The car steering Bible - how steering works including rack and pinions, pitman arms, power steering, passive and active 4-wheel steering, tilt-and-slide steering wheels and much more.
Steering : essential to driving
Latest blog entry
|03/03/2014 07:00 AM|
|Rotating your tyres - things to be aware of.|
It's common wisdom that rotating your tyres is A Good Thing. It helps even out wear especially on front-wheel-drive or rear-wheel-drive where one pair of tyres will typically wear quicker. There's a couple of things you need to be aware of when having a tyre rotation done though. Unidirectional tyres must be kept on the same side of the car - typically the front and rear lefts will be swapped, and the front and rear rights will be swapped.
If you don't have unidirectional tyres, then front-left to back-right and vice versa is just fine.
But there's a slight wrinkle in the scheme with a lot of modern cars : TPMS. For the average family car, the TPMS system (the tyre pressure monitors built into each wheel) is a single-point-of-failure system, meaning that if any one sensor registers low pressure, the car will present a single warning light on the dash and it's up to you to find the tyre in question. For higher end vehicles though - Range Rovers, BMWs, Mercedes and the like - they sometimes have TPMS that will tell you which tyre needs attention. This makes tyre rotation more complicated because you either have to take the tyres of the rims to ensure each rim and sensor stays on the correct corner of the car. Or you need to have the car re-learn the location of each sensor afterwards. Generally speaking this isn't easy to do - it normally requires access to the computer via the OBD-II port and access to someone who knows that they're doing.
There is one manufacturer - and I can't remember who it is - who made this easy though. They have a mode where you, as the owner, can put the TPMS into 'learn' mode and then it shows you on the dash which sensor it wants to learn, in order. To 'teach' it which sensor is where, you bleed a little air out of the tyre. So when the car indicates "left front", you go and blip a little air out of the left front tyre, and it then associates that sensor (the one that changed) with the front left corner.
Anyway - the point is this - by all means rotate your tyres, but be aware of the little pitfalls when it comes to tread directionality and TPMS.
Elsewhere on this site you can learn about all the other stuff that makes a car go and stop, so this page is where you'll learn about how it goes around corners. More specifically, how the various steering mechanisms work.
Like most things in a car, the concept of steering is simple - you turn the steering wheel, the front wheels turn accordingly, and the car changes direction. How that happens though is not quite so simple. Well - it used to be back in the days when cars were called horseless carriages, but nowadays, not so much.
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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, which we'll go into further down the page, 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 the steering geometry. The tie rod lengths can normally be changed to achieve these different geometries.
The Ackermann Angle : your wheels don't point the same direction.
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 you go. Except that by doing this, you end up with tyres scrubbing, loss of grip and a vehicle that 'crabs' around the corner. So why is this? Well, it's the same thing you need to take into consideration when looking at transmissions. 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 (see the Transmission Bible), but in the case of steering, it's why you need the front wheels to actually point in different directions. On the left is the diagram from the Transmission Bible. You can see the inside wheels travel around a circle with a smaller radius (r2) than the outside wheels (r1).
In order for that to happen without causing undue stress to the front wheels and tyres, they must point at slightly different angles to the centreline of the car. The diagram to the left shows the same thing only zoomed in to show the relative angles of the tyres to 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 (right).
This particular technology was first introduced in 1758 by Erasmus Darwin, father of Charles Darwin, in a paper entitled "Erasmus Darwin's improved design for steering carriages--and cars". It was never patented though until 1817 when Rudolph Ackermann patented it in London, and that's the name that stuck.
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 tyres 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 20° and the front wheels only turn 1°, that gives a steering ratio of 20:1. 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. For example, if a car has a steering ratio of 18:1 and the front wheels have a maximum deflection of 25°, then at 25°, the steering wheel has turned 25°x18, which is 450°. That's only to one side, so the entire steering goes from -25° to plus 25° giving a lock-to-lock angle at the steering wheel of 900°, or 2.5 turns (900° / 360).
This works the other way around too of course. If you know the lock-to-lock turns and the steering ratio, you can figure out the wheel deflection. For example if a car is advertised as having a 16:1 steering ratio and 3 turns lock-to-lock, then the steering wheel can turn 1.5x360° (540°) each way. At a ratio of 16:1 that means the front wheels deflect by 33.75° each way.
For racing cars, the steering ratio is normally much smaller than for passenger cars - ie. closer to 1:1 - as the racing drivers need to get fuller deflection into the steering as quickly as possible.
The turning circle of a car is the diameter of the circle described by the outside wheels when turning on full lock. There is no hard and fast forumla to calculate the turning circle but you can get close by using this:
turning circle radius = (track/2) + (wheelbase/sin(average steer angle))
The numbers required to calculate the turning circle explain why a classic black London taxi has a tiny 8m turning circle to allow it to do U-turns in the narrow London streets. In this case, the wheelbase and track aren't radically different to any other car, but the average steering angle is huge. For comparison, 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 : Pitman arm types
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, so for the sake of completeness, I've documented some common types. 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 centre 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. The example here shows a compound link (left).
Most of the steering box mechanisms that drive the pitman arm have a 'dead spot' in the centre 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.
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.
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.
Worm and nut or recirculating ball
This is by far the most common type of steering box for pitman arm systems. In a recirculating 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 recirculating channel within the nut where they are fed back into the worm drive again. Hence recirculating. 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, hence why it's used the most. The example below shows a recirculating ball mechanism with the nut shown in cutaway so you can see the ball bearings and the recirculation channel.
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.
Steering System designs : Rack and pinion
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. This is rare though.
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. It really is that simple. The diagrams here show an example rack and pinion system (left) as well as a close-up cutaway of the steering rack itself (right).
Variable-ratio rack and pinion steering
This is a simple variation on the above design. 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 centre 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 oversteering at speed. As the teeth get further away from the centre, they increase in spacing slightly so that the wheels turn more for the same turn of the steering wheel towards full lock. Simple.
Vehicle dynamics and steering - how it can all go very wrong
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. In racing, the aerodynamic wings, air splitters and undertrays help to maintain an even balance of the vehicle in corners along with the position of the weight in the vehicle and the supension setup. The two most common problems you'll run into are understeer and oversteer.
Understeer is so called because the car steers less than you want it to. Understeer 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 tyres 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. In racing, that normally involves going off the outside of the corner into a catch area or on to the grass. In normal you-and-me driving, it means crashing at the outside of the corner. Getting out of understeer can involve letting off the throttle in front-wheel-drive vehicles (to try to give the tyres chance to grip) or getting on the throttle in rear-wheel-drive vehicles (to try to bring the back end around). It's a complex topic more suited to racing driving forums but suffice to say that if you're trying to get out of understeer and you cock it up, you get.....
The bright ones amongst you will probably already have guessed that oversteer is the opposite of understeer. With oversteer, 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. Oversteer 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 (see below) the end result in racing is that the car will spin and end up going off the inside of the corner backwards. In normal you-and-me driving, it means spinning the car and ending up pointing back the way you came.
Counter-steering is what you need to do when you start to experience oversteer. 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 oversteer by directing the nose of the car out of the corner. In drift racing and demonstration driving, it's how the drivers are able to smoke the rear tyres and power-slide around a corner. They will use a combination of throttle, weight transfer and handbrake to induce oversteer into a corner, then flick the steering the opposite dirction, honk on the accelerator and try to hold a slide all the way around the corner. It's also a widely-used technique in rally racing. Tiff Needell - a racing driver who also works on some UK motoring programs - is an absolute master at counter-steer power sliding.