Patent Publication Number: US-2022219662-A1

Title: Braking arrangement for a vehicle

Description:
BACKGROUND AND SUMMARY 
     The present invention relates to a braking arrangement for a vehicle wherein brakes that function differently are provided on different axles of the vehicle. 
     In vehicles such as tandem axles trucks and tractors, it is sometimes desirable to provide one type of brake, such as drum brakes, on one axle, such as an auxiliary axle, and another type of brake, such as disc brakes, on another axle, such as a rear axle. This is usually done for cost reasons. 
     As the commercial vehicle industry has grown rapidly in advancing control systems and safety, vehicles have improved to travel faster and stop much quicker. There has been enormous growth in anti-lock brake system (ABS) technology. However, the market has not carried this technology growth uniformly in all axles. Usually for cost reasons, some vehicles are not provided with ABS on all axles, such as auxiliary axles, but are provided with ABS on other axles. 
     In the case of drum brakes versus disc brakes, there will tend to be a braking torque imbalance provided by the two brake types at the same brake demand. Drum brakes tend to provide more braking torque and do more work toward stopping a vehicle than disc brakes and will tend to wear much faster than disc brakes.  FIG. 8  shows the shape of illustrative curves of braking torque provided versus braking demand for conventional drum brakes (curve Y) and disc brakes (curve C). Use of braking arrangements that function differently on different axles of a vehicle, such as drum brakes on an auxiliary axle and disc brakes on a rear axle, tends to cause several problems: 
     First, for a similar brake size, drum brakes have a higher brake factor than disc brakes due to their geometrical construction. This difference tends to be linear. 
     Second, drum brakes have a tendency to get self-energized during braking and stick more towards the brake shoe, thereby tending to provide an additional braking torque. In contrast, disc brakes behave opposite. In disc brakes, the piston in the caliper has a tendency to be pushed away from the rotor. The difference in brake torque between drum brakes and disc brakes due to self-energization tends to be non-linear. 
     Third, drum brakes tend to have poor heat dissipation compared to disc brakes. This phenomenon again leads to a non-linear difference between the coefficient of friction in the friction materials of the brake types. In addition, this introduces the uneven brake wear. It tends to be necessary to change drum brakes very often relative to disc brakes which is costly and requires maintenance during which the vehicle cannot be operated. 
     Yet another problem is that, as vehicles move faster, the need for them to have high braking forces so that they will be able to stop more quickly has grown has given rise to issues in connection with axles not provided with ABS. When high braking forces are provided, the wheels on axles without ABS tend lock up prematurely while the wheels on axles with ABS are brought to a stop in a more controlled fashion. The locking of the wheels on the axles without ABS tends to create a flat spot on the tires which wears out the tire rapidly and also creates tire pollution. 
     It is desirable to provide a braking arrangement in which the curves of braking torque versus braking demand for braking arrangements that function differently can be caused to have more similar shapes such that brake factor, tendency to self-energize, and heat dissipation for the different braking arrangements will be more similar, and brakes such as drum brakes will not tend to wear at a substantially different rate from brakes such as disc brakes on the same vehicle but different vehicle axles. It is also desirable to provide a braking arrangement in which the tendency for wearing of flat spots on and tire pollution due to tires on axles not being provided with ABS when other axles are provided with ABS can be overcome. 
     In accordance with an aspect of the present invention, a braking arrangement for a vehicle comprises a source of pressurized air, a first wheel with a first pneumatic brake arrangement including a first brake, the first brake being arranged to be engaged when connected to the source of pressurized air and disengaged when disconnected from the source of pressurized air, a second wheel with a second pneumatic brake arrangement including a second brake, the second brake being arranged to be engaged when connected to the source of pressurized air and disengaged when disconnected from the source of pressurized air, and a proportional valve between the source of pressurized air and the first brake, opening of the proportional valve being proportional to pressure in a line between the source of pressurized air and the proportional valve, wherein the first pneumatic brake arrangement and the second pneumatic brake arrangement function differently. 
