Stable skateboard truck

Not applicable.

Not applicable.

Not applicable.

Not applicable.

This application relates to the skateboard industry and specifically to skateboarding as a form of transportation, exercise, and high speed downhill skateboarding. During higher speeds, the skateboard operator typically travels longer distances and in a relatively straight line. Additionally, this skateboard provides a means of pumping to propel oneself, with quick turns, while remaining stable at higher speeds.

Some of the preferred embodiments include a skateboard with a truck similar to the cited prior art for steering, this truck will be referred to as a steering truck and another truck which is constrained to two degrees of freedom. The steering truck has offset axes which allow the skateboard to turn, and in some cases, provide propulsion. This propulsion is explained in prior art U.S. Pat. No. 8,454,038 O'Rourke column 6 lines 44 thru 54. The latter truck restricts the axle compliance to vertical displacement of each wheel independent of each other and rotationally about an axis perpendicular to this vertical displacement. The combination of this truck with the steering truck mentioned above, uniquely allows for propulsion and stability at higher speeds.

Current solutions exist addressing the instability. One solution is referred to as a zero angle truck and is specifically designed with a pivot angle of zero degrees. However, this solution relies on the skateboard platform to provide all structural support. By providing both structure and the compliance needed for performance, this invention eliminates weight, complexity and cost from the skateboard system.

Also noteworthy is that the bushings that provide spring back in this traditional truck type of design are mechanically disadvantaged as the pivot angle goes to zero and is prone to wear more quickly when the deck is tilted repeatedly for turning.

It has been found that at higher speeds the axle in the steering truck can start to oscillate about a vertical axis causing the steering truck to turn back and forth in an uncontrollable manner. One can understand that if all trucks on the skateboard are allowed to react in this fashion the operator would tend to lose control. Therefore, understanding this undesired problem the inventor has provided a truck which allows a steering truck to operate as required for control but reduces steering within its own mechanics, thus providing a more stable experience especially at higher speeds.

An embodiment includes a rear axle which is compliant in two degrees of freedom. This compliance is characterized with flex and spring in the degrees of freedom. One embodiment includes an axle support that has vertical compliance and rotational compliance about an axis parallel to the direction of travel. The approximate location and orientation of these degrees of freedom will be discussed in more detail below. Additionally, the rotational degrees of freedom are described as having directions in the form of vector components this will be explained in more detail with reference to FIG. 7 in the detailed description.

The prior art cited in U.S. Pat. No. 8,454,038 O'Rourke teaches a bidirectional propulsion caster assembly which enables a rider to generate bi-directional motion from a ride-on device (page 1 abstract). This patent teaches the benefits of an angularly offset wheel axis as stated in column 4 line 41 thru 43 of the detailed description. This patent goes on to tout the ability of the offset axis to create an obtuse or acute angle of the caster shaft, see column 5 line 3 thru 13 of the detailed description. It is understood that this device also has an undesirable axle oscillation effect at higher speeds.

Other prior art is cited which includes U.S. Pat. No. 7,341,260 Hosoda. This patent teaches a single offset axis arrangement and the use of a torsion spring. This torsion spring is situated such that it prevents jamming of the offset axis during steering of the skateboard. As stated in the detailed description, the rotational axis of the wheels and the offset axis connected to the base of the skate board are not perpendicular to the plane of the base, see column 2 lines 45 thru 54. This truck device is understood to have stability issues at higher speed, caused again, by axle oscillation.

FIGS. 23, 24, 25 and 26 shows a zero degree truck 100 . The zero degree truck 100 , allows the axle 104 to pivot about an axis 101 . FIG. 24 shows the axle 104 which is supported by the axle housing 105 . Each end of the axle 104 has two bearings 103 A, 103 B, 103 C and 103 D; these bearings support the wheels 102 A and 102 B in the usual manner.

