Performing a Precise Wheel Alignment Assures Maximum Tire Life, Vehicle
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Pg_EDIT_Align:Pg_EDIT_ 5/19/15 3:49 PM Page 1 ncorrect wheel alignment con- ditions affect tire wear and can cause drifting and/or pulling during cruise, acceleration and PERFORMANCE braking, plus poor directional PERFORMANCE control. For the performance- minded customer, the need for precise Iwheel alignment becomes more pro- nounced, due to a number of factors: •He expects crisp handling and maxi- mum grip/traction. WHEEL •Aftermarket upgrades—such as WHEEL moving to wider tires, different wheel offset, shorter sidewall due to plus-siz- ing—are common. •Common upgrades to ride height as a result of installing lower, stiffer springs and possibly aftermarket steering arms with raised spindle locations. ALIGNMENTALIGNMENT The goal, regardless of any potential aftermarket upgrades, is to retain the original wheel alignment specifications, as closely as possible for street-driven BY JIM GIBSON vehicles. If a vehicle is intended only for off-road use, deviating from stock set- tings will be necessary in order to maxi- Performing a precise mize the tire contact patch during hard cornering and to enhance turn-in re- wheel alignment sponse. However, since the majority of assures maximum alignment jobs, even those termed “per- formance,” will involve street driving, tire life, vehicle stability we’ll focus on this aspect. The majority of your alignment jobs and control. Alignment will involve daily street drivers and un- modified vehicles, in which case you want assumes even greater to align the wheels (within manufacturer specs) for maximum tire life and direc- importance as vehicle tional stability. For the performance- performance increases. minded driver, a performance alignment is preferred, which simply means that you should take advantage of the vehicle maker’s tolerance range of wheel angles to choose the settings that will maximize Photoillustration: Harold A. Perry; images: Hunter Engineering & Thinkstock the tires’ performance. This would in- volve using the manufacturer’s maximum A toe-in condition (also referred to as ally offer rear wheel toe adjustment. negative camber, maximum positive cast- a positive toe angle) is present when the The toe angle is critical in terms of er and preferred toe settings. In this way, two wheels on the same axle are closer tire tread life. Ideally, the front steer you enhance handling without wander- together at the front and wider apart at wheels need to be parallel while cruis- ing beyond the vehicle maker’sspecs. the rear. A toe-out condition (also called ing to avoid tread scrub. However, toe Toe angle compares the distance be- a negative toe angle) is present when the can also be used to alter a vehicle’s han- tween the center of the front of the wheels are farther apart at the front and dling traits. An increased toe-in setting tires to a distance between the centers closer together behind the axle center- can help to reduce an oversteer condi- of the rear of the tires on the same line. All front suspensions, regardless of tion in turns, and will improve a vehi- axle. A zero-toe angle exists if the dis- design, feature toe angle adjustment, at cle’s high-speed directional stability. An tance between the front of the wheels a location on the steering tie rods/tie rod increase in toe-out can reduce an un- (ahead of axle centerline) is identical to ends. Live rear axles will feature no toe dersteer tendency and will enhance ini- the distance between the wheels be- angle adjustment, since this is a fixed an- tial turn-in during cornering. But, a toe- hind the axle centerline. gle. Independent rear suspensions usu- out can also result in a darty, less deci- 34 June 2015 Pg_EDIT_Align:Pg_EDIT_ 5/19/15 3:50 PM Page 2 likely won’t change as much, so you may be able to set front toe closer to zero, or a bit toward the preferred side, depending on driver requirements. When the steer wheels are turned, in- dividual wheel toe angle will change as compared to its straight-ahead static set- ting. For example, when the steering wheel is turned to the left, the left front wheel will exhibit greater toe-out as com- pared to the number of degrees that the right front wheel toes in. This design fea- ture reduces the tendency of tire scrub during turns and reduces the turning ra- dius of the outboard wheel, reducing the car’s tendency to turn-in too quickly, while providing reduced recovery effort when the vehicle direction changes. The inside wheel must turn in a tighter radius than the outside wheel to allow a smoother turn and reduce tire scrubbing. While static toe (with wheels