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     The theory that the tractor's weight is carried on either the raised hub flange shoulder or the bolts, and the theory of the need for a hubcentric ring are both erroneous and misleading. 

      In the first case, the rim is bolted securely (75-85 ft.lbs, up to 150-175 ft.lbs, depending on bolt size) to the spacer and the spacer similarly to the hub flange.  The bolts simply provide this clamping effect making the rim, spacer, and hub flange into a single unit which transfers the driven torque thru the axle and wheel assembly to the ground with the machine weight being carried by the wheel bearings.  The raised shoulder on the hub flange or hubcentric ring are both quite irrelevant in this regard and do NOT carry the weight, nor, for the same reasons, do the studs/bolts carry the weight.  The axle hub bearings carry the weight. 

      In the second case, the proponents of "hubcentricity" also cite "lack of vibration" as an advantage created by a centering ring, but such is absurd unless you are considering high RPM applications.  This proposal is a marketing sham.  The wheel RPM for tractors is so low that no amount of eccentricity will create rotational vibration.  Even if the wheels were oval instead of perfectly round, instead of this inducing rotational "vibration", there will simply be a gentle "up and down" movement as the eccentric wheel rolls. 
      In the absence of a "raised shoulder centering ring" as is on the hub flange, the rim is still effectively centered on the wheel spacer to within a radius of 20/1000" (the width of a match book cover) by the holes in the rim that have an only slightly larger ID than the OD of the mounting bolts.  Any eccentricity thus imparted is insignificant to the point of irrelevance considering that rubber tires are typically out of round, and thus out of balance, in excess of the amount imparted by this "bolt to hole size" differential, both when new, and certainly after a bit of wear has occured.
     These issues are both a sales gimmick and a failures to understand mechanical physics.   

     As to construction materials, another "out of context" claim that is advanced by the proponents of aluminum wheel spacers is the assertion that "aluminum is stronger than steel", but absent any further elaboration.  Such an idea is contrary to common sense and is little more than another sales gimmick.  It must be appreciated that on a "weight to strength" basis, cobwebs are "stonger than steel" as well, but would you really want your wheel spacers to be made out of cobwebs? 
      While one school of thought holds that aluminum spacers (as opposed to those in steel) may be okay for automotive applications where weight and torque is less than with tractors, the opposing concern is that with tractor applications where stress and torque are an operational expectation, the thinned web where the bolts fasten the aluminum spacer to the hub may be prone to stress failure over extended use, especially with the wider applications. Typically forestry and agricultural tractor spacers are steel, and probably for valid reasons.

      My spacers are individually fabricated by hand here in the USA (personally by me) and are designed to withstand the operational conditions required for commercial, agricultural, and forestry applications, not just the less demanding automotive applications where aluminum is the construction norm.
      With these spacers you get all the quality design, workmanship, and materials you expect, thereby giving you the best of what is available in the market!