     In accordance with another aspect of the present invention, a braking arrangement for a vehicle comprises a source of pressurized air, a first wheel with a first pneumatic brake arrangement including a first brake, the first brake being arranged to be engaged when connected to the source of pressurized air and disengaged when disconnected from the source of pressurized air, a proportional valve between the source of pressurized air and the first brake, opening of the proportional valve being proportional to pressure in a line between the source of pressurized air and the proportional valve, a second wheel with a second pneumatic brake arrangement including a second brake, the second brake being arranged to be engaged when connected to the source of pressurized air and disengaged when disconnected from the source of pressurized air, a first axle on which the first wheel is mounted, and a second axle on which the second wheel is mounted, wherein a curve of braking torque provided by the first brake versus braking demand is more similar in shape to a curve of braking torque provided by the second brake versus braking demand than those curves would be in the absence of the proportional valve. 
     In accordance with yet another aspect of the present invention, a braking arrangement for a vehicle comprises a source of pressurized air, a first wheel with a first pneumatic brake arrangement including a first brake, the first brake being arranged to be engaged when connected to the source of pressurized air and disengaged when disconnected from the source of pressurized air, a proportional valve between the source of pressurized air and the first brake, opening of the proportional valve being proportional to pressure in a line between the source of pressurized air and the proportional valve, a second source of pressurized air, a second proportional valve between the first proportional valve and the first brake, opening of the second proportional valve being proportional to pressure in a line between the second source of pressurized air and the second proportional valve, a second wheel with a second pneumatic brake arrangement including a second brake, the second brake being arranged to be engaged when connected to the source of pressurized air and disengaged when disconnected from the source of pressurized air, a first axle on which the first wheel is mounted, and a second axle on which the second wheel is mounted. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and advantages of the present invention are well understood by reading the following detailed description in conjunction with the drawings in which like numerals indicate similar elements and in which: 
         FIG. 1  is a schematic view of a vehicle including a braking arrangement according to a first embodiment of the present invention; 
         FIG. 2  is a schematic view of a vehicle including a braking arrangement according to a second embodiment of the present invention; 
         FIG. 3  is a schematic view of a vehicle including a braking arrangement according to a third embodiment of the present invention; 
         FIG. 4  is a schematic view of a vehicle including a braking arrangement according to a fourth embodiment of the present invention; 
         FIG. 5  is a schematic view of a vehicle including a braking arrangement according to a fifth embodiment of the present invention; 
         FIG. 6  is a schematic view of a vehicle including a braking arrangement according to a sixth embodiment of the present invention; 
         FIG. 7  is a graph showing how delivery pressure versus service pressure in a brake can be varied according to an aspect of the present invention; 
         FIG. 8  is a graph showing how brake torque versus service brake demand can be varied according to an aspect of the present invention by providing a proportional valve associated with a brake; and 
         FIG. 9  is a graph showing how brake torque versus service brake demand can be varied according to an aspect of the present invention by providing a proportional valve associated with a brake and another proportional valve associated with a suspension airbag. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of braking arrangements for a vehicle  21  are shown schematically in  FIGS. 1-6 . The vehicle  21  can be any type of vehicle, however, the present invention is believed to have particular application to vehicles such as trucks, tractors, and tractor-trailer combinations that use pneumatic brakes, such as disc brakes and/or drum brakes, for wheels on multiple axles and/or that provide anti-lock braking technology in connection with some but not all of the wheels on the vehicle&#39;s axles. The vehicle  21  illustrated in  FIGS. 1-6  is a tractor-trailer combination comprising a tractor  23  and a trailer  25 . Components that can be (but are not necessarily) identical in the various embodiments shown in  FIGS. 1-6  are provided with the same reference numbers in the various figures. It will also be appreciated that the use of different reference numbers in the various figures does not necessarily mean that components in one figure are different from components in other figures. 
