WO2018224236A1

WO2018224236A1 - Vehicle brake system and method of assembling

Info

Publication number: WO2018224236A1
Application number: PCT/EP2018/061991
Authority: WO
Inventors: Stefan Mischo; Eberhard Kurz; Louis Damanti; John Gatza; Michael Jacobs; James TIMPA; Ryan KUHLMAN; Douglas Patterson; Frederic HEASLEY; Robert Kaster; Michael Meier; Martin Langsch
Original assignee: Robert Bosch Gmbh
Priority date: 2017-06-07
Filing date: 2018-05-09
Publication date: 2018-12-13

Abstract

A brake system (120,220,320,420,620) comprising a driver unit (242) with brake pedal (245) to receive user input, a user input sensor (246A,246B), and front and rear axle brake units (232A,232B). The front axle brake unit (232A) is fluidly isolated from the driver unit (242) and includes an electronically-controlled fluid pressure generating unit (244A) and at least one control valve (248,250), operable to supply fluid pressure to at least one front wheel cylinder (224). The rear axle brake unit (232B) is fluidly isolated from the driver unit (242) and includes an electronically-controlled fluid pressure generating unit (244B) and at least one control valve (248,250), operable to supply fluid pressure to at least one rear wheel cylinder (228). The front and rear axle brake units (232A,232B) include separate housings spaced at disparate locations of the vehicle, and each include their own dedicated fluid reservoir (RSV,236) such that the front and rear axle brake units (232A,232B) operate with supplies of fluid separate from each other and uncoupled to the driver unit (242). Further a method of installing such a brake system (120,220,320,420,620) is claimed. Another brake system (520) according to the present invention comprises ECU-controlled electro-mechanical rear brakes (570) as rear axle brake unit.

Description

VEHICLE BRAKE SYSTEM AND METHOD OF ASSEMBLING

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present invention claims priority to U.S. Provisional Patent Application No. 62/516,314, filed on June 7, 2017, the entire contents of which are incorporated by reference herein.

FIELD OF THE INVENTION

[0002] The present invention relates to vehicles and vehicle braking systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0003] Fig. 1 is a schematic representation of an earlier-conceived brake system for a four- wheeled vehicle with two hydraulic wheel cylinders on each of two axles.

[0004] Fig. 2 is a schematic representation of another earlier-conceived brake system for a four-wheeled vehicle with two hydraulic wheel cylinders on each of two axles.

[0005] Fig. 3 schematically illustrates a vehicle brake system according to one embodiment of the present invention.

[0006] Fig. 4 schematically illustrates a vehicle brake system according to yet another embodiment of the present invention.

[0007] Fig. 5 schematically illustrates a vehicle brake system according to yet another embodiment of the present invention.

[0008] Fig. 6 schematically illustrates a vehicle brake system according to yet another embodiment of the present invention.

[0009] Fig. 7 schematically illustrates a vehicle brake system according to yet another embodiment of the present invention. [0010] Fig. 8 schematically illustrates a vehicle brake system according to yet another embodiment of the present invention.

SUMMARY

[0011] In one aspect, the invention provides a brake system for a four-wheel vehicle, the brake system comprising a driver unit including a brake pedal operable to receive a user input, at least one user input sensor operable to detect the user input, and front and rear axle brake units. The front axle brake unit is fiuidly isolated from the driver unit and includes an electronically- controlled fluid pressure generating unit and at least one control valve. The front axle brake unit is operable to supply fluid pressure to at least one front axle wheel cylinder in response to the at least one user input sensor. The rear axle brake unit is fiuidly isolated from the driver unit and includes an electronically-controlled fluid pressure generating unit and at least one control valve. The rear axle brake unit is operable to supply fluid pressure to at least one rear axle wheel cylinder in response to the at least one user input sensor. The front and rear axle brake units include separate housings that are spaced at disparate locations of the vehicle. The front and rear axle brake units each include their own dedicated fluid reservoir such that the front and rear axle brake units operate with supplies of hydraulic fluid that are separate from each other and uncoupled to the driver unit.

