Wet disc brake

A brake pad includes a friction material with a braking surface to contact a rotor disc. The braking surface has a leading edge relative to rotation of the rotor disc, an opposed trailing edge, and inner and outer circumferentially extending edges joining the leading and trailing edges. A plurality of grooves are formed in the friction material and open onto the braking surface. The grooves are angled from a leading end of the grooves rearwardly relative to a radial line and are curved from their leading end to their trailing end, and form substantially the arc of a circle when viewed in a direction perpendicular to the braking surface.

The present invention relates to improvements in disc brake assemblies and 
components, particularly discs and friction pad configurations. 
Conventional vehicle brakes most commonly comprise a rotor disc with a pair 
of friction material pads respectively acting on opposed braking surfaces 
of the rotor disc in a dry environment. Such systems have a number of 
significant disadvantages in that both the disc and particularly the 
friction material pads wear during use allowing wear particles to be 
released into the atmosphere which is polluting and may well be a health 
risk. Moreover the performance of such brakes is not uniform and varies 
considerably over time, particularly, as a result of friction pad wear and 
external factors such as whether water or other liquids get onto the 
braking surfaces during use. 
Forms of wet disc brake are also known where a plurality of friction 
material discs are provided within a sealed brake housing containing oil. 
Such wet disc brakes have usually utilised oils of 7-8 or higher 
centistokes mm.sup.2 /sec at 100.degree. C. viscosity. Such oils normally 
also had a low viscosity index of 100 which caused substantial viscosity 
alterations as the oil temperature changed. The discs are provided such 
that some carrying grooved friction material rotate and other plain discs 
are stationary and braking effect is achieved by axially forcing the discs 
together. The oil is provided primarily as a coolant and for this purpose 
oil circulates through the grooves in the rotating discs. Oil of 7-8 (or 
higher) centistokes was, however, considered necessary to promote 
hydrodynamic lubrication and resistance to pressure "squeeze out" in the 
interengaged friction zone to suppress stick slip, wear, and localised 
overheating of the brake friction discs and plates. The drawback of 
utilising hydrodynamic lubrication regime is that a lower friction 
coefficient results in the interengaged region. Thus, because these oils 
cause an adverse effect on friction levels, it is necessary to have the 
much greater braking surface area provided by the plurality of discs. 
Moreover the configuration of these systems are generally considerably 
different to those of the dry friction systems described in the preceding 
paragraph making it difficult to interchange the respective assemblies. 
In an earlier filed International Patent Application No. PCT/AU92/00540, 
there is also disclosed a brake assembly for a vehicle of the type 
comprising a single rotor disc adapted to rotate with an axle end and a 
stationary housing enclosing the rotor disc with one or more seals acting 
to provide a substantially sealed but pressure equalised zone surrounding 
at least a brake effect region of said rotor disc. At least one pair of 
friction pads is provided adapted to be engaged against opposed braking 
faces of a peripheral zone of the rotor disc, and the housing is formed so 
that it can contain a liquid in at least part of the braking zone. In one 
part of its disclosure, the aforesaid International Application indicates 
the liquid may be maintained at a level above the seals so as to prevent 
ingress of contaminants Into the interior of the housing and particularly 
in the area of the braking surfaces and friction pads. The aforesaid 
International Patent Application further discloses friction pads including 
backing plates with friction material secured thereto, the friction 
material having rotor disc engaging faces with one or more grooves formed 
therein of particular configuration. The aforementioned brake arrangement 
will, however, benefit from a minimum of liquid or lubrication and the 
grooves are provided for the purpose of oil skimming and draining from the 
braking surfaces in contrast to other oil immersed brake systems where 
grooves are provided to assist with oil circulation. 
The objective of the present invention is to provide improved friction 
material products, particularly brake pads and other components, capable 
of use in brake assemblies of the type shown in the aforesaid 
International Patent Application. 