     In all of the embodiments shown in  FIGS. 1-6 , the braking arrangement includes a source of pressurized air  27  such as a reservoir or a compressor. 
     In the embodiment shown in  FIG. 1 , the braking arrangement further comprises a first wheel  29  on a first axle A 1  with a first pneumatic brake arrangement  31  including a first brake  33 . There are ordinarily two first brakes  33  (left and right) and at least two wheels  29  (left and right) on the first axle A 1 . As is common in truck and tractor braking systems, the first brake  33  is arranged to be engaged, i.e. deliver braking torque, when connected to the source of pressurized air  27  and disengaged when disconnected from the source of pressurized air. 
     The braking arrangement further comprises a second wheel  35  on a second axle A 2  with a second pneumatic brake arrangement  37  including a second brake  39 . Again, there are ordinarily two second brakes  39  (left and right) and at least two wheels  35  (left and right) on the second axle A 2 . The second brake  39  is also arranged to be engaged, i.e. deliver braking torque, when connected to the source of pressurized air  27  and disengaged when disconnected from the source of pressurized air. In a typical embodiment of a tractor having a tandem axle arrangement as shown, the first axle A 1  will often be an auxiliary axle, and the second axle A 2  will be a powered rear axle. The present invention is not, however, limited to such axle arrangements including an auxiliary axle and a powered rear axle, or to tandem axle vehicles. 
     Additional braking arrangements (not shown) may be provided for front wheels  41  and wheels  43  on other axles  45  (shown in phantom) such as axles on trailers  25 . 
     In the embodiment shown in  FIG. 1 , a proportional valve  47  is provided between the source of pressurized air  27  and the first brake  33 . Opening of the proportional valve  47  and, therefore, the extent to which the first brake  33  performs a braking function, is proportional to pressure in a line  49  between the source of pressurized air  27  and a supply port  51  of the proportional valve  47 . A proportional valve that is well-suited for use as the proportional valve  47  is a Bendix BP-R1 Bobtail Proportioning Relay Valve, available from Bendix Commercial Vehicle Systems LLC, 901 Cleveland Street, Elyria, Ohio 44035. While the configuration of lines associated with the proportional valve  47  may vary depending upon the particular proportional valve used, in a presently preferred embodiment, a control line  53  is provided between the line  49  and a control port  55  in the proportional valve  47  so that pressure in the line  49  and, thus, the line  53 , determines the extent to which the proportional valve  47  opens. The line  49  between the source of pressurized air  27  and the supply port  51  of the proportional valve  47  is part of a line including a line  49 ′ between a delivery port  59  of the proportional valve and the first pneumatic brake arrangement  31 . 
     A conventional brake pedal arrangement  57  that includes or is associated with a valve (not shown) in the line  49  is provided and opens or closes flow from the source of pressurized air  27  to the supply port  51  of the proportional valve  47  depending upon whether the valve is depressed or not. The extent to which a brake pedal (not shown) of the brake pedal arrangement  57  is depressed corresponds to braking demand and determines the extent to which the valve of the brake pedal arrangement opens and, consequently, the pressure in the line  49 . Between minimal, usually zero, pressure in the line  49  when the brake pedal is not depressed and maximum pressure in the line equal to maximum pressure available from the source of pressurized air  27  when the brake pedal is fully depressed, there is ordinarily a substantially linear, direct relationship between the extent to which the brake pedal is depressed, the valve is opened, and the pressure in the line  49 . When, for example, braking demand is minimal as when a user applies a light touch to the brake pedal, the valve of the brake pedal arrangement opens a small amount and pressure in the line  49  is relatively low. When braking demand is greater, as when the user applies more force to the brake pedal, the valve of the brake pedal arrangement opens more so that pressure in the line  49  increases and, as the valve of the brake pedal arrangement becomes fully open, pressure in the line  49  can approach or equal the pressure provided by the source of pressurized air  27 . 
