CENTERING

4 Wheel & Off Road - - WHEEL & TIRE TECH 101 -

The other el­e­ment that af­fects di­rectly whether a wheel can be bolted onto a car is hub­cen­tric­ity. Long ago, in the deep mists of time, wheels were lo­cated by the ta­per of the lug nuts or bolts. This could lead to all sorts of prob­lems, but they can be sum­ma­rized by say­ing centering was li­able to be less than per­fect, and the sheer stress on wheel bolts or studs could be enor­mous. We’re not aware of any pas­sen­ger car wheels now made that are not hub­cen­tric.

Hub­cen­tric wheels have a hole at their cen­ter that fits closely over a round fea­ture on the hub, serv­ing to cen­ter the wheel on the axis of the spin­dle, as well as bear the ver­ti­cal weight of the ve­hi­cle. The wheel bolts or studs then serve sim­ply to hold the wheel onto the hub, and are loaded only in ten­sion, where they are strong. If the studs were re­quired to ab­sorb ver­ti­cal forces, they would be loaded in sin­gle shear, the weak­est ar­range­ment for any fas­tener. Fac­tory wheels are all machined to fit their spe­cific ap­pli­ca­tion ex­actly, and some of the bet­ter af­ter­mar­ket wheels are, too.

How­ever, many af­ter­mar­ket wheels rely on centering rings. This means that, in­stead of ma­chin­ing wheels specif­i­cally for each OE centering hole di­am­e­ter, the wheel man­u­fac­turer ma­chines all wheels to one size, and then uses in­serts to give a centering sur­face of the di­am­e­ter re­quired for each ap­pli­ca­tion. This is ob­vi­ously eas­ier to do, and makes in­ven­to­ry­ing a com­plete wheel line much sim­pler and less costly.

If you buy wheels that use cen- ter­ing rings, be sure that the rings fit snugly in the wheels. If they’re loose enough to fall out, how ac­cu­rately can they be lo­cat­ing your wheel? Some tire shops au­to­mat­i­cally re­move centering rings to bal­ance a wheel, just to make sure that there is no slop to make their bal­anc­ing in­ac­cu­rate.

The fact that a wheel phys­i­cally bolts onto a car doesn’t nec­es­sar­ily mean that it “fits.” The centering sur­face could be too large, in which case there’s essen­tially no centering. Just as im­por­tantly, the off­set could be wrong.

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