A further preferred objective is to provide an improved brake arrangement 
capable of advantageously using the aforesaid friction material products 
and other brake assembly components. It is also a preferred objective that 
the brake arrangement according to the present invention be such that it 
is relatively easily interchangeable with the dry friction disc brake 
assemblies now commonly in use. 
With regard to the friction material products, it is recognised in the 
aforesaid International Patent Application that the friction material 
needs to be provided with skimming and drainage grooves to enable liquid 
removal from the braking surfaces, however, the general positioning of 
these grooves has been found to be important in relation to braking 
performance. Moreover, it has been found that optimum performance is 
generally a balance between the number and width of grooves relative to 
the remaining friction surface areas or lands between the grooves. That 
is, there must be sufficient land areas remaining to achieve desired 
braking characteristics. Used throughout this specification "leading" and 
"trailing" or similar have reference to the brake pad and the direction of 
rotation of the rotor disc. 
According to a first aspect of the present invention, there is provided a 
brake pad including a friction material having a braking surface adapted, 
in use, to contact a rotor disc of a brake assembly, said braking surface 
having a leading edge relative to rotation of said rotor disc, an opposed 
trailing edge, and inner and outer circumferentially extending edges 
joining said leading and trailing edges, said friction material further 
including a plurality of grooves formed in said friction material with 
said grooves opening onto said braking surface and being angled from a 
leading end of the grooves rearwardly relative to a radial line, at least 
one of said grooves having a said leading end opening onto the leading 
edge of said braking surface and exiting from said friction material at 
either said trailing edge or said outer circumferential edge, said at 
least one of said grooves having a widened mouth portion at the leading 
edge of said braking surface and a zone of decreasing width inwardly from 
said leading edge of the braking surface. Conveniently, the leading edge 
is substantially radially disposed in use. The trailing edge may also be 
radially disposed in use for the sake of convenience In manufacture. 
Preferably, at least two said grooves are provided with a leading end 
opening onto the leading edge of the friction material. Conveniently, each 
said groove having a leading end opening onto the leading edge has a 
substantially uniform width from said zone of decreasing width to its exit 
from the friction material. Conveniently, a plurality of said grooves are 
provided spaced over the length of the brake pad from the leading edge to 
the trailing edge with said grooves being equally spaced. In one preferred 
arrangement, the leading ends of the grooves having leading ends located 
rearwardly of the leading edge of the brake pad, are located radially 
outwardly of the Inner peripheral edge of the friction material, each of 
these grooves having trailing ends opening either on the outer peripheral 
edge or the trailing edge of the brake pad friction material. In a second 
preferred arrangement, the leading ends of the grooves having leading ends 
located rearwardly of the leading edge of the friction material of the 
brake pad are located opening onto the inner peripheral edge of the 
friction material. Conveniently, each of the grooves are substantially 
parallel to one another and are curved when viewed perpendicular to the 
braking surface, the curvature being such as to present a convex side of 
the grooves towards the leading edge of the friction material. 
The advantage that is achieved by arranging one or more grooves leading 
from a leading edge of the pad friction material and exiting from 
preferably the outer peripheral edge is that an improved flow of liquid 
away from the braking surface region is achieved and thereby Improved 
braking performance particularly at low clamp pressures. Generally, it is 
believed the amount of oil on a rotating disc will increase as the radial 
distance increases from the axis of rotation. Thus, with the leading edge 
substantially radially disposed, the area with greatest liquid levels 
(i.e. further away from the axis of rotation) has the shortest groove and 
liquid will thus flow more quickly so that any increased volume of liquid 
is balanced by a shorter exit duct or groove. If the grooves do not open 
onto the inner peripheral edge their wear debris may be centrifugally 
deposited at this edge and cause scoring of the rotor disc braking 
surfaces. Allowing liquid to flow through this edge tends to eliminate 
rotor scoring and improve the useable life of the brake assembly as well 
as making braking more predictable. 
