Performance Bikes — March 2018

MARCH 2018|PERFORMANCEBIKES.CO.UK 83

For more modulation...

youneedasmallerpiston

For less input vs power...

youneedasmallerpiston

To maintain power under

sustained hard braking...

you need a larger piston

With greater caliper

piston area...

you need a larger piston

Thanks to

AP RACINGapracing.com

VENHILLvenhill.co.uk

CHANGES

MADE

SIMPLE

THE GARAGE / MASTER CYLINDERS

cylinder – what does change is the leverage ratio
between the brake lever and the piston. On the AP, the
piston is a pull rather than the more familiar push type.
By making the lever’s pivot adjustable, the pivot point
can be moved nearer or closer to the piston. If the pivot
point is moved away from the piston towards the
handlebar end, there is less lever travel for a given
volume of fluid. This reduces leverage while increasing
the quantity of fluid pushed down the line which,
recalling what we talked about in the section on
hydraulic multiplier effect, is the same as having a
larger bore size. On the flip side, if the pivot is moved
close to the piston, leverage is increased, which is the
same as having a reduced piston size.
The Brembo RCS Kar fitted has a push-type piston
but also offers alternative ‘effective’ bore sizes by
changing the pivot point. The Brembo has a choice of
two distances between the lever fulcrum point and the
piston – 18 and 20mm. This model master cylinder
combines the virtues of two MCs offered by Brembo –
the 19 x 18 (19 representing the bore in mm, 18 being
the pivot distance) and the 19 x 20. The 20 variant
offers more instant braking effect while the 18
promises smoother progression.

So what do I want?
The luxury of having an adjustable master cylinder to
suit our set-up and riding style is a nice one to have.
However, if you are simply looking to match a master
cylinder to different calipers then the rule of thumb is
that the more piston area you have, the larger the
master cylinder you require. The issue is not the
number of pistons but their area, and more precisely
the volume that has to be shifted to put the pads firmly
into contact with the discs. For example, a single disc,

Brick-wall braking

small two-piston caliper set-up on a trailie might deliver

the goods with a 12 or 13mm conventional master

cylinder, but for a dual-disc, big multi-piston caliper

arrangement you’ll want to be getting closer to 20mm

or maybe even more.

With those parameters in mind, you can tailor your

bike’s set-up to suit both the hardware and your own

firmness of fist.

Joe Bennett, marketing engineer at AP Racing says:

“It’s all about displacement. The smaller the bore the

less fluid is moved but the longer the travel. The issue

arises when the brakes get hot and everything expands;

the smaller cylinder cannot shift enough fluid, hence a

long lever and no brakes...

“For example, the original Yamaha YZF-R1 had a

14mm master cylinder, to give riders that ‘two-finger’

braking feel. The problem was when these bikes took to

the track this small-bore cylinder couldn’t cope when

the brakes got hot. We sold many CP4215-26s for these

• as well as all the other bikes with small cylinders –
  which solved this problem. Yamaha also fixed it by
  fitting a larger bore 16mm master cylinder later on.
  “The best mix of ‘feel’ and travel on race bikes is
  typically with a 19mm bore and it is also able to shift
  enough fluid when the brakes get hot.”
  Bennett offers the following advice on selecting a
  radial replacement for a regular master cylinder:
  “There is no absolute rule for a bike cylinder size. You
  only have work out if the cylinder has enough volume
  to shift fluid when the brakes are hot but soft enough to
  give the rider some feel. The benchmark figures for
  twin-disc front set-ups for radial master cylinders are
  19mm or 22mm bore for race bikes and 16mm for road
  bikes. If you find yourself running out of brakes under
  sustained hard braking then you need a larger bore.”

With the effective bore of the master cylinder increased, less stroke (and therefore

input force) is required in order to move the equivalent volume of hydraulic fluid.

This translates into light-switch braking performance, but at the expense of fine

modulation of a rider’s braking input.

Input force

Pivot

PISTON Hydraulic force