FIG. 25 shows a vertical pin 107 which is supported by compliant bushings 106 A and 106 B and also is rotationally attached to the axle housing 105 . The axle housing 105 supports the axle 104 . By twisting the base 110 , the compliant bushings 106 A and 106 B are loaded against the pin 107 , the axle housing 105 and the washer 109 . The axle housing 105 includes a pivot support feature 105 A which allows the axle housing 105 to pivot inside a wear bushing 108 . In this embodiment the spring back function resisting the twisting is provided by twisting the compliant bushings 106 A and 106 B. This device however does not reduce cost, decrease the weight and does not provide for strong spring back characteristics. For instance as the compliant bushings 106 A and 106 B wear the spring back function is adversely effected causing for a less enjoyable experience.

Other prior art exists which includes the use of a skateboard truck that is modified to prevent the wheels from turning as the base is twisted. No actual documentation is available; however, this type of truck is very similar to the truck of U.S. Pat. No. 7,341,260.

Many of the above cited prior art includes skateboard trucks that include two pivoting axes that require additional hardware. This additional hardware has some undesirable consequences such as increased cost and weight and tended to increase the height of the skateboard base. The increased height of the base has been determined to cause a less comfortable experience for the user.

Another problem with the cited prior art is the response of the skateboard to “spring back” to the untwisted or horizontal orientation. This spring back is addressed in cited prior art of U.S. Pat. No. 7,341,260 to Hosoda. This patent teaches the inclusion of a spring (labeled item 22 ) which provides spring back into a horizontal position of the skateboard base.

The instant invention addresses the stability, spring back, cost and weight issues of the above prior art. Many of the embodiments combine spring back and compliance into the structure of the skateboard base. This is accomplished by structural members instead of additional pivoting mechanisms. The use of these compliant structural members increases the overall higher speed skateboarding experience. Additionally, the trailing wheels are positioned away from the trailing end of the skateboard base such that the wheels do not contact the base even during maximum compliance. This interference is typically referred to as “wheel bite”.

Some of the preferred embodiments address the issue of stability at higher speeds of a skateboard that allows steering and propulsion. Additionally, each and every issue in the prior art is not addressed by each embodiment. In fact, some embodiments may not address any of the prior art issues mentioned above.

In view of the previously mentioned prior art, the present invention discloses an improved skateboard truck. The present invention provides a more stable skateboard at higher speeds while still providing for lower weight, less cost, responsive spring back and lower height.

In some of the embodiments the stable truck mechanism is fixed to the base of the skateboard. This provides structural support for the rider, spring suspension in the vertical direction and rotation compliance and torsion spring return about an axis in the direction of travel. Specifically, the rotation compliance and torsion spring return (or spring back) allows for easier propulsion sometimes referred to as pumping.

In one embodiment a compliant member in the form of a cantilever beam is constructed as an integral part of the skateboard base. As discussed below, this embodiment allows one end of the board (i.e. skateboard) to pivot while restricting rotation perpendicular to the direction of travel. A steering truck is attached to the opposite end of the skateboard and provides a means of turning the skateboard by twisting.

In another embodiment a U-shaped rod is affixed to the underside of the skateboard. In this embodiment the U-shaped rod is the compliant member and each end of the rod allows independent vertical and rotational compliance. This embodiment can be easier to manufacture and provides for reduced upfront manufacturing costs. Again this embodiment allows the adjacent end of the skateboard to pivot while restricting rotation perpendicular to the direction of travel. This embodiment easily allows adjustment of both the twisting compliance and vertical compliance by changing how far the cantilevered portion is extended from the base. Notice that additional mounting holes 720 A, 720 B, 720 C and 720 D are provided. However, this concept could easily be applied to any of the other embodiments.

In yet another embodiment, a channel is affixed to the underside of the skateboard. This channel is further shaped into a fork construction. This construction again provides for compliance of the axle vertically and allows pivoting of the axle. Again the rotation perpendicular to the direction of travel is restricted.