In accordance with a further aspect of the present invention, there is 
provided a brake pad including a friction material having a braking 
surface adapted, in use, to contact a rotor disc of a brake assembly, said 
braking surface having a leading edge relative to rotation of said rotor 
disc, an opposed trailing edge, and inner and outer circumferentially 
extending edges joining said leading and trailing edges, said friction 
material further including a plurality of grooves formed in said friction 
material with said grooves opening onto said braking surface and being 
angled from a leading end of the grooves rearwardly relative to a radial 
line, at least one of said grooves opening onto the leading edge of said 
braking surface and said grooves further being curved from their leading 
end to their trailing end with a convex side of said grooves facing 
towards the leading edge of the braking surface, said grooves further 
forming substantially an arc of a circle when viewed in a direction 
perpendicular to the braking surface. Conveniently, lines tangential to 
the groove at the inlet end and at the outlet end form substantially equal 
angles with the inner and outer circumferential edges. Preferably, the 
grooves are substantially parallel. Preferably the aforesaid substantially 
equal angles have an angle between 30.degree. and 40.degree.. 
Conveniently, the angle is about 35.degree.. Preferably, the grooves all 
have their inlet ends on a leading edge or the inner peripheral edge of 
the friction material and have their outlet ends on either the outer 
peripheral edge or a trailing edge of the friction material. 
In accordance with a particularly preferred embodiment, the brake pad has a 
plurality of primary grooves formed as aforesaid with one or more 
secondary grooves intersecting the primary grooves at an acute angle 
thereto. Conveniently, the secondary grooves have inlet ends opening onto 
the leading edge of the friction material. Conveniently, friction material 
between the inlet end of a primary groove and a secondary groove at the 
leading edge is at least partially removed to provide a mouth region of 
decreasing width as it approaches a first intersection point between the 
primary and secondary grooves. Conveniently, the aforesaid first 
intersection point has a throat width greater than that which would be 
formed by intersection between the primary and secondary grooves. 
Preferably the aforesaid primary grooves have a width between 1.5 and 4 mm, 
preferably 2 mm. Conveniently, the area of friction material land left 
after forming the grooves is between 60 and 80% of the braking surface 
area without grooves. Preferably the land area is about 75% of the braking 
surface area without grooves. Conveniently, the land width between primary 
grooves is about 5 to 6 mm which ensures sufficient mechanical strength. 
In accordance with a further aspect of the present invention, a brake 
arrangement for a vehicle is provided adapted to be connected to an axle 
end, said arrangement comprising a single rotor disc adapted, in use, to 
rotate with said axle end and a stationary housing enclosing said rotor 
disc with one or more seals to provide a substantially self contained zone 
surrounding at least a braking effect region of said rotor disc, at least 
one friction pad located adjacent a braking surface of said rotor disc in 
said braking effect region so as to operationally engage said braking 
surface during a vehicle braking operation and liquid filling means 
provided to permit at least part of said substantially self contained zone 
to be filled with a liquid medium, said brake arrangement being 
characterised in that said braking surface is radially disposed and 
annular in shape having an inner circumferential edge and an outer 
circumferential edge, the inner circumferential edge defining a 
circumferentially extending liquid collection region adapted to collect 
liquid moving outwardly on surfaces of said rotor disc inwardly of inner 
circumferential edge and at least one outwardly directed passage leading 
from said liquid collection region to an outer periphery of said rotor 
disc. 
Conveniently said liquid collection region is located axially inwardly of 
an overhang lip forming the inner circumferential edge of the braking 
surface. Preferably a plurality of said passages are provided leading 
radially outwardly through the body of said rotor disc. Conveniently, 
opposed radially directed annular braking surfaces are provided on either 
side of the rotor disc. 