     If the proportional valve  47  were not present or were always completely open, the relationship between pressure supplied from the source of pressurized air  27  through the brake pedal arrangement  57  (i.e. the pressure arriving at the supply port  51  of the proportional valve) and pressure delivered to the first pneumatic brake arrangement  31  (i.e. the pressure delivered from a delivery port  59  of the proportional valve) will ordinarily be expected to be substantially linear, as shown by line X in  FIG. 7 . For certain types of brakes, such as drum brakes, if a proportional valve  47  is not present or is always completely open, the relationship between brake demand and brake torque applied may have a shape similar to that shown illustratively by line Y in  FIG. 8 , it being understood that the line Y is merely illustrative and that different brakes may have different brake demand and brake torque curves. 
     Ordinarily, the proportional valve  47  is closed, such as by an ordinarily closed proportioning piston (not shown) that is urged to a closed position by a structure such as a spring (not shown) when the brake pedal is not depressed and pressure in the line  53  is zero. When the brake pedal is depressed, the proportional valve  47  opens as pressure in the line  53  increases and overcomes the force of the structure, such as a spring, that urges the proportioning piston to the closed position. 
     The extent to which the proportional valve  47  opens depends upon the pressure in the line  49  and, thus, the line  53 . By providing sufficient resistance to opening of the proportional valve  47 , when there is relatively low pressure in the lines  49  and  53  the proportional valve will tend to remain mostly closed until pressure in the lines increases and the proportional valve opens more fully or completely. The relationship between pressure in the lines  49  and  53  and the extent to which such a proportional valve  47  opens is not linear but, rather, follows one of the curves A in  FIG. 7 . Thus, by providing such proportional valve  47  and control line  53  downstream of the brake pedal arrangement  57 , pressure at the supply port  51  of the proportional valve  47  versus pressure in a line  49 ′ between a delivery port  59  of the proportional valve and the first pneumatic brake arrangement  31  is not linear and, for a brake of the type where the brake torque corresponds to pressure in the line  49 ′, the curve of brake torque versus brake demand will be non-linear in a shape similar to that shown illustratively by curve B in  FIG. 8 . 
     A relay valve  61  is provided in a line  63  between the brake pedal arrangement  57  and the second pneumatic brake arrangement  37 . The extent to which the brake pedal is depressed may be directly proportional to the pressure in the line  63  and, for purposes of discussion, will be assumed to be directly proportional to the pressure in the line  63 . Depending upon the nature of the second pneumatic brake arrangement  37 , the extent to which the brake pedal is depressed will ordinarily correspond to the extent to which the second brake  39  engages, although not necessarily in a linear manner. For certain types of brakes, such as disc brakes, the extent to which the second brake  39  engages, i.e. the brake torque delivered, as the result of increased pressure in the line  63  due to depression of the brake pedal, i.e. brake demand may have more of a curved shape as shown by curve C in  FIG. 8 . 
     According to an aspect of the invention, the first pneumatic brake arrangement  31  and the second pneumatic brake arrangement  37  function differently. For example, the first pneumatic brake arrangement  31  and the second pneumatic brake arrangement  37  may function differently in that the first brake  33  may be a drum brake and the second brake  39  may be a disc brake as shown in  FIG. 1 . Alternatively, as seen, for example, in  FIG. 4 , the first pneumatic brake arrangement  31 ′ and the second pneumatic brake arrangement  37 ′ may function differently in that a second brake  39 ′ including an anti-lock braking system may be associated with the second pneumatic brake arrangement and not with the first brake  33 ′ associated with the first pneumatic brake arrangement  31 ′. The particular types of first and second brakes  33 ′ and  39 ′ associated with the first pneumatic brake arrangement  31 ′ and the second pneumatic brake arrangement  37 ′ are not necessarily otherwise different. For example, the first and second brakes  33 ′ and  39 ′ may both be drum brakes or disc brakes, however, they may be different, such as by providing drum brakes for the first brakes  33 ′ and disc brakes for the second brakes  39 ′. 
