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This page is where you'll learn about how it goes around corners with the use of various steering mechanisms. Steering is simple, you turn the steering wheel, the front wheels turn accordingly, and the car changes direction.

Basic steering components

99% of the world's car steering systems are made up of the same three or four components. The steering wheel, which connects to the steering system, which connects to the track rod, which connects to the tie rods, which connect to the steering arms. The steering system can be one of several designs, but all the designs essentially move the track rod left-to-right across the car. The tie rods connect to the ends of the track rod with ball and socket joints, and then to the ends of the steering arms, also with ball and socket joints. The purpose of the tie rods is to allow suspension movement as well as an element of adjustability in steering geometries. The tie rod lengths can normally be changed to achieve these different geometries.

Ackermann Angle

In the simplest form of steering, both the front wheels always point in the same direction. You turn the wheel, they both point the same way and around the corner.  When a car goes around a corner, the outside wheels travel further than the inside wheels. In the case of a transmission, it's why you need a differential, but in the case of steering, it's why you need the front wheels to actually point in different directions. In order for that to happen without causing undue stress to the front wheels and tires, they must point at slightly different angles to the centerline of the car.  It's all to do with the geometry of circles. This difference of angle is achieved with a relatively simple arrangement of steering components to create a trapezoid geometry (a parallelogram with one of the parallel sides shorter than the other). Once this is achieved, the wheels point at different angles as the steering geometry is moved. Most vehicles now don't use 'pure' Ackermann steering geometry because it doesn't take some of the dynamic and compliant effects of steering and suspension into account, but some derivative of this is used in almost all steering systems.

Steering Ratios

Every vehicle has a steering ratio inherent in the design. If it didn't you'd never be able to turn the wheels. Steering ratio gives mechanical advantage to the driver, allowing you to turn the tires with the weight of the whole car sitting on them, but more importantly, it means you don't have to turn the steering wheel a ridiculous number of times to get the wheels to move. Steering ratio is the ratio of the number of degrees turned at the steering wheel vs. the number of degrees the front wheels are deflected. So for example, if you turn the steering wheel 12° and the front wheels only turn 1°, that gives a steering ratio of 12:1. For most modern cars, the steering ratio is between 12:1 and 20:1. This, coupled with the maximum angle of deflection of the wheels gives the lock-to-lock turns for the steering wheel.
 

Turning circles

The turning circle of a car is the diameter of the circle described by the outside wheels when turning on full lock. The average steering angle is huge.  A typical passenger car turning circle is normally between 11m and 13m with SUV turning circles going out as much as 15m to 17m.

Steering System Designs

There really are only two basic categories of steering system today; those that have pitman arms with a steering 'box' and those that don't. Older cars and some current trucks use pitman arms. Newer cars and unibody light-duty trucks typically all use some derivative of rack and pinion steering.

Pitman arm mechanisms have a steering 'box' where the shaft from the steering wheel comes in and a lever arm comes out - the pitman arm. This pitman arm is linked to the track rod or center link, which is supported by idler arms. The tie rods connect to the track rod. There are a large number of variations of the actual mechanical linkage from direct-link where the pitman arm is connected directly to the track rod, to compound linkages where it is connected to one end of the steering system or the track rod via other rods.

Most of the steering box mechanisms that drive the pitman arm have a 'dead spot' in the center of the steering where you can turn the steering wheel a slight amount before the front wheels start to turn. This slack can normally be adjusted with a screw mechanism but it can't ever be eliminated. The traditional advantage of these systems is that they give bigger mechanical advantage and thus work well on heavier vehicles. With the advent of power steering, that has become a moot point and the steering system design is now more to do with mechanical design, price and weight. The following are the four basic types of steering box used in pitman arm systems.

     1) Worm and Sector

In this type of steering box, the end of the shaft from the steering wheel has a worm gear attached to it. It meshes directly with a sector gear (so called because it's a section of a full gear wheel). When the steering wheel is turned, the shaft turns the worm gear, and the sector gear pivots around its axis as its teeth are moved along the worm gear. The sector gear is mounted on the cross shaft which passes through the steering box and out the bottom where it is splined, and the the pitman arm is attached to the splines. When the sector gear turns, it turns the cross shaft, which turns the pitman arm, giving the output motion that is fed into the mechanical linkage on the track rod. The following diagram shows the active components that are present inside the worm and sector steering box. The box itself is sealed and filled with grease.

