Abstract:
A snowmobile frame ( 106 ) is described, with a tunnel ( 108 ) having an inverted U-shape. An engine cradle ( 110 ) is attached to a forward portion of the tunnel. A suspension assembly ( 112 ) is disposed forwardly of the engine cradle. At least one support member ( 142 ) supports the suspension assembly on the engine cradle. The at least one support member forms a crumple zone between the suspension assembly and the engine cradle. The at least one support member resists plastic deformation in response to the first predetermined force exerted on the suspension assembly. The at least one support member undergoes plastic deformation in response to a second predetermined force being exerted on the suspension assembly. The second predetermined force is greater in magnitude than the first predetermined force. The tunnel resists plastic deformation in response to the second predetermined force.

Description:
CROSS-REFERENCE 
       [0001]    The present application is the United States National Stage of PCT/US2008/085059, filed Nov. 28, 2008, entitled ‘Snowmobile Frame’, the entirety of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to snowmobile frames. 
       BACKGROUND OF THE INVENTION 
       [0003]    Snowmobiles, particularly utility snowmobiles, are sometimes used in areas without groomed trails. In such conditions, it is not uncommon for the front skis of the snowmobile to strike an object hidden from view by the snow, such as a rock or a tree stump. As such, the frames of these snowmobiles must be of a sufficiently rigid construction to withstand these impacts. 
         [0004]    Referring to  FIG. 1 , a prior art snowmobile frame  10  includes a tunnel  12  made of stamped sheet metal having an inverted U-shaped cross section. An endless drive track (not shown) is disposed in part inside the tunnel  12 . An engine cradle  14  is rigidly attached to a forward part of the tunnel  12 , for example with bolts, by welding, or by forming both together as a single part. The engine cradle  14  supports an internal combustion engine  16  that powers the drive track via a transmission (not shown) to propel the snowmobile. The inherent rigidity of the engine  16  contributes additional rigidity to the engine cradle  14 . A front suspension assembly  18  includes a cross member  20  that also forms a forward part of the engine cradle  14 . The cross member  20  supports a pair of front skis  22 , each via a shock absorber  24 . The rigidity of the frame  10  may be further enhanced by the addition of a pyramidal support structure (not shown) connected to the tunnel  12 , the engine cradle  14  and the front suspension assembly  18 . 
         [0005]    In the event that one of the skis of the snowmobile strikes an object, the frame  10  absorbs the forces differently depending on the severity of the impact. Light impacts are absorbed primarily by the shock absorber  24 . Heavier impacts are sufficiently strong to fully compress the shock absorber, with the result that the majority of the impact is borne by the rigid portions of the frame  10 , namely the tunnel  12 , the engine cradle  14  and the front suspension assembly  18 . In most cases, the frame  10  is rigid enough to withstand these impacts without sustaining permanent damage. However, it is possible for the impact to be severe enough to cause plastic deformation of the frame  10 . The plastic deformation occurs at the portion of the frame  10  that is structurally the weakest, generally either a part of the tunnel  12  or side walls of the engine cradle  14  that form part of the connection between the tunnel  12  and the engine cradle  14 . As a result, plastic deformation of the frame  10  generally causes a misalignment between the engine cradle  14  and the tunnel  12 , and therefore between components of the transmission that transfer power from the engine  16  to the endless track, potentially rendering the snowmobile inoperable. 
         [0006]    Attempting to repair the snowmobile by restoring the shape of the existing frame  10  can result in a less than perfect alignment of transmission components, because the bends cannot always be completely removed. In these cases, it is impossible to adequately repair the vehicle without replacing the entire frame  10 , resulting in high repair costs. 
         [0007]    Therefore, there is a need for a snowmobile frame having reduced repair costs from sustaining severe impacts. 
       SUMMARY OF THE INVENTION 
       [0008]    It is an object of the present invention to ameliorate at least some of the inconveniences present in the prior art. 
         [0009]    It is also an object of the present invention to provide a snowmobile frame having a crumple zone between the front suspension assembly and the engine cradle. 
