Abstract:
Two and four point truck cab suspensions comprise combined spring/damping element units at at least the rear corners of the cab, and preferably at all four corners of the cab.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to secondary suspension systems for trucks and, more particularly, to a truck cab suspension system providing a high degree of noise and shock isolation. 
     2. Description of the Prior Art 
     The load carried by a truck is supported by a frame which is resiliently carried by suspension springs from the truck&#39;s axles. The primary axle suspension springs usually have very high spring rates which make the ride jarring to the operator and to structures supported by the frame. To dampen the vibration and shock transmitted to the cab, secondary suspension systems have been employed utilizing conventional suspension components such as air springs to isolate the cab structure from shock and shock absorbers to dampen motion of the cab, respectively. Shock and vibration isolation improves occupant comfort and reduces stress fatigue to the cab structure. 
     Secondary suspension systems connect the cab to, and support the cab from, the vehicle chassis. The secondary suspension system must support the cab&#39;s weight, guide vertical, lateral and fore to aft movement of the cab, improve ride comfort by isolating the cab from vibrations and dampening those vibrations, absorb collision energies, and if desired, provide cab leveling. 
     Early cab suspension systems placed hard rubber element directly between the cab and the vehicle frame. This sort of minimal suspension system proved uncomfortable for the vehicle&#39;s occupants and could place undesirable force levels on the cab promoting premature structural failure. 
     Later cab suspensions provided differentiated forward and aft suspension sub-systems making use of a rear suspension sub-system having air springs or coil springs, a guide element based on a panhard rod or other lateral locators, and a dampening element. The air spring and damping elements of the rear suspension sub-system do not provide fore to aft constraints to cab movement. This forces the use of very stiff forward rubber elements which exhibit poor noise isolation. 
     Europe has favored cab over engine designs due in part to the tight space constraints of many European roads. Typical linehaul European trucks have increasingly been equipped with a cab suspension having suspensive and damping elements located outside of the chassis siderails, that is, toward the outside edges of the truck cab. The suspension element, e.g. the spring, and the damping element, e.g. the shock absorber, are provided as a combined strut, one strut being located at each of the four outer corners of the cab. The top of the strut is attached to the bottom of the cab by a rubber bushing. See Gross, et al.,  Development of a  4- point - Air Cab Suspension System for Conventional Heavy Trucks  (SAE Technical paper 2001-01-2708). However, the weight of the cab is suspended through the primary springs and the bushings, placing the bushings under compression. This has, in practice, required the provision of higher spring rate bushings than would be required if the bushings were not under constant compressive loading. 
     SUMMARY OF THE INVENTION 
     According to the invention there is provided a vehicle having a suspended cab. The vehicle has a chassis with elongated side rails. The truck cab suspension comprises a plurality of support points for the truck cab positioned on an underside of the truck cab, including two forward support points and two aft support points located near corners of the truck cab. At least two spring-strut units, one each attached to the two aft support points provide for support of the aft portion of the cab. Each spring-strut unit comprises a spring element, a shock dampening element, a yielding bushing interposed between the shock dampening element and the respective aft support point for the spring-strut unit, and a non-yielding brace element interposed between the spring element and the support point. For a four point suspension the truck cab suspension additionally comprises the two forward support points to the truck cab being disposed on the underside of the truck cab substantially forward from the rear support points and toward outside side edges of the truck cab. Two forward spring-damper strut units are attached, one to each of the forward support points. The spring-damper strut units each having a spring element, a shock dampening element, a bushing interposed between the shock dampening element and a respective forward support point, and a non-yielding mount interposed between the spring and the respective forward support point. 
     Additional effects, features and advantages will be apparent in the written description that follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a perspective view of a truck cab suspended in accordance with a first embodiment of the invention. 
     FIG. 2 is a perspective view of a truck cab suspended in accordance with a second embodiment of the invention. 
     FIG. 3 is a perspective view of a cab suspension element. 
     FIGS. 4 and 5 are cross sectional views of the two embodiments of a suspension-strut element. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the figures and in particular to FIG. 1, a truck cab  12  disposed on a truck chassis  14  is depicted. Truck chassis  14  comprises two longitudinal siderails  16  and  18  running the lengths of the left side (L) and the right side (R) of a truck or tractor unit. Truck cab  12  is suspended from the side rails at four support points  20 ,  22 ,  26  and  28  on the underside  24  of the cab. The four support points are located generally toward the corners of cab underside  24 . Support points  20 ,  22 ,  26  and  28  are preferably provided along a section of a beam or channel forming a portion of the frame for the cab superstructure. 