     By providing the proportional valve  47  in the line  49 ,  49 ′ between the source of pressurized air  27  and the first pneumatic brake arrangement  31  as shown in  FIG. 1 , the shape of a brake torque versus brake demand curve of the first pneumatic brake arrangement  31  that would ordinarily be substantially linear in the fashion of curve Y in  FIG. 8  can be caused to follow a curve C that approximates the shape of the brake torque versus brake demand curve B of the second pneumatic brake arrangement  37 . In this way, torque imbalance and uneven brake wear that tends to occur when one type of brake, such as a drum brake, is provided on one vehicle axle and a different type of brake, such as a disc brake, is provided on another can be minimized. 
     The vehicle  21 ′ shown in  FIG. 4  is identical to the vehicle  21  shown in  FIG. 1  except that, in the first pneumatic brake arrangement  31 ′ in the tractor  23 ′ of the vehicle  21 ′, the first brakes  33 ′ are not provided with an anti-lock braking system (ABS) while the second brakes  39 ′ of the second pneumatic brake arrangement  37 ′ are provided with ABS. Other than the first brakes  33 ′ not having ABS while the second brakes  39 ′ have ABS, the first brakes may be the same type of brake as the second brakes (e.g., both drum brakes or both disc brakes) or may be different types of brakes (e.g., first brakes drum brakes and second brakes disc brakes). 
     Ordinarily, wheels with brakes that do not have ABS will tend to lock before wheels with brakes that do have ABS, which can tend to result in flat spots on the tires of the locked wheels. As in the vehicle  21  shown in  FIG. 1 , a line  49  extends between the source of pressurized air  27  and the supply port  51 ′ of a proportional valve  47 ′, and a control line  53  branches off the line  49  and leads to a control port  55 ′ of the proportional valve. A line  49 ′ extends from a delivery port  59 ′ of the proportional valve  47 ′ to the first pneumatic brake arrangement  31 ′. By providing the proportional valve  47 ′ in the line  49 ,  49 ′ between the source of pressurized air  27  and the first pneumatic brake arrangement  31 ′ in the vehicle  21 ′ as shown in  FIG. 4  where the first brakes  33 ′ are not provided with ABS will tend to reduce the tendency of those first brakes to lock and to more closely approximate the generally non-locking braking characteristics of the second brakes  39 ′ of the second pneumatic brake arrangement  37 ′ that are provided with ABS. This will, thus, tend to reduce flat spots on tires due to locking of the first brakes  29 . 
     It will be appreciated that the first pneumatic brake arrangement and the second pneumatic brake arrangement may function differently in other ways than in the illustrative examples expressly described herein. With respect to all embodiments of the present invention, regardless however the first pneumatic brake arrangement and the second pneumatic brake arrangement might function differently, providing a proportional valve or proportional valves according to aspects of the present invention can facilitate causing the first and second pneumatic brake arrangements to behave more like each other and reduce or avoid torque imbalance and uneven brake wear and/or reduce or avoid the tendency of certain brakes to lock while others do not so that occurrence of flat spots on tires is reduced. 
       FIGS. 2 and 5  show vehicles  121 ,  121 ′, respectively, with tractors  123 ,  123 ′, respectively, similar to those shown in  FIGS. 1 and 4  except that, instead of a proportional valve configured to alter the curves of brake torque versus brake demand in response to pressure in a control line that corresponds the extent to which an operator depresses a brake pedal, a proportional valve  147  ( FIG. 2 ) or  147 ′ ( FIG. 5 ) is configured to alter the curves of brake torque versus brake demand in response to the extent to which the vehicle is loaded. With reference to  FIG. 2  for purposes of discussion, a control line  153  connects a suspension airbag  165  to a control port  155  of the proportional valve  147 . As in  FIG. 1 , a line  49  connects the source of pressurized air  27  to the supply port  151  of the proportional valve  147 , and a line  49 ′ connects the delivery port  159  of the proportional valve to the first pneumatic brake arrangement  131 . 
