Steering

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In an RC car, steering is a system of components, linkages, and other parts that allows a driver to control the direction of the vehicle. The most conventional steering arrangement uses a servo that transfers the driver's motion on the wheel of the radio transmitter to the wheels of the car. The steering servo uses a horn which is linked to rods and pivots that allow the driver to change the direction of the front wheels.

Overview of the steering system

RC car steering system explained

Translating the RC driver's action into motion of the wheels starts with the servo. A servo is a the device that converts electrical commands from the receiver into physical movement using an electric motor. It is connected to a servo horn which connects to a steering link by a ball joint and the steering link itself is attached on the other end to the steering plate. The steering plate is connected to two metal arms called the tie rods which are attached to the steering knuckles of the RC car wheels.

All these components and sometime others form the steering system, and moving one moves the entire structure so that a force applied by the servo turns the wheels of the car left and right.

Overview of the main components

There may be particulars to an RC car steering system but most make use of the same components to operate:

  • Servo: converts electrical commands from the receiver into physical movement on the car. It contains a small motor that rotates in both directions, a gearbox to amplify the motion, a potentiometer to measure the position of the output gear and an electronic circuit that controls the motor to make the output gear move to the desired position.

  • Servo horn: a plastic or metal part that attaches to the top of the splined servo output shaft. They sit between the servo and the steering link and transfer the motion from one to the other. Servo horns come in a variety of shapes and sizes. They sometimes embed a servo saver using springs or similar mechanism to protect the servo gears.

  • Steering link: a plastic or metal rod that ideally forms a 90° angle with the servo horn and transfers the motion of the servo horn either to the servo saver or directly to the steering plate.

  • Servo saver: an assembly of different components usually including a spring and that protects the steering servo by absorbing shocks that might otherwise cause internal gear damage.

  • Steering plate: also called the steering arm, it is a plastic or metal part that sits in the center of the steering system and that transfers the motion from the steering link to both tie rods.

  • Tie rod: a plastic or metal rod that connects the steering plate to the steering knuckle and transfers the motion of the steering plate directly to the wheels.

  • Steering knuckle: a plastic or metal part that is mounted as a pivot point on the wheels and allows them to turn. Steering knuckles come in pair each one being mounted on a wheel and when one is pulled by a tie rod, the the one is pushed and vice versa, turning the wheels simultaneously in the same direction.

Steering system operation and tuning

The main purpose of the steering system is to steer the car with optimal precision and stability. By making adjustments to the steering system, it is possible to greatly improve the car's manoeuvrability and cornering abilities. The issue with steering tuning is that there are many components, some outside of the steering system, that can impact the steering efficiency of a car.

For that reason, adjustments to the steering system can be made in many ways:

  • Servo: Improving to a fast and strong servo for a better feel and response. See the Servo article for detailed information.

  • Tires: change or improve tires and fit the car with tires which are best suited for the track or temperature. See the Tires article for detailed information.

  • Improve suspension: front tires of a car need to be in optimal contact with the road for the most efficient steering. See the Suspension article for detailed information.

  • Transmitter: Set the EPA to avoid damaging or prematurly aging the servo. See the Radio Communication article for detailed information.

  • Caster: the angle to which the steering pivot axis is tilted from vertical impacts the cornering ability of a car. See below for Caster adjustment.

  • Bump steer: changing a car’s toe angle as the suspension compresses improves connection to the road and steering. See below for bump Steer adjustment.

  • Ackermann angle: changes in toe angle when wheels are turned dramatically impacts steering. See below for Ackermann angle adjustment and see the Ackermann article for detailed information.

Caster

Caster is the angle to which the steering pivot axis is tilted forward or rearward from vertical, as viewed from the side:

  • When the pivot axis is tilted backward (when the top pivot is positioned further rearward than the bottom pivot), then the caster is positive
  • When it's vertical to the lower pivot point then the caster is zero.
Less Caster

More Off power steering into the corner

Less steering out of the corner

Less straight-line stability

More Caster

Less steering into the corner

More ON POWER steering out of the corner

More straight-line stability

The benefits of tuning caster are to:

  • Maximizes tire contact patch during roll, braking, and acceleration
  • Increase directional stability
  • Improved steering "feel" and self-center
  • Improves turn-in response
  • Increases dynamic negative camber during turn in

Caster Vs Camber

Some of the adjustments provided by tuning caster could be achieved by tuning camber. But camber tuning presents limitations when it comes to steering since it:

  • does not improve turn-in when positive caster does
  • does not improve directional stability
  • wears on tire more
  • adversely effects braking and acceleration.

Bump steer

Bump steer is the change in a car’s toe angle as the suspension compresses. As one pushes down on a car, it is possible to notice that the toe-in changes and this effect is called bump steer. The amount of bump steer a car has can be adjusted by changing the vertical angle of the steering links, either at the inside (rack) or at the outside (steering arm).

Changing a car's camber link locations, toe, or kick-up may require re-adjusting bump steer to keep the handling characteristics consistent.

Less bump steer

Increases toe-in (decreases toe-out) when suspension is compressed

More bump steer

Decreases toe-in (increases toe-out) when suspension is compressed

It is recommended to adjust the front wheel alignment for a little toe in since both tires are trying to turn slightly even when going in a straight line. Adding toe in will help when cornering forcing the wheel on the outside of the turn to have a more positive lock and turn sharper.

Ackermann angle

Ackermann describes the angle of the inside tire in relation to the outside tire when the wheels are turned to full lock. This property of a car's suspension causes the car's toe angle to change as the wheels are turned farther to each side.

In most RC cars, the Ackermann angle is positive meaning the inside wheel in the turn has less toe-in (or more toe-out) than when the wheels are straight. The car's Ackermann can be adjusted by moving the inside or outside of the steering link either forward or back.

Depending on the car it can be done by:

  • Using a different hole for the ball stud either on the rack (inside end of the link) or the steering arm (outside end of the link)
  • Changing the number of spacers at the rack (inside of the link). This only applies when the ball stud is aligned horizontally, if it is vertical, the spacers control bump steer.
Lower Ackermann angle (links straighter, more perpendicular to chassis)

More on-power steering

Decreases toe-out when wheels are turned

Higher Ackermann angle (links more angled)

More off-power steering

Increases toe-out when wheels are turned

See the Ackermann article for detailed information.

Let's Talk Steering

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