Tuesday, July 9, 2019

Bike Friday HaD: Deore XT 4 Piston Brake Components

On a cargo bike, the effective choice of brakes would be hydraulic disc brakes. This is for safety, as it is necessary to have sufficient stopping power when hauling heavy items or people. Especially when traveling downhill, it is critical to be able to stop properly, since the whole bike is dependent on the 2 small brake levers to apply sufficient brake power.

Mechanical disc brakes are less powerful, but they are easier to set up as it just needs a brake inner cable and brake outer casing, with no need for bleeding of the hydraulic hoses. Rim brakes are even less effective, and I don't recommend using it on a cargo bike, since disc brakes are easily available.

Brake power depends on a few factors, which are mainly the size of the wheels, the size of the brake rotors, and the type of brake caliper. Brake levers are not so critical as even the entry level hydraulic brake levers can work well and effectively.

For the Bike Friday Haul-A-Day cargo bike, it uses custom built 20" wheels, and can accept hydraulic disc brakes as there are disc brake caliper mounting points on the frame and fork.

I want to have as much brake power as reasonably feasible, as it is important to be able to brake effectively when rolling down a hill, with an additional load on the bike. A rough estimate of total bike + rider + cargo weight would be 20 kg (bike) + rider (70 kg) + additional rider (70 kg), giving a total of 160 kg. This is quite a lot and as explained earlier, there are only 2 small brake levers to operate the brakes, so the brake system had better be effective!

My plan is to use a 4 piston brake caliper, as compared to the usual 2 piston brake caliper. 4 piston brake calipers are usually only used for downhill bikes, such as on the Saint or Zee series of components. Recently, 4 piston hydraulic brake calipers were added to Deore XT, but they are actually just rebranded Saint brake calipers, since the shape of the brake calipers look exactly the same. These 4 piston brake calipers are meant for e-MTB, which are heavier due to the additional motor and battery pack.

The good thing is, these Deore XT 4 piston brake calipers are compatible with standard hydraulic brake levers, which are usually used with 2 piston brake calipers. For best performance and best compatibility, the same Deore XT series brake lever should be used. The only trade off of using a 4 piston instead of 2 piston brake caliper is that the brake lever stroke is now slightly longer, as slightly more hydraulic fluid needs to be pushed at the lever side, in order to activate 4 pistons instead of 2.

Deore XT M8000 brake levers!

Tool-free lever reach adjustment, with free stroke adjustment on top.

Clamp band area, with Servo-Wave design.

Bleed port at the top of the master cylinder (right side).

Dimpled aluminium brake lever for extra grip. Not carbon fibre like the top level XTR brake levers.

One side weighs just 106 grams, which is really nice.

As for the brake calipers, I decided to get the powerful Deore XT 4 piston brake calipers. It is claimed that these offer a 20% increase in brake power over the standard 2 piston type.

Deore XT M8020 4 piston brake calipers. Essentially rebranded Saint brake calipers.

It has 4 pistons (2 pairs), of diameter 15 mm and 17 mm. A standard 2 piston (1 pair) type has diameter 22 mm pistons.

The white pistons (ceramic?) can be seen on the inside of the brake caliper.

If you calculate the surface area of the pistons (2 vs 4), you will find that the total area is actually quite similar.

Area of diameter 22 mm piston = PI * (22/2)^2 = 380 mm^2
Area of diameter 15 mm + 17 mm pistons = PI * (15/2)^2 + PI * (17/2)^2 = 404 mm^2

The main contributor to the higher brake power is not extra piston area, it is actually the larger area of the brake pad used in the 4 piston brake caliper. As the 4 piston brake pad is longer and thus has a bigger area than the 2 piston brake pad, it can contact the rotor with a larger surface area for higher braking power.

2 piston brake pad on the left, 4 piston brake pad on the right. The difference in area is very obvious.

Bleeding block for the 4 piston brake caliper.

Brake pads and associated hardware.

Weight of 1 brake caliper plus the brake pads, which is the actual weight when installed.

Brake mount adapters, for the front and rear.

Left side adapter is to convert the front fork IS mount to Post Mount. The right side adapter is to convert 160 mm to 180 mm brake caliper mounting, for the rear.

Front brake adapter weighs 33 grams.

Rear brake adapter weighs 35 grams.

The biggest headache for installing a hydraulic brake system on this cargo bike is the extra long brake hose needed for the rear brake. When you buy a standard brake kit, it comes with a 1000 mm hose for the front brake, and a 1700 mm hose for the rear brake. This is sufficient for standard bikes, but not for the Haul-A-Day with an extra long chain stay length.

