Get a bet­ter un­der­stand­ing of the wheels on your car

Truckin - - TRUCK ON ROAD -


Off­set is the lo­ca­tion of the flat mount­ing sur­face of a wheel rel­a­tive to the wheel’s cen­ter­line. Neg­a­tive off­set means that the mount­ing sur­face is to­ward the cen­ter of the car, pos­i­tive off­set means that it’s to­ward the out­side of the car, or the wheel is “pulled in” to­ward the cen­ter. Off­set af­fects many things other than just whether the wheel has the ap­pear­ance of “stick­ing out” past the fender. The wrong off­set can cause rub­bing prob­lems when the sus­pen­sion is com­pressed or the wheel is turned. Off­set af­fects the steer­ing ge­om­e­try’s


scrub ra­dius, pos­si­bly lead­ing to prob­lems with torque steer or self-cen­ter­ing char­ac­ter­is­tics. Off­set also af­fects the sus­pen­sion’s mo­tion ra­tio, which di­rectly de­ter­mines the ef­fec­tive spring and damper rates. Po­ten­tially, in a very heav­ily loaded ve­hi­cle, or with ex­treme changes in off­sets, wheel bear­ing life can be af­fected, but this is more of­ten talked about by truck peo­ple than by small car en­thu­si­asts. It’s very, very im­por­tant that the proper off­set wheels be used. While not di­rectly a mat­ter of


off­set, brake caliper clear­ance is a re­lated is­sue. If you have or plan to have big brakes on your car, be sure that your wheels, or the wheels that you’re go­ing to use will fit over the calipers. Spac­ers are avail­able to solve the prob­lem if they don’t, but it’s best to get a wheel with enough dish to meet your off­set specs and still fit your brakes. Con­sult­ing the wheel and brake man­u­fac­tur­ers ahead of time is wise. Many af­ter­mar­ket brake com­pa­nies even have tem­plates of their brakes avail­able that you can eas­ily check against any wheel.


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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