In yet another embodiment, a beam member is affixed to the underside of the skateboard. This embodiment twists this beam member to provide the twisting compliance for turning. This beam member is also considerably stiff to resist the rotation perpendicular to the direction of travel.

An additional embodiment allows for adjustment of the twisting compliance for turning. This embodiment is based on the channel embodiment mentioned above. However, this concept could easily be applied to any of the other embodiments.

In a final embodiment the use of compliant blocks and a U-shaped compliant member are combined to support the axle providing a compliant skateboard truck. This embodiment greatly decreases the cost and complexity of a stable skateboard truck. Two compliant blocks, one on each end of the U-shaped compliant member, are also attached to the axle. Under load the U-shaped compliant member and the compliant blocks again allow for twisting of the stable skateboard truck without adding additional mechanisms or raising the base of the skateboard.

FIG. 1 is a top view of a skateboard with a trailing truck constructed of a U-shaped compliance member during a turn.

FIG. 2 is a backside view of the skateboard with a trailing truck constructed of a U-shaped compliance member.

FIG. 3 is a top view of a skateboard constructed of the prior art during a turn.

FIG. 4 is a top view of a skateboard with integrated stable truck.

FIG. 5 is a cross sectional view of the integrated stable truck.

FIG. 6 is an enlarged detail area of FIG. 5 .

FIG. 7 is a diagram supporting a description of a rotation or twist as a vector.

FIG. 8 is a bottom view of a skateboard with a U-shaped compliance member stable truck.

FIG. 9 is a side view of a skateboard with a U-shaped compliance member stable truck.

FIG. 10 is a cross sectional view of a U-shaped compliance member stable truck, this cross section is taken thru the axle.

FIG. 11 is a side view of a channel embodiment of a stable truck.

FIG. 12 is a bottom view of a channel embodiment of a stable truck.

FIG. 13 is a cross sectional view of a channel embodiment of a stable truck, this cross section is taken thru the axle.

FIG. 14 is an enlarged detail of one area of FIG. 13 .

FIG. 15 is a side view of a compliant beam stable truck.

FIG. 16 is a bottom view of a compliant beam stable truck.

FIG. 17 is a cross sectional view of a compliant beam stable truck, this cross section is taken thru the axle.

FIG. 18 is a side view of a stable truck with adjustable compliance.

FIG. 19 is a bottom view of a stable truck with adjustable compliance.

FIG. 20 is a cross section view of a stable truck with adjustable compliance; this cross section is taken thru the axle.

FIG. 21 is an enlarged detail of an area of FIG. 20 .

FIG. 22 is a side view of an adjustable compliance bushing used in the stable truck with adjustable compliance.

FIG. 23 is a side view of a zero degree truck and is understood to be prior art.

FIG. 24 is a sectional view of a zero degree truck thru the truck axle.

FIG. 25 is a sectional view of a zero degree truck thru the axle housing.

FIG. 26 is a top view of a compliant block truck.

FIG. 27 is a sectional view of a compliant block truck thru the axle.

FIG. 28 is a side view of a compliant block truck.

Most embodiments are an integral part of a skateboard. The skateboard typically includes a steering truck, a base, a compliant member and a wheel assembly attached to the compliant member.

A quick discussion of twist and angular direction will be helpful. In several engineering texts, vectors are used to describe rotations and angular moment. This vector description traditionally uses the “right hand” rule to determine a vector which is perpendicular to the plane of rotation or twist. Referring to FIG. 7 , the twist TW is described as a vector VT. For additional information regarding describing a twist or rotation in terms of vectors can be found in several statics and dynamics engineering textbooks. This type of description is used repeatedly within this document to explain rotation and twist directions.