In accordance with a still further aspect of the present invention, a brake 
arrangement is provided for a vehicle adapted to be connected to an axle 
end, said arrangement comprising a single rotor disc adapted, in use, to 
rotate with said axle end and a stationary housing enclosing said rotor 
disc with one or more seals to provide a substantially self contained zone 
surrounding at least a braking effect region of said rotor disc, at least 
one friction pad located adjacent a braking surface of said rotor disc In 
said braking effect region so as to operationally engage said braking 
surface during a vehicle braking operation and liquid filling means 
provided to permit at least part of said substantially self contained zone 
to be filled with a liquid medium, said brake arrangements being 
characterised by heat exchange means to alter the temperature of said 
liquid medium within said housing to maintain said temperature within a 
predetermined range. 
In accordance with a still further aspect, the present invention provides a 
rotor disc for use in a vehicle brake assembly which in use contains 
liquid through which said rotor disc passes, said rotor disc being 
characterised by at least one radially disposed annular braking surface 
having a radially inner circumferential edge and a radially outer 
circumferential edge, the inner circumferential edge defining a 
circumferentially extending liquid collection region adapted to collect 
liquid moving outwardly on surfaces of said rotor disc inwardly of inner 
circumferential edge and at least one outwardly directed passage leading 
from said liquid collection region to an outer periphery of said rotor 
disc. 
By the brake arrangements described above, it is possible to achieve a 
minimisation of liquid on the braking surfaces adjacent the friction pad 
or pads when a braking operation is required to be carried out. 
In accordance with a still further aspect of the present invention, a brake 
arrangement for a vehicle is provided adapted to be connected to an axle 
end, said arrangement comprising a single rotor disc adapted, in use, to 
rotate with said axle end and a stationary housing enclosing said rotor 
disc with one or more seals to provide a substantially sealed zone within 
at least part of said stationary housing such that at least a braking 
effect region of said rotor disc, in use, passes through said 
substantially sealed zone, at least one friction pad located adjacent a 
braking surface of said rotor disc in said braking effect region so as to 
operationally engage said braking surface during a vehicle braking 
operation and liquid filling means provided to permit at least part of 
said substantially sealed zone to be filled with a liquid medium, said 
brake arrangement being characterised by filter means to remove friction 
material particles from said liquid at least in the vicinity of where the 
or each said friction pad engages said braking surface or surfaces. 
Conveniently, said filter means may comprise compartment means located in 
a lower region of the stationary housing configured to entrap and retain 
said friction material particles. By the arrangement described above, the 
liquid in the vicinity of the friction pads is maintained clean and 
unlikely to contaminate the disc rotor surface with films of smeared wear 
debris thereby ensuring uniform braking performance. 
In accordance with yet another aspect, the present invention provides a 
brake arrangement for a vehicle adapted for connection to an axle end, 
said arrangement comprising a single rotor disc adapted, in use, to rotate 
with said axle end and a stationary housing enclosing said rotor disc with 
one or more seals providing a substantially sealed zone within at least 
part of said stationary housing such that at least a braking effect region 
of said rotor disc, in use, passes through said substantially sealed zone, 
at least one friction pad located adjacent a braking surface of said rotor 
disc in said braking effect region so as to operationally engage said 
braking surface during a vehicle braking operation, and said stationary 
housing containing a liquid medium within at least said substantially 
sealed zone, said brake arrangement being characterised in that said 
liquid medium has a viscosity of less than 7 centistokes mm.sup.2 /sec at 
100.degree. C. Preferably said liquid medium has a viscosity in the range 
of 2 to 5 centistokes mm.sup.2 /sec at 100.degree. C. Preferably said 
liquid medium has a viscosity index of between 135 to 145, preferably 
about 140 whereby changes in its viscosity with temperature changes is 
minimised. The advantages of utilising low viscosity oils and preferably 
low viscosity synthetic mineral oils as the liquid medium is that 
hydrodynamic lubrication effects are suppressed in favour of a mixed 
lubrication regime. Mixed lubrication regimes provide much higher friction 
levels than hydrodynamic lubrication resulting in improved performance of 
the brake. The brake assembly enables the pads to act partly in oil 
stripping mode and partly in oil shear mode thereby combining thick film 
and ultrathin film conditions. A much higher friction pad to disc 
engagement pressures, friction pad oil skimming, grooving and ultra low 
lubricant viscosities are combined to result in mixed film lubrication 
which sees hydrodynamic lubrication suppressed in favour of higher 
friction micro elastohydrodynamic and boundary lubrication regimes.

