A Complete Guide to Wheel Alignment

Summary:

1. Car Alignment: Open or Pinched

2. Wheel Alignment Adjustments and Constraints

2.1 Front Wheel Alignment

2.2 Rear Wheels: Consequences of Parallelism

2.3 Difference Between Traction and Propulsion

3. Car Alignment: A Notion of Compromise

4. Causes of the Wrong Alignment of Your Car

5. Wrong Car Parallelism: What Are the Consequences?

Intro

Wheel alignment or car parallelism is one of the concepts involved in the adjustment of a vehicle’s suspension geometry. In contrast to camber, which evaluates the angle formed by the wheels in relation to the ground (front view), parallelism is the angle formed by the axis of the wheels in relation to the direction of travel of the vehicle (top view).

It is measured independently for the front and rear wheels. It is expressed in degrees or millimeters and generally represents a deviation of between 0,2 and 1,5 mm per wheel (obtained by subtracting the distance between the front of the two wheels from the distance between the rear of the two wheels). Even if the measurement seems small, it is an adjustment not to be neglected since it can lead to pronounced mechanical stress, driving discomfort and asymmetrical tyre wear. Explanations below.

Please note: This is done by hand; it is checked with a string encircling the four tires to measure alignment deviations. Professionals use pre-calibrated benches or laser beam meters.

1. Car Parallelism: Open or Pinched

Positive parallelism indicates that the wheels are pinched inward (as in snowplow position with skis). Conversely, negative parallelism indicates that the wheels are open (duckwalk):

2. Alignment Adjustments and Constraints

Dave Walker, 341st Logistics Readiness Squadron, performs a wheel alignment on June 10. (U.S. Air Force Photo/ Beau Wade)

Generally speaking, neutral parallelism would see the four wheels parallel to each other. This adjustment does not cause any loss of power or premature mechanical (or pneumatic) wear:

  • On a pinched (positive) alignment, the two wheels force towards each other and therefore wear the tire on the outside.
  • Whereas an open (negative) alignment causes the wheels to pull away from each other and wears out the inside of the tires.

2.1 Front Wheel Alignment

Pinched parallelism creates an inward thrust:

  • It gives more stability at high speeds but makes the vehicle less directional.
  • Open parallelism gives more incisive steering but with less stability at sustained speed.
  • It also puts more stress on the steering mechanics (ball joints and rack and pinion).

2.2 Rear Wheels: Consequences of Parallelism

Pinching the rear wheels gives even more stability but may promote understeer (front of the vehicle skidding), especially if coupled with a pinched front wheel alignment.

Opening the rear wheel alignment gives more agility but makes the vehicle oversteer (the rear of the vehicle is skidding).

2.3 Difference Between Traction and Propulsion

As a reminder, traction is a vehicle whose driving wheels are at the front while they are at the rear for a car with a propulsion system:

  • In both cases, when accelerating, the drive wheels will tend to pinch (because of the torque that pulls the wheels forward while they are held by the suspension arms).
  • The non-driven wheels of a traction drive, at the rear, have a relatively neutral behavior.
  • The non-driven wheels of a propulsion drive, at the front, undergo much more load transfer and will tend to open under acceleration.
  • In addition to being relieved under acceleration, the front wheels of a rear-wheel-drive system will be made very directional but relatively unstable, unlike the rear wheels of a rear-wheel-drive system, which receive more load under acceleration and therefore more grip.

3. Car Alignment: A Notion of Compromise

For all these reasons, common settings have been established on consumer vehicles to correct the inherent characteristics of each technology.

In this way, a vehicle will have the most neutral parallelism possible when driving. However, each correction has its drawbacks; there is no perfect choice:

– For traction, parallelism slightly open at the front counteracts the tendency to accelerate. It makes the vehicle more directional, which is necessary due to the overweight of the engine and transmission at the front. At the rear, the wheels are slightly pinched for added stability.

– For rear-wheel drive, it’s a matter of countering the effects of acceleration: the rear will be open, the front slightly pinched to maintain stability when braking.

– A 4-wheel drive vehicle has the properties of both types (propulsion and traction), but the settings can be very different because of the possible uses (city, road, or crossing). These specificities will have to be taken into account because although they have the same transmission scheme, an SUV will have a very different setting than a supersport GT. In this case, please refer to the manufacturer’s data.

Good to know: beyond these characteristics, you should know that the alignment must also be modified because of other geometry settings – in particular the camber (which is not, however, a setting accessible to all vehicles).

4. Causes of the Wrong Alignment of Your Car

The difference in parallelism is subtle in the conventional use of your vehicle. Unlike on a racetrack, alterations in behavior can go unnoticed.
In addition to the wear of the suspension and steering structures, the parallelism of an axle can be altered by these events:

– rolling vibrations;
– unbalanced tire;
– indelicate towing / troubleshooting;
– the difference in inflation between the two tires of the same axle (thus modifying the suspension geometry);
– shocks (pothole, sidewalk…);
– repeated and asymmetrical forces (roundabouts);
– off-road crossing, etc.

5. What Are the Consequences of the wrong car alignment?

In the short term, asymmetrical parallelism can cause an increase in fuel consumption, steering discomfort such as the feeling of being swept to the right or left (the car is said to “pull” to the side), the steering wheel may move from its horizontal neutral position while you are driving straight, or the vehicle no longer corresponds to your daily driving habits.

In the long term, the tires will wear faster and asymmetrically, the mechanical components of the ground and steering will be subjected to additional stress and will, therefore, last less than expected.

In any case, the long-term effects here are mechanical repercussions that are relatively difficult to quantify except for the tires. Premature wear can be as much as 20 or 30%, meaning that a tire can reach its legal limit on an outer or inner tread while the rest of the structure is only two-thirds worn.

Caution: Wrong alignment can also cause the stabilizers and rack end to cut off from the wheel! 

Conclusion

For all these reasons, it is advisable to regularly check the alignment as well as the overall geometry (if the vehicle allows it) at a professional auto repair and maintenance shop. The technical inspection is a good reminder, but do not hesitate to do it proactively after a major impact on one of the axles or if your vehicle behaves in an unusual way.

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