[0012] In another aspect, the invention provides a method of installing a vehicle brake system. A driver unit is installed to the vehicle, the driver unit including a brake pedal operable to receive a user input. A front axle brake unit and a rear axle brake unit are provided, each of the front and rear axle brake units being transported to a location of the vehicle installation in a fluid-filled and pre-sealed state. The front axle brake unit is installed to the vehicle, the front axle brake unit including an electronically-controlled fluid pressure generating unit and at least one control valve in fluid communication with at least one front axle wheel cylinder. The front axle brake unit is installed at a first position in the vehicle proximate a front axle position and spaced from the driver unit. The rear axle brake unit is installed to the vehicle, the rear axle brake unit including an electronically-controlled fluid pressure generating unit and at least one control valve in fluid communication with at least one rear axle wheel cylinder. The rear axle brake unit is installed at a second position in the vehicle proximate a rear axle position and spaced from both the driver unit and the front axle brake unit. The front and rear axle brake units are installed with entirely independent supplies of hydraulic fluid, and both the front and rear axle brake units are maintained in fluid isolation from the driver unit.

[0013] In yet another aspect, the invention provides a brake system for a four-wheel vehicle having first and second axles spaced from each other in a fore-aflt vehicle direction. A driver unit includes a brake pedal operable to receive a user input. At least one user input sensor is operable to detect the user input. A first axle brake unit includes an electronically-controlled fluid pressure generating unit and at least one control valve. The first axle brake unit is operable to supply fluid pressure to at least one first axle wheel cylinder in response to the at least one user input sensor. A second axle brake unit includes a motor-on-caliper unit operable to apply a braking torque to a second axle wheel in response to the at least one user input sensor. The first and second axle brake units include separate housings that are spaced at disparate locations of the vehicle, and the first axle brake unit operates with a supply of hydraulic fluid that is not shared with the second axle brake unit.

DETAILED DESCRIPTION

[0014] Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways.

[0015] Figs. 1-2 schematically illustrate two types of brake systems preceding the present invention. In Fig. 1, the brake system 1020, which is capable of hydraulically actuating braking at front axle wheel cylinders 1024 and rear axle wheel cylinders 1028, is organized into two units, not counting the electrical power supply and the wheel cylinders 1024, 1028 themselves. The first unit is a driver unit 1032 including a driver input member, such as a brake pedal 1036, an electro-mechanical booster 1040, and a tandem master cylinder 1044. The second unit is an ABS hydraulic unit 1048 including at least a plurality of wheel-specific inlet and outlet valves, a plurality of motor-driven pump elements, and an electronic control unit operable to control the pumping and valving of hydraulic fluid to control the level of hydraulically-actuated braking to each wheel cylinder 1024, 1028.

[0016] In Fig. 2, the brake system 1120, which is also capable of hydraulically actuating braking at front axle wheel cylinders 1 124 and rear axle wheel cylinders 1128, is organized into a single unit 1130. Sometimes referred to as an integrated power brake unit, the single unit includes a master cylinder 1144 operable to receive an input from a driver input member, such as a brake pedal 1136. The master cylinder 1144 is switchable to be in fluid communication with either a pedal feel simulator 1140 or a plurality of wheel-specific inlet and outlet valves. The master cylinder 1144, along with the pedal feel simulator 1140, a pedal feel simulator valve, and a pressure sensor constitute a driver sub-unit 1132, which is not physically separate, but provided within the unit 1 130. As actuated by an electronic control unit, a motor-driven plunger 1 146 supplies the requisite hydraulic fluid pressure to the two circuits containing the wheel cylinders 1124, 1128 through a pair of valves 1150, 1152 that are in fluid communication without an outlet end of the motor-driven plunger 1146.