         [0010]    It is also an object of the present invention to provide a snowmobile frame that maintains alignment between the engine cradle and the tunnel when experiencing an impact severe enough to damage the frame. 
         [0011]    It is also an object of the present invention to provide a snowmobile frame that resists plastic deformation of the tunnel when experiencing an impact severe enough to damage the frame. 
         [0012]    In one aspect, the invention provides a snowmobile frame comprising a tunnel having an inverted U-shape. An engine cradle for supporting an engine with a crankshaft oriented transversely is attached to a forward portion of the tunnel and extends forwardly from the tunnel. A suspension assembly is disposed forwardly of the engine cradle. At least one support member supports the suspension assembly on the engine cradle. Each of the at least one support member has a rearward portion connected to the engine cradle and a forward portion connected to the suspension subassembly. The at least one support member forms a crumple zone between the suspension assembly and the engine cradle. 
         [0013]    In a further aspect, the at least one support member is at least one generally triangular support member. 
         [0014]    In a further aspect, the at least one support member includes bent sheet metal. 
         [0015]    In a further aspect, the at least one support member comprises two support members. 
         [0016]    In a further aspect, each of the at least one support member has a generally rearwardly facing base attached to the engine cradle and a generally forwardly facing apex attached to the suspension assembly. The apex is disposed laterally outwardly of the base with respect to a longitudinal center line of the frame. 
         [0017]    In a further aspect, a pyramidal support structure is disposed generally above the tunnel and the engine cradle. The pyramidal support structure has two first members connected to the tunnel. The pyramidal support structure has two second members connected to corresponding ones of the two support members. The second members are disposed forwardly of the first members. 
         [0018]    In a further aspect, the apex of each of the at least one support member is disposed laterally outwardly of the engine cradle with respect to the longitudinal center line of the frame. 
         [0019]    In a further aspect, the at least one support member undergoes plastic deformation in response to a force of between 17,000 N and 22,000 N exerted on the suspension assembly. The force is oriented in a vertical plane perpendicular to an axis of rotation of the crankshaft and angled upwardly and rearwardly at an angle of between 62 degrees and 68 degrees from horizontal. 
         [0020]    In a further aspect, the force is about 22,000 N. The force is oriented at an angle of about 65 degrees from horizontal. 
         [0021]    In a further aspect, the suspension assembly includes a generally transverse cross member. Left and right telescopic struts are attached to the cross member. The at least one support member is attached to the suspension assembly via the cross member. 
         [0022]    In a further aspect, the suspension assembly is spaced apart from the engine cradle. 
         [0023]    In an additional aspect, the invention provides a snowmobile frame comprising a tunnel having an inverted U-shape. An engine cradle for supporting an engine with a crankshaft oriented transversely is attached to a forward portion of the tunnel and extends forwardly from the tunnel. A suspension assembly is disposed forwardly of the engine cradle. The suspension assembly includes at least one shock absorber. The at least one shock absorber has a bottom-out position. At least one support member supports the suspension assembly on the engine cradle. The at least one shock absorber reaches the bottom-out position in response to a first predetermined force being exerted on the suspension assembly. The at least one support member resists plastic deformation in response to the first predetermined force being exerted on the suspension assembly. The at least one support member undergoes plastic deformation in response to a second predetermined force being exerted on the suspension assembly. The second predetermined force is greater in magnitude than the first predetermined force. The tunnel resists plastic deformation in response to the second predetermined force being exerted on the suspension assembly. 
         [0024]    In a further aspect, the at least one support member is at least one generally triangular support member. 
         [0025]    In a further aspect, the at least one support member includes bent sheet metal. 
         [0026]    In a further aspect, the at least one support member comprises two support members. 
         [0027]    In a further aspect, each of the at least one support member has a generally rearwardly facing base attached to the engine cradle and a generally forwardly facing apex attached to the suspension assembly. The apex is disposed laterally outwardly of the base with respect to a longitudinal center line of the frame. 
