TECHNICAL GUIDE / 00
Alignment Measurement Methods and Why We Use the String Method
There are several ways to measure alignment, particularly toe.
Full-size wheel alignment systems, instruments attached to tyres or wheels, and strings stretched along both sides of the vehicle all use different measurement principles, provide different information and require different equipment.
At 813 Alignment Tools, we use the string method because it allows the operator to check the measurement setup directly and recreate the same conditions consistently.
What Matters in Alignment Measurement
Alignment figures are not an end in themselves.
In practice, setting up a vehicle involves changing camber and toe, then assessing driver feedback, lap times, tyre temperatures and wear to find settings that suit the vehicle.
Being able to repeat measurements before and after a change using the same reference is therefore more important than taking a single high-resolution reading.
However, repeatability does not mean that the actual values do not matter. The values must also be valid if results are to be related to those obtained using other equipment or methods.
Alignment measurement requires both: the ability to measure the same condition repeatedly, and values that properly represent the actual direction of the wheels.
1. Wheel Alignment Systems
A wheel alignment system uses cameras, lasers or similar technology to read targets or sensors attached to the wheels and measure the alignment of each wheel.
These systems can measure camber, toe, caster and thrust angle together and output the results as data, making them well suited to diagnosing vehicle alignment.
However, they require dedicated equipment and installation space and cannot readily be taken to a circuit or home garage. Measurements are also taken with the tyres and wheels fitted, so their condition forms part of the result.
2. Direct Measurement Methods
These methods use instruments attached to the tyres or wheels to measure toe from the relative positions of the left and right wheels or their front and rear measurement points.
Examples include laser-and-mirror systems and methods that directly measure the distance between the tyres at their front and rear edges.
The equipment is relatively compact and portable, but its mounting and measurement positions must be consistent each time.
When total toe is calculated from the difference between these distances, the total alone cannot tell you the direction of each wheel relative to the vehicle's longitudinal axis.
Even if total toe matches the target, an uneven left-to-right distribution can leave the steering wheel off-centre.
3. The String Method
The string method uses strings stretched along both sides of the vehicle as reference lines. Distances are measured from each string to the front and rear measurement points of each wheel.
Because the strings and measurement points are visible, the operator can check both the reference being used and exactly which distance difference is being measured.
The method measures not only total toe but also the direction of each wheel relative to its reference line. Checking total toe and its left-to-right distribution during adjustment also makes it easier to maintain the steering wheel's centre position.
However, simply stretching strings beside a vehicle does not guarantee a correct measurement. Their direction relative to the vehicle and the references used to position them are essential.
Why 813 Uses the String Method
813 Alignment Tools does not use the string method because it is the only correct way to measure alignment.
We use it because the operator can inspect the measurement setup directly and recreate the same references at a circuit or in a home garage.
In conventional string setups, tyres, wheels or wheel arches may be used as positioning guides. However, tyre condition changes with pressure, temperature, load and wear. Body panels are not necessarily symmetrical or parallel to the vehicle's centreline or hub positions either.
Front and rear track widths and wheel offsets may also differ. Positioning each string at an equal distance from the outside of the tyres or wheels can therefore leave it angled relative to the vehicle's longitudinal axis.
At 813 Alignment Tools, we instead use the vehicle's hubs as the starting point for measurement, rather than its tyres or body panels.
We calculate the front and rear hub mounting-face positions from the manufacturer's published track widths and original wheel offsets, then position the strings to account for the difference.
By attaching Dummy Wheels or Hub Stand PLUS units directly to the hubs and measuring at fixed points on the tools, we can check each wheel's toe using references that are less affected by tyre condition or body panel position.
Measurement Values and Repeatability
A wheel alignment system and a Dummy Wheel setup use different measurement references and include different elements in their results.
A wheel alignment system measures the vehicle with its tyres and wheels fitted. A Dummy Wheel setup measures directly from the vehicle's hubs, without the tyres and wheels in between.
The displayed values may therefore not match exactly, even on the same vehicle. A difference alone does not establish that one method is correct and the other is wrong.
However, if each method faithfully measures the quantity it defines, and setup, mounting and reading errors are reduced, the results would be expected to converge towards values representing the same wheel condition.
At a minimum, a major discrepancy, such as one method showing clear toe-in and another clear toe-out, calls for the measurement references and working conditions to be checked.
Valid measurement values are needed to relate results from different equipment and share setup data. In practical setup work, repeatability is then essential: using the same method and conditions to reliably identify the difference before and after a change.
Summary
There are several alignment measurement methods, each providing different information and suiting different applications.
813 Alignment Tools uses the string method because the operator can inspect the measurement references directly and recreate the same setup in a garage or at a circuit.
We establish reference lines from the vehicle's hubs rather than its tyres or body panels, then consistently measure values that properly represent wheel direction under the same conditions.
Relating those readings to driver feedback and lap times, checking the effect of setup changes, and being able to reproduce the desired condition: this is how 813 Alignment Tools approaches alignment measurement.
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