     2) Worm and Roller

The worm and roller steering box is similar in design to the worm and sector box. The difference here is that instead of having a sector gear that meshes with the worm gear, there is a roller instead. The roller is mounted on a roller bearing shaft and is held captive on the end of the cross shaft. As the worm gear turns, the roller is forced to move along it but because it is held captive on the cross shaft, it twists the cross shaft. Typically in these designs, the worm gear is actually an hourglass shape so that it is wider at the ends. Without the hourglass shape, the roller might disengage from it at the extents of its travel.

     3) Worm and Nut (Re-circulating Ball)

This is by far the most common type of steering box for pitman arm systems. In a re-circulating ball steering box, the worm drive has many more turns on it with a finer pitch. A box or nut is clamped over the worm drive that contains dozens of ball bearings. These loop around the worm drive and then out into a re-circulating channel within the nut where they are fed back into the worm drive again. As the steering wheel is turned, the worm drive turns and forces the ball bearings to press against the channel inside the nut. This forces the nut to move along the worm drive. The nut itself has a couple of gear teeth cast into the outside of it and these mesh with the teeth on a sector gear which is attached to the cross shaft just like in the worm and sector mechanism. This system has much less free play or slack in it than the other designs.

     4) Cam and lever

Cam and lever steering boxes are very similar to worm and sector steering boxes. The worm drive is known as a cam and has a much shallower pitch and the sector gear is replaced with two studs that sit in the cam channels. As the worm gear is turned, the studs slide along the cam channels which forces the cross shaft to rotate, turning the pitman arm. One of the design features of this style is that it turns the cross shaft 90° to the normal so it exits through the side of the steering box instead of the bottom. This can result in a very compact design when necessary.

This is by far the most common type of steering you'll find in any car today due to it's relative simplicity and low cost. Rack and pinion systems give a much better feel for the driver, and there isn't the slop or slack associated with steering box pitman arm type systems. The downside is that unlike those systems, rack and pinion designs have no adjustability in them, so once they wear beyond a certain mechanical tolerance, they need replacing completely.
In a rack and pinion system, the track rod is replaced with the steering rack which is a long, toothed bar with the tie rods attached to each end. On the end of the steering shaft there is a simple pinion gear that meshes with the rack. When you turn the steering wheel, the pinion gear turns, and moves the rack from left to right. Changing the size of the pinion gear alters the steering ratio.

Variable-Ratio Rack and Pinion Steering

All the components are the same, and it all works the same except that the spacing of the teeth on the rack varies depending on how close to the center of the rack they are. In the middle, the teeth are spaced close together to give slight steering for the first part of the turn - good for not over-steering at speed. As the teeth get further away from the center, they increase in spacing slightly so that the wheels turn more for the same turn of the steering wheel towards full lock.

The two tie rods are connected to the center link and through adjusting sleeves to the tie rod ends. The sleeves are threaded to allow lengthening and shortening of the tie rod assembly when the toe angle is adjusted during a wheel alignment.

Generally speaking, when you turn the steering wheel in your car, you typically expect it to go where you're pointing it. At slow speed, this will almost always be the case but once you get some momentum behind you, you are at the mercy of the chassis and suspension designers.  The two most common problems you'll run into are under-steer and over-steer.

Under-steer

Under-steer is so called because the car steers less than you want it to. Under-steer can be brought on by all manner of chassis, suspension and speed issues but essentially it means that the car is losing grip on the front wheels. Typically it happens as you brake and the weight is transferred to the front of the car. At this point the mechanical grip of the front tires can simply be overpowered and they start to lose grip (for example on a wet or greasy road surface). The end result is that the car will start to take the corner very wide. Getting out of under-steer can involve letting off the throttle in front-wheel-drive vehicles (to try to give the tires chance to grip) or getting on the throttle in rear-wheel-drive vehicles (to try to bring the back end around).

Over-steer

With over-steer, the car goes where it's pointed far too efficiently and you end up diving into the corner much more quickly than you had expected. Over-steer is brought on by the car losing grip on the rear wheels as the weight is transferred off them under braking, resulting in the rear kicking out in the corner. Without counter-steering  the car will spin and end up going off the inside of the corner backwards. In normal driving, it means spinning the car and ending up pointing back the way you came.

Counter-steering

Counter-steering is what you need to do when you start to experience over-steer. If you get into a situation where the back end of the car loses grip and starts to swing out, steering opposite to the direction of the corner can often 'catch' the over-steer by directing the nose of the car out of the corner. They will use a combination of throttle, weight transfer and handbrake to induce over-steer into a corner, then flick the steering the opposite direction, honk on the accelerator and try to hold a slide all the way around the corner.