87389424 the Car Steering Basics

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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 wheelsand much more.

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Translated versions of this site: Svenska

Steering : essential to driving

Latest blog entry

07/25/2011 07:05 AM

Toyota wants to steer for you

Toyota recently demonstrated an accident-avoidance system that not only brakes for you, but steers for you too. It uses a series of sensors

hooked up to its onboard computer to determine if you're about to be involved in an accident. It then takes control of the steering and

brakes and attempts to avoid the accident for you.

The key word there is "attempts".

This is another in a long line of R&D projects that we don't want and don't need. This constant dumbing-down of everything is going to

be the death of our civilisation. If this technology becomes commonplace, then drivers will have even less reason to concentrate whilst

driving, and there will be even more accidents. Remember, computer systems are not infallible. If there's a bug in the code, and duringthe attempt to avoid what it "thinks" is an impending accident, your car actually causes another one, then the lawyers will get involved

and everyone will pay.Why don't Toyota spend this money on driver education instead? Driving schools, safety programs, driver education and other such

things? Surely it's better to attempt to treat the cause of the problem rather than the symptoms?And for those who relish the idea of their vehicle driving itself, here's a suggestion: use a taxi, bus or train. They exist right now.

Toyota wants to steer for you.

Chris -www.carbibles.com

Elsewhere on this site you can learn about all the other stuff thatmakes a car go and stop, so this page is where you'll learn abouthow it goes around corners. More specifically, how the varioussteering mechanisms work.Like most things in a car, the concept of steering is simple - youturn the steering wheel, the front wheels turn accordingly, andthe car changes direction. How that happens though is not quiteso simple. Well - it used to be back in the days when cars werecalled horseless carriages, but nowadays, not so much.

Find Mustang steering wheels at AmericanMuscle.

Basic steering components

99% of the world's car steering systems are made up of the samethree or four components. The steering wheel, which connects tothe steering system, which connects to the track rod, whichconnects to the tie rods, which connect to the steering arms. The
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steering system can be one of several designs, which we'll go intofurther down the page, but all the designs essentially move thetrack rod left-to-right across the car. The tie rods connect to theends of the track rod with ball and socket joints, and then to theends of the steering arms, also with ball and socket joints. Thepurpose of the tie rods is to allow suspension movement as wellas an element of adjustability in the steering geometry. The tierod lengths can normally be changed to achieve these differentgeometries.

The Ackermann Angle : your wheels don'tpoint the same direction.

In the simplest form of steering, both the front wheels alwayspoint in the same direction. You turn the wheel, they both pointthe same way and around the corner you go. Except that by doingthis, 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 samething you need to take into consideration when looking attransmissions. When a car goes around a corner, the outsidewheels travel further than the inside wheels. In the case of atransmission, it's why you need a differential (seethe Transmission Bible), but in the case of steering, it's why youneed the front wheels to actually point in different directions. Onthe left is the diagram from the Transmission Bible. You can seethe inside wheels travel around a circle with a smaller radius (r2)than the outside wheels (r1).
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In order for that to happen without causing undue stress to thefront wheels and tyres, they must point at slightly different anglesto the centreline of the car. The diagram to the left shows the

same thing only zoomed in to show the relative angles of thetyres to the car. It's all to do with the geometry of circles. Thisdifference of angle is achieved with a relatively simple

arrangement of steering components to create a trapezoidgeometry (a parallelogram with one of the parallel sides shorterthan the other). Once this is achieved, the wheels point atdifferent angles as the steering geometry is moved. Most vehiclesnow don't use 'pure' Ackermann steering geometry because itdoesn't take some of the dynamic and compliant effects ofsteering and suspension into account, but some derivative of thisis used in almost all steering systems (right).

Why 'Ackermann'?

This particular technology was first introduced in 1758 byErasmus Darwin, father of Charles Darwin, in a paperentitled"Erasmus Darwin's improved design for steeringcarriages--and cars". It was never patented though until 1817when Rudolph Ackermann patented it in London, and that's thename that stuck.