[0017] The term "unit" can refer to a group of components housed together in a single housing. However, in some constructions multiple housings or cases can be provided for a single unit, and the unit(s) are defined by the fact that each requires its own dedicated mounting to the vehicle frame or sub-frame, typically at separated locations for multiple units. In terms of vehicle assembly, separate units of a brake system are those components that get mounted separately to the vehicle frame or sub-frame, and may be separately catalogued for assembly at the same or different steps along a vehicle assembly line. In the case of Figs. 1 and 2, either brake system 1020, 1120 is capable of adjusting, during operation, a relationship between the brake pedal 1036, 1 136 and the hydraulic circuit(s) that actively apply fluid pressure to the wheel cylinders, and this makes the systems particularly useful in full electric drive vehicles, electric hybrid vehicles, and vehicles having powertrains with no/low natural vacuum for a conventional booster. Although the driver unit 1032 or driver sub-unit 1132 may be fluidly de -coupled from the wheel cylinders during at least some operational states, each of the master cylinders 1044, 1144 has a fluid connection with the wheel cylinders that can be selectively enabled by the actuation of valves. In the system 1020 of Fig. 1 , fluid connections must be made at the time of vehicle assembly between the driver unit 1032 and the ABS hydraulic unit 1048, and additionally, fluid connections must be made by brake lines extending from the ABS hydraulic unit 1048 to each of the four wheel cylinders 1024, 1028. In the system 1 120 of Fig. 2, the brake unit 1130 can be located at any convenient place in the vehicle, but separate fluid connections must be made by brake lines extending to each of the four wheel cylinders 1124, 1128. Thus, with either system of Figs. 1-2, there is a need for fluid connections spanning the front and rear axles of the vehicle.

[0018] As shown in Fig. 3, a first construction of the invention provides a brake system 120 in which hydraulic units 132 are separately provided for wheel cylinders 124 of a front axle and wheel cylinders 128 of a rear axle as separate standalone systems having capability for only electronic communication, with no capability for fluid connection therebetween. Each hydraulic unit 132 can have associated therewith its own dedicated fluid reservoir 136 and dedicated electronic control unit (ECU) 140. For example, the dedicated fluid reservoir 136 and/or dedicated ECU 140 can be physically- integrated with the hydraulic unit 132 so as to mounted in the vehicle jointly at a shared location. Although the ECUs 140 separately control the hydraulic units 132, the ECUs 140 are coupled for electronic communication therebetween to work in conjunction with each other to provide a prescribed overall braking effect to the vehicle. In other constructions, the brake system 120 has an ECU 140 that is shared between the two hydraulic units 132. Within each hydraulic unit 132, an electronically-controlled pressure generating unit 144, such as a motor-driven plunger, is operable by the ECU 140 to fluidly actuate the two wheel cylinders 124 or 128 coupled therewith. The ECU 140 actuates the motor-driven plunger in a manner according to an input sensed from a driver unit 142, including a driver input member such as a brake pedal 145, so that the braking is carried out to correspond to the driver's requested amount, unless conditions require additional intervention. Sensing of the input to the brake pedal 145 by the driver can be by a pressure sensor and/or travel sensor 146 A, 146B. Wheel-specific inlet and outlet valves 148, 150 enable anti-lock braking operation as well as stability control and traction control in accordance with known practice. In some constructions, the two hydraulic units 132 that make up the brake system 120 can be identical to each other for component flexibility. In other constructions, the two hydraulic units 132 can be constructed as identical circuits, but having components of different capacity (e.g., differently-sized motor- driven plungers). [0019] By providing the brake system 120 in two individual units 132 that are not hydraulically connected whatsoever, fewer fluid connections are required on the vehicle assembly line and brake lines need not extend between the front and rear axles of the vehicle. Each hydraulic unit 132 is effectively a self-contained unit having no external fluid connections except those to the associated wheel cylinders 124, or 128. In some constructions, each hydraulic unit 132 can be pre-charged with fluid and held fluidly sealed throughout assembly into the vehicle so that fluid filling and/or system bleeding on the vehicle assembly line is/are eliminated. For example, each hydraulic unit 132 can be stored, transported, and supplied to the automotive assembly line as a pre-sealed unit.