Next, a brief discussion of the prior art is provided. This discussion is best understood by referring to FIG. 3 . As shown in FIG. 3 , as the base 304 of the skateboard 300 is twisted the leading and trailing steering trucks 312 and 311 respectively turn the wheels 310 A, 310 B, 301 A and 301 B at an angle Z″ from the direction of travel illustrated by the arrow ZZ. This causes the skateboard 300 to turn about an arc of radius R′. This is helpful when trying to turn the skateboard 300 , however steering trucks 311 and 312 constructed in this manner can have adverse characteristics at higher speeds.

When the skateboard 300 is moving in a straight line, the leading and trailing steering trucks 312 and 311 can start to vibrate rotationally. This is worsened at higher speed as the roughness of the ground causes a vibration force on the wheels 301 A, 301 B, 310 A and 310 B. This causes the skateboard front 303 and rear 302 to vibrate. These vibrations hinder control of the skateboard 300 . Several of the embodiments address this effect.

Referring to FIGS. 1 and 2 an understanding of the mechanics is provided. FIG. 2 shows the skateboard 4 from the trailing end. Here the base 1 is tilted at an angle A relative to the ground 3 . This causes the steering truck 2 to turn to an angle T as shown in FIG. 1 . This turning causes the front of the skateboard 4 to follow the steering truck 2 . The skateboard 4 begins to follow an arc of radius R.

The following truck 5 , again, with the base 1 tilted to an angle A, does not rotate to an angle T. Instead the flexible supports 6 A and 6 B comply and the following truck 5 continues in a straight line. However, when the skateboard 4 is operated at higher speed, and only needs to make smaller turns, the lack of turning ability in the following truck 5 provides for better control.

Several embodiments are now described in detail and are not considered an exhaustive list, but demonstrate different mechanisms for providing a following truck 5 which allows the operation of a steering truck 2 while maintaining the desired stability of the skateboard 4 at higher speeds.

FIG. 4 shows an embodiment where the skateboard base 403 is a continuous structure which includes a flexible region 401 which consists of two leg features 401 A and 401 B. These leg features 401 A and 401 B provide support for the axle 404 . Refer now to FIG. 5 , the axle 404 in turn supports bearings 405 A, 405 B, 405 C and 405 D which are attached to and provide rotation for the wheels 409 A and 409 B. A spacer 407 A is located on the axle 404 and provides for space between the wheel 409 A and the leg feature 401 A. An additional spacer 407 B is located on the axle 404 and provides for space between the wheel 409 A and the leg feature 401 B. Support for the axle 404 is provided by bushings 408 A and 408 B. FIG. 6 shows bushing 408 A in contact with the axle 404 , this bushing 408 A includes two chamfered surfaces 410 and 411 which interface with the axle 404 . These chamfered surfaces 410 and 411 provide clearance around the axle 404 when the leg feature 401 B complies.

Additionally, FIG. 6 shows the bushing 408 A includes a spherical surface 408 A′, this spherical surface 408 A′ interacts with a second spherical surface 407 B′ on the spacer 408 B. During compliance this interaction allows for relative movement so that the compliance does not cause undesired wear and restriction.

The compliance provided by the leg features 401 A and 401 B allows the base 403 of the skateboard 400 to twist. This compliance allows the steering truck 402 to turn as discussed above, but prevents the following axle 404 from turning and so turns the skateboard 400 .

FIGS. 8 and 9 show another embodiment of the invention. This embodiment includes a steering truck 700 on the base 701 and a compliant structure 702 on the other end. This compliant structure 702 includes a U-shaped rod 703 which is attached to the base 701 . This attachment is provided by an upper plate 704 , lower plates 705 A and 705 B. The U-shaped rod 703 is clamped between the upper plate 704 and lower plates 705 A and 705 B using fasteners 706 A, 706 B, 706 C and 706 D. These fasteners could be rivets, screws and nuts or a number of solutions for attaching the U-shaped rod 703 to the base 701 .

The U-shaped rod 703 could be simply two rods without connection to each other; this U-shaped configuration is simply a preferred implementation.

