Front End Alignment: Toe Measurement Explained

When it comes to ensuring your vehicle tracks straight and true, a proper front-end alignment is paramount. One of the most commonly discussed, and often misunderstood, aspects of this procedure is the measurement of 'toe'. This refers to the angle of the wheels relative to the vehicle's longitudinal axis. A common question that arises, particularly for those undertaking DIY alignments or seeking to understand garage practices, is whether to measure the toe-in from the edges of the wheel rim or from the widest points on the tyres. The answer to this question is not as straightforward as it might seem, as both methods exist, and understanding the nuances is key to achieving accurate results. Furthermore, understanding the correct specifications for your specific vehicle, such as the 'total toe-in' for a classic like a 1973 MGB Roadster, and the adjustability of other alignment parameters like camber and caster, can make a significant difference to your car's handling and tyre wear.

Understanding the 'Toe' Measurement: Rim vs. Tyre

The fundamental purpose of measuring toe is to ensure that the front wheels are pointing precisely forward, or with a slight inward (toe-in) or outward (toe-out) angle, as specified by the manufacturer. This precise angle is critical for stability, steering feel, and even tyre longevity. However, the method of measurement can introduce variables that lead to different readings.

Measuring from the Rim Edges

Many professional alignment systems utilise fixtures that mount directly to the wheel rim. These systems often employ lasers or precise measuring tools that reference the rim's circumference. The advantage of this method is that the rim is generally a rigid and consistent surface. Modern alloy wheels, in particular, are manufactured to tight tolerances, providing a stable reference point. When using rim-mounted fixtures, the measurement is taken at specific points on the rim, often equidistant from the centre line of the wheel. This method is favoured in professional settings due to its speed and the accuracy achievable with sophisticated equipment. The key here is that the fixture itself is designed to be perfectly perpendicular to the wheel's hub mounting surface, effectively measuring the angle of the wheel itself, not necessarily the tyre's contact patch.

Measuring from the Widest Points on the Tyre

Conversely, many DIY enthusiasts and some older alignment procedures suggest measuring from the widest points of the tyre. This approach often involves using a simple tape measure or a dedicated toe-in gauge that clips onto the tyres. The rationale behind this is that the tyre's contact patch with the road is the ultimate determinant of how the car will track. However, tyres are not perfectly uniform. They can have slight variations in their construction, wear patterns, and even ballooning effects at speed. Identifying the absolute widest point on a tyre can also be subjective. Furthermore, the tyre can be slightly out of round, or the tread pattern might not be perfectly symmetrical. This can lead to inconsistencies in measurements, especially if the tyre is not perfectly clean or if the measurement is taken at a different point on the tyre's circumference for each wheel. The 'widest point' can also change depending on the tyre's pressure and load.

Why the Discrepancy?

The discrepancy between these two methods arises from the fact that the tyre is a flexible component mounted on a rigid rim. Even a perfectly aligned wheel can have a tyre that appears to have a slight toe-out or toe-in if measured from its sidewalls, due to the tyre's shape and tread design. Professional alignment systems that measure from the rim are, in essence, measuring the geometry of the wheel hub, which is what the suspension components are directly connected to. While the tyre's contact patch is what interacts with the road, the alignment is set based on the wheel's orientation. The assumption is that the tyre will then deform appropriately to create the correct contact patch. Measuring from the tyre's widest point can be a useful indicator, but it's less precise than a rim-based measurement, especially when dealing with aftermarket wheels or tyres with aggressive tread patterns.

Key Alignment Specifications: Total Toe-in for a '73 MGB Roadster

You've hit upon a crucial point regarding specific vehicle requirements. For your 1973 MGB Roadster, you're looking for the 'total toe-in' specification. You're right to question the Bentley manual; sometimes, specific details can be elusive! Generally, for classic British sports cars like the MGB, a slight amount of toe-in is desirable for straight-line stability. You've mentioned a figure of 3/32" to 1/8" (approximately 2.4mm to 3.2mm). This range is quite typical for many vehicles requiring a small amount of toe-in.

Let's break down what 'total toe-in' means and how it's measured:

• Toe-in: The front edges of the front wheels are closer together than the rear edges.
• Toe-out: The front edges of the front wheels are further apart than the rear edges.
• Total Toe: This is the sum of the toe on the left wheel and the toe on the right wheel. If both wheels have 1/16" of toe-in, the total toe-in is 1/8".

The exact specification for a 1973 MGB Roadster can vary slightly depending on the source and the era of the manual. However, a common recommendation for MGBs is indeed around 1/16" to 1/8" (2mm to 3mm) of total toe-in. This slight toe-in helps to counteract the tendency of wheels to want to toe-out under braking or acceleration due to play in the steering system or suspension components. It provides a more stable feel on the open road.