Steering ratios

Every vehicle has a steering ratio inherent in the design. If itdidn't you'd never be able to turn the wheels. Steering ratio gives

mechanical advantage to the driver, allowing you to turn the tyreswith the weight of the whole car sitting on them, but moreimportantly, it means you don't have to turn the steering wheel aridiculous number of times to get the wheels to move. Steeringratio is the ratio of the number of degrees turned at the steeringwheel vs. the number of degrees the front wheels are deflected.So for example, if you turn the steering wheel 20 and the frontwheels only turn 1, that gives a steering ratio of 20:1. For most
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modern cars, the steering ratio is between 12:1 and 20:1. This,coupled with the maximum angle of deflection of the wheels givesthe lock-to-lock turns for the steering wheel. For example, if a carhas a steering ratio of 18:1 and the front wheels have a maximumdeflection of 25, then at 25, the steering wheel has turned25x18, which is 450. That's only to one side, so the entiresteering goes from -25 to plus 25 giving a lock-to-lock angle atthe steering wheel of 900, or 2.5 turns (900 / 360).This works the other way around too of course. If you know thelock-to-lock turns and the steering ratio, you can figure out thewheel deflection. For example if a car is advertised as having a16:1 steering ratio and 3 turns lock-to-lock, then the steeringwheel can turn 1.5x360 (540) each way. At a ratio of 16:1 thatmeans the front wheels deflect by 33.75 each way.For racing cars, the steering ratio is normally much smaller thanfor passenger cars - ie. closer to 1:1 - as the racing drivers needto get fuller deflection into the steering as quickly as possible.

Turning circles

The turning circle of a car is the diameter of the circle describedby the outside wheels when turning on full lock. There is no hardand fast forumla to calculate the turning circle but you can getclose by using this:

turning c ircle radius = (track/2) + (wheelbase/sin(average steerangle))

The numbers required to calculate the turning circle explain why aclassic black London taxi has a tiny 8m turning circle to allow it todo U-turns in the narrow London streets. In this case, thewheelbase and track aren't radically different to any other car,but the average steering angle is huge. For comparison, a typicalpassenger car turning circle is normally between 11m and 13mwith SUV turning circles going out as much as 15m to 17m.

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Steering System designs : Pitman arm types

There really are only two basic categories of steering systemtoday; those that have pitman arms with a steering 'box' andthose that don't. Older cars and some current trucks use pitmanarms, so for the sake of completeness, I've documented somecommon types. Newer cars and unibody light-duty trucks typicallyall use some derivative of rack and pinion steering.


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Pitman arm mechanisms have a steering 'box' where the shaftfrom the steering wheel comes in and a lever arm comes out - thepitman arm. This pitman arm is linked to the track rod or centrelink, which is supported by idler arms. The tie rods connect to thetrack rod. There are a large number of variations of the actualmechanical linkage from direct-link where the pitman arm isconnected directly to the track rod, to compound linkages where itis connected to one end of the steering system or the track rodvia 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 canturn the steering wheel a slight amount before the front wheelsstart to turn. This slack can normally be adjusted with a screw

mechanism but it can't ever be eliminated. The traditionaladvantage of these systems is that they give bigger mechanicaladvantage and thus work well on heavier vehicles. With theadvent of power steering, that has become a moot point and thesteering system design is now more to do with mechanical design,price and weight. The following are the four basic types ofsteering box used in pitman arm systems.

Worm and sector

In this type of steering box, the end of the shaft from the steeringwheel has a worm gear attached to it. It meshes directly with asector 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 movedalong the worm gear. The sector gear is mounted on the crossshaft which passes through the steering box and out the bottomwhere it is splined, and the the pitman arm is attached to the
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splines. When the sector gear turns, it turns the cross shaft,which turns the pitman arm, giving the output motion that is fedinto the mechanical linkage on the track rod. The followingdiagram shows the active components that are present inside theworm and sector steering box. The box itself is sealed and filledwith grease.

Worm and roller

The worm and roller steering box is similar in design to the wormand sector box. The difference here is that instead of having asector gear that meshes with the worm gear, there is a rollerinstead. The roller is mounted on a roller bearing shaft and is heldcaptive on the end of the cross shaft. As the worm gear turns, theroller is forced to move along it but because it is held captive on

the cross shaft, it twists the cross shaft. Typically in thesedesigns, the worm gear is actually an hourglass shape so that it iswider at the ends. Without the hourglass shape, the roller mightdisengage from it at the extents of its travel.

Worm and nut or recirculating ball
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This is by far the most common type of steering box for pitmanarm systems. In a recirculating ball steering box, the worm drivehas many more turns on it with a finer pitch. A box or nut isclamped over the worm drive that contains dozens of ballbearings. These loop around the worm drive and then out into arecirculating channel within the nut where they are fed back intothe worm drive again. Hence recirculating. As the steering wheelis turned, the worm drive turns and forces the ball bearings topress against the channel inside the nut. This forces the nut tomove along the worm drive. The nut itself has a couple of gearteeth cast into the outside of it and these mesh with the teeth ona sector gear which is attached to the cross shaft just like in theworm and sector mechanism. This system has much less free playor slack in it than the other designs, hence why it's used themost. The example below shows a recirculating ball mechanismwith the nut shown in cutaway so you can see the ball bearingsand the recirculation channel.

