Abstract:
A wheel cover retention system for a wheel having a plurality of lug studs extending through the wheel. The wheel cover includes a base having an outboard surface and an inboard surface opposite the outboard surface, and also having lug towers formed integrally therein and projecting inboard from the inboard surface. The wheel cover includes thermal isolator itegrally fixed coaxially and apically to each of the lug towers, each lug stud extending through each lug tower and each thermal isolator. A lug nut threads to each lug stud to fasten the wheel cover between the wheel and a shoulder on the lug nut. The thermal isolator thermally isolates the base of the wheel cover from heat transfer from a brake component through the lug stud and nut.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to plastic wheel covers for a vehicle wheel. More particularly, this invention relates to bolt-on wheel covers that are integrally retained to, but thermally isolated from, lug nuts of a vehicle wheel. 
     2. Description of the Prior Art 
     Wheel covers have been used on vehicle wheels for many decades for purposes of aesthetic design, reduction in drag coefficient, and for improvement of brake cooling. Wheel covers have been fastened to wheels with a variety of devices including clips, springs, and integral retaining features. In addition, traditional materials for forming wheel covers include steel and aluminum. Plastic, however, has increasingly been used in order to reduce cost and weight, and to increase design flexibility. 
     Use of “high-temperature” plastic for vehicle wheel covers is well known in the art. In contrast, use of “low-temperature” plastic is less known, but is becoming more popular because it can be chrome plated for aesthetic value. A popular technique for retaining a wheel cover to the wheel involves connecting the wheel cover to the lug nuts of the wheel. The lug nuts, however, tend to be prohibitively hot due to heat transfer generated from a brake component to which the lug nuts are indirectly connected. Under severe conditions, brake lining temperatures are known to exceed 1000° F. while wheel temperatures are known to exceed 500° F. Therefore, care must be taken not to subject the low-temperature cover to the hot lug nuts of the wheel. Accordingly, there are several different wheel covers having various lug nut retention devices to select from, that could be described as: cap retained, bolt-on, integral press-on, and retainer press-on wheel covers. 
     Cap retained covers involve a cover body that mounts to the wheel and includes lug towers that loosely circumscribe the lug nut or stud. A separate cap retainer interlockingly engages the lug nut or stud to sandwich an apical shoulder of the lug tower between the retainer and the wheel. Several examples include: U.S. Pat. No. 4,895,415 to Stay et al., U.S. Pat. No. 4,998,780 to Eshler, U.S. Pat. No. 5,181,767 to Hudgins et al., U.S. Pat. No. 5,667,281 to Ladouceur, and U.S. Pat. No. 5,842,749 to DiMarco. With some of these designs, the cover is spaced away from the wheel by the cap retainer. These designs, however, require use of several cap retainers thus adding to part count and assembly steps—thus increasing costs. Additionally, the cap retainers have been known to work loose and fall off, thereby resulting in loss of or damage to the wheel cover itself. Finally, the retainers have been known to slacken due to temperature creep, resulting in a slack condition between the cover and the wheel. This slack condition also results in noise generated by the loose wheel cover rattling against the wheel. 
     Bolt-on covers involve a cover body having open lug towers for mounting over lug studs on a wheel. Subsequently, a lug nut mounts to the lug stud to trap an apical shoulder of the lug tower therebetween. An example of this is illustrated in U.S. Pat. No. 5,520,445 to Toth. Here the apical shoulder of the lug tower comes in direct contact with the hot lug nut. In another embodiment, a skirt on the lug nut spaces the cover away from the lug nut. The skirt, however, is taught as a device for retaining the lug nut to the cover for disassembly purposes, and not for thermal isolating purposes. In fact, such skirts are typically metallic and therefore there is no reason to believe that the skirt provides any thermal insulation at all. 
     Integral press-on wheel covers typically involve a cover body provided with integral tubular extensions, or lug towers, each having an integral projection for engaging a shoulder of a lug nut. Several examples include: U.S. Pat. No. 4,382,635 to Brown et al., U.S. Pat. No. 4,707,035 to Kondo et al., U.S. Pat. No. 5,071,197 to Webster et al., and U.S. Pat. No. 5,163,739 to Stanlake. With this configuration, the wheel cover is aligned to the wheel and pressed thereto such that each lug tower expands open over each lug nut shoulder and snaps shut behind each shoulder to retain the cover to the wheel. Unfortunately, traditional integral press-on covers come in direct contact with the hot lug nuts and therefore necessitate use of only non-platable high-temperature plastic. 
     Finally, retainer press-on covers involve a cover body including lug towers each having an intermediate retainer therein for gripping a lug nut. For example, U.S. Pat. No. 4,842,339 to Roulinson addresses the problem of having to use a non-standard lug nut having a special bulge for engaging the wheel cover. Roulinson teaches use of an expandable ring within each lug tower for gripping a standard lug nut. Unfortunately, the expanding ring is metal and therefore conducts heat from the lug nut to the cover. In addition, the expanding ring is an extra part that adds part count and assembly time, thereby increasing costs. 