Each end of the U-shaped rod 703 has a spherical bushing 707 A and 707 B attached, these bushings provide connection to the axle 708 . The axle 708 in turn rotationally supports the wheels 709 A and 709 B. These connections are discussed in more detail by referring to FIG. 10 .

FIG. 10 shows a cross section of the axle 708 and other components. Since the base 701 is twisted to turn the steering truck 700 , the compliant structure 702 allows the axle 708 to remain parallel to the ground. This compliance causes the U-shaped rod 703 to twist. Spherical bushings 707 A and 707 B connect to the axle 708 but allow the U-shaped rod 703 to twist. The wheels 709 A and 709 B are attached to bearings 712 A, 712 B, 712 C and 712 D which are supported by the axle 708 . A spacer 710 A is positioned axially on the axle 708 between a spherical bushing 707 A and a bearing 712 B. A similar arrangement is provided using spacer 710 B, axle 708 , spherical bushing 707 B and bearing 712 C. The axle 708 includes a head 708 B on one end and a threaded portion 708 A on the other. A nut 713 is engaged with the threaded portion 708 A. As this nut 713 is tightened the head 708 B pulls the wheels 709 A, 709 B, the bearings 712 A, 712 B, 712 C and 712 D, the spacers 710 A and 710 B and spherical bushings 707 A and 707 B towards each other. This provides a solid structural assembly. Also note the spacers 710 A and 710 B interface with spherical bushings 707 A and 707 B and have spherical surfaces 710 A′ and 710 B′ for reducing restriction during compliance.

The compliance and spring force provided by the U-shaped rod 703 allows the base 701 to tilt. This compliance allows the steering truck 700 to turn as discussed above but prevents the following axle 708 from turning.

Another embodiment is shown in FIGS. 11, 12, 13, and 14 . This embodiment includes the steering truck 800 , a base 801 and a compliant channel member 802 as part of the compliant truck 805 . The compliant channel member 802 is attached to the base 801 by fasteners 804 A, 804 B, 804 C and 804 D which clamp the base 801 the spacers 803 A and 803 B and the channel base 802 C. Again several different fastening methods could be used to implement this attachment such as bolts and nuts, rivets, pins etc. As can be seen in FIG. 12 the compliant member 802 includes two legs 802 A and 802 B, these legs support the compliant truck 805 .

FIG. 13 shows a cross section of the compliant truck 805 at the axle 806 . The axle 806 supports bearings 809 A, 809 B, 809 C and 809 D which are attached to, and provide rotation for, the wheels 807 A and 807 B. A spacer 810 A is located on the axle 806 and provides for space between the wheel 807 A and the leg feature 802 A. An additional spacer 810 B is located on the axle 806 and provides for space between the wheel 807 B and the leg feature 802 B. Support for the axle 806 is provided by bushings 811 A and 811 B. FIG. 14 shows bushing 811 B in contact with the axle 806 , this bushing 811 A includes two chamfered surfaces 813 B and 814 B which interface with the axle 806 . These chamfered surfaces 813 B and 814 B provide clearance around the axle 806 when the leg feature 802 B complies.

Additionally, FIG. 14 shows the bushing 811 B includes a spherical surface 812 B. This spherical surface 812 B interacts with a second spherical surface 810 B′ on the spacer 810 B so that restriction is reduced. During compliance this interaction allows for relative movement so that the compliance doesn't cause undesired wear and restriction.

The compliance provided by the compliant channel member 802 including legs 802 A and 802 B allows the base 801 to tilt. This compliance allows the steering truck 800 to turn as discussed above but prevents the following axle 806 from turning. The mounting of the compliant channel member 802 also plays a role in the compliance of the truck. FIG. 12 shows additional holes 820 A and 820 B which can be used to for fasteners 804 C and 804 D or for fasteners 804 A and 804 B. This allows for changing the support characteristics of the compliant channel member 802 and so adjusting the compliance of the compliant truck 805 .