Referring now to FIGS. 1 and 2, a rotor disc 50 is illustrated adapted to 
be secured to a vehicle axle end by any suitable means. The rotor disc has 
radial annular and opposed braking surfaces 51, 52. A stationary housing 
53 (only partially shown in FIG. 2) defines a substantially sealed braking 
effect zone 54 surrounding the braking surfaces 51, 52, the zone being 
created by seals not shown. The housing 531 in use, contains a liquid such 
as a low viscosity oil. In PCT/AU92/00540 it is suggested that the oil 
volume should be relatively high but it has now been found that this is 
not necessary and preferably all that is required is sufficient liquid to 
keep the seals wet in use, i.e. by splashing or the like. Conveniently 
about one quarter of the vertical height of the brake assembly has been 
found to be satisfactory. The positioning of the brake pads adapted to 
engage the opposed braking surfaces 51, 52 may be as shown at 67, 68 
(approximately half way up the height of the brake assembly) or may 
alternatively be located in the upper quarter zones 80, 81 (FIG. 1). 
As shown in FIGS. 1 and 2, the inner radial edges 55 of the braking 
surfaces 51, 52 is preferably undercut to form circumferential liquid 
collection zones 56, 57 to collect centrifugally directed liquid on 
surfaces inwardly of the edges 55. This collected liquid can then flow 
through radial passages 58 to minimise the amount of liquid on the 
surfaces 51, 52. 
It is generally desirable to minimise the amount of liquid at the interface 
between the braking surfaces of the rotor disc and the braking surface of 
the friction pad. The foregoing is relevant in this regard. Other factors 
include using a low viscosity oil as the liquid within the housing as 
maintaining low viscosity enables a low coefficient of friction between 
the braking members when braking force is not applied but permits a high 
coefficient of friction when force is applied to the pads during braking. 
While oils of a viscosity up to ISO 15 at 40.degree. C. may be used, it is 
more preferred to use oils in the range of ISO 3 to ISO 7 which provide 
improved performance. More particularly, synthetic mineral oils having 
viscosities less than 7 (preferably 2 to 5) centistokes mm.sup.2 /sec at 
100.degree. C. are preferred. A disadvantage of utilising ultra low 
viscosity oils such as 2 centistokes in the past has been that 
conventional oils of ultra low viscosity had high volatilities and low 
flash points. This problem has now been overcome by the so-called 
synthetic mineral oil based oils such as the Shell Oil Company trademarked 
Extra High Viscosity Index Oils i.e. XHVII and XHVIII oils. These oils 
provide flash points and volatilities such that they can be utilised for 
their ultra low viscosity in the present braking system without flash 
points or volatility problems. 
It is well recognised that oil viscosity increases as temperature decreases 
and decreases as temperature increases. To overcome problems that may 
arise with operation at severe temperatures, (either hot or cold) it is 
proposed, as shown in FIGS. 1 and 2 to incorporate a heat exchanger 70 
within the oil in the brake housing. The heat exchanger 70 may be formed 
by a metallic tube (such as stainless steel) adapted to convey engine 
cooling water (normally at about 50.degree. C.) therethrough to heat the 
oil in the housing when necessary. Suitable valving and temperature 
sensors may be provided to supply this heating capability only when 
necessary to maintain temperatures of the oil in the brake assembly within 
an acceptable range. If the brake assembly is to be utilized in 
particularly hot environments, it may also be necessary to convey a 
cooling fluid through the heat exchanger 70. This fluid might be cooled by 
any suitable means including facilities such as air conditioning equipment 
that might be already on board the vehicle or by some purpose built 
device. If desired the heat exchanger 70 might be formed by passages cast 
into the brake assembly housing. 