[0020] Optionally, the driver unit 142 of the brake system 120 that receives the input from the brake pedal 145 can include an amount of hydraulic fluid, for example, in communication with a pedal feel simulator to provide driver feedback. Hydraulic fluid of the driver unit 142 can be shared with one of (e.g., front) the hydraulic units 132, or can be entirely separate from the hydraulic fluid of both hydraulic units 132. In yet other constructions, forced feedback to the brake pedal 145 can be provided by non-fluid means in a controlled manner, e.g., by an electric motor.

[0021] As shown in Fig. 4, a second construction of the invention provides a brake system 220 in which hydraulic units 232A, 232B are separately provided for wheel cylinders 224 of a front axle and wheel cylinders 228 of a rear axle as separate standalone systems having capability for only electronic communication, with no capability for fluid connection therebetween. Each hydraulic unit 232A, 232B can have associated therewith its own dedicated fluid reservoir 236 and dedicated electronic control unit (ECU) 240. In other constructions, the brake system 220 has an ECU 240 that is shared between the two hydraulic units 232. Within each hydraulic unit 232A, 232B, an electronically-controlled pressure generating unit 244A, 244B is operable by the ECU 240 to fluidly actuate the two wheel cylinders 224 or 228 coupled therewith. In one of the hydraulic units 232 (e.g., the front axle unit), the electronically- controlled pressure generating unit 244A can be a motor-driven plunger as with the hydraulic units 132 of Fig. 3, while the other hydraulic unit 232B (e.g., the rear axle unit) includes an electronically-controlled pressure generating unit 244B in the form of one or more pump elements 254 driven by a motor 258. A separation valve 260 (e.g., a normally-open solenoid actuated 2-position valve) is provided between the reservoir 236 and a pressure line 262 that is arranged to extend from outlets of the pump elements 254, through the inlet valves 248 to both of the wheel cylinders 228 of the hydraulic unit 232B. Thus, for braking by the action of the electronically-controlled pressure generating unit 244B, the separation valve 260 is closed to block high pressure fluid from being pumped to the reservoir 236. Each ECU 240 actuates the respective electronically-controlled pressure generating unit 244A, 244B in a manner according to an input sensed from a driver unit 242, including a driver input member such as a brake pedal 245, so that the braking is carried out to correspond to the driver's requested amount, unless conditions require additional intervention. Sensing of the input to the brake pedal 245 by the driver can be by a pressure sensor and/or travel sensor 246 A, 246B. Wheel-specific inlet and outlet valves 248, 250 enable anti-lock braking operation as well as stability control and traction control in accordance with known practice.

[0022] By providing the brake system 220 in two individual units 232A, 232B that are not hydraulically connected whatsoever similar advantages as those of the brake system 120 can be realized, such as fewer fluid connections on the vehicle assembly line and the lack of brake lines extending between the front and rear axles of the vehicle. In some constructions, each hydraulic unit 232A, 232B can be pre-charged with fluid and held fluidly sealed throughout assembly into the vehicle so that fluid filling and/or system bleeding on the vehicle assembly line is/are eliminated.

[0023] As shown in Fig. 5, a third construction of the invention provides a brake system 320 that is generally similar to the brake system 220 of Fig. 4, with the exception of the features noted below. Although renumbered with reference numbers formatted as "3~", the first hydraulic unit 332A is similar to the hydraulic units 132, 232A. Whereas the second hydraulic unit 232B of Fig. 3 includes one electronically-controlled pressure generating unit 244B providing a single-channel output (e.g., pressure line 262) that requires the additional inlet and outlet valves 248, 250 (i.e., four valves for the two wheel cylinders 228), the second hydraulic unit 332B of Fig. 5 includes two electronically-controlled pressure generating units 344B providing two independent channels for supplying fluid pressure to the two wheel cylinders 328. For example, the two electronically-controlled pressure generating units 344B can include independent pump elements 354 driven by a common motor 358 (or separate motors) to generate fluid pressure at two independent, uncoupled pressure lines 362. Although not shown, a separate driver unit similar to the driver units 142, 242 is included in the brake system 320.

