either flips over if this is the front wheel or the rear wheel starts dragging, locked making the bike unstable and uncontrollable. These brakes must be squeezed gently, slowly, they react fast enough anyway, so this is how you will achieve this nice feeling of keeping this force under control. The good news is this will not be just feeling, this is the real power mastered to serve you under your control.

Safety tip:

Always maintain your brakes in top condition, in the city, you never know, how other people, or intensive traffic will surprise you, and when. Being able to stop before is to late, means that you will spare yourself a lot of trouble. If you do not maintain your brakes in proper condition, you may ride fast and ram the surprise, this may cause you and other people being injured, further damaging the bike and if you hit a car, you can damage the car as well as your bike, injuring yourself in the process.