82 PERFORMANCEBIKES.CO.UK|MARCH2018BRAKE MOD WISDOM
more power for less effort thanks to the hydraulic
multiplier effect, and its longer stroke means greater
lever travel across which to exploit the available range
of braking power. But it doesn’t take long for the
practical maximum to be exceeded as the stroke soon
becomes so great that the lever is back at the bar before
full braking force is realised because of the volume of
fluid that has to be shifted.
Hence, while a small a master cylinder is capable of
pushing enough fluid for a single-caliper small piston
area set-up, you’ll need a larger one for practical use
with a multi-piston, dual-caliper arrangement.Putting it all together
Bike manufacturers are always looking at compromise.
The usual thing is to err on the side of smaller bore
master cylinders for a longer stroke and more power for
the same or less effort than would be required with a
larger bore. Key to the calculation of the MC size is the
volume of fluid that will actually need to be moved for
the caliper pistons to force the pads into contact with
the discs. Typically the pads will need to move only a
millimetre or so. Therefore the whole volume of the
pistons is not considered, rather just the total area of
the pistons multiplied by the pad gap. Usually, caliper
pistons are around 30 to 36mm each, while master
cylinders range from 13 to 20mm.
Then there’s a further variable – the issue of leverage
dictated by the fulcrum or pivot point of the lever,
which takes us on to different types of MCs themselves.Radial departure
Back in 1985, Brembo patented the first radial master
cylinder design, primarily motivated by racing
requirements of less space and weight with the same orbetter efficiency as an axial master cylinder. The
following year, Eddie Lawson and his YZF Yamaha
equipped with a Brembo radial MC were 500GP world
champions. The concept evolved and eventually arrived
on road bikes with the 2002 Aprilia RSV1000. Today,
the radial master cylinder is pretty much standard
fitment for serious performance bikes.
The radial master cylinder we are now so familiar
with has the piston axis parallel to the pull of the lever
and perpendicular to the handlebar. Effectively, the
direction of the rider’s hand movement is the same as
that of the piston so there are lower mechanical and
friction losses compared to the axial design.
British braking firm AP, now a subsidiary of Brembo,Better modulation
‘The radial master cylinder design
evolved from Eddie Lawson’s 1986 GPbike. It arrived on road bikes in 2002...’
Lessening the effective bore via the adjustable pivot on this ISR set-up gives a longer lever
stroke. This translates into better modulation of braking force across a wider range of
movement. However, it does mean that under very hard use you’ll find the lever comes back
to the bars sooner, and will require adjustment to the pivot to increase the effective bore.Pivotdeveloped an adjustable master cylinder when
carbon-fibre discs first appeared on race bikes, to meet
the varying demands of racers when It came to getting
the right amount of ‘feel’ or ‘modulation’ to prevent
them landing on their arses when they hit the brakes.
Today, across three different models, AP are able to
offer equivalent bore sizes from 24.38mm down to
11.79mm. The CP4125-29 goes from 21.16 to 14.60mm
giving it a good range of adjustment for the typical
dual-disc road bike. There are models either side of this
offering larger and smaller effective bores. We say
‘effective’ because the actual diameter of the bore
doesn’t change when you fiddle with the masterPISTON Hydraulic forceInput force