MX Restoration
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Getting the most out of drum brakes
Vintage racers have no choice but to use the stock type drum brakes. Though our
old bikes are fast, light and turn well, they just won’t stop like a new one. Here are
some ideas that when applied will make you have new faith in those old drums.
Basic principal of mechanical drum brakes:
Hey, we know how they work. Two shoes with friction material on hopefully the
same radius as the drum surface. When the cam turns the shoes press against the
drum and the bike stops. Why do some work much better than others? What the
hell is brake fade?
There are ways to improve your vintage brakes.
Number 1 is CLEAN CLEAN CLEAN! NEVER use petroleum solvents on shoes or
drums. Use only brake parts cleaner. Even paint thinners and other solvents leave
residue.
Spray the drum so the cleaner flushes out of it. Allow to dry; don’t wipe them.
Never touch shoes or the drum surface with your fingers after cleaning. Even a
fingerprint can damage a brake drum. The oils in skin can cause that spot to wear
slower than the rest of the drum. Over time you’ll have brakes that pulse.
Number 2 is friction material selection. A local clutch and brake shop will have a
large selection of brake lining material and can bond it to your shoes for a modest
cost. Industrial supply houses such as McMaster Carr sells it in sheets and in
various thickness and coefficient of friction ratings. The key here is sizing the
radius of the shoe to your individual drum. Since most motorcycle shoes are cast
and are not flexible the shoes may have to be mounted to the backing plate and
machined in a lathe.
Number 3 is work to prevent “fade”. Many things can cause fade. Most believe it’s
heat. Heat is not the problem it’s the cause of other problems. First and foremost
when the temperature in an enclosed drum brake rises gases are released from
the friction material and produce a barrier between the shoes and drum and the
shoe starts floating like an air hockey puck. The harder you apply the brakes the
worse it gets. That makes for a bad day. Another common cause of fade is the
accumulation of brake dust. Like tiny ball bearings the dust particles get between
the shoes and drum and reduce stopping power. Road racers can vent the
backing plates and scoop air onto the drums but mx racers can’t because of dirt,
rocks and so on. The simplest way to reduce these problems is to put grooves in
the shoes.
The NASCAR stock cars and road race cars such as the Corvette Grand sport of
the '60s had brake shoes with four pucks on them with about a quarter inch
between them rather than one continuous piece of friction material. This gives
ample area for gases and dust to escape. Over adjusting can lead to quick failure
as can inadequate return spring tension. It's best to have a return spring on the
actuator lever itself rather than just rely on the internal springs on the shoes. Any
drag at all will lead to quick fade. Learn more about fade here
Number 4 is float those backing plates.
Two types of drum brakes are common to vintage MX bikes.
Single leading shoe and double leading shoe.
Single Leading Double Leading
The leading shoe is the one on the side of the cam in the direction of the drum
rotation. The leading shoe is actually drawn into the drum by it's own friction and
the other shoe is being pushed back into the cam and offers little stopping power.
The double leading concept puts both shoes to work. This too has limitations. Both
single and double systems rely on the chance that everything is machined perfect
and the shoes are perfectly concentric to the drum. As mentioned earlier, turning
the shoes to the correct diameter on a lathe by mounting from the backing plate
center hole is best you can do. Or is it? Remember the full floating rear brakes of
the late '70s? The strut used to prevent backing plate rotation was connected to
the frame and the backing plate would rotate on a bushing that was through the
center. Connecting the strut to the frame was unnecessary. The true advantage
came from the fact that the backing plate was loose and was able to move and self
center when the brakes were applied.
More to come..........
Vintage racers have no choice but to use the stock type drum brakes. Though our
old bikes are fast, light and turn well, they just won’t stop like a new one. Here are
some ideas that when applied will make you have new faith in those old drums.
Basic principal of mechanical drum brakes:
Hey, we know how they work. Two shoes with friction material on hopefully the
same radius as the drum surface. When the cam turns the shoes press against the
drum and the bike stops. Why do some work much better than others? What the
hell is brake fade?
There are ways to improve your vintage brakes.
Number 1 is CLEAN CLEAN CLEAN! NEVER use petroleum solvents on shoes or
drums. Use only brake parts cleaner. Even paint thinners and other solvents leave
residue.
Spray the drum so the cleaner flushes out of it. Allow to dry; don’t wipe them.
Never touch shoes or the drum surface with your fingers after cleaning. Even a
fingerprint can damage a brake drum. The oils in skin can cause that spot to wear
slower than the rest of the drum. Over time you’ll have brakes that pulse.
Number 2 is friction material selection. A local clutch and brake shop will have a
large selection of brake lining material and can bond it to your shoes for a modest
cost. Industrial supply houses such as McMaster Carr sells it in sheets and in
various thickness and coefficient of friction ratings. The key here is sizing the
radius of the shoe to your individual drum. Since most motorcycle shoes are cast
and are not flexible the shoes may have to be mounted to the backing plate and
machined in a lathe.
Number 3 is work to prevent “fade”. Many things can cause fade. Most believe it’s
heat. Heat is not the problem it’s the cause of other problems. First and foremost
when the temperature in an enclosed drum brake rises gases are released from
the friction material and produce a barrier between the shoes and drum and the
shoe starts floating like an air hockey puck. The harder you apply the brakes the
worse it gets. That makes for a bad day. Another common cause of fade is the
accumulation of brake dust. Like tiny ball bearings the dust particles get between
the shoes and drum and reduce stopping power. Road racers can vent the
backing plates and scoop air onto the drums but mx racers can’t because of dirt,
rocks and so on. The simplest way to reduce these problems is to put grooves in
the shoes.
The NASCAR stock cars and road race cars such as the Corvette Grand sport of
the '60s had brake shoes with four pucks on them with about a quarter inch
between them rather than one continuous piece of friction material. This gives
ample area for gases and dust to escape. Over adjusting can lead to quick failure
as can inadequate return spring tension. It's best to have a return spring on the
actuator lever itself rather than just rely on the internal springs on the shoes. Any
drag at all will lead to quick fade. Learn more about fade here
Number 4 is float those backing plates.
Two types of drum brakes are common to vintage MX bikes.
Single leading shoe and double leading shoe.
Single Leading Double Leading
The leading shoe is the one on the side of the cam in the direction of the drum
rotation. The leading shoe is actually drawn into the drum by it's own friction and
the other shoe is being pushed back into the cam and offers little stopping power.
The double leading concept puts both shoes to work. This too has limitations. Both
single and double systems rely on the chance that everything is machined perfect
and the shoes are perfectly concentric to the drum. As mentioned earlier, turning
the shoes to the correct diameter on a lathe by mounting from the backing plate
center hole is best you can do. Or is it? Remember the full floating rear brakes of
the late '70s? The strut used to prevent backing plate rotation was connected to
the frame and the backing plate would rotate on a bushing that was through the
center. Connecting the strut to the frame was unnecessary. The true advantage
came from the fact that the backing plate was loose and was able to move and self
center when the brakes were applied.
More to come..........
