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working disc brake in action > |
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The do's &
Don'ts for disc brakes > |
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 When you pull
the lever on the disc brake hydraulic fluid is pressurized
by a master cylinder. This pressure is dissipated by moving
another cylinder located in a U- shaped calliper. The
cylinder pushes the brake pads onto the rotor, and friction
is developed. This slows the rotor and the wheel attached to
it. Most mountain bike disc brakes have two pistons, one
either side of the rotor, but now there are a few available
with four pistons.
Hope's
X-C 4 and Shimano's XT. A four piston brake is more powerful
for the simple reasons that it has greater mechanical
advantage but also because it uses a longer pad, which
generates more friction. There are two types of hydraulic
systems used in disc brakes- open and closed.
Closed system:
Like the one
used on Hope's X-C 4 and Giant's MPH, means the position of
the brake pads and the `bite point` can be altered. We like
this feature because it allows us to run a lot of lever
travel before the brake comes on - good for technical
descents where we want to squeeze the levers rather than
grab them. The Hope adjuster is on top of the master
cylinder next to the lever and it turns clockwise to move
the pads closer to the rotor, anti - clockwise moves them
away. It can be used on the move or before descending.
This adjuster
is also handy when setting the brake up for the first time
because it eliminates the drag of the rotor rubbing on the
pads. The down side is there's nowhere for the oil to expand
into when it heats up, apart from pushing against the
pistons. These push the pads against the rotor and lock it
solid this is called ( Pump Up) Backing off the
adjuster creates a space for the expanding oil and while
doing this is not a problem out on the trail, it's bloody
hard during a downhill race.
Open system:
Like Hayes, the
adjuster is replaced by an expansion reservoir. When the
brake heats up the oil expands and pushes up the hose into
the reservoir. Like a safety valve, the problem with open
system brakes is the pads run a little closer to the rotor
so there's more likelihood of them rubbing and the bite
point is actually set close to the beginning of the levers
travel.
The pads are
not adjustable, there are no hard and fast rules on buying
disc systems, but if you're into downhill / dual stuff then
generally an open system makes sense. |
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DISC BRAKES . . DO'S & DON'TS |
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DON'T
Bleed your new
hydraulic system unless it is really necessary, the best
working brakes are often the ones that have never been
touched.
DON'T
Touch the
disc rotor directly after use they get very hot,
remember this when removing a wheel for tyre repairs.
DON'T
Contaminate the
pads or rotors with oil, grease or hydraulic fluid, as
it will drastically reduce braking power. Slight
contamination of pads, try filing top layer off.
DO
Read
manufacturer's fitting instructions, paying attention to
which hydraulic fluid systems to run on.
DO
Give your new
disc brakes a couple of hours riding to bed in, a wet
ride off-road will do the trick with plenty of mud.
DO
Clean the
rotors with an alcohol based disc brake cleaner after
contamination.
DO
Bleed the
system if the action at the lever feels spongy.
Top of page
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1
Disc Rotor |
2
Brake Hose |
3
Calliper Unit |
4
Rotor Bolt |
5
Bleed Nipple |
6
Pad Insert |
7
Fork Adaptor |
8
Mounting Bolt |
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The workings of
a disc brake are based on 3 principals:
1) Leverage
2) Hydraulic multiplication
3) Friction |
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As you
can see in the animation, the hand-force is
multiplied by the brake lever. Next hydraulic
multiplication increases the pressure on the disc
brake pistons. (Size difference between the pistons
from the brake lever and - calliper).
When
the brake-pads are pushed against the rotor, the
friction between these two and the tires and road,
makes your bike slow down or stop.
Top of page |
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FIX IT
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SYMPTOM |
CAUSE |
REMEDY |
Not enough brake power
No pressure point |
Brake not run in
Leaking / air in system |
Bed in use good old
mud
Check hose & connections |
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Bad vibrations when
braking |
Rotor bolts loose,
or calliper bolts loose |
Stop ! check all bolts
are tight,
remove bolts and reinsert with Loctite |
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Brake fluid on pipe |
Leaking connections
or cut to hose |
Check all hose
connections
& tighten if necessary.
Damaged drake hose cut and replace |
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Wheel will not mount
properly |
Lever blade was pulled
when the wheel was out |
Push back pads with
tool part
Or use a flat head screwdriver |
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Poor braking |
Contaminated rotor or
disc pads.
Disc pads maybe worn out |
Clean disc rotor
with degreaser. Replace worn out pads with new.
Replace contaminated pads with new |
Rubbing sound coming
from the rotor
when applying the brake,
or even free wheeling. |
Rock damage or bending
the rotor when removing or replacing the wheel.
Or heat build-up warping rotor. |
Check rotor disc for
alignment, rotor disc maybe out of true.
Remove and straighten |
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FROM NOW ON ! |
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Do not
contaminate rotors with oily hands.
keep a
check on the rotor bolts.
Check for
pad wear every couple of months.
Check for
pad wear after a long downhill ride.
Check pad
wear after wet/muddy rides.
Don't let
cables get fouled on the bike. |
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'THE BEST PLACE FOR CYCLING PARTS . . . .
. ON THE INTERNET'
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