If the hose joins to the brake caliper with a normal bolt, it will be easy, as I just need to measure the correct length from the bulk packaging of the brake hose, and cut to the length I need. It does not matter even if the hose is long as the bulk packaging hose length can cover any bike requirements.

Bulk packaging of hydraulic brake hose. Can be cut to any length you need.

However, as the brake hose is mounted to the 4 piston brake caliper using a banjo bolt, I cannot use the hydraulic hose from the bulk packaging box. The banjo bolt is crimped to one end of the hose at the factory, and it is not possible to install it myself on the desired hose length.

The only choice is to buy the extra long hose kit, which is actually the Saint type. Luckily there is an option for a 2000 mm long hose, which should be enough from my estimate. If not, I also have no idea how to make it work.

Extra long hydraulic hose kit, that comes in 2000 mm length for this cargo bike. I can trim off excess length (if any) from the end without the banjo bolt.

The 2000 mm hydraulic hose kit, with all the parts shown here weighs 52 grams.

As you can already tell from the choice of brake adapters shown earlier, I plan to use 180 mm rotors for extra stopping power. Some people prefer to use a smaller rotor at the rear to balance out the braking force, but I prefer to use the same size for simplicity sake.

XTR SM-RT99 brake rotors, in 180 mm diameter.

Each rotor plus lock ring weighs 138 grams, about 15 grams more than the XTR RT99 160 mm brake rotor.

From the choice of all the brake components, you might infer that I want to achieve as much brake power as possible. However, my objective is not to achieve super high brake power, but to enable sufficient braking force, with as little input at the brake lever as possible. In other words, to achieve good (sufficient brake power), while only pulling lightly on the brake lever.

For my set up, brake power will be determined by 3 factors. Wheel size, brake rotor size, and type of brake caliper (2 or 4 piston). This is assuming that all other factors are the same (such as brake lever, rotor and brake pad grade, brake hose grade, tire friction, etc).

Wheel Size:
A smaller wheel will have greater brake power than a bigger wheel, with all other factors being constant.
Comparing a 20" wheel to a 700C (28") wheel, the 20" wheel will have 28/20 = 1.4 times more brake power. This is exactly the phenomenon that I experienced on the Ascent Bolt mini velo, with hydraulic disc brakes on the 20" wheels.

Brake Rotor Size:
A 180 mm diameter brake rotor will be more powerful than a 160 mm brake rotor.
180/160 = 1.125 times more brake power.

Brake Caliper Type (2 or 4 piston):
The 4 piston brake caliper is claimed to be 20% more powerful than the standard 2 piston type, which means it will be 1.2 times more powerful.

Let's set the brake power of the standard bike (700C wheels, 160 mm rotor, 2 piston) as 100%. An example would be the Canyon Endurace with these specifications.

If all these brake enhancing factors are multiplied together, the total comparative brake power will be 1.4 x 1.125 x 1.2 = 1.89 times, or 89% more powerful!

This is a big difference, and I am not too sure if it is too powerful or not. In any case, there are some interference issues which forced me to downsize the brake rotor to 160 mm. The details will be shared during the other blog post on the wheelset and brake caliper installation.

In summary, the final brake rotor used will be 160 mm in diameter. Total brake power is thus 1.4 x 1.2 = 1.68 times, or 68% more, which is still plenty of brake power!

The 160 mm diameter brake rotor that I chose is the new Ultegra RT800 brake rotor, which looks exactly the same as the Dura-Ace RT900 brake rotor, except for no black paint on the cooling fins.

Ultegra RT800 Ice-Tech brake rotor, 160 mm diameter, with Centerlock mounting.

Weighs 133 grams with the lock ring, just 5 grams lesser than the XTR 180 mm rotor.

Size and appearance comparison. Very different design used for these brake rotors.

Final choice of brake components:
Brake Lever: Deore XT BL-M8000
Brake Caliper: Deore XT BR-M8020 4 Piston
Brake Rotor: Ultegra SM-RT800 160 mm
Brake Hose: SM-BH90 High Power brake hose

This collection of brake components should result in really nice brake performance for this cargo bike! Installation of the wheelset and these brake components will be in another blog post.

2 comments:

  1. Hi,

    Can ask where you purchase your
    Shimano brake?

    Online or local bike shop?

    Which online sites or local
    Bike store?

    Thanks

    Rgds

    ReplyDelete
    Replies
    1. I got it before online order was stopped. But now I think you can still get stuff from Merlin Cycles.

      Delete