Toe measurement is fundamental to wheel alignment for any road or race car because it changes handling, performance and even tyre wear.
- Learn what Toe is
- How to tune it, to improve performance
- Ways to measure toe
- How to adjust toe
What is Toe?
If you imagine you’re looking at the vehicle from a birds-eye view with the front of the car at the top. If the tips of the wheel are pointing in, this is toe-in. If the wheels are parallel, this is zero toe. If the wheels are pointing out, this is toe-out. The toe geometry refers to the symmetrical angle the wheel creates with the longitudinal axis of the vehicle, which is usually measured when the vehicle is static.
Literally, point your toes
An easy way to remember this, is if you are standing, look down at your feet. If your toes are pointing in, this equates to toe in. Then, if you rotate them to point outwards – it’s toe-out. If they are not pointed in or out, you are parallel this equates to zero toe. Hence the name toe in, out etc.
The quick thinkers may be looking down at their feet and wondering what happens if both feet “wheels” are in different directions. If this happens on a car, then your alignment is out, the car won’t drive straight, and you need the geometry fixing (wheel alignment).
How Toe Affects Performance – Toe Tuning
Toe can be used to enhance chassis setup
- Making the car agile
- Turning sharply or increasing stability
- Inspiring confidence in high-speed corners
Using the 3 different toe angles have very different effects depending on if the wheels are the front axle or rear axle. Also, if the wheels provide the driving momentum, this causes different effects for front-wheel drive FWD, rear-wheel drive RWD or all-wheel drive AWD.
The following sections will break down these effects, so you understand the dynamics and what can improve the handling of your vehicle.
Toe In Front Wheels
Having the front wheels toe-in will make the car feel stable and planted through higher speeds corners. As the wheels require a larger input from the driver before a direction change. This high-speed stability due to toe in provides will reduce agility and nimbleness, through tight low speed corners.
The tyre will also heat up fast because toe-in increases the tyres slip angle.
Heating the tyre increases adhesion, making the tyre sticky, thus enhancing grip. As you may have noticed, driving on a cold slick tyre is like driving on ice and the tyres only work once they are warm. Hence having green flag lap in circuit racing.
Slip angle tangent
We need to go into some detail regarding slip angle, so we can fully cover all aspects of this subject. We will return to toe in after this section.
Slip angle is the difference between the direction in which a wheel is pointing and the direction of moment a vehicles is travelling in. (see image below)
Do not get the wrong idea about increasing slip angle to generate more grip. Having the wheels pointing at a ridiculous 45 degrees won’t help.
There is a window where the tyres maximum friction and deformation from slip angle provide peak grip. This is the window we are aiming to work in with toe settings. Lean more about tyres in an article coming soon.
Back to toe in on the front axle…
Increasing tyre heat to aid with adhesion will only help to a point.
The consequence of heating the tyre faster though deformation, is increased tyre wear; thus reducing tyre life from the stresses on the rubber.
Nobody wants a stressed rubber!
The greater the amount of toe-in applied the greater the reduction to top speed. Excessive Toe in can lead to uneven tyre wear with the outside edge getting worn away fastest.
If the front wheels are driven such as in FWD and AWD vehicles the increased tyre slip angle results in increased grip. As the driven wheel has more grip, acceleration improves.
Running toe in on RWD will reduce straight-line speed and acceleration as the grip generated by undriven wheels increases drag.
Parallel, Zero Toe Front Wheels
Top speed can be improved by running zero-toe on the front wheel as this reduces resistance by minimising tire stress. Minimising tire stress, this improves tire life, making zero toe or very close to popular with road cars, where tire life is very important. Endurance racing may also use a front wheel set close to zero-toe to maximise tire life & increasing distance between pit stops.
With zero-toe, the driver steering inputs are the most neutral, neither enhancing nor reducing high-speed corner stability or low-speed corner stability.
Zero front toe on the driven wheels such as AWD and FWD can reduce acceleration as there is a reduction in slip angle that generates grip.
Toe Out Front wheels
A toe-out set up on the front wheel will make the vehicle turn-in better on tight lower speed corners, as less steering input is required. Resulting in an agile feel, Ideal for tight tracks or hill climb courses with a lot of direction changes.
Through high-speed corners, toe-out can result in a less stable, twitchy car.
Similar to the above paragraphs, the tyre heats up fast as toe-out increases tyre slip angle. The improved adhesion from hot tyres makes them sticky, enhancing grip.
Heating the tyre faster reduces tyre life due to stresses on the rubber.
The greater the amount of toe-out applied the greater the reduction to top speed. A Lot of toe-out can lead to uneven tyre wear with the inside edge getting worn away fastest.
As front wheels are driven on FWD and AWD vehicles the increased type slip angle results in increased grip. As the driven wheel has more grip it improves acceleration.
Toe out on the front wheels of a RWD will reduce straight-line speed and acceleration as the grip generated by undriven front wheels causes drag.
Toe In Rear Wheels
Toe-in on the rear axle will make the vehicle more stable during high-speed corners. During the corner, weight transfers to the outside tyre. As toe in points inwards, the loaded tyre in the direction you’re turning works against the steering reducing rotation and oversteer.