Because the brake assembly is sealed, wear debris from the pads and the 
rotor disc is retained within the housing. This debris contained in the 
oil may cause wear debris/oil smear films on the rotor disc braking 
surfaces and can cause poor braking characteristics. Thus, it is desirable 
to remove this wear debris from at least the region of the mating surfaces 
of the pads and rotor disc. Low viscosity oils also tend to enable 
settling out of this wear debris more easily and quickly than higher 
viscosity oils. 
The pad wear debris can be effectively separated to acceptable levels by 
integral gravity settling and integral centrifuging techniques. Thus the 
housing 53 may include one or several "settling chambers" 59 in the sump 
of the brake housing. The settling chambers 59 may be cast into either the 
inner or the outer housing parts 60, 61 or may be formed by a wall member 
62, 63 bolted at 64 to the housing parts. The bottom edge of the wall 
members 62, 63 may be sealed if desired to the bottom wall surface of the 
respective housing part by provision of a machined or cast recess. 
Magnetic plugs 65, 66 might be used to attract metallic particles into the 
settling chambers 59 and upon removal enables the wear debris to be 
removed from these settling chambers without discarding the bulk of the 
oil. If the settling chamber or chambers is/are provided with a positive 
flow oil current by choosing its location to coincide with the pad oil 
discharge grooves, the respective chamber will always remain full and 
during periods of brake disc rotation, a slow but continuous flow of oil 
through the settling chambers will occur. When the disc is at rest or 
rotating , a substantial portion of the sump oil will be captured in the 
settling chamber for variable periods depending upon brake usage patterns. 
In the case of "slow flow" through the settling chamber or chambers 59, 
larger and heavier particles are collected. During periods such as 
overnight or other idle times when the rotor is stationary, the 
gravitational settling of pad wear debris continues particularly for the 
finer particles which may otherwise stay in suspension. To further promote 
retention of wear debris, the chambers 59 may include one or more spaced 
pads 69 of a material such as steel wool. Where the brake does not 
experience regular idle periods following brake use an integral or 
external pump can be used to circulate the oil containing wear debris 
through an external filter. Such an arrangement can also include 
components to either cool or heat the scavenged oil at the same time as 
the purification process. 
Referring now to FIGS. 1A and 1B, a lower section of the brake housing 53 
is illustrated. A pair of baffles 82, 83 and 84,85 are provided on either 
side of the housing 53 just above the liquid medium surface level 86. Each 
baffle 82, 83, 84, 85 is located so as to provide minimal spacing between 
the baffles and the rotor braking surfaces 51, 52 so as to minimise bulk 
splashing of the liquid medium or forcing of the liquid medium into upper 
zones of the housing 53 by action of the rotor 50 during use. Further, 
each of the bolt receiving bosses 88 having bores 90 adapted to receive 
the bolts 87 to hold the housing parts 60, 61 together, are formed with at 
least one curved surface 89 adapted to promote turbulent conditions in the 
liquid medium when the brake assembly is in use. 
