BACK­WARDS BACKSPACING

4 Wheel & Off Road - - NUTS & BOLTS -

Q I’m in the mar­ket for new tires and wheels and want to make sure they don’t stick out past the fend­ers, be­cause here in Penn­syl­va­nia they’re pretty strict about tire cov­er­age. What I don’t un­der­stand is how to fig­ure out what I need. Is backspacing and off­set the same thing or different? Why do wheel com­pa­nies use different mea­sur­ing tech­niques? One man­u­fac­turer will say their wheel has 41⁄2 inches of backspacing, and the next one will say -12 mm. How am I sup­posed to compare these two mea­sure­ments?

MIKE C. Via nuts@4wor.com

A We feel your pain. Once upon a time is was easy be­cause just about ev­ery wheel com­pany used backspacing, but as the met­ric sys­tem be­came more and more stan­dard in the rest of the world, so too did us­ing wheel off­set. These days they use one

or the other, or both. It can get hor­ri­bly con­fus­ing, but maybe this will help clear things up.

Wheel backspacing and off­set are two ways of mea­sur­ing the same thing: the po­si­tion of the wheel’s mount­ing face in re­la­tion to the wheel’s outer shell. Backspacing is the dis­tance be­tween the mount­ing face and the in­ner lip of the wheel (usu­ally ex­pressed in inches), while off­set is the dis­tance of the mount­ing face rel­a­tive to the cen­ter of the wheel (usu­ally in mil­lime­ters).

Pic­ture a wheel in­stalled on a ve­hi­cle. If the wheel mount­ing face is po­si­tioned be­fore the cen­ter of the wheel (tire is pushed away from the ve­hi­cle) it’s a neg­a­tive num­ber, and if the mount­ing face is po­si­tioned af­ter the cen­ter of the wheel (tire pushed to­wards the ve­hi­cle) it’s a pos­i­tive num­ber. So a wheel with a width of 9 inches and 41⁄2 inches of backspacing has an off­set of -12 mm, and one with 5 3⁄4 inches of backspacing has an off­set of 18 mm.

Wheel backspacing/off­set is im­por­tant for sev­eral rea­sons. One is that it de­ter­mines how far the tires stick out (or don’t) from the fend­ers, but it also can make or break whether or not a wheel clears brake calipers or other sus­pen­sion com­po­nents. This is less of a con­cern on solid axle ve­hi­cles, but backspacing is crit­i­cal on most IFS ap­pli­ca­tions. Most mod­ern IFS lifts leave the up­per con­trol arms in the stock lo­ca­tion but lower the lower con­trol arm as­sem­bly. A new steer­ing knuckle spans the in­creased dis­tance be­tween the two, and the new knuckle usu­ally re­quires a different wheel off­set than stock to avoid wheel in­ter­fer­ence. For this rea­son, lift com­pa­nies will of­ten spec­ify a max­i­mum backspacing or off­set. Also, wheel off­set can make or break whether or not a cer­tain tire size makes con­tact with fend­ers. It changes the scrub ra­dius of a ve­hi­cle, which is the dis­tance be­tween the cen­ter­line of the ball joints or king­pins and the cen­ter of the tire’s con­tact patch with the ground. What scrub ra­dius does is more than what we re­ally want to get into here, but the take­away for this dis­cus­sion is that the tire’s depth in the wheel­well changes de­pend­ing on steer­ing an­gle, and this change can mean the dif­fer­ence be­tween a tire and wheel pack­age hit­ting or clear­ing sheet­metal.

One last thing to con­sider is that wheel width will have an im­pact on the fi­nal po­si­tion of a tire in the wheel­well. If there are two different wheels with 41⁄2 inches of backspacing, the one that is 8 inches wide will pull the tire deeper into the wheel­well than the one that is 10 inches wide. The same goes for off­set.

This is one of those things that seem re­ally easy on the sur­face but get com­pli­cated quickly. It’s also a good rea­son to con­sult your lo­cal off-road shop for some ex­pert ad­vice. You might pay a lit­tle more, but you’ll know that what you buy is right the first time, not to men­tion sup­port­ing the lo­cal econ­omy.

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