         [0028]    In a further aspect, a pyramidal support structure is disposed generally above the tunnel and the engine cradle. The pyramidal support structure has two first members connected to the tunnel. The pyramidal support structure has two second members connected to corresponding ones of the two support members. The second members are disposed forwardly of the first members. 
         [0029]    In a further aspect, the apex of each of the at least one support member is disposed laterally outwardly of the engine cradle with respect to the longitudinal center line of the frame. 
         [0030]    In a further aspect, the magnitude of the second predetermined force is between 17,000 N and 22,000 N. The second predetermined force is oriented in a vertical plane perpendicular to an axis of rotation of the crankshaft and angled upwardly and rearwardly at an angle of between 62 degrees and 68 degrees from horizontal. 
         [0031]    In a further aspect, the magnitude of the second predetermined force is about 22,000 N. The second predetermined force is angled upwardly and rearwardly at about 65 degrees from horizontal. 
         [0032]    In a further aspect, the suspension assembly includes a generally transverse cross member. Left and right telescopic struts are attached to the cross member. The at least one support member is attached to the suspension assembly via the cross member. 
         [0033]    In a further aspect, the suspension assembly is spaced apart from the engine cradle. 
         [0034]    For the purposes of this application, directional terms such as “horizontal” and “vertical” in relation to a part of a snowmobile refer to the orientation of the part when placed on a snowmobile that is steered straight on level ground, with a 170 lb driver seated thereon in a normal driving position, and should be understood as they would be perceived by the driver while seated in the normal driving position. 
         [0035]    Embodiments of the present invention each have at least one of the above-mentioned objects and/or aspects, but do not necessarily have all of them. It should be understood that some aspects of the present invention that have resulted from attempting to attain the above-mentioned objects may not satisfy these objects and/or may satisfy other objects not specifically recited herein. 
         [0036]    Additional and/or alternative features, aspects, and advantages of embodiments of the present invention will become apparent from the following description, the accompanying drawings, and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]    For a better understanding of the present invention, as well as other aspects and further features thereof, reference is made to the following description which is to be used in conjunction with the accompanying drawings, where: 
           [0038]      FIG. 1  is a perspective view, taken from a front, left side, of a snowmobile frame according to the prior art; 
           [0039]      FIG. 2  is a perspective view, taken from a front, right side, of a snowmobile; 
           [0040]      FIG. 3  is a top plan view of a frame and skis of the snowmobile of  FIG. 2 ; 
           [0041]      FIG. 4  is a front elevation view of the frame and skis of the snowmobile of  FIG. 2 ; 
           [0042]      FIG. 5  is a side elevation view of a support member; 
           [0043]      FIGS. 6A and 6B  are perspective views, taken from a front, right side, of left and right support members respectively; and 
           [0044]      FIG. 7  is a perspective view, taken from a front, left side, of a support member attached to a snowmobile frame. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0045]    As shown in  FIG. 2 , a snowmobile  100  according to the present invention includes a forward portion  102  and a rearward portion  104  which are defined consistently with a forward travel direction of the vehicle. As best seen in  FIGS. 3 and 4 , the snowmobile  100  includes a frame (also known as a chassis)  106  which includes a rear tunnel  108 , an engine cradle  110  (seen in  FIG. 3 ) attached to a forward portion of the tunnel  108  and extending forwardly therefrom, and a front suspension assembly  112  disposed forwardly of the engine cradle  110  and attached thereto in a manner that will be discussed below in further detail. An engine  114  (shown schematically) is carried by the engine cradle portion  110  of the frame  106  which forms part of an engine compartment. The engine  114  is oriented such that the crankshaft (not shown) is transverse to the normal direction of travel of the snowmobile  100 . Two skis  116  are positioned at the forward portion  102  of the snowmobile  100  and are attached to the frame  106  via the front suspension assembly  112 . Each ski  116  is operatively connected to a steering assembly which includes a steering column (not shown) connected to a handlebar  124 . The handlebar  124  is used to rotate the skis  116  in order to steer the vehicle. 