     Another example, as set forth in U.S. Pat. No. 5,249,845 to Dubost, is directed to the problem of difficulty in removing wheel covers from a wheel for servicing purposes. Dubost teaches use of an intermediate sliding sleeve retainer within each lug tower for a more compliant grip on the lug nut. Unfortunately, the sleeve adds to part count and assembly time, thereby increasing costs. In addition, the Dubost design involves moving parts instead of a more robust static state design. Further, Dubost does not teach use of a thermally insulating plastic for the sleeve. 
     Lastly, U.S. Pat. No. 5,297,854 to Nielsen et al., involves a plastic cover body having lug towers with tulip retainers fastened thereto for gripping the lug nuts. The tulip retainers expand open over the lug nut and collapse behind a shoulder on the lug nut for retention thereto. Also, Nielsen et al. is directed to the problem of poor grip of integral press-on architecture and does not teach thermal isolation of the cover body from hot wheel lugs. Unfortunately, each tulip retainer must be assembled to the cover body, thereby adding unwanted assembly time and expense. Additionally, Nielsen et al. appears to be applicable to only hub-style wheel covers and not full face style wheel covers. Full face style wheel covers are highly desirable and ordinarily require a more positive retention method, like the bolt-on retention configuration, to resist continual tension on the retaining area of the cover created by pre-load across the face of the cover. In addition, the Nielsen et al. retaining method is more susceptible to theft of the cover than the bolt-on configuration. Finally, Nielsen et al. also does not apply to a cover with open lug towers that permit display of decorative lug nuts. 
     Therefore, what is needed is a plastic wheel cover that overcomes the shortcomings of the prior art. Firstly, the cover should be primarily composed of chrome platable “low-temperature” plastic that is thermally isolated from the lug nuts. Secondly, the cover should have integral isolators that serve to isolate the cover from the lug nuts and that are easily assembled or are easily molded in place. Thirdly, the cover should include a robust retaining architecture to permit use of full-face wheel covers, enable open lug towers for display of decorative lug nuts, and provide better theft deterrence. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a plastic wheel cover composed of low-temperature plastic that is integrally retained on, but thermally isolated from the lug nuts of a wheel. The cover includes integral isolators that are easily assembled or are easily molded in place. 
     According to the present invention, a wheel cover retention system for a wheel having lug studs is provided. The retention system includes a wheel cover having a base with an outboard surface and an oppositely disposed inboard surface. The base also has lug towers formed integrally therein and projecting inboard from the inboard surface. The wheel cover also has isolators integrally fixed, coaxially and apically, to the lug towers. Each lug stud corresponds to and extends through each lug tower and each isolator. Lug nuts, each including a shoulder, are also provided. Each lug nut threading to each lug stud and fastens the wheel cover between the wheel and the lug nuts to pre-load the wheel cover against the wheel. 
     Further according to the present invention, a wheel assembly for a vehicle is provided. The wheel assembly includes a wheel having lug stud apertures and lug studs extending through the lug stud apertures. A wheel cover overlays at least a portion of the wheel and includes a base having an outboard surface and an oppositely disposed inboard surface. The base also has lug towers extending axially inward from the inboard surface, each terminating in an apical end and each having a passage therethrough. The wheel cover also includes isolators integrally fixed, coaxially and apically, to the apical end of each lug tower. The isolators thermally isolating the wheel cover from heat transfer through the lug studs that extend through the lug towers and isolators. Lug nuts engage the lug studs with each lug nut having a shoulder fastening each isolator between each lug nut and the wheel thereby pre-loading the wheel cover against the wheel. 
     Again, according to the present invention, a wheel cover is provided including a base having an outboard surface, an oppositely disposed inboard surface, and lug towers extending axially inward from the inboard surface. Isolators are integrally fixed to the lug towers to thermally isolate the base from hot lug nuts of a wheel. 
     Accordingly, it is an object of the present invention to provide a wheel cover that includes integrally fastened or molded-in isolators that thermally isolate a platable base of the wheel cover from high temperature lug nuts of a wheel. 
     It is another object to provide a wheel cover that can be bolted onto a vehicle wheel with at least one lug nut to deter theft of the wheel cover. 
     It is yet another object to provide a wheel cover that covers the full face of a vehicle wheel. 
     It is still another object to provide a wheel cover that permits display of decorative lug nuts. 
     It is a further object to provide a plastic wheel cover that is capable of being chrome plated. 
     It is yet a further object to provide a wheel cover that permits use of different length isolators to achieve different draw-down or pre-load across the face of the wheel cover. 