aimed straight ahead) allows both front wheels to rotate at the same speed and parallel to each other, during a turn the individ- ual wheel toe angles differ when the steering wheel is turned more than 20°. This is referred to as the Ackerman principle, allowing the inside wheel to turn in a tighter radius while the outside wheel turns at a larger radius. Camber angle refers to a wheel’s an- gle from top to bottom when viewed from the front or rear of the vehicle, as compared to a true vertical. If the wheel leans out at the top, this is positive cam- ber. If the wheel leans inward at the top, this is negative camber. If the wheel is set at a true vertical, this is zero camber. For high-performance driving, a neg- ative camber angle is preferred, to com- pensate for the lateral forces experi- enced in cornering. Dialing in more negative camber serves to compensate, sive straight-ahead condition at speed, Commonly, a rear-drive vehicle will placing the load more evenly across the especially in wet or slippery conditions. call for a slight toe-in setting, and a front- tire’s tread area. With an excessive toe setting (in or drive vehicle will call for a slight toe-out For a performance setting, you need out), each front tire is pointed in a di- setting. This is intended to compensate to consider how the vehicle is to be rection other than straight ahead. When for front tire and front suspension bush- driven (re the driver’s expectations) to the tires encounter a road surface with ing deflection as the vehicle is driven for- achieve an acceptable balance between diminished traction (water, snow or ice), ward. As a rear-drive vehicle moves for- cornering traction and tire tread wear. the tire that hits the less tractive side of ward, the front wheels may tend to try to As you increase negative camber, the in- the road loses its grip, favoring the op- crawl away from each other, while a side tread area will tend to wear faster posite tire on the same axle, which can front-wheel-drive vehicle’s front wheels than the rest of the tread when the ve- tend to pull the vehicle in the direction may try to crawl inboard. If the suspen- hicle is driven in a straight line; but if of the toe angle. For the street, it’s best sion has been modified with the use of the driver is aggressive in turns, insuffi- to stay within the limit range specified stiffer, less compliant bushings, the cient negative camber will allow the tire by the vehicle maker. amount of toe change under acceleration to “roll” excessively, reducing the tire June 2015 35 Pg_EDIT_Align:Pg_EDIT_ 5/19/15 3:50 PM Page 3 PERFORMANCE WHEEL ALIGNMENT contact patch. The goal is to manufacturer’s caster spec to create an acceptable com- Toe-Out obtain the best compromise promise between tread wear between tight-turn and high- and cornering grip. In simple speed control. terms, we dial in more nega- Front caster angle may or tive camber to allow the tire may not be readily ad- to “stand up straight” during justable, again, depending on hard cornering. suspension design. If the Factory camber specs are front suspension features up- usually biased toward overall per and lower control arms, tire tread life for so-called the upper arm will likely be normal driving. The perfor- Toe-In adjustable, either via the ad- Illustrations courtesy Hunter Engineering mance-minded driver will During turns, the inside wheel turns at a greater toe-out angle, dition or removal of shims benefit from a setting that’s turning the wheel to follow a tighter radius than the outside (between the upper arm and negative. Keep in mind that wheel. This reduces tire scrub during a turn, allowing the wheels frame) or via eccentric bush- excessive negative camber to rotate during the turn with less rolling resistance. ings. If an upper/lower con- (excessive for the street, that trol arm system is featured, is) can result in reduced straight-line er ball joint. A more positive caster an- the two anchoring locations (where the high-speed stability and make the vehi- gle contributes to directional stability at upper arm attaches to the frame) can be cle a bit darty. speed, and aids in steering wheel re- adjusted (again, with shims or ec- Caster angle settings for street-driven turn, helping the steering to return to a centrics). To alter camber, the adjust- vehicles should provide a good compro- straight-ahead position after a turn. A ment must be performed equally at mise to achieve an acceptable steering zero caster angle, where the lower pivot front and rear attachment points, to effort, confident high-speed stability is directly below the upper pivot, would move the upper arm pivot inboard or and turning/cornering performance.