Several preferred embodiments of friction pad arrangements will now be 
described with reference to FIGS. 3 to 10. The positioning of these pads 
in the brake arrangement shown in FIGS. 1 and 2 may be, for example, at 67 
and 68 and these may be operated by any known conventional actuating 
system. Referring first to FIG. 3, the brake pad 10 comprises a backing 
plate 11 of conventional configuration and a friction material 12 secured 
thereto. The friction material 12 has a leading edge 13, a trailing edge 
14, an inner peripheral edge 15 and an outer peripheral edge 16. A 
plurality of primary grooves 17 are formed into the friction material with 
outlet ends 18 opening onto the outer peripheral edge 16 or the trailing 
edge 14. A number of the primary grooves 17 have inlet ends 19 opening 
directly onto the leading edge 13 and each of these inlet ends 19 have 
mouth regions 20 of decreasing width adapted to funnel liquid flow into 
the grooves 17. This arrangement provides an effective means of collecting 
and removing bulk liquid from interengaging brake surfaces during a 
braking operation. The grooves 17 rearwardly of the above discussed group 
of grooves have inlet ends 21 located outwardly of the inner peripheral 
edge 15 so that a sealing land 22 of friction material is formed as 
discussed in preceding parts of this specification. Each of the grooves 17 
have a width of 1.5 to 4 mm and an intervening land 24 width of about 5 to 
6 mm. Conveniently, each of the grooves are straight and parallel to each 
other. FIG. 4 illustrates a modification to the configuration shown in 
FIG. 3 in that secondary grooves 25 are provided each forming an acute 
angle with the primary grooves 17. Conveniently at least some of the 
secondary grooves 25 have their inlet ends coincident with an inlet end of 
the primary grooves 17 at the leading edge 13. If desired the throat area 
of this intersection point might be enlarged to improve the liquid flow 
pattern. 
Reference will now be made to the embodiments of FIGS. 6 to 10. In FIG. 6, 
the primary grooves 17 are similarly disposed to the embodiment of FIG. 3 
except that the grooves are curved from their inlet ends to the outlet 
ends and that each groove 17 has an inlet end opening onto an edge of the 
friction material 12. Conveniently, the curvature of each of the grooves 
approximates an arc of a circle and for each of the grooves, the radius of 
curvature is substantially equal. Conveniently, each of the grooves 17 are 
parallel to one another. Further, it is desired that the angle a at the 
inlet 26 to each of the grooves 17 opening onto the inner peripheral edge 
15, being the angle between the tangent to the groove curve and the 
tangent to the curve of the edge 15 at the inlet 26, is substantially 
equal for all such grooves. Moreover, this angle is also substantially 
equal to the same angle at the outlet ends of the grooves opening onto the 
outer peripheral edge 16. Conveniently, this angle may be between 
20.degree. and 40.degree. but is preferably about 30.degree. which is 
desired for optimum performance for a conventional vehicle wheel 
travelling initially at the allowed road speed limit i.e. 100-110 km/h. 
This corresponds to a rotor disc rpm of about 800-1000. 
FIGS. 7 and 8 show embodiments Including secondary grooves 27 each 
intersecting the primary grooves 17 at an acute angle. The secondary 
grooves 27 may be curved as illustrated or they may be straight as 
indicated in FIG. 4. Further, the secondary grooves 27 may pass through 
substantially all the friction material as shown in FIG. 7 or they may be 
terminated at their intersection with an intermediate primary groove 17 as 
shown in FIG. 8. 
Conveniently, in each case, the primary grooves opening onto the leading 
edge 13 of the friction material have mouth regions 28 of wider 
proportions which conveniently funnel liquid flow into the adjacent 
grooves 17. Preferably the radially inner wall of the groove 17 forms one 
wall 29 of the widened mouth region. As illustrated in FIG. 8, the throat 
30 of the intersection between the grooves 17 and 27 at the mouth region 
may be widened, for example by removal of the friction material indicated 
in dotted outline 31, so as to improve liquid flow into the two grooves 17 
and 27 extending from this throat. 
FIG. 9 shows a cross sectional view along lines B--B of FIGS. 8 and 10. 
FIG. 10 shows an embodiment similar to FIG. 9 except that the grooves 17 
are more widely spaced towards a trailing end of the load. 
FIG. 11 is a graph showing a vehicle stopping distance against brake 
pressure for a 2.5 tonne Toyota Land Cruiser utilising wet disc brakes as 
described in the foregoing specification and comparing ungrooved pads with 
brake pads produced according to a configuration as shown in FIG. 8 
hereof. It will be apparent from this comparison, that the brake system 
and particularly the brake pads according to the present invention out 
perform ungrooved pads in an otherwise similar configuration.