     These objects and other features, aspects, and advantages of this invention will become apparent after a reading of the following detailed description, appended claims, and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial cross-sectional view of a vehicle wheel assembly; 
         FIG. 2  is an enlarged exploded cross-sectional view of the vehicle wheel assembly of  FIG. 1  showing a preferred lug nut retention system; 
         FIG. 3  is enlarged cross-sectional view of the lug nut retention system of  FIG. 2 , as assembled; 
         FIG. 4  is an enlarged cross-sectional view of another lug nut retention configuration; 
         FIG. 5  is an enlarged cross-sectional view of an isolator snapped onto a lug tower; and 
         FIG. 6  is an enlarged cross-sectional view of an isolator threaded onto a lug tower. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring in detail to  FIG. 1 , the wheel cover retention system of the present invention is a wheel  20  that includes a hub  22  that receives lug studs  24  that extend axially outward from an axle flange  26 , through a brake disc  28 , and along an axis of rotation  30  of the wheel  20 . As will be apparent to one skilled in the art, the wheel  20  may be composed of steel, an aluminum alloy, or any suitable composite material and is mounted on the lug studs  24  and an axle flange hub  32 . Lug nuts  40  are secured on the lug studs  24  to retain the wheel  20  to the axle flange  26 . 
     A wheel cover  60  according to the present invention is provided that includes a base portion  61  having an outboard surface  62  that provides aesthetic value, and an oppositely disposed inboard surface  64 . The outboard surface  62  of the wheel cover  60  can be painted or plated, for example with an attractive chrome finish. A plurality of open tubular extensions, or lug towers  66 , axially extend inward from the inboard surface  64  of the wheel cover  60 . The wheel cover  60  may be made of any appropriate wheel cover material that can be injection molded, preferably from a chrome-platable plastic. Chrome-platable plastic is typically composed of a low-temperature plastic, such as polycarbonate/acrylonitrile-butadiene-styrene (PC/ABS). 
     Referring now to  FIGS. 2 and 3 , each of the lug towers  66  has an outboard aperture  68  at an outboard end  70  and an inboard aperture  72  at an apical end  74 . An inner passage  76  extends between the outboard and apical ends  70  and  74 . At least one of the lug towers  66  has an interlocking feature such as a tongue, projection, or tab  78  of continuous or interrupted annulus, for interlocking with an annular groove  94  on the isolator  90 . 
     The isolator  90  may be made of any appropriate material, but is preferably composed of a thermoset, such as PC/PBT produced from a compression molding, glass matte transfer molding, or bulk molding process. The isolator  90  has an external taper  92  to provide a gradual snap fit assembly to the tab  78  of the wheel cover  60 . As described above, the isolator  90  also has an engagement feature such as the annular groove  94  for interlocking with the tab  78  of the lug tower  66  of the wheel cover  60 . The annular groove  94  of the isolator  90  is axially bounded by the external taper  92  and a shoulder  96  on the wheel cover lug tower  66 . The external taper  92  and shoulder  96  axially locate the isolator  90  to the wheel cover  60 . The shoulder  96  also acts to pilot the isolator  90  within the inner passage  76  of the lug tower  66 . 
     The lug nut  40  engages an internal taper  98  of the isolator  90  to locate, positively and axially, the wheel cover  60  to the lug nut  40  on the wheel  20 . The lug nut  40  includes a head  42  and an outer annular surface  44  extending between the head  42  and a wheel-engaging surface  46 . The outer annular surface  44  includes a flange  48  having an external taper  50  for engaging the internal taper  98  of the isolator  90  to provide positive axial positioning of the wheel cover  60  to the wheel  20 . The isolator  90  may be pre-assembled to the wheel  20  to provide a one-piece wheel cover  60  for ease of assembly. The isolator  90  must be fixedly secured to at least one lug tower  66 , but preferably, multiple isolators  90  are fixedly secured, one each, to multiple lug towers  66  for optimal retention. Finally, the positive axial positioning of the wheel cover  60  may be varied by using a thicker isolator  190 , as shown in FIG.  4 . Here, the isolator  190  is thicker in cross section to provide more drawdown or pre-load on the wheel cover  60 . 
     Referring again to  FIG. 1 , drawdown is a measure of how much the wheel cover  60  is drawn inboard toward the wheel  20  as the lug nuts  40  are torqued down to pre-load the wheel cover  60  against the wheel  20 . First, the wheel cover  60  is aligned with the wheel  20  such that each lug tower  66  circumscribes each lug stud  24 , and only the wheel cover periphery locates against the wheel  20  in an initial state of rest—as exaggerated in phantom line. Then, the lug nuts  40  are torqued down on the lug studs  24  to engage the isolators  90  and thereby pull the lug towers  66  and the rest of the wheel cover  60  inboard toward the wheel  20 , as indicated by the arrow A. The displacement of the wheel cover  60  toward the wheel  20  from the initial state of rest is defined as the drawdown on the wheel cover  60 . This drawdown ensures constant positive axial engagement of the wheel cover periphery to the wheel  20  at all times, by imparting a pre-load across the wheel cover  60 . 
