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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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 generating minimum drag. This reduction in drag force makes the car faster in a straight line. 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 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.
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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 Since the front wheels are 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 and reach its 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 more nimble and allows it to 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.
Rear Wheel Drive: Front Wheels - Toe In
Running toe in will also 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 is also reduced due to the increased stresses on the rubber. However, the tire will warm up faster getting to operating temperature sooner in the race.
on the front wheels of a rear wheel drive car has the same effects on acceleration, top speed, tire life, and tire performance as running toe out on the front wheels of a rear wheel drive car. The dynamic effect of toe in is that the car feels more stable at high speeds and during high speed cornering. This is due to the car requiring larger steering inputs from the driver to turn, giving the car a more planted feel. This does reduce the agility through tight, slow corners but the trade-off is especially worth it for new or unconfident drivers.
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Running zero toe will increase the straight line acceleration and top speed of the car. It will also prolong the life of the tires out on track or on the road. The disadvantage to this set up means that the tire takes longer to warm up to operating temperature to generate maximum grip.
Running zero toe will make the car feel relatively stable in a straight line at high speeds. It will also make the car feel more neutral when taking long sweeping corners and slower tight corners. It leaves the feeling of the steering and the responsiveness of the steering in the hands of the Ackermann geometry as well, which can be a good thing for a race car.
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Toe out on the rear wheels of a rear wheel drive car will improve the acceleration of the car but decreases decrease the top speed due to the increased grip and drag force. It will also reduce the life of the tire due to the increased slip angle. The acceleration is increased as there is more grip available from the tire due to the toe putting a greater slip angle into the rubber. This allows more power to be transferred to the ground without spinning the wheels, allowing the car to accelerate faster. The tire will also heat up faster getting to its most effective level of grip faster.
Toe out on the rear wheels will make the car more nimble and faster through tight sections, making it feel as though it has a shorter wheel base. This effectively makes the car have a higher tendency to oversteer so is a set up used by smooth drivers who can be light on the throttle. Toe out will also make the car feel less stable through high speed corners due to the feeling of oversteer occurring at the rear wheels.
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Toe in on the rear wheels improves the acceleration of the car due to the increased grip levels allowing more power to be put through the tire without spinning the wheels. However, the top speed of the car is reduced due to the increased drag from the tire. The tire heats up faster when running toe in as the rubber is stressed, generating heat, getting the tire to operating temperature much sooner in the race; this in turn reduces the lifetime of the tire.
Having toe in at the rear wheels will increase the high speed stability of the car round long fast corners and makes the car feel as though it has a longer wheel base. The increase in high speed stability is comforting for new drivers who are not used to a nimble car that is sensitive to steering inputs. Toe in does mean that more steering effort is required from the driver through tight corners which can slow the car down through tight sections. Toe in on the rear wheels can also give the car a tendency to understeer due to fighting the want to oversteer and increasing increased rear-end grip which can overcome the grip of the front wheels.
Rear Wheel Drive: Rear Wheels - Zero Toe
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.
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Like bump steer, lateral compliance steer is not something which should be eliminated completely from a suspension system. Some level of understeer is good in a suspension system. Cars people drive on a daily basis have inherent understeer designed in. This is a behaviour we are all familiar with and have become accustomed to. Therefore driving something with no understeer or inherent oversteer would feel unnatural, too agile and too responsive. Lateral compliance steer is also key to ensure the car feels stable and less nervous at high speed. This means that even high performance sports cars with >200mph top speeds have relatively high levels of lateral compliance steer.
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