         [0046]    An endless drive track  126  is positioned at the rear portion  104  of the snowmobile  100  under the tunnel  108 . The endless drive track  126  is operatively connected to the engine  114  through a belt transmission system (not shown). The endless drive track  126  is driven to run about a rear suspension assembly  128  to propel the snowmobile  100 . 
         [0047]    At the front portion  102  of the snowmobile  100 , a cowling  130 , made of multiple parts, encloses the engine  114  and the belt transmission system, thereby providing an external shell that not only protects the engine  114  and the belt transmission system, but can also be decorated to make the snowmobile  100  more aesthetically pleasing. Typically, the cowling  130  includes a hood and one or more side panels. At least one part of the cowling  130  can be opened or removed to allow access to the engine  114  and the belt transmission system, for example for inspection or maintenance thereof. A windshield  132  is connected to the cowling  130  near the front portion  102  of the snowmobile  100 . Alternatively, the windshield  132  could be connected directly to the handlebar  124 . The windshield  132  acts as a wind screen to lessen the force of the air on the rider while the snowmobile  100  is moving. 
         [0048]    A straddle seat  134  for a driver, and optionally one or more passengers, is positioned atop the tunnel  108  and extends from the rear portion  104  of the snowmobile  100  to the cowling  130 . Two footrests  136  are positioned on opposite sides of the snowmobile  100  below the seat  134  to accommodate the driver&#39;s and passenger&#39;s feet. 
         [0049]    Additional aspects and features of a snowmobile will be recognized by persons skilled in the art, and will not be described in detail herein. 
         [0050]    Referring now to  FIGS. 3 and 4 , the attachment of front suspension assembly  112  to the frame  106  of the snowmobile  100  will be described in further detail. 
         [0051]    The front suspension assembly  112  consists of left and right telescopic shock absorbers  138  connected to opposite ends of a transverse cross member  140 . It is contemplated that other known types of suspension may alternatively be used, such as a double A-arm suspension. The front suspension assembly  112  is spaced apart from the engine cradle  110  and supported thereon by two support members  142  which will be described below in further detail. It is contemplated that a single support member, or more than two support members, may alternatively be used. A pyramidal support structure  144  is disposed generally above the frame  106  and serves to add rigidity to the frame  106 . The support structure  144  includes two rearward members  146  attached to the tunnel  108  at their base and extending upwardly, forwardly and laterally inwardly therefrom to the apex of the support structure  144 . Two forward members  148  of the support structure  144  extend downwardly, forwardly and laterally outwardly from the apex of the support structure  144  and are connected to the support members  142 . 
         [0052]    Referring to  FIGS. 5 ,  6 A,  6 B and  7 , one of the support members  142  will be described in detail. It should be understood that the other support member  142  is a mirror image thereof and functions in a substantially similar manner. The support member  142  is made from stamped sheet metal, and has a generally triangular shape. An aperture  150  is provided in the support member  142  by removing a middle portion thereof so as to reduce its weight. The rigidity of the support member  142  is enhanced by the bent outside edges  152  of the support member  142 , the bent edges  154  of the aperture  150 , and the grooves  156  formed in the body of the support member  142 . It is contemplated that the support member  142  may be constructed in any other shape, so long as the support member  142  has the appropriate structural rigidity as will be discussed below in further detail. 
         [0053]    Referring now to  FIG. 7 , an apex  158  of the support member  142  is oriented forwardly and is connected to the front suspension subassembly  112  via a flange  160  of the cross member  140 . A base  162  of the support member  142  is disposed rearwardly of the apex  158 . The base  162  is connected at a lower portion  164  and a middle portion  166  thereof to the engine cradle  110  via corresponding flanges on the support member  142  and the engine cradle  110 . An upper portion  168  of the base  162  of the support member  142  is also connected to the forward member  148  of the support structure  144 . It is contemplated that the support member  142  may alternatively be connected only to the front suspension assembly  112  and the engine cradle  110 . The flange  160  is located laterally outwardly of the engine cradle  110 , such that the apex  158  is disposed laterally outwardly of the engine cradle  110  and the base  162  with respect to the longitudinal center line of the vehicle, to provide additional rigidity. 