This is excellent at reducing the snappiness of a short wheelbase car, giving the diver the feel of a longer wheelbase.
The downside of a stable rear-end the car is that the car becomes less agile and nimble through tight slow-speed corners requiring greater steering inputs.
The increased slip angle of toe-in increases tyre grip, heating the tyre faster, however, reducing tyre life.
On an FWD having more grip at the rear creates more drag, this resistance reduces acceleration and top speed. As FWD are harder to get heat into the rear tyres. Using toe in to help generate heat can aid an inexperienced driver.
Ever been at a race meeting and seen a car lose it and spin out on the green flag lap?
Now you know why… an inexperienced driver running out of talent attempting to heat their rear tyres.
When the rear wheels are driven in RWD and AWD applications, extra grip from increased slip angle aids acceleration but reduces top speed.
Something to be considered is increasing rear grip too much will increase understeer particularly on RWD because the rear axle is pushing the front straight resisting the driver’s steering inputs.
Drift cars having a stable grippy rear, which can make the car easier to drive and drift better at high speeds. The stability from toe-in makes the rear more predictable allowing the driver to control the drift on the throttle. The extra grip that the rear provides, helps drive the car forward during the drift & reduces the chance of spinning out. High power drift cars are often grip limited and with more grip and power comes higher speed drifts with more smoke.
Parallel Zero Toe Rear wheel
Running zero toe is neutral neither enhancing nor reducing stability in high-speed corner or low-speed corners. The reduction in slip angle lowers tire stress having a few direct effects improving tire life, reducing heat, reducing rear grip and drag, the reduction of drag increased top speed and fuel range for long-distance races
FWD acceleration can increase as there is less resistive drag from the rear axle to overcome.
RWD acceleration is reduced with rear grip lessened. Zero toe points the wheels straight ahead, which can stop the rear end from wandering or shifting about during straight driving, particularly at high speeds.
Toe Out Rear wheels
Running toe-out on the rear axle makes for an “interesting” drive. It improves rotation making the rear of the car lively. Improving performance in tight corners as it will turn faster and tighter. If rotation is not controlled properly, it’s prone to become oversteer.
To an inexperienced driver a very lively rear end can result in occasionally spinning out, their heart beating very fast and small brown stains on their underwear.
During corners the weight transfers to the outside wheel. As toe-out point’s the wheel out, this aids in rotating the vehicle around the corner. Requiring less steering input to turn the car. This give the driver an agile feeling.
The car may feel unstable through high-speed corners.
Yes unstable and prone to oversteer in the fast scary corners, leading to some of those brown stain moments.
The increase to slip angle results in more tyre stress, thus reducing tyre life but generating heat-enhancing adhesion and grip.
When the rear wheels are driven in RWD and AWD this improves acceleration.
If undriven in FWD the extra grip results in more drag-reducing acceleration. All cars will experience a reduction in top speed to the greater the angle of toe applied.
For any drifter’s in low powered cars toe out on the rear can help initiate a drift.
How To Measure Toe
The toe measurement is usually taken from the outside lip of the wheel rim to a parallel axis line to the chassis. The difference in distance between the front and rear edge of the wheel to the axis gives the toe reading in mm. This is most visually clear when you can see the string line alignment method. Other measurement methods have equipment clamps to the wheels to measure the angle of the wheels. Providing readouts in degrees or maybe minutes.
Add string alignment picture here?
Here is a helpful conversion table to equate toe readings from degree to mm for different wheel sizes.
ADD CONVERSION TABLE HERE
Before any wheel alignment check for buckled wheels. The buckle in the wheels will provide a false reading causing the alignment to be set wrong.
Another little tip. Setting all the tyres to the same pressure increases the consistency of measurement.
String Line Alignment
As old fashioned and low tech as the name suggests, string alignment is most popular in the motorsport industry. It provides portability & is easy for the race team to set up trackside. It can also provide an accurate reading for each individual wheel. There is a variety of equipment used for this method but it all comes down to these key features.
Two strings are set to run parallel along the outside of the chassis. The height of the sting is set to the middle of the wheels. Then measurements are taken from the front and rear lip of each wheel to the string. If the larger number is at the rear lip you have to toe out, if the larger number is at the front lip, you have toe in. The difference between the number is the amount of toe given in mm.
Higher-level motorsport use another piece of equipment with a sting kit – Set up wheels. These are usually machined blocks that bolt to the hubs instead of the wheels giving two ..
- It improves accuracy by removing potential buckled wheels to prevent false readings.
- It allows mechanics to have easy access to the hubs and suspension for any adjustments to the car while at ride height.
Another simple method to measure toe – A straight plate is placed at the outside of each wheel, a measurement is then taken between the back edge of the left plate to the back of the right. Then between the front of the left then right plate. The difference between these numbers gives the total axle toe measure in mm. (Opposite to the string alignment). The larger number at the rear wheel edge is toe in, the greater number at the front is toe out.
