Brake Force And Vehicle Stability; Wheel Slip And Hydraulic Control - Yamaha R1 Series 2015 Service Manual

• Slip ratio:
When the brakes are applied, slipping occurs between the tires and the road surface. This causes a
difference between the wheel speed and the chassis speed.
Slip ratio is the value that shows the rate of wheel slippage and is defined by the following formula.
Slip ratio = (Chassis speed – Wheel speed)/Chassis speed
0 %: There is no slipping between the wheel and the road surface. The chassis speed is equal to the
wheel speed.
100 %: The wheel speed is "0", but the chassis is moving (i.e., wheel lock).

Brake force and vehicle stability

When the brake pressure is increased, wheel speed is reduced. Slipping occurs between the tire and
the road surface and brake force is generated. The limit of this brake force is determined by the friction
force between the tire and the road surface and is closely related to wheel slippage. Wheel slippage is
represented by the slip ratio.
Side force is also closely related to wheel slippage. See figure "A". If the brakes are applied while keep-
ing the proper slip ratio, it is possible to obtain the maximum brake force without losing much side force.
ABS allows full use of the tires' capabilities even on slippery road surfaces or less slippery road surfac-
es. See figure "B".
A
a
c
a. Friction force between the tire and road sur-
face
b. Brake force
c. Side force
d. Slip ratio

Wheel slip and hydraulic control

The ABS ECU calculates the wheel speed of each wheel according to the rotation signal received from
the front and rear wheel sensors. In addition, the ABS ECU calculates the vehicle chassis speed and
the rate of speed reduction based on the wheel speed values.
The difference between the chassis speed and the wheel speed calculated in the slip ratio formula is
equal to the wheel slip. When the wheel speed is suddenly reduced, the wheel has a tendency to lock.
When the wheel slip and the wheel speed reduction rate exceed the preset values, the ABS ECU de-
termines that the wheel has a tendency to lock.
If the slip is large and the wheel has a tendency to lock (point "A" in the following figure), the ABS ECU
reduces the hydraulic pressure in the brake caliper. Once the ABS ECU determines that the tendency
of the wheel to lock has diminished after the hydraulic pressure is reduced, it increases the hydraulic
pressure (point "B" in the following figure). The hydraulic pressure is initially increased quickly, and then
it is increased gradually.
b
d
(%)
100 (%)
B
a
e. Less slippery road surface
f. Controlling zone
g. Slippery road surface
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FEATURES
e
f
g
d
(%)