Cam and lever

Cam and lever steering boxes are very similar to worm and sectorsteering boxes. The worm drive is known as a cam and has amuch shallower pitch and the sector gear is replaced with twostuds that sit in the cam channels. As the worm gear is turned,the studs slide along the cam channels which forces the crossshaft to rotate, turning the pitman arm. One of the designfeatures of this style is that it turns the cross shaft 90 to thenormal so it exits through the side of the steering box instead ofthe bottom. This can result in a very compact design whennecessary.
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Steering System designs : Rack and pinion

This is by far the most common type of steering you'll find in anycar today due to it's relative simplicity and low cost. Rack andpinion systems give a much better feel for the driver, and thereisn't the slop or slack associated with steering box pitman armtype systems. The downside is that unlike those systems, rack

and pinion designs have no adjustability in them, so once theywear beyond a certain mechanical tolerance, they need replacingcompletely. This is rare though.In a rack and pinion system, the track rod is replaced with thesteering rack which is a long, toothed bar with the tie rodsattached to each end. On the end of the steering shaft there is asimple pinion gear that meshes with the rack. When you turn thesteering wheel, the pinion gear turns, and moves the rack fromleft to right. Changing the size of the pinion gear alters thesteering ratio. It really is that simple. The diagrams here show anexample 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 s imple variation on the above design. All the componentsare the same, and it all works the same except that the spacing ofthe teeth on the rack varies depending on how close to the centreof the rack they are. In the middle, the teeth are spaced closetogether to give slight steering for the first part of the turn - goodfor not oversteering at speed. As the teeth get further away fromthe centre, they increase in spacing slightly so that the wheelsturn more for the same turn of the steering wheel towards fulllock. 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
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speed, this will almost always be the case but once you get somemomentum behind you, you are at the mercy of the chassis andsuspension designers. In racing, the aerodynamic wings, airsplitters and undertrays help to maintain an even balance of thevehicle in corners along with the position of the weight in thevehicle and the supension setup. The two most common problemsyou'll run into are understeer and oversteer.

Understeer

Understeer is so called because the car steers less than you wantit to. Understeer can be brought on by all manner of chassis,suspension and speed issues but essentially it means that the caris losing grip on the front wheels. Typically it happens as youbrake and the weight is transferred to the front of the car. At this

point the mechanical grip of the front tyres can simply beoverpowered and they start to lose grip (for example on a wet orgreasy road surface). The end result is that the car will start totake the corner very wide. In racing, that normally involves goingoff 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 ofthe corner. Getting out of understeer can involve letting off thethrottle in front-wheel-drive vehicles (to try to give the tyreschance to grip) or getting on the throttle in rear-wheel-drivevehicles (to try to bring the back end around). It's a complextopic more suited to racing driving forums but suffice to say thatif you're trying to get out of understeer and you cock it up, youget.....

Oversteer
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The bright ones amongst you will probably already have guessedthat oversteer is the opposite of understeer. With oversteer, thecar goes where it's pointed far too efficiently and you end updiving into the corner much more quickly than you had expected.Oversteer is brought on by the car losing grip on the rearwheelsas the weight is transferred off them under braking, resulting inthe rear kicking out in the corner. Without counter-steering (seebelow) the end result in racing is that the car will spin and end upgoing off the inside of the corner backwards. In normal you-and-me driving, it means spinning the car and ending up pointing backthe way you came.

Counter-steering

Counter-steering is what you need to do when you start toexperience oversteer. If you get into a situation where the backend of the car loses grip and starts to swing out, steering

opposite to the direction of the corner can often 'catch' theoversteer by directing the nose of the car out of the corner. Indrift racing and demonstration driving, it's how the drivers areable to smoke the rear tyres and power-slide around a corner.They will use a combination of throttle, weight transfer andhandbrake to induce oversteer into a corner, then flick thesteering the opposite dirction, honk on the accelerator and try tohold a slide all the way around the corner. It's also a widely-used
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technique in rally racing. Tiff Needell - a racing driver who alsoworks on some UK motoring programs - is an absolute master atcounter-steer power sliding.

Topics still to come...

Hydraulic and electronic power steering

Speed-sensitive power steering

4-wheel steering - passive and active

Drive-by-wire steering

Tilt / slide steering wheels and collapsible steeringcolumns

These pages were last updated on 12 th July 2011.

Copyright Chris Longhurst 1994 - 2011 unless otherwise noted.

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87389424 the Car Steering Basics