Toe plates are easy and simple to use but have a large flaw. They can only provide measurements across the axle, not individual wheels. If both wheels are not symmetrical it could cause the car to pull to one side.
None Symmetrical Example: The front axle might read 4mm toe out tota on the plates, however, the wheels might be 2mm toe-in and the other 6mm toe out. Give a false reading of 4mm out This would cause the vehicle to pull to the side with 6mm out. Know if both wheels are running true you will be able to adjust the wheels to 2mm toe out each, 4mm out total across the axle. Giving a symmetrical reading and straight driver car.
Laser wheel alignment
These systems use equipment that hangs on the outside of the wheel rim. By pointing lasers across the axle to the other opposite side (left to right), equipment it can give total axle toe readings. By pointing lasers to the equipment mounted on wheels on the same side but opposite axle (front to back) it will tell the user if the wheel is running true/symmetrical.
4 Wheel Alignment Machine
Most road car garages will use a large 4-wheel alignment machine to adjust wheel alignment. These machines are often fitted to 4 post ramps. They attach equipment to each wheel. Dependant on the brand a mixture of lasers, mirrors and sensors are used to check each wheel’s geometry. Which give a reading in degrees/minutes to a display screen.
How to Adjust Toe
There are a variety of ways to adjust toe geometry depending on suspension setup and vehicle style. A specially built single-seater racer will be different from a fast road car.
Common methods covered
- Track Rod
- Eccentric washer’s
- Adjustable arms
The track rod is the most common way front toe is adjusted, its actually the same for road cars as most purpose-built race cars. They are sometimes referred to as tie rods, but I will continue to refer to them as track rods as they are used to adjust the cars tracking.”
The tack rods consist of two parts,
1 track rod end, connected to the hub with a ball joint. A female threaded hole that connects to part two.
2 inner track Rod, this is a partly threaded bar that connects the track rod end to the steering rack with bearing.
Having bearing joints at both ends allows for suspension movement.
To adjust the toe, it’s simply an act of threading the inner track rod bar in or out of the female threaded track rod end, making the total track rod length longer or shorter. As it is connected to the hub on one side, the lengthening or shortening will toe the wheels in or out.
Popular on-road cars, the eccentric washer system utilises a bolt and washer with the hole offset to one side. As the bolt and washers rotate, the offset hole moves the position of the bolt in or out. Applied to the toe arm suspensions pickup points, this movement can be used to adjust the wheel’s toe. However adjustment is limited because of size and as the washers are round, once it’s adjusted to its max point it will decrease in adjustment.
The use of toe shims is very common on solid axle vehicles. For some odd reason FWD cars with solid rear axle people tend to say it can’t be adjusted; this is false. you will now learn how it works and to do it.
A solid shim or plate is used to space the hub from the axle at the required angle. The shim plates are precision machined with a tapered face, when the hub is solidly bolted to the axle the shim plate sits in between, spacing the hub to the toe angle required.
Once set it is very unlikely for the geometry to become unaligned, even with some apex curb strikes on track.
If it does move out of alignment this usually means the axle is bent. If the axle is bent this usually means the car has hit something hard and solid like a wall. If this happens, I would suspect you have more urgent priorities!
The use of shims is not limited to solid axles. Some purpose build race cars will use these shims with double wishbones suspensions. In this application the hub is adjustable. The shims are used between the arm’s mounting points, giving full adjustability without effecting the suspensions system’s geometry eg bump-steer.
An excellent replacement for OEM suspension arms/toe links are adjustable arms. As they provide a much larger range of adjustability compared to eccentric washers. These adjustable arms replace the stock suspension link. Similarly to the track rod system, they are made with a threaded section that can be screwed out to lengthen, or in to shorten the arm’s length, thus changing the wheel alignment.
These are an excellent upgrade if you’re converting road cars to push the limits on the track. Many motorsports vehicles like TCR cars a factory-built race car that use adjustable arms as they provide excellent adjustability.
Road Car Adjustment Tip
As road cars are used in all weathers conditions, adjusters are prone to rust and therefore seizing. Applying some penetrating fluid, serious heat from a torch to the nut and some lubricants to a bushing, will reduce hours of stress and swearing.
There are other ways to release a nut and stress with lube and penetration, but this definitely isn’t the website for that.
The greater the toe angle the greater the increase to wear.
Toe In – results in more wear to the outside of the tyre. As the vehicle drives straight, the moment is directly forward but the wheels are slightly pointed in, this causes wear to the outside leading edge of the tyre. The inverse is true for toe out.
Toe Out – will cause increased wear to the inside edge of the tyre. When tuning your wheel alignment set up, be mindful negative camber also increases wear to the inside edge, the compounding effect from both of these is not to be overlooked if tyre life is a performance factor.
Now you’ve learned the effect of changing the wheel’s toe, where to measure it, and how to adjust it. You can apply it to enhance your driving with a custom wheel alignment.
Another things to consider is the wheel camber and how that can be optimised to enhance grip, learn how it work and how to tune it here
Camber Geometry – Tuning Wheel Alignment