     The wheel cover  60  requires drawdown to be induced during assembly to compensate for a loss of preload stress across the wheel cover  60  due to the effects of heat transfer from the brake disc  28 . Accordingly, drawdown is needed to induce sufficient preload stress across the wheel cover  60  to keep the wheel cover periphery tight to the wheel  20  so that the wheel cover  60  will not rattle during vehicle operation. When the wheel cover  60  is pre-loaded beyond its natural state of rest —such as under a drawdown condition—it will tend to seek an alternative state of rest when heat is applied. In other words, the wheel cover  60  seeks to relieve the drawdown stress when heat is applied. Therefore, a target preload stress is designed in to the assembly of the wheel cover  60  to the wheel  20  that exceeds the amount of stress that the wheel cover  60  can relieve naturally due to thermal effects. For example, typical loss of preload stress in a plastic wheel cover  60  due to heat translates into approximately 3 mm of displacement of the wheel cover  60 . Therefore, the amount of drawdown induced is typically engineered to be in the range of about 5 mm, thereby yielding a safety dimension of 2 mm. 
     Chrome plating a wheel cover  60  increases the stiffness of the wheel cover  60 , thereby yielding a desired preload stress through less drawdown. Plastic wheel covers lack this increased stiffness, and therefore, require relatively more drawdown than chrome-plated wheel covers. Since painted wheel covers require more drawdown than a chrome-plated wheel covers, the thicker isolator  190  of  FIG. 4  is employed to induce additional drawdown to achieve the desired target preload stress. 
       FIG. 5  illustrates an alternative isolator  290  that snap fits to an apical end  274  of a lug tower  266 . Here the isolator  290  has an interlocking feature, or internal barb  292 , that interlocks with an external barb  280  on the apical end  274  of the lug tower  266 . The isolator  290  is pressed onto the apical end  274  of the lug tower  266 . Likewise,  FIG. 6  illustrates another alternative isolator  390  that threads to an apical end  374  of a lug tower  366 . The isolator  390  includes internal threads  392  that mate to external threads  382  of the apical end  374  of the lug tower  366 . As above, isolators having varying axial thicknesses may be used in combination with a single plastic wheel cover to achieve different drawdown for different applications. 
     In general, the retention system of the present invention provides a positive location of the wheel cover relative to the lug nuts and the wheel through an integrally fastened isolator having an internal taper that acts as stop against an external taper on the lug nuts. The isolator is composed of a high-temperature material to isolate the base of the wheel cover from the detrimental thermal effects of heat transfer from the brakes. Therefore, a chrome-platable low-temperature plastic wheel cover base may be retained on hot lug nuts of a standard wheel. 
     From the above, it can be appreciated that a significant advantage of the present invention is that the wheel cover includes the low-cost, functionality, and quality of a traditional one-piece wheel cover, and also includes the ability to be painted or chrome-plated. 
     Another advantage is that a common wheel cover may be used with different isolators to achieve different drawdown dimensions. This facilitates use of one common wheel cover that may be painted, thereby requiring relatively more drawdown than a chrome-plated wheel cover, or may be chrome-plated thereby requiring relatively less drawdown. Therefore, the expense of having to tool two wheel cover molds is avoided. 
     Yet another advantage is that the design of the present invention permits installation of full wheel covers, not just hub covers. Also, the lug towers may be open at either end to permit display of decorative lug nuts. Finally, this design promotes theft deterrence of the wheel cover by requiring removal of at least one lug nut to remove the entire wheel cover. 
     Still another advantage is that the isolator of the present invention is a relatively cheap and easy piece to manufacture compared with the tulip retainer of the Nielsen et al. reference that requires use of progressive die tooling to stamp the tulip. In addition, while the present invention does rely on continual tension across the face of the wheel cover, it does not rely on continual tension of the isolator for retention, as in the Nielsen et al. reference. Rather, the isolator is positively trapped between the lug nut and wheel cover in its natural state. 
     While the present invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. For example, the isolator may be insert molded to the wheel cover instead of integrally fastened. Additionally, the isolator may include a tab or tongue feature that interlocks with a groove feature on the lug tower to attach the isolator thereto. Finally, the wheel cover may include a quantity of isolators fewer in number than the number of correspondingly available lug nuts. For example, the wheel cover may include only three isolators and lug towers that attach to a wheel having five lug nuts. Accordingly, the scope of the present invention is to be limited only by the following claims.