     When the suspension airbag  165  is unloaded or at a minimal load such that there is no or minimal pressure in the line  153 , the proportional valve  147  is closed, such as by an ordinarily closed proportioning piston (not shown) that is urged to a closed position by a structure such as a spring (not shown). When the load on the suspension airbag  165  is increased, the proportional valve  147  opens as the load and pressure in the line  153  increases and overcomes the force of the structure, such as a spring, that urges the proportioning piston to the closed position. The extent to which the proportional valve  147  opens depends upon the pressure in the suspension airbag  165  (which is a function of the load of the vehicle on the suspension airbag) and, thus, the line  153 . 
     If the vehicle  121  is unloaded or lightly loaded and pressure in the suspension airbag  165  is relatively low, when an operator depresses the brake pedal of the brake pedal arrangement  157 , the proportional valve  147  will be less open than at higher loads and the pressure in the line  149  between the source of pressurized air  127  and the proportional valve will be less than the pressure in the line  149 ′ between the proportional valve and the first brake  133  of the first pneumatic brake arrangement  131  so that braking torque provided is less than would otherwise be provided without the proportional valve. At higher vehicle loads, as pressure in the suspension airbag  165  increases, the proportional valve  147  will open to a greater extent than at lower loads and the pressure in the line  49 ′ between the proportional valve and the first brake  133  of the first pneumatic brake arrangement  131  will approach or equal the pressure in the line  49  between the source of pressurized air  27  and the proportional valve so that braking torque provided approaches or equals that which would otherwise be provided without the proportional valve. 
     In  FIG. 2 , as in  FIG. 1 , the first brake  133  can be a brake such as a drum brake that, ordinarily (in the absence of any proportional valve), has a relationship between brake demand and brake torque that might follow a curve Y as shown in  FIGS. 8 and 9 , and the second brake  139  of the second pneumatic brake arrangement  137  can be a brake such as a disc brake that tends to have a relationship between brake demand and brake torque that might follow a curve such as curve C in  FIGS. 8 and 9 . By providing the proportional valve  147  between the line  49  between the source of pressurized air  27  and the proportional valve and the line  49 ′ between the proportional valve and the first brake  133 , at lower vehicle loads, the first brake will provide less brake torque in response to a particular brake demand than it otherwise would without the proportional valve while, at the same time, the second brake  139  will provide the brake torque that it ordinarily does in response to that brake demand. At full load, i.e. when the proportional valve  147  is open 100%, the first brake  133  will tend to have a brake torque versus brake demand curve the same as or similar to the curve Y. As load decreases, the curve will tend to be shallower, particularly when there is low brake demand. At high brake demand, it will ordinarily be desirable for the curve to approach the curve Y, even at low loads. When brake demand is lower, the shallower curve of brake torque versus brake demand of the brake  133  of the first pneumatic brake arrangement  131  can more closely approximate the brake torque versus brake demand provided curve C of the second pneumatic brake arrangement  137  and torque imbalance and uneven brake wear that tends to occur when one type of brake, such as a drum brake, is provided on one vehicle axle and a different type of brake, such as a disc brake, is provided on another can be minimized. 
     The vehicle  121 ′ shown in  FIG. 5  is identical to the vehicle  121  shown in  FIG. 2  except that, in the first pneumatic brake arrangement  131 ′ in the tractor  123 ′ of the vehicle  121 ′, the first brakes  133 ′ are not provided with an anti-lock braking system (ABS) while the second brakes  139 ′ of the second pneumatic brake arrangement  137 ′ are provided with ABS. Other than the first brakes  133 ′ not having ABS while the second brakes  139 ′ of the second pneumatic brake arrangement  137 ′ have ABS, the first brakes may be the same type of brake as the second brakes (e.g., both drum brakes or both disc brakes) or may be different types of brakes (e.g., first brakes drum brakes and second brakes disc brakes). 
