What is Toe?
Toe is how much the wheels of a vehicle are turned in or out from a straight-ahead position. The amount of toe can be expressed as: the difference between the track widths measured from the leading and trailing edges of the tires, degrees, or fractions of an inch.
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Effects of Toe-in and Toe-out
The effects of different types of toe on the front and rear wheels are different depending on whether a car is front, rear or four wheel drive. Since our solar car is very likely not going to be four wheel drive, we will focus on the effects on the front and rear wheels of front and rear wheel drive cars.
Front Wheel Drive
Front Wheel Drive: Front Wheels - Toe Out
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The dynamic effects of toe out will give the car faster steering, meaning that the car turns in faster with less steering input from the driver, which . This is ideal for a tight, winding course with multiple direction changes. This The increased response means that the high speed stability of the car is reduced and can feel twitchy in long fast corners.
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Running toe in on the front wheels of a front wheel drive car will again increase the grip within the front tires due to the increased slip angle in the tire. This increases the acceleration of the car due to the increased grip in the driven tires. The tire heats up faster due to the slip angle, which means the tire gets to its most grippy point faster than if no toe was installed. However, running toe in will decrease the life of the tire due to the increased stress on the rubber.
The dynamic effect of toe in reduces the responsiveness of the steering. This means that the car requires a larger steering input from the driver for the car to change direction. This makes the car more stable at high has the same effects on grip, acceleration, and tire wear as toe out. The dynamic effect of toe in reduces the responsiveness of the steering. This means that the car requires a larger steering input from the driver for the car to change direction. This makes the car more stable at high speeds and makes the car feel planted through long fast corners. This set up is perfect good for new drivers within motorsport or for drivers who do not have fast reaction times.
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Running zero toe on the front wheels will increase the life of the tire. Therefore, this set up is commonly seen within road cars where tire life is important. That said, tire life can be of great importance within long races such as endurance racing, so running a set up close to zero toe can be beneficial as your tires last longer on track. Furthermore. Furthermore, the top speed of the car is increased due to having less drag to overcome the power of the car.
The dynamic effect of running zero toe on the front wheels is that the car is no less or more stable in different corners and will respond evenly in short or fast corners and the . The behaviour of the car will be more dependent on other aspects of set up and geometry.
Front Wheel Drive: Rear Wheels - Toe Out Running toe out on the rear wheels of a front wheel drive car will reduce the acceleration and the top speed of the car. This is because these wheels are not driven so are being pulled around the track. Therefore, any extra grip in the rear tires translates into increased drag, therefore slowing the car down in a straight line. However, the slip angle does generate heat within the tire faster, generating maximum grip faster, which is ideal for short races.
The dynamic effect of running - Toe Out
Running toe out on the rear wheels effectively shortens the wheel base of the car. This happens due to the toe out, rotating the car on corner entry allowing the car to turn in faster and tighter. This effect feels similar to oversteer, without the loss of grip, and makes the car more nimble, ideal for tracks with multiple quick direction changes. However, this effect does make the car feel more unstable through high speed corners.
Front Wheel Drive: Rear Wheels - Toe In
Running toe in on the rear wheels of a front wheel drive car will also reduce the acceleration and the top speed of the car. This is the same as above due to these wheels not being driven so are being pulled around the track. Therefore, any extra grip in the rear tires translates into increased drag, slowing the car down in a straight line. However, the slip angle does generate heat within the tire faster, generating maximum grip faster, which is ideal for short races.
The dynamic effect of running of a front wheel drive car will reduce the acceleration and the top speed of the car. This is because these wheels are not driven and are being pulled around the track. Any extra grip in the rear tires translates into increased drag, which slows the car down in a straight line. However, the slip angle does generate heat within the tire faster. This generates maximum grip faster, which is ideal for short races.
The dynamic effect of running toe out on the rear wheels effectively shortens the wheel base of the car. This happens due to the toe out rotating the car on corner entry allowing the car to turn in faster and tighter. This effect feels similar to oversteer, without the loss of grip, and makes the car more nimble. This is ideal for tracks with multiple quick direction changes. However, this does make the car feel more unstable through high speed corners.
Front Wheel Drive: Rear Wheels - Toe In
Running toe in on the rear wheels of a front wheel drive car has the same effects on acceleration, top speed, and grip as running toe out on the rear wheels. Running toe in on the rear wheels lengthens the effective wheel base of the car. This is because the loaded rear wheel during cornering is already pointing in the direction of the corner. This means it works , causing it to work against the steering input and rotates . This makes the car rotate more slowly. This increase in increases control and makes the car more stable through long high speed corners. It is especially effective when used on a sort short wheel base car.
