How to do a string line wheel alignment
Wheel alignment is an important part of a race car set up as it has a direct impact on the handling of the car, particularly in cornering, and roll resistance which translates to speed. In simple terms a wheel alignment ensures your wheels are adjusted to the best angle’s – both vertically and laterally – to deliver the best handling and straight-line speed.
There are several elements to doing a full and complete wheel alignment including:
Camber – the angle that your wheels are tilted (vertically) either in or out
Castor – the slope of the steering axis, determine by drawing a vertical line through the upper and lower ball joint
Toe – the angle that the leading edge of your wheels point inwards towards the center line of your vehicle or point outwards and away from the center line of your vehicle
When doing a wheel alignment, it is essential that Caster and Camber be adjusted to the desired settings first as this will have an impact on Toe. It is also necessary that all adjustments be made with driver weight onboard as this too will have an impact on your alignment settings.
In this article we will focus on the sting line wheel alignment used to measure Toe but before we start on this let’s have a brief look at Camber and Caster.
Camber is the angle that your wheels are tilted (vertically) either in or out. When your wheels are tilting inwards at the top this is referred to as negative camber and conversely when the top of your wheel is tilting outwards, this is known as positive camber.
Camber is an adjustment used to ensure that the optimum tire patch is in contact with the road when cornering to deliver maximum grip, handling and stability. The g-forces exerted on your car and tires when corning results in your tires being distorted and this is where camber ensures optimum surface contact is maintained. To measure camber you will need other tools and techniques not covered in this article.
Formula Vee’s run negative camber in the range or negative 1 – 1.5 degrees on both the front and rear.
Caster is the slope of the steering axis, determine by drawing a vertical line through the upper and lower ball joint as seen below.
Positive caster is when the bottom of the steering axis line is in front of the tire's contact patch. Zero caster is when the steering axis is at 0 degrees. All manufacturers recommend positive caster (as shown above) as this ensures good stability, helps maintain straight direction and promotes wheel self-centering. To measure caster you will need other tools and techniques not covered in this article.
Formula Vee's generally run 4.5 degrees positive camber.
The String line Box & Measuring Toe
Using a simple string line to do a wheel alignment may sound rudimentary and inaccurate but rest assured that even the most experienced teams at the most elite levels of motorsport still use the string line method on a regular basis.
When setting up the string line box ensure that your car is sitting on level ground to ensure the suspension on all wheels is the same and remember to have the driver sitting in the car both for weight and to hold the steering wheel straight and centered.
Once on flat ground and your driver is onboard you will need two rods longer than the width of your car – one to sit in front of your car and one behind. These rods will need to be set up at axle height of your car, a couple of car stands and clamps should do the job. The next step is to run a string line on both sides of your car between the two rods – in order to get these parallel to your car there will be some measuring, adjusting and remeasuring to be done before they are set in the right position.
To ensure the string lines are parallel to your car you will need to measure the distance from the center of the axle hub to the string and adjust your string until the distance between the hub and string line is the same distance for both front wheels and the same distance for both rear wheels. The distance between the hub and the string line will be different between the front wheel and the rear wheel due to different wheel tracks between the front and rear of the car.
Once your string line is parallel with your car its time to measure and adjust the toe as required. In order to measure your toe you will need to take a measurement from the front of your rim (or Tyre, but wherever you measure from make sure you are consistent around the entire car) to the string line and write this number down, you will then need to measure from the rear of your rim (or Tyre) to the string line, again write this number down. In order to calculate the toe in or toe out subtract the front measurement from the rear measurement. For example, if your front measurement was 60mm and rear was 50mm you would have +10mm or 10mm TOE IN. If the numbers were reversed with 50mm at the front and 60mm at the rear, you would have -10mm or 10mm TOE OUT.
Some people prefer to convert these numbers into degrees, and the easiest way to do this is to use an online conversion calculator. When using this calculator, you can use any unit of measurement providing the preferred unit (mm, cm, in) is used throughout (ie don’t mix mm with cm or inches). Below you can see an example of this conversion where the front wheels have zero toe and the rear wheels have 1.43* TOE IN.
Recommended Toe for a Formula Vee is 1mm TOE OUT on the front wheels and 1mm TOE IN on the rear wheels.
Additional information relating to Formula Vee set up can be found on the Formula Vee Australia website covering issues such as tyres, suspension, dry v wet track set up and testing.