Performance Bikes — March 2018

MARCH 2018|PERFORMANCEBIKES.CO.UK 81

THE GARAGE / MASTER CYLINDERS

Radial master cylinder

On an aftermarket radial master cylinder, not only are frictional and mechanical losses

mitigated by the fact that input force travels in the same direction as hydraulic force, but the

pivot is also adjustable without recourse to a toolkit (although this Magura system doesn’t

have the easiest to operate adjuster if you happen to be wearing gloves at the time).

Input force

Pivot

Hydraulic force

ends of the set-up. The job of your master cylinder is to

convert the force of the lever into fluid pressure. You

might think a larger bore MC would generate more

pressure, but as we’ve seen above the opposite is true

for the same lever force; a smaller bore creating more

pressure. This also impacts on the stroke of the master

cylinder. Less stroke is required with a large bore

master cylinder to move the same amount of fluid and

more stroke is needed to move the equivalent volume in

a smaller bore. However, in the case of a big-bore/

short-stroke MC, more effort force is required to

generate the same amount of pressure than a small

bore/long-stroke system. Put another way, it requires

less force on the lever of a smaller bore master cylinder

to produce the same amount of pressure in the line as it

would in a large bore one, however the volume shifted

is smaller. You can see it in the table (left) supplied by

AP. With the CP4125-26 radial master cylinder set to an

equivalent bore of 24.38mm it requires 85.6N of

pressure at the lever end to generate 250psi in the line.

Spin the adjuster out to an equivalent bore of 16.7mm

and you need only 40.2N on the lever to get 250psi, but

a longer lever stroke.

Don’t fight it, feel it

It’s the combination of these factors that contribute to

that most ineffable of qualities – braking feel.

Too big a bore at the master cylinder end and the feel

becomes increasingly wooden and less predictable. This

isn’t because they’ve made the brakes stiffer; it’s down

to the fact that the greater force required, along with

the shorter stroke required to push the pads into contact

with the discs, gives the impression of stiffness.

A quality we greatly value when it comes to braking

hard but in control is modulation. An on/off brake with

minimal lever travel makes modulation all but

impossible. A smaller bore master cylinder gives us

Number of
turns on ratio
adjuster wheel
towards
handlebar
0

5

10

2

7

12

1

6

11

3

8

13

4

9

14

15

Tot al lever

ratio at typical

working

stroke

6.88 : 1

8.42 : 1

10.72:1

7.43:1

9.23:1

11.99:1

7.14 : 1

8.81 : 1

11.32:1

7.74:1

9.68 : 1

12.73 : 1

8.06:1

10.18 : 1

13.56:1

14.45 : 1

Equivalent

cylinder bore size

for 9.4/1 lever ratio

24.38

22.04

19.53

23.46

21.05

18.46

23.93

21.54

19.00

22.99

20.55

17.92

22.51

20.04

17.36

16.70

Load (N) at bend

of lever to achieve

250psi pressure

85.6

69.9

54.9

79.3

63.8

49.1

82.5

66.8

52.0

76.1

60.8

46.3

73.0

57.9

43.4

40.2

AP CP4 125-26

AP Racing

CP4125-26 radial

master cylinder

PISTON

AP’s calculations of lever ratio vs equivalent bore ratio

and input required to achieve 250psi line pressure. As

the effective bore lessens, so does the effort required