The next embodiment is shown in FIGS. 15, 16 and 17 . The skateboard 900 in this embodiment includes a steering truck 901 , base 902 and a beam member 905 as part of a compliant truck 903 . A block 904 is attached to the base 902 . This base 902 supports a beam member 905 which in this embodiment is a cylindrical rod however this beam member 905 could be a tube of various cross sectional geometry and need not be restricted to a single cross sectional geometry along its length. An axle 907 is attached at the distal end of the beam member 905 . The block 904 , beam member 905 and axle 907 could be a single piece or combination of several other parts which provide structural support and twisting compliance for the wheels 906 A and 906 B.

Support for the wheels 906 A and 906 B is best explained with reference to FIG. 17 . Axle 907 includes a head feature 907 B, a threaded portion 907 A and is attached to the beam member 905 , the nut 908 threads onto the threaded portion 907 A to clamp the head feature 907 B to hold the bearings in place. Bearings 909 A, 909 B, 909 C and 909 D are located concentrically with the axle 907 and are rotationally attached to the wheels 906 A and 906 B.

In this embodiment the beam member 905 allows for twist without redirecting the wheels 906 A and 906 B. This compliance allows the steering truck 901 to turn as discussed above but prevents the following axle 907 from turning. The compliance of this truck embodiment is adjustable by sliding the beam member 905 into or out of the block 904 . This increases or decreases the length of the beam member 905 and so changes the compliance of the compliant truck 903 .

The following embodiment is shown in FIGS. 18, 19, 20, 21 and 22 . The skateboard 200 of this embodiment again includes a steering truck 201 , a base 202 and a compliant truck 204 to be discussed in detail below. A compliant beam 203 is attached to the base 202 using fasteners 208 A, 208 B, 208 C and 208 D and standoffs 209 A, 209 B, 209 C and 209 D, this compliant beam 203 is very similar to the compliant member 802 above however, additional features are included and provide an additional compliance mechanism.

The compliant beam 203 provides support for the wheels 205 A and 205 B. This support is further explained by referring to FIGS. 20, 21 and 22 . The axle 207 is in contact with compliant bushings 206 A, 206 B, 211 A and 211 B. These compliant bushings 206 A, 206 B, 211 A and 211 B are inserted over the axle 207 thru an adjacent hole 206 B″ shown in FIG. 22 . Also shown in FIG. 22 is a flat surface 206 B′, this surface 206 B′ is assembled to be in intimate contact with a foot feature 203 BB of the compliant beam leg 203 B. A large opening 203 B′ is provided in the compliant beam leg 203 B, this assures that the bushing 206 B carries the supporting load instead of the surface of the hole 203 B′. Spacers 211 A and 211 B are directly positioned between the compliant legs 203 A and 203 B and the bearings 210 B and 210 C to provide space between the compliant legs 203 A and 203 B and the wheels 205 A and 205 B. The bearings 210 A, 210 B, 210 C and 210 D provide rotational support for the wheels 205 A and 205 B.

A compliance adjustment will now be explained. In FIG. 20 the axle 207 includes a head feature 207 B and a threaded portion 207 A. A nut 212 is threaded onto this threaded portion 207 A. By tightening the nut 212 the bushings 206 A and 206 B are compressed since a rigid spacer 220 is located between the two bushings 206 A and 206 B. The bushings 206 A and 206 B follow the well understood Poisson's effect of elasticity and so as the bushings 206 A and 206 B are compressed they expand in a radial direction, this causes the surface 206 A′ to push against the foot feature 20366 shown in FIG. 21 . This preloads the bushings 206 A and 206 B such that the compliance is changed. Since the axle head 207 B and nut 212 load both bushings 206 A and 206 B both compliant legs 203 A and 203 B are adjusted.