     As in the vehicle  121  shown in  FIG. 2 , a line  49  extends between the source of pressurized air  27  and the supply port  151 ′ of a proportional valve  147 ′, and a control line  153 ′ extends from a suspension airbag  165 ′ and leads to a control port  155 ′ of the proportional valve. A line  49 ′ extends from a delivery port  159 ′ of the proportional valve  147 ′ to the first pneumatic brake arrangement  131 ′. Providing the proportional valve  147  in the line  49 ,  49 ′ between the source of pressurized air  27  and the first pneumatic brake arrangement  131 ′ in the vehicle  121 ′ as shown in  FIG. 5  where the first brakes  133 ′ are not provided with ABS will tend to reduce the tendency of those first brakes to lock and approximate the generally non-locking braking characteristics of the second brakes  139 ′ of the second pneumatic brake arrangement  137 ′ that are provided with ABS and will, thus, tend to reduce flat spots on tires due to locking of the first brakes. 
       FIGS. 3 and 6  show vehicles  221 ,  221 ′, respectively, with tractors  223 ,  223 ′, respectively, similar to those shown in  FIGS. 1 and 4  and  FIGS. 2 and 5  except that, in addition to a proportional valve  247   a  ( FIG. 3 ) and  247   a ′ ( FIG. 6 ) configured to alter the curves of brake torque versus brake demand in response to pressure in a control line that corresponds the extent to which an operator depresses a brake pedal as in  FIGS. 1 and 4 , a proportional valve  247   b  ( FIG. 3 ) or  247   b ′ ( FIG. 6 ) is also provided in series with the proportional valves  247   a  and  247   a ′ and is configured to alter the curves of brake torque versus brake demand in response to the extent to which the vehicle is loaded. 
     With reference to  FIG. 3  for purposes of discussion, a control line  253   a  is provided between the line  49  and a control port  255   a  in the proportional valve  247   a  so that pressure in the line  49  and, thus, the line  253   a , determines the extent to which the proportional valve  247   a  opens. The line  49  between the source of pressurized air  27  and the supply port  251   a  of the proportional valve  247   a  is part of a line including a line  49 ″ between a delivery port  259   a  of the proportional valve  247   a  and a supply port  251   b  of the proportional valve  247   b , and a line  49 ′ between a delivery port  259   b  of the proportional valve  247   b  and the first pneumatic brake arrangement  231 . 
     As in the embodiment of  FIGS. 1 and 4 , the pressure at the delivery port  259   a  of the proportional valve  247   a  will depend upon brake demand, usually the extent to which an operator depresses a brake pedal of the brake arrangement  57 . In the vehicle  221  of  FIG. 3 , the first brakes  233  of the first pneumatic brake arrangement  231  can be brakes such as drum brakes that tend to have a higher brake factor than, a greater tendency to self-energize, and to dissipate heat less effectively than disc brakes, and the second brake arrangement  237  functions differently from the first brake arrangement, such as by including brakes such as disc brakes. 
