Steering 101 - 2024

Vehicle steering systems can typically be broken up into two parts: the upper steering and lower steering systems. The upper steering system usually consists of the steering column/shaft that transmits rotary motion from steering wheel input along the length of the shaft, and any intermediary u joints to change the direction of the rotary motion. The lower steering system includes the steering rack itself which transmits rotary motion from the upper steering system into translational motion. Tie rods are connected to the rack and link it to the wheels themselves through the suspension uprights. This allows the front wheels on the vehicle to move and steer the car.

Open

In the lower steering system, typically you might consider a vehicle’s steering system where the front wheels turn parallel with each other. Imagine a four wheeled vehicle of trackwidth T and wheelbase L, with two imaginary wheels at the midpoint between the front wheels and back wheels. When the vehicle is travelling in a straight line, all the wheels will be lined up parallel with each other, allowing the vehicle to travel straight.

Open

Now lets say the vehicle wants to turn. In a parallel steering system, this means that all front tires will rotate at the same angle to turn the car. We can represent this by drawing the path each wheel might take through a full circle. Remember, because each tire is turning at the same rate, the radius of the path that each tire tracks is the same.

Notice that the tracks of each tire do not line up. This results in a situation where each tire wants to keep going in a circle, but is unable to go in the same circle. This means that the tires will want to either get closer together or further away from each other when driving the circle. This situation does not work in real life, and means that the tires will slip through the turn and puts added stress on the joints connected to the tires.

We can help solve this issue by using Ackermann Geometry. By drawing two perpendicular lines, one from the imaginary central front tire and one from the imaginary central back tire, the intersection of these two lines determines the radius of our turn from the rear tire. Notice that due to geometry, the imaginary front tire will trace a circle that is slight different to that of the rear tire. Also note that by changing the track width of the vehicle, we can manipulate the minimum turning radius of the vehicle. If we do the same thing for two other front tires, we can see that they will both track a different radius as well, with the inner tire tracking a smaller radius and the outer tire a larger radius. This means that the inside tire will have to turn at a greater angle and the outside tire will have to turn at a shallower angle to track these ideal paths. This model represents a scenario with zero tire slip at a given turning radius.

We can represent the ideal Ackermann angle for both wheels as a function of the Track Width T and wheelbase L:

Note: The wider the track width the more stable the car is (less body roll)

As mentioned before, the upper steering is mainly responsible for transmitting driver input in the rotation of the steering wheel down into the steering rack through a series of shafts. Often due to packaging constraints, the use of a u joint or some other mechanism of changing the direction of the shafts is required to physically fit the upper steering mechanism in the car. U joints, or universal joints are able to change the angle of an output relative to an input shaft, however is limited in its range of motion. As the angle of the u joint increases, so to does the difference in velocity between the input and the output, resulting in a sinusoidal output speed. This particular quirk means that often u joints will be used in pairs where by phasing them correctly, two u joints can cancel out the variance in their output. It is theoretically possible to achieve the connection without the need to change the direction of the steering shaft, however this will likely result in a very high angle steering wheel and be uncomfortable to drive. Because of the significant issues we had with steering compliance and slop in the previous design, we are looking into using a bevel gearbox to transfer the direction of the shafts instead of u joints. When designing steering shafts, it is also important that they will be able to withstand certain forces, in this case the maximum amount of torque the driver can put on the wheel at any moment.