[0024] With respect to maintaining complete fluid separation between the front and rear axles of the vehicle, the brake system 320 provides similar benefits and advantages as those described above with respect to Figs. 3 and 4. However, the second hydraulic unit 332B (e.g., for the rear axle) allows a drastic reduction in the number of valves required compared to the second hydraulic unit 232B of Fig. 4. When a need arises for the hydraulic fluid pressures to be different between the two wheel cylinders 328 coupled to the second hydraulic unit 332B, the separation valves 360 cannot be left closed, and instead one or both must be modulated independently, since separate inlet and outlet valves are not present. Fluid is also returned to the reservoir 336 through the open separation valves 360 at the completion of braking.

[0025] As shown in Fig. 6, a fourth construction of the invention provides a brake system 420 that is generally similar to the brake system 320 of Fig. 5, with the exception of the features noted below. Although renumbered with reference numbers formatted as "4~", the first hydraulic unit 432A is similar to the hydraulic units 132, 232A, 332A. Also, though not shown, the brake system 420 can include a driver unit similar to the driver units 142, 242 discussed above. Rather than providing the two independent channels in a single housing (second hydraulic unit 332B) for fluidly supplying the two wheel cylinders 328 as in Fig. 5, the brake system 420 divides the functional components of the second hydraulic unit 332B into two separate units 432B, 432C for the two wheel cylinders 428 (e.g., of the rear axle). Thus, for the wheel cylinders 428, there is no central hydraulic unit and the separate hydraulic units 432B, 432C can be positioned on the vehicle even closer to the individual wheel cylinders 428 at the opposing ends of the axle. All three hydraulic units 432A, 432B, 432C maintain complete separation of hydraulic fluid.

[0026] As shown in Fig. 7, a fifth construction of the invention provides a brake system 520 that is generally similar to the brake systems 120, 220, 320, 420 discussed above, with the exception of the features noted below. The brake system 520 is provided with a hydraulic unit 532 for a first pair of wheel cylinders 524 for one axle (e.g., front axle) of the vehicle, and the hydraulic unit 532 can include the features of the hydraulic units 132, 232A discussed above. Thus, the hydraulic unit 532 includes an electronically-controlled pressure generating unit 544 pairs of inlet and outlet valves 548, 550, and hydraulic fluid that does not serve brakes for the opposite axle (e.g., rear axle) of the vehicle. Rather than providing one or more separate hydraulic units for the opposite axle, no hydraulic brakes are provided for the opposite axle. In lieu of a hydraulic system that selectively actuates braking through wheel cylinders, wheel braking at the opposite axle is accomplished with individual motor-on-caliper (MoC) units 570, each of which includes an electric motor operable by an ECU 574 to modulate braking torque to the respective wheels. Because the MoC units 570 are not fluid-based devices, the brake system 520 eliminates the need to extend any hydraulic brake lines beyond the single axle having the two wheel cylinders 524 served by the single hydraulic unit 532. Each MoC unit 570 directly associated with a particular wheel of the vehicle thus can be considered its own brake unit or self-contained brake unit, as it does not require connections or assembly with the braking components of the hydraulically-braked first axle, nor any connections or assembly with any other components of the brake system 520, except for the requisite electrical connections required for communication and/or power.