     Cab  12  is suspended from siderails  16  and  18  by forward and aft suspension subsystems, the aft suspension subsystem comprising two spring-strut units  30  and  32 , and the forward suspension subsystem comprising rubber block assemblies  33  and  34 . Spring-strut units  30  and  32  are mounted with respect to siderails  16  and  18 , respectively, by mounting arms attached to the outside face of the siderails (only mounting arm  31  for spring-strut  30  is visible). The mounting arms may be attached in any conventional manner, for example by bolting, welding, screwing, and the like. Spring-strut units  30  and  32  are attached to aft support points  20  and  22  on truck cab  12 , respectively. Rubber block assemblies  33  and  34  are of types well known in the art and are not described in detail. They provide suspension, damping and guiding functions for the suspended weight and are attached both to the siderails  16  and  18  and to support points  26  and  28  by open brackets which are linked by a rubber bushing. The type of suspension illustrated in FIG. 1 is termed a  2 -point suspension in the art. It otherwise provides an inferior ride to 4-point suspensions due to the poor shock isolation through the rubber blocks. An alternative  4- point suspension is illustrated in FIG.  2 . Forward rubber block assemblies  33  and  34  have been replaced by spring-strut units  36  and  40 . Spring-strut units  36  and  40  are identical to the aft spring strut units. They are attached to siderails using mounting arms (with mounting arm  38  for left forward spring-strut unit  36  being visible). The preferred locations for the spring-strut assemblies are outside of the siderails  16  and  18  toward the horizontal perimeter of the cab  12 . 
     In FIG. 3 the location and positioning of an exemplary spring-strut unit  50  is illustrated. Unit  50  is located between the underside  24  of a cab  12  one of the major longitudinal siderails  11  of a vehicle. Extending from the outward side of siderail  11  is a mounting arm  59 , which is a generally A-shaped piece, attached at its feet to siderail  11 . A rod or pillar  55  is firmly fixed in the head of the “A” above the cross-arm and supports a co-axial assembly  57  of the spring and shock dampening strut of the unit-on a base  60 . Co-axial assembly  57  abuts a support point  52  formed at the intersection of two frame support beams  51  and  53  forming part of the structure of cab  12 . 
     Spring-strut units  50  can be based on a metal coil spring or an air spring. FIGS. 4 and 5 detail differences in construction of the two types of spring-strut units  50  in cross sectional views. Located between base  60  and an exemplary support point  52 , spring-strut unit  50  can have either a coil spring  257  or an air spring  157  as a suspension element. Either type of suspension unit is arranged around a shock dampening element  64  to be co-axial with the shock dampening element on strut  72 . Spring-strut unit  50  is mated to a support point  52  using a bushing assembly  80 . Bushing assembly  80  comprises an outer sleeve  65  having out turned flange  66 , an inner barrel  68 , and a rubber sleeve  70  located between the inner barrel and the outer sleeve and fused to each. The rubber sleeve  70  has different spring rates in different directions to achieve cab stability. Inner barrel  68  has some freedom of movement as a result of the compressibility of rubber sleeve  70 . The suspension element (springs  157  or  257 ) bypass the rubber sleeve  70  so that the bushing does not carry the weight of the cab and compromising the compression characteristics of the rubber. 
     Strut  72  has one end which is disposed in and can move up and down with respect to shock dampening element  64 . The opposite end is firmly retained barrel  68  by a bolt  74  which extends upwardly through an opening in the barrel end allowing a retaining washer  76  and bolt  78  to be applied thereto. Outer sleeve  65  is attached to support point  52 , preferably by spot welding of flange  66  to the support point. Sleeve  65  provides a support area for receiving one end of either air spring  157  or coil spring  257 . Thus, strut  72 , and the shock dampening element of the unit are isolated from the cab by a rubber bushing, while the shock isolation provided by the springs is not so isolated. 
     The invention provides an advantageous cab suspension that allocates load to the several support points of the cab fairly evenly. With struts solidly attached to the frame, fore/aft as well as lateral, yaw and pitching forces are more readily shared by the front and the rear of the cab structure. Adequate vertical movement is still allowed for providing a soft ride. The various compression rates of the rubber bushings can be varied to provide variable amounts of freedom of movement in various directions. 
     While the invention is shown in only one of its forms, it is not thus limited but is susceptible to various changes and modifications without departing from the spirit and scope of the invention.