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A wheel clamp attaches to a vehicle wheel for performing a wheel alignment on the vehicle. The wheel clamp has a body for supporting a target or measuring head, and three extendable arms slidably mounted to the body. Each arm has a gripping portion for gripping the tire's tread surface and a sidewal

A wheel clamp attaches to a vehicle wheel for performing a wheel alignment on the vehicle. The wheel clamp has a body for supporting a target or measuring head, and three extendable arms slidably mounted to the body. Each arm has a gripping portion for gripping the tire's tread surface and a sidewall contact portion for contacting the tire sidewall such that the clamp body is substantially parallel to the vehicle wheel and the wheel clamp has no contact with the rim. A self-centering linkage has enmeshed gears and link arms respectively connecting the arms to the gears, such that when the gears are rotated, the arms simultaneously proportionally slide relative to the clamp body, so the gripping portions of the arms engage and grip the tire tread surface and tighten the clamp onto the tire, while the sidewall contact portions of the arms contact the tire sidewall.

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1. A wheel clamp for attaching to a vehicle wheel for performing a wheel alignment on the vehicle, the wheel comprising a tire and a rim, the wheel clamp comprising: a clamp body for supporting an apparatus used in performing the wheel alignment;three extendable arms, each slidably mounted to the cl

1. A wheel clamp for attaching to a vehicle wheel for performing a wheel alignment on the vehicle, the wheel comprising a tire and a rim, the wheel clamp comprising: a clamp body for supporting an apparatus used in performing the wheel alignment;three extendable arms, each slidably mounted to the clamp body such that they extend outward radially from a central point, each extendable arm having a gripping portion for gripping a tread surface of the vehicle tire and a sidewall contact portion for contacting a sidewall of the tire when the gripping portion is gripping the tire tread surface; anda self-centering linkage mechanism comprising a plurality of gears rotatably mounted to the clamp body and engaged with each other, and a plurality of link arms, each link arm movably connecting one of the extendable arms to one of the gears such that when the gears are rotated, the extendable arms simultaneously proportionally slide relative to the clamp body;wherein the linkage mechanism is for moving the extendable arms such that the gripping portions of the extendable arms engage and grip the tire tread surface and tighten the clamp onto the tire, while the sidewall contact portions of the extendable arms contact the tire sidewall; andwherein the extendable arms are for spacing the wheel clamp away from the wheel rim when the gripping portions are gripping the tire tread surface and the sidewall contact portions are contacting the tire sidewall, such that the clamp body is substantially parallel to the vehicle wheel and the wheel clamp has no contact with the rim. 2. The wheel clamp of claim 1, wherein the linkage mechanism comprises: an orbital gear rotatably mounted to the clamp body; anda manually rotatable worm gear engaged with the orbital gear for rotating the orbital gear;wherein each link arm has a first end pivotably attached to the orbital gear and a second end pivotably attached to one of the extendable arms such that when the orbital gear is rotated by the worm gear, the link arms cause the extendable arms to slide relative to the clamp body. 3. The wheel clamp of claim 2, further comprising a knob attached to the worm gear for manually rotating the worm gear, and a clutch between the knob and the worm gear to limit a torque applied to the worm gear to a predetermined maximum torque when the wheel clamp is tightened onto the tire. 4. The wheel clamp of claim 2, wherein the clamp body has a plurality of slots, and the second ends of the link arms respectively engage and slide in the slots. 5. The wheel clamp of claim 1, wherein the clamp body is for supporting the apparatus used in performing the wheel alignment at the central point. 6. The wheel clamp of claim 1, wherein each of the extendable arms comprises: an arm support slidably mounted to the clamp body and movably attached to one of the link arms;a main arm including the gripping portion and the sidewall contact portion, adjustably mounted to the arm support for adjusting a length of the extendable arm. 7. The wheel clamp of claim 6, wherein the main arm is slidably mounted to the arm support in a telescoping manner, and the wheel clamp further comprises a lock for maintaining a position of the main arm relative to the arm support. 8. The wheel clamp of claim 7, wherein the lock comprises a ball detent mechanism between the main arm and the arm support. 9. The wheel clamp of claim 1, wherein a first one of the extendable arms is spaced more than 90 degrees clockwise from a second one of the extendable arms, and a third one of the extendable arms is spaced more than 90 degrees counterclockwise from the second one of the extendable arms. 10. The wheel clamp of claim 2, further comprising a handle attached to the clamp body for gripping by a user for holding the clamp while the worm gear is manually rotated. 11. The wheel clamp of claim 10, wherein the handle is substantially vertical when gripped by a user, one of the extendable arms is proximal the handle, and each of two other of the extendable arms are offset more than 90 degrees from the one of the extendable arms, measured relative to the central point. 12. The wheel clamp of claim 1, wherein the linkage mechanism comprises: three drive gears rotatably mounted to the clamp body, each drive gear associated with one of the extendable arms and including a crank arm;a connecting gear train rotatably mounted to the clamp body and engaging all of the drive gears for synchronizing motion of the drive gears; anda handle attached to one of the drive gears for manually rotating the drive gears;wherein each link arm has a first end pivotably attached to an end of the crank arm of one of the drive gears, and a second end pivotably attached to one of the extendable arms, such that when the drive gears are rotated, the link arms cause the extendable arms to slide relative to the clamp body. 13. The wheel clamp of claim 12, comprising a second handle attached to a second one of the drive gears, for manually rotating the drive gears. 