[0027] As shown in Fig. 8, a sixth construction of the invention provides a brake system 620 that is generally similar to the brake system 420 of Fig. 6, with the exception of the features noted below. Although renumbered with reference numbers formatted as "6~", the hydraulic units 632B, 632C for the individual wheel cylinders 628 (e.g., of the rear axle) are similar to the hydraulic units 432B, 432C of Fig. 6. Also, though not shown, the brake system 620 can include a driver unit similar to the driver units 142, 242 discussed above. Rather than providing one additional hydraulic unit with pairs of inlet and outlet valves for the remaining wheel cylinders 624 (e.g., of the front axle), the brake system 620 is provided with two additional, independent hydraulic units 632A, 632D. In some constructions, the four hydraulic units 632A-D can all be of identical construction so that the vehicle assembly process simply requires one component or part type, provided in a quantity of four. Regardless of whether all the hydraulic units 632A-D are identical or have internal or external differences, the brake system 620 avoids the need for any brake lines to extend between axles (as with the other constructions of Figs. 3-7), and further avoids the need for any brake lines to extend across the length of either axle between the separate wheel cylinders of that axle. Thus, the total length of brake lines and the total amount of hydraulic fluid required is reduced, as are the inherent performance drawbacks. Each and every hydraulic unit 632A-D is dedicated to a particular wheel cylinder and may be mounted in a location on the vehicle in the nearest reasonable proximity to the wheel cylinder, such as on a wheel housing of the vehicle frame.

Claims

CLAIMS What is claimed is:

1. A brake system for a four-wheel vehicle comprising:

a driver unit including a brake pedal operable to receive a user input;

at least one user input sensor operable to detect the user input;

a front axle brake unit fluidly isolated from the driver unit and including an

electronically-controlled fluid pressure generating unit and at least one control valve, wherein the front axle brake unit is operable to supply fluid pressure to at least one front axle wheel cylinder in response to the at least one user input sensor; and

a rear axle brake unit fluidly isolated from the driver unit and including an electronically- controlled fluid pressure generating unit and at least one control valve, wherein the rear axle brake unit is operable to supply fluid pressure to at least one rear axle wheel cylinder in response to the at least one user input sensor;

wherein the front and rear axle brake units include separate housings that are spaced at disparate locations of the vehicle, and

wherein the front and rear axle brake units each include their own dedicated fluid reservoir such that the front and rear axle brake units operate with supplies of hydraulic fluid that are separate from each other and uncoupled to the driver unit.

2. The vehicle brake system of claim 1 , wherein the rear axle brake unit is a first rear axle brake unit operable to supply fluid pressure to a first rear axle wheel cylinder, the vehicle brake system further comprising a second rear axle brake unit operable to supply fluid pressure to a second rear axle wheel cylinder,

wherein the first and second rear axle brake units include separate housings that are spaced at disparate locations of the vehicle, and

wherein the first and second rear axle brake units operate with entirely independent supplies of hydraulic fluid.

3. The vehicle brake system of claim 2, wherein the second rear axle brake unit includes its own dedicated fluid reservoir.

4. The vehicle brake system of claim 2, wherein the front axle brake unit is a first front axle brake unit operable to supply fluid pressure to a first front axle wheel cylinder, the vehicle brake system further comprising a second front axle brake unit operable to supply fluid pressure to a second front axle wheel cylinder,

wherein the first and second front axle brake units include separate housings that are spaced at disparate locations of the vehicle, and

wherein the first and second front axle brake units operate with entirely independent supplies of hydraulic fluid.

5. The vehicle system of claim 4, wherein the second front axle brake unit includes its own dedicated fluid reservoir.

6. The vehicle brake system of claim 2, wherein each of the first and second rear axle brake units includes a single switchable valve.

7. The vehicle brake system of claim 1, wherein the front axle brake unit is a first front axle brake unit operable to supply fluid pressure to a first front axle wheel cylinder, the vehicle brake system further comprising a second front axle brake unit operable to supply fluid pressure to a second front axle wheel cylinder,

8. The vehicle brake system of claim 1, wherein the electronically-controlled pressure generating unit of the front axle brake unit is a motor-driven plunger.

9. The vehicle brake system of claim 1, further comprising a sensor operable to provide the signal to the controller, wherein the sensor is one of a pressure or travel sensor.