Crucially, when measuring, ensure the steering is held perfectly straight, and the suspension is loaded as it would be when driving. For a classic car, it's also important to ensure there's no significant play in the steering rack or worn bushes, as this can make achieving and maintaining precise alignment very difficult.

Adjustability of Camber and Caster

Your suspicion about camber and caster adjustability is astute. While many modern cars have fixed camber and caster angles from the factory, some vehicles, particularly older ones or those with specific suspension designs, do offer adjustability. You mentioned shims, and indeed, shims are a common method for adjusting camber and caster, especially on vehicles with wishbone or stub axle suspension setups.

Camber

Camber is the angle of the wheel relative to the vertical when viewed from the front of the car. Negative camber means the top of the wheel is tilted inwards, and positive camber means it's tilted outwards.

• Fixed Camber: Many MacPherson strut designs have the camber angle determined by the relationship between the strut and the steering knuckle, which is not typically adjustable without modification.
• Adjustable Camber: On vehicles with double wishbone suspension, camber is often adjusted by adding or removing shims between the suspension arms (or their mounting points) and the chassis, or by using eccentric bolts. The MGB's front suspension, a classic independent setup, does have provisions for camber adjustment, typically via shims at the trunnion or stub axle mounting points. It's a more involved process than toe adjustment and requires careful measurement and adjustment to achieve the desired angle, usually a slight negative camber for improved cornering grip.

Caster

Caster is the angle of the steering axis relative to the vertical when viewed from the side of the car. Positive caster means the steering axis is angled backward, and negative caster means it's angled forward.

• Fixed Caster: Similar to camber, caster can be fixed on some suspension designs.
• Adjustable Caster: On many wishbone-type suspensions, caster is adjusted by shims at the front or rear of the upper or lower wishbone mounting points. Adjusting caster significantly affects steering stability and feel. More positive caster generally leads to a heavier steering feel and increased self-centring action, which helps the steering wheel return to centre after a turn and improves straight-line stability at higher speeds. The MGB's caster is also adjustable, often through the same shim points used for camber, or via specific shims in the wishbone mounting brackets.

Important Note on Adjustments: When adjusting camber and caster, it's vital to understand that changing one can affect the other, and both can influence the 'thrust angle' (the direction the rear axle is pointing relative to the car's centreline). Therefore, it's a delicate balancing act. For a classic car, it's often recommended to set the toe first, then adjust camber and caster, and then re-check and fine-tune the toe, as adjustments can slightly alter toe angles.

Practical Considerations and Tips

Here are some practical tips for performing a front-end alignment, whether you're a seasoned mechanic or a dedicated enthusiast:

• Level Surface: Always perform alignment measurements and adjustments on a perfectly level and flat surface. Even a slight slope can throw off your readings.
• Tyre Pressure: Ensure all tyres are inflated to the manufacturer's recommended pressure before taking any measurements.
• Suspension Check: Thoroughly inspect all suspension and steering components for wear, including ball joints, tie rod ends, bushings, and wheel bearings. Any play in these components will render alignment efforts futile.
• Steering Wheel Centred: Ensure the steering wheel is perfectly centred before you start adjusting the toe.
• Lock it Down: Once you've achieved the desired toe setting, ensure the tie rod ends are securely tightened and the locking mechanisms (if any) are engaged.
• Professional Equipment: While DIY is possible, professional equipment offers superior accuracy. If you're aiming for precision, especially for performance driving or if you're experiencing persistent issues, consider using a professional alignment service.

Frequently Asked Questions

Q1: Which method of toe measurement is considered more accurate?
A1: Measuring from the rim edges using professional equipment is generally considered more accurate as it references the rigid wheel hub, not the flexible tyre.

Q2: Can I measure toe-in with just a tape measure?
A2: You can get an approximation, but it's difficult to achieve the precision required for optimal alignment due to tyre variations and subjectivity in finding the widest point.

Q3: What happens if my car has too much toe-in or toe-out?
A3: Too much toe-in or toe-out can lead to rapid and uneven tyre wear, poor straight-line stability, and a tendency for the vehicle to pull to one side.

Q4: Is it normal for camber and caster to be adjustable on older cars like my MGB?
A4: Yes, many classic cars with independent suspension systems, like the MGB, have provisions for adjusting camber and caster, often using shims.

Q5: If I adjust camber or caster, do I need to re-check the toe?
A5: Absolutely. Adjusting camber and caster can slightly alter the toe angle, so it's essential to re-measure and adjust toe as a final step.

By understanding these principles and specifications, you can be better equipped to diagnose alignment issues or ensure that your next alignment is performed correctly, leading to improved handling, safety, and tyre life for your vehicle.