14. The wheel clamp of claim 12, further comprising: a cover attached to the clamp body for covering the drive and connecting gears, the cover having a first frictional surface;second frictional surface for engaging the first frictional surface, the second frictional surface being attached to the handle such that it is constantly engaged with the handle and selectively engaged with the first frictional surface;wherein when the first and second frictional surfaces are not engaged, the handle is rotatable to move the extendable arms such that the gripping portions of the extendable arms engage and grip the tire tread surface and tighten the clamp onto the tire, and when the first and second frictional surfaces are engaged, the drive gears are locked to prevent loosening of the clamp. 15. The wheel clamp of claim 14, comprising a spring between the handle and the second frictional surface for biasing the second frictional surface into engagement with the first frictional surface when the handle is not gripped by a user; wherein when the handle is gripped by the user, the first and second frictional surfaces are not engaged. 16. A wheel clamp for attaching to a vehicle wheel for performing a wheel alignment on the vehicle, the wheel comprising a tire and a rim, the wheel clamp comprising: a clamp body;three elongated arms, each arm having a proximal end pivotally mounted to the clamp body such that the arm extends outward from the clamp body, each arm having a distal end with a gripping portion for gripping a tread surface of the vehicle tire and a sidewall contact portion for contacting a sidewall of the tire when the gripping portion is gripping the tire tread surface; anda self-centering mechanism comprising an arm gear attached to the proximal end of each of the arms coaxial with the pivot point of each respective arm, and a central connecting gear engaged with all the arm gears for synchronizing the movement of each arm with respect to the other arms, such that when any of the gears are rotated, all the arms simultaneously proportionally pivot relative to the clamp body;wherein the self-centering mechanism is for moving the arms such that the gripping portions of the arms engage and grip the tire tread surface and tighten the clamp onto the tire, while the sidewall contact portions of the arms contact the tire sidewall; andwherein the arms are for spacing the wheel clamp away from the wheel rim when the gripping portions are gripping the tire tread surface and the sidewall contact portions are contacting the tire sidewall, such that the clamp body is substantially parallel to the vehicle wheel and the wheel clamp has no contact with the rim. 17. The wheel clamp of claim 16, wherein each arm comprises a handle for grasping by a user such that when the user grasps two of the handles and rotates two of the arms, the gripping portions of all the arms engage and grip the tire tread surface and tighten the clamp onto the tire. 18. The wheel clamp of claim 16, comprising a lock for preventing rotation of the arm gears and the central connecting gear to prevent loosening of the clamp after it has been tightened onto the tire. 19. The wheel clamp of claim 18, wherein the lock comprises a ratchet mechanism selectively engageable with the central connecting gear. 20. The wheel clamp of claim 16, comprising a lock assist knob attached to one of the arm gears for rotating the arm gears and the central connecting gear for tightening the clamp onto the tire. 21. The wheel clamp of claim 16, comprising a spindle rotatably mounted to the clamp body for supporting an apparatus used in performing the wheel alignment, and a set of position maintenance gears for linking the movement of one of the arms to movement of the spindle, for maintaining an angular orientation of the spindle constant relative to the one of the arms. 22. The wheel clamp of claim 21, wherein the apparatus is a wheel alignment target. 23. The wheel clamp of claim 12, further comprising: a cover attached to the clamp body for covering the drive and connecting gears, the cover having a first set of ratchet teeth;a ratchet ring having a second set of ratchet teeth for mating with the first set of ratchet teeth, the ratchet ring being attached to the handle such that the ratchet ring is constantly engaged with the handle and selectively engaged with the first set of ratchet teeth;wherein when the first and second set of ratchet teeth are not engaged, the handle is rotatable to move the extendable arms such that the gripping portions of the extendable arms engage and grip the tire tread surface and tighten the clamp onto the tire, and when the first and second set of ratchet teeth are engaged, the drive gears are locked to prevent loosening of the clamp. 24. The wheel clamp of claim 23, comprising a spring between the handle and the ratchet ring for biasing the second set of ratchet teeth into engagement with the first set of ratchet teeth when the handle is not gripped by a user; wherein when the handle is gripped by the user, the first and second set of ratchet teeth are not engaged. 25. The wheel clamp of claim 10, wherein the handle comprises a level indicator for indicating when the handle is vertical. 26. A wheel clamp for attaching to a vehicle wheel for performing a wheel alignment on the vehicle, the wheel comprising a tire and a rim, the wheel clamp comprising: a clamp body for supporting an apparatus used in performing the wheel alignment;three extendable arms, each slidably mounted to the clamp body such that they extend outward radially from a central point, each extendable arm having a gripping portion for gripping a tread surface of the vehicle tire and a sidewall contact portion for contacting a sidewall of the tire when the gripping portion is gripping the tire tread surface; anda linkage mechanism comprising a plurality of gears rotatably mounted to the clamp body and engaged with each other, and a plurality of link arms, each link arm movably connecting one of the extendable arms to one of the gears such that when the gears are rotated, the extendable arms simultaneously slide relative to the clamp body;wherein the linkage mechanism is for moving the extendable arms such that the gripping portions of the extendable arms engage and grip the tire tread surface and tighten the clamp onto the tire, while the sidewall contact portions of the extendable arms contact the tire sidewall; andwherein the extendable arms are for spacing the wheel clamp away from the wheel rim when the gripping portions are gripping the tire tread surface and the sidewall contact portions are contacting the tire sidewall, such that the clamp body is substantially parallel to the vehicle wheel and the wheel clamp has no contact with the rim.