         [0054]    The structure of the support members  142 , as well as the number of support members  142  and their arrangement on the frame  106 , are selected such that the support members  142  provide a crumple zone between the front suspension assembly  112  and the engine cradle  110 . The crumple zone is designed such that if a force exerted on the front suspension assembly  112  would ordinarily be great enough to cause plastic deformation of the tunnel  108 , the engine cradle  110  or the support structure  144 , the support members  142  will preferentially undergo plastic deformation, thereby absorbing most or all of the force of the impact and either preventing the force of the impact from being transmitted to the tunnel  108 , the engine cradle  110  or the support structure  144 , or reduce the force of the impact on the tunnel  108 , the engine cradle  110  or the support structure  144  to a level that can be withstood by these components without sustaining structural damage. The crumple zone is also designed such that if a force exerted on the front suspension assembly  112  would be too small to cause plastic deformation of the tunnel  108 , the engine cradle  110  or the support structure  144 , the support members  142  will maintain their shape. The support members  142  can preferably withstand a force as close as possible to the minimum force that would damage the tunnel  108 , the engine cradle  110  or the support structure  144 , to provide as rigid a frame  106  as possible while still undergoing plastic deformation in preference to the tunnel  108 , the engine cradle  110  or the support structure  144  when experiencing a severe impact. It should be understood that the support members  142  may not completely prevent damage to the tunnel  108 , the engine cradle  110 , or the support structure  144  in every case. It is possible that some impacts might be so severe that even after a portion of the force is absorbed by the plastic deformation of the support members  142 , the remaining force transmitted to the tunnel  108 , the engine cradle  110 , or the support structure  144  will still be sufficient to cause some damage thereto. 
         [0055]    The operation of the snowmobile  100  of  FIG. 2  will now be discussed by way of example. It should be understood that different snowmobiles will have different frames with different rigidity and other properties, and the maximum force designed to be borne by the support members  142  should be adjusted accordingly, by varying the shape and dimensions of the support members. If the right ski  116  strikes an object  170 , a force F is exerted on the front suspension assembly  112 . It was determined through experimentation that the force F is typically exerted at an angle θ of 62-68 degrees from horizontal, and in a vertical plane perpendicular to the transverse axis of the crankshaft, due to the geometry of the snowmobile front suspensions and the conditions in which they are typically used. If the force F exceeds about 1600 Newtons, the force will be sufficient to fully compress the right shock absorber to a bottom-out position. If the force F exceeds about 17,000-22,000 N, the support members  142  will structurally fail and undergo plastic deformation, thereby absorbing the impact and, in the majority of cases, protecting the tunnel  108 , the engine cradle  110  or the support structure  144  from the damage the tunnel  108 , the engine cradle  110  or the support structure  144  would normally sustain from such a large force. The snowmobile  100  can later be conveniently and inexpensively repaired by replacing the support members  142 . The support members  142  for this particular snowmobile may therefore be designed to withstand a force of about 22,000 N exerted on the front suspension assembly  112  at a 65 degree angle from horizontal before undergoing plastic deformation. Through experimentation conducted by applying a static force to an isolated snowmobile frame to simulate the effects of a severe impact, it was determined that this 22,000 N threshold provided sufficient rigidity so the rider would not notice a difference in the overall structural properties of the snowmobile  100  compared to prior art snowmobiles, while still protecting the tunnel  108 , the engine cradle  110  or the support structure  144  from damage. 
         [0056]    Modifications and improvements to the above-described embodiments of the present invention may become apparent to those skilled in the art. The foregoing description is intended to be exemplary rather than limiting. The scope of the present invention is therefore intended to be limited solely by the scope of the appended claims.