10. The vehicle brake system of claim 1, wherein the driver unit further comprises feel simulator coupled to the brake pedal to receive the user input.

11. A method of installing a vehicle brake system comprising:

installing a driver unit to the vehicle, the driver unit including a brake pedal operable to receive a user input;

providing a front axle brake unit and a rear axle brake unit, each of the front and rear axle brake units being transported to a location of the vehicle installation in a fluid-filled and pre- sealed state;

installing the front axle brake unit to the vehicle, the front axle brake unit including an electronically-controlled fluid pressure generating unit and at least one control valve in fluid communication with at least one front axle wheel cylinder, the front axle brake unit being installed at a first position in the vehicle proximate a front axle position and spaced from the driver unit;

installing the rear axle brake unit to the vehicle, the rear axle brake unit including an electronically-controlled fluid pressure generating unit and at least one control valve in fluid communication with at least one rear axle wheel cylinder, the rear axle brake unit being installed at a second position in the vehicle proximate a rear axle position and spaced from both the driver unit and the front axle brake unit;

wherein the front and rear axle brake units are installed with entirely independent supplies of hydraulic fluid, and both the front and rear axle brake units are maintained in fluid isolation from the driver unit.

12. The method of claim 11 , wherein the rear axle brake unit is a first rear axle brake unit in fluid communication with a first rear axle wheel cylinder, the method further comprising installing a second rear axle brake unit to the vehicle, the second rear axle brake unit including an additional electronically-controlled fluid pressure generating unit in fluid communication with a second rear axle wheel cylinder, the second rear axle brake unit being installed at a third position in the vehicle proximate the rear axle position, wherein the first and second rear axle brake units include separate housings and are not in fluid communication with each other.

13. The method of claim 12, wherein the front axle brake unit is a first front axle brake unit in fluid communication with a first front axle wheel cylinder, the method further comprising installing a second front axle brake unit to the vehicle, the second front axle brake unit including an additional electronically-controlled fluid pressure generating unit in fluid communication with a second front axle wheel cylinder, the second front axle brake unit being installed at a fourth position in the vehicle proximate the front axle position, wherein the first and second front axle brake units include separate housings and are not in fluid communication with each other.

14. The method of claim 11 , wherein the front axle brake unit is a first front axle brake unit in fluid communication with a first front axle wheel cylinder, the method further comprising installing a second front axle brake unit to the vehicle, the second front axle brake unit including an additional electronically-controlled fluid pressure generating unit in fluid communication with a second front axle wheel cylinder, the second front axle brake unit being installed at a third position in the vehicle proximate the front axle position, wherein the first and second front axle brake units include separate housings and are not in fluid communication with each other.

15. A brake system for a four-wheel vehicle having first and second axles spaced from each other in a fore-aflt vehicle direction, the brake system comprising:

a driver unit including a brake pedal operable to receive a user input;

at least one user input sensor operable to detect the user input;

a first axle brake unit including an electronically-controlled fluid pressure generating unit and at least one control valve, wherein the first axle brake unit is operable to supply fluid pressure to at least one first axle wheel cylinder in response to the at least one user input sensor; and

a second axle brake unit including a motor-on-caliper unit operable to apply a braking torque to a second axle wheel in response to the at least one user input sensor;

wherein the first and second axle brake units include separate housings that are spaced at disparate locations of the vehicle, and

wherein the first axle brake unit operates with a supply of hydraulic fluid that is not shared with the second axle brake unit.

16. The brake system of claim 15, further comprising a further second axle brake unit including a further motor-on-caliper unit operable to apply a braking torque to an additional second axle wheel in response to the at least one user input sensor.

17. The brake system of claim 15, wherein the first axle brake unit includes a physically- integrated dedicated fluid reservoir.

18. The brake system of claim 15, wherein the supply of hydraulic fluid of the first axle brake unit is not shared with the driver unit.

19. The brake system of claim 15, wherein the first axle brake unit is a self-contained unit having no external fluid communication other than with the at least one first axle wheel cylinder.