Front Wheel Drive: Rear Wheels - Zero Toe
Running zero toe on the rear wheels of a front wheel drive car will increase the acceleration and the top speed of the car. This is because the tire is rolling in its most efficient direction and will therefore generate generating minimum drag. This reduction in drag force can be used to propel makes the car forwards instead making it faster in a straight line. Furthermore, the The tire life will be increased due to the reduced stress. However, the tire will take longer to heat up, so will not generate as much grip as early on in the race.
The dynamic effects of zero toe are that the car will feel neutral to control through tight and long fast corners meaning that the driver does not have to alter their driving style if there is a mixture of styles of corner on the track.
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Rear Wheel Drive
Rear Wheel Drive: Front Wheels - Toe Out
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Running toe out on the front wheels of a rear wheel drive car will decrease the acceleration and top speed of the car in a straight line. This is due to the front wheels not being driven so any extra grip generated by toe will increase the drag on the system. The life of the tire will also be reduced which is not ideal for endurance racing. However, the tire will warm up faster getting to operating temperature sooner in the race.
The dynamic effects of toe out will increase the agility of the car, making it turn in faster and sharper with less steering input from the driver. This makes the car navigate through a tight; twisting course faster makes it feel nimble. Toe out also means that the car feels less stable at high speeds and though high speed corners due to the twitchy nature of the set up.
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Zero toe on the rear wheels will reduce the acceleration capabilities of the car but will increase the top speed of the car due to the tire rolling in its most efficient direction. This also means that the life time of the tire is increased but it takes longer to heat up to its operating temperature.
Zero toe makes the car more stable in a straight line at high speed and stops the rear end of the car from shifting around. It also gives the car a similar feel through fast and slow corners and leaves the dynamic feeling of the car down to other aspects of set up and geometry.
How is Toe Adjusted on a Car
Toe is adjusted by modifying the length of the tie rod. The tie rod is a linkage that connects the wheel hub to the steering rack. The body of the tie rod can be threaded or unthreaded to increase or decrease its length which adjusts the toe of the car.
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Measuring Toe
A simple way to measure toe is by using a string and line kit. Two equal length poles are mounted parallel to the axles at either end of the car with string joining both ends. The string should be coincident with the centre point of the wheels. A ruler or tape measure is then used to measure the distance between the string and front and back of each wheel, giving a toe measurement in units of length.
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Sample Toe Values/Configurations
Solar Cars
MSXII: 0 toe
MSXIV: ?
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The front tires of consumer cars are usually toe-in to improve straight line stability since cornering performance does not matter as much. Typical toe-in specs vary from 1/32” (0.79mm) to 1/8” (3.175mm) depending on the vehicle. Some manufacturers calibrate their cars with a slight degree of toe that goes to zero once the car is on the road.
Other Factors that Change Toe
Bump Steer
Bump steer is a change in the steering angle of the wheel when the wheel is in bump or droop without turning of the steering wheel or lateral movement in the steering rack. When a car drives over a bump, the wheels, control arms, and tie rods displace vertically (each by the same distance). Since the control arms and tie rods pivot around a fixed point, this displacement follows the path of an arc. The fixed point the linkages pivot around is called the instantaneous center (IC). If the tie rod is configured in such a way so that it does not rotate around the same IC, the control rods and tie rods travel about different arcs, and horizontal displacement occurs. This horizontal displacement causes unwanted steering, or bump steer. Bump steer can also occur if the tie rods are not the correct length. The tie rods should have a length that falls along an imaginary line passing through the control arm connection points on the chassis and the upright. If the tie rod is too short, it will have a more severe arc. If the tie rod is too long, it will not arc enough. If the tie rods pass through the same IC as the control arms and are the correct length, bump steer is minimized. If the steering rack is positioned behind the wheel spindle, the tire with toe out on bumps. If the steering rack is positioned in front of the spindle, the tire with toe in on bumps.
Bump steer is highly undesirable within racing especially on tracks or surfaces that contain bumps. If the suspension is constantly transitioning between bump and droop, then a car with bump steer will constantly be shifting the steering angle of the car and will make the front end very unstable at high speeds. The bump steer in the system will affect the steering geometry and will make steering more unpredictable for the driver.
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