The spacers 211 A and 211 B are also used to adjust the compliance. Again, as the nut 212 is tightened these spacers 211 A and 211 B are compressed between the bearing 210 B and compliant leg 203 B and bearing 210 C and compliant leg 203 B respectively. Since the twisting of the compliant beam 203 is resisted by these spacers 211 A and 211 B the compliance is also adjusted.

In this embodiment the compliance allowing for twist without redirecting the wheels 205 A and 205 B is provided by three devices, a compliant beam 203 , compliant bushings 206 A and 206 B and spacers 211 A and 211 B. This compliance allows the steering truck 201 to turn as discussed above but prevents the following axle 207 from turning. Additionally, the stiffness and spring back is also adjusted allowing for better control at higher speeds and based on rider preference.

Additionally, compression of the bushings 206 A and 206 B and spacers 211 A and 211 B could be provided in a number of other ways including a locking wedge, a cam mechanism similar to a bicycle axle clamp, simply a screw that engages one end of the axle such that as the screw is tightened the distance from the axle head 207 B and the screw is modified, and several other common means.

A final embodiment is shown in FIGS. 26, 27 and 28 . This embodiment includes a steering truck 501 on the base 500 and a compliant structure 502 on the other end. This compliant structure 502 includes a U-shaped rod 503 which is attached to the base 500 . This attachment is provided by an upper plate 504 , lower plates 505 A and 5056 . The U-shaped rod 503 is clamped between the upper plate 504 and lower plates 505 A and 5056 using fasteners 506 A, 506 B, 506 C and 506 D. These fasteners could be rivets, screws and nuts or a number of solutions for attaching the U-shaped rod 503 to the base 500 .

The U-shaped rod 503 could be simply two rods without connection to each other; this U-shaped configuration is simply a preferred implementation. Additionally, the U-shaped rod can be extended and retracted to adjust the compliance of the compliant truck.

Each end of the U-shaped rod 503 has compliant blocks 507 A and 507 B attached. These compliant blocks provide connection to the axle 508 . The axle 508 in turn rotationally supports the wheels 509 A and 509 B with bearings 512 A, 512 B, 512 C and 512 D. These connections are discussed in more detail by referring to FIG. 27 . This embodiment with 2 compliant blocks is one implementation. A single compliant block attached to both ends is also a viable embodiment.

FIG. 27 shows a cross section of the axle 508 and other components. Since the base 500 is twisted to turn the steering truck 501 , the compliant structure 502 allows the axle 508 to remain parallel to the ground. This compliance causes the U-shaped rod 503 to twist. Compliant blocks 507 A and 507 B connect to the axle 508 but allow the U-shaped rod 503 to twist. The wheels 509 A and 509 B are attached to bearings 512 A, 512 B, 512 C and 512 D which are supported by the axle 508 . A spacer 510 A is positioned axially on the axle 508 between a compliant block 507 A and a bearing 512 B. A similar arrangement is provided using spacer 5108 , axle 508 , compliant block 507 B and bearing 512 C. The axle 508 includes a head 508 B on one end and a threaded portion 508 A on the other. A nut 513 is engaged with the threaded portion 508 A. As this nut 513 is tightened the head 508 B pulls the wheels 509 A, 509 B, the bearings 512 A, 512 B, 512 C and 512 D, the spacers 510 A and 5108 and compliant blocks 507 A and 507 B towards each other. This provides a solid structural assembly.

The compliance and spring force provided by the U-shaped rod 503 and the compliant blocks 507 A and 507 B allows the base 500 to tilt. This compliance allows the steering truck 501 to turn as discussed above but prevents the following axle 508 from turning.

It will be understood that while the invention has been described in conjunction with specific embodiments thereof, the foregoing description and examples are intended to illustrate, but not limit the scope of the invention. Other aspects, advantages and modifications will be apparent to those skilled in the art to which the invention pertains, and these aspects and modifications are within the scope of the invention, which is limited only by the appended claims.