Lotus 2004 Eleven Service Notes page 248

Lotus Service Notes
JJ.1 - GENERAL DESCRIPTION
The Lotus Elise, when fitted with anti-lock brakes, comprises ventilated discs all round with AP Racing 2-
piston fixed callipers on the front and Brembo single piston sliding callipers on the rear. A tandem master
cylinder, with vacuum servo assistance, operates the callipers via a front/rear split hydraulic circuit with anti-
lock control by a Kelsey-Hayes microprocessor based system. The centrally mounted parking brake lever,
operates the rear calliper pistons by control cables.
The AP Racing front callipers, have lightweight alloy bodies housing two opposed pistons, and are mounted
via two bolts directly to the rear of the forged steel hub carriers. The rear brakes use Brembo cast iron, single
piston callipers, sliding on pins mounted directly to the forged steel hub carriers. A ventilated cast iron brake
disc with curved internal vanes and cross drilling is common to all four wheels, and is sandwiched between the
road wheels and hubs, retained for convenience by a single countersunk screw.
The tandem master cylinder is made by FTE, and incorporates a front section to supply both front brakes,
and a rear section to supply the rear brakes. A translucent fluid reservoir is mounted on a bracket above the
master cylinder, and has separate compartments connected to the front and rear cylinder sections by flexible
hose. A third hose connects the rear reservoir compartment with the clutch master cylinder to supply that
system's needs. A fluid level sensor in the filler cap will light a fascia tell tale lamp if the level becomes
dangerously low.
The parking brake ratchet lever is mounted between the seats and uses a short primary cable and balanc-
ing yoke to actuate a single control cable linked to each rear calliper. Adjustment of the calliper mechanism to
compensate for brake pad wear is automatic. The parking brake should be applied by pulling up the lever with
high effort, and engaging the highest ratchet setting attainable. When parking the car on a slope, the additional
precaution should be taken of leaving the transmission in first or reverse gear and steering the wheels towards
the kerb. If the parking brake is applied when the brakes are hot (e.g. after prolonged or vigorous braking),
special care should be taken to ensure that the parking brake is securely engaged in order to allow for any
potential affect on brake performance as the discs cool.
The braking system is designed to enhance brake performance during high speed driving, with good fade
and pad wear characteristics, and have a higher friction level when heated to normal working temperature than
when cold. Required pedal effort will reduce as cold brakes become heated to normal working temperature,
and the braking efficiency will increase significantly as new discs or pads become 'bedded in'. After fitting new
brake components, maximum braking efficiency will be achieved if, for the first few hundred miles, needless
heavy braking is avoided, and the brake pads are allowed to 'bed in' fully before being used to their full
potential.
A Kelsey-Hayes Antilock Brake System (ABS) is used to optimise brake performance in extreme condi-
tions and reduce the tendency of any wheel to lock up. Under most conditions, the maximum braking force is
provided by a wheel which is rotating at about 90% of road speed. Apart from the likelihood of increasing the
stopping distance, a locked wheel provides no steering force, such that with both front wheels locked, move-
ment of the steering wheel has no effect on vehicle direction. With the antilock system, even panic braking
results in controlled deceleration and the retention of steering response and is especially advantageous when
braking on slippery road surfaces and in bad driving conditions. The ABS control system is self monitoring and
has the capability of switching itself off if a fault is detected, allowing the base brake system to operate without
anti-lock control.
Under normal circumstances, the hydraulic power brake system of the vehicle operates without input
from the ABS, with brake pressures governed by the force applied to the brake pedal. The ABS microprocessor
receives signals from wheel speed sensors at each of the four wheel hubs, and interprets this data to determine
if any wheel is tending to lock up. If imminent lock up is determined, the microprocessor commands solenoid
valves in an electro-hydraulic unit to reduce the pressure in that particular brake circuit in order to restore wheel
speed to that providing the maximum braking force consistent with continued wheel rotation.
When the ABS is operating, indication to the driver is provided by a 'pulsing' sensation felt at the brake
pedal as fluid is pumped between the master cylinder and hydraulic control unit, and also by audible clicking of
the relays and switches. These signals indicate to the driver that maximum retardation is being approached,
and that driving style should be modified to suit the conditions. The minimum stopping distance is achieved by
applying the brakes firmly and steadily, and allowing the ABS to modulate hydraulic pressure. The driver
should not attempt to emulate this process by 'pumping' the brake pedal, as modulation at the pedal will treat
all four wheels similarly, rather than the individual wheel control allowed by the electronics.
During ABS operation, the wheels may appear to lock momentarily as the wheel speed changes rapidly,
and some tyre noise (intermittent screeching) may be heard. This noise is normal and will vary with road and
Section JJ
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