     A control line  253   b  connects a suspension airbag  265  to a control port  255   b  of the proportional valve  247   b . When there is no pressure in the suspension airbag  265  is unloaded or at a minimal load such that there is no or minimal pressure in the line  253   b , the proportional valve  247   b  is closed or open to a minimal degree, such as by an ordinarily closed proportioning piston (not shown) that is urged to a closed position by a structure such as a spring (not shown). When the load on the suspension airbag  265  is increased, the proportional valve  247   b  opens as the load and pressure in the line  253  increases and overcomes the force of the structure, such as a spring, that urges the proportioning piston to the closed position. The extent to which the proportional valve  247   b  opens depends upon the pressure in the suspension airbag  265  (which is a function of the load of the vehicle on the suspension airbag) and, thus, the line  253 . Thus, the pressure at the delivery port  259   b  of the proportional valve  247   b  will depend upon the load of the vehicle as well as upon the pressure delivered to the supply port  251   b  of the proportional valve  247   b  via the line  49 ″ from the delivery port  259   a  of the proportional valve  247   a , which depends upon brake demand, normally the extent to which an operator depresses the brake pedal of the brake arrangement  57 . In this way, as in the illustrative graphs shown in  FIG. 9 , at full vehicle load, the proportional valve  247   b  can be assumed to be fully open and the brake torque versus brake demand provided curve B of the first pneumatic brake arrangement  231  can more closely approximate the brake torque versus brake demand provided curve C of the second pneumatic brake arrangement  237  and torque imbalance and uneven brake wear that tends to occur when one type of brake, such as a drum brake, is provided on one vehicle axle and a different type of brake, such as a disc brake, is provided on another can be minimized. At lower vehicle loads, the brake torque versus brake demand curves for the first pneumatic brake arrangement are shifted so that less braking torque is provided as braking demand decreases. For example, as seen in  FIG. 9 , at zero load (usually meaning that the pressure in the suspension airbags is due entirely to the weight of the vehicle), the brake torque versus brake demand curve E will be shallower at low brake demand than the brake torque versus brake demand curve D when the vehicle is at 50% of maximum load, and shallower still at low brake demand than the brake torque versus brake demand curve B for the vehicle at 100% of maximum load. As brake demand increases, braking torque will ordinarily increase toward a maximum braking torque available through the first brake arrangement. 
     The vehicle  221 ′ shown in  FIG. 6  is identical to the vehicle  221  shown in  FIG. 3  except that, in the first pneumatic brake arrangement  231 ′ in the tractor  223 ′ of the vehicle  221 ′, the first brakes  233 ′ are not provided with an anti-lock braking system (ABS) while the second brakes  239 ′ of the second pneumatic brake arrangement  237 ′ are provided with ABS. Other than the first brakes  233 ′ not having ABS while the second brakes  239 ′ of the second pneumatic brake arrangement  237 ′ have ABS, the first brakes may be the same type of brake as the second brakes (e.g., both drum brakes or both disc brakes) or may be different types of brakes (e.g., first brakes drum brakes and second brakes disc brakes). As in the vehicle  221  shown in  FIG. 3 , a line  49  extends between the source of pressurized air  27  and the supply port  251   a ′ of a first proportional valve  247   a ′, and a control line  253   a  branches off the line  49  and leads to a control port  255   a ′ of the first proportional valve. A line  49 ″ extends from a delivery port  259   a ′ of the first proportional valve  247   a ′ to the supply port  251   b ′ of a second proportional valve  251   b ′, a control line  253   b ′ extends from a suspension airbag  265 ′ to a control port  255   b ′ of the second proportional valve, and a line  49 ′ extends from the delivery port  259   b ′ of the second proportional valve to the first pneumatic brake arrangement  231 ′. Providing the proportional valves  247   a ′,  247   b ′ in the line  49 ,  49 ′,  49 ″ between the source of pressurized air  27  and the first pneumatic brake arrangement  231 ′ in the vehicle  221 ′ as shown in  FIG. 6  where the first brakes  233 ′ are not provided with ABS will tend to reduce the tendency of those first brakes to lock and can more closely approximate the generally non-locking braking characteristics of the second brakes  239 ′ of the second pneumatic brake arrangement  237 ′ that are provided with ABS and will, thus, tend to reduce flat spots on tires due to locking of the first brakes. 
     In the present application, the use of terms such as “including” is open-ended and is intended to have the same meaning as terms such as “comprising” and not preclude the presence of other structure, material, or acts. Similarly, though the use of terms such as “can” or “may” is intended to be open-ended and to reflect that structure, material, or acts are not necessary, the failure to use such terms is not intended to reflect that structure, material, or acts are essential. To the extent that structure, material, or acts are presently considered to be essential, they are identified as such. 
     While this invention has been illustrated and described in accordance with a preferred embodiment, it is recognized that variations and changes may be made therein without departing from the invention as set forth in the claims.