Steering geometry

Often when asking questions about chassis and steering settings, the usual answer is simply to say changing some setting or other causes an increase or decrease in grip. There is never an explanation of the physical principles involved in causing these changes. I hope in this article to explain the physical forces involved in driving a kart, along with how those forces are generated by the steering, and what the effect is on the track of changing the various parameters available as setup.

Although a kart may seem to be rather simple device, it is perhaps a more difficult subject to explain than an equivalent car. Both vehicles have many parts and principles in common but there two major differences, which account for a large divergence in design and in setting up. These differences are the karts lack of differential, and also its lack of any suspension components. A good knowledge of the forces involved can help greatly when setting up a kart - giving the mechanic some knowledge as to what should happen when a parameter is changed.

This should result in considerably less time spent on the track testing. Steering Geometry. The steering geometry can be regarded at the movement and displacement of the front wheels as the steering wheel is turned. This movement is quite complex, and involves a number of different settings.

There is one thing in common though, and that is the reason why we need a complicated geometry - We MUST lift the inside wheel while cornering. The inside wheel lift is what enables a kart to go round a corner without using a differential. Because of this lack of a differential, a karts natural direction of travel, forwards, is very difficult to change.

This is down to the differing radii of turn experienced by the inner and outer rear wheels while turning a corner. The inside wheel is actually travelling a shorter distance than the outside, so therefor is needs to take fewer revolutions to go round the corner.

steering geometry

However, the two rear wheels are attached by a solid axle, and must therefor move together, so in order to turn, one of the wheels need to skid over the track surface.

In a car, the differential will allow the wheels to turn at different rates, without this skidding action. This skidding action, or indeed the lack of it, is what make a stationary kart so difficult to turn round - you have to overcome the grip of one of the tyres, and with the sticky tyres used in many kart classes this can expend a lot of energy. This is the reason for lifting the inside wheel and it effectively turns the kart into a tricycle during the cornering process!

The steering geometry causes the inner rear wheel to lift off the ground while cornering, which means the wheel can rotate faster than it is passing over the ground.

The rear inner wheel is no longer touching the track, and we therefor no longer need to overcome the grip from that tyre in order to turn. In fact, depending on the power of the engine, we may be able to allow some scrub. For example, while a Prokart may need to entirely lift the inner wheel, because it does not have enough power to overcome the scrub, a more powerful kart may have power to spare in the corner, meaning that the power loss to scrub can be overcome. However, any scrub will start to cause understeer when entering a corner, so even though the engine may be powerful, it may still be necessary to completely lift the inner rear to maintain decent handling.

However, we haven't yet explained how the front geometry can affect the rear wheel lift, and in order to do this, lets define a few terms used when describing front end geometry. This is the degree to which the front wheel lean in or out from each other. A camber setting of 0 means that both tyres sit flat on the track. Maximising the amount of rubber on the track is one of the aims of kart setup.

This is the angle of the kingpin, which is the point around which the stub axles rotate. This is one of the most important settings for inducing wheel lift during cornering.

Figure 1. This is the angle at which the front wheels either point in towards each other, or away from each other. Scrub Radius. This is distance from the centre of the tyre to the point where a line down the kingpin axis intersects the ground. Along with caster this affects wheel lift during cornering. Scrub radius is set using spacers on the stub axle.

Figure 2. King Pin Inclination. This is the inward lean of the kingpin, and it modifies the amount of camber change caused by the caster when steering.We are all familiar with the idea that turning the steering wheel in a car rotates the front wheels, causing the car to steer into a turn.

But why do we normally steer with the front wheels rather than the rear wheels or all four wheels, as shown below? Do both front wheels turn the same amount, or is something more complicated going on?

When a rigid body turns in a circle, the four corners of the body all move with different velocities and speeds. If we think of a car as a single rigid body with wheels at the corners, this means the four wheels move with different speeds and also different directions.

Car designers can choose different steering arrangements, as shown below, where the steering can be with the front two wheels only conventional carswith the rear two wheels only forkliftsor with all four wheels in some high-performance cars.

Forklifts typically have fixed front wheels and steer with the rear wheels. This gives them great control over the position of the front lifting forks, but makes steering harder for the operator.

Rear-wheel steering is a type of non-minimum phase control system. For example, to turn right in a forklift we have to first swing the back out to the left. This makes it very easy to accidentally run into obstacles, and is also unstable at higher speeds. Sometimes in cars it would be convenient to also steer with the rear or trailing wheels, for example when trying to park in a tight spot.

We actually have a mechanism for converting a car to trailing-wheel steering, which is to drive backwards, such as when parallel parking. In a conventional car, the rear two wheels are fixed to point straight ahead, while the front two wheels must turn at different but matched angles. To achieve the correct front wheel angles, cars typically use a four-bar linkage as shown below.

Ackermann steering geometry

By correctly choosing the lengths of the linkage rods, the car steering will automatically produce nearly correctly matched front wheel angles to make a perfect turn with any radius of curvature. Adjustable Ackerman steering geometry. Three wheeled vehicles with two wheels in back and one in front a delta or tricycle configuration avoid the need for complicated steering geometry, leading Karl Benz to use this system in the world's first automobile. Benz Patent-Motorwagen Nr. Image credit: Wikimedia Commons public domain full-sized image.

Episode 4 of Season 9 of Top Gear featured a segment on the Reliant Robina three-wheel car produced in the s in the United Kingdom. This Top Gear episode clearly demonstrates some of the reasons that single-wheel steering systems for cars have not achieved long-lasting commercial success.

Three-wheeled cars were rather popular with British manufacturers in the s and s. As well as single-wheel-in-front models like the Reliant Robin, there were also production cars with a single wheel in the back a tadpole configuration. An example of this is the smallest production car ever made, the Peel 50shown in another Top Gear episode Series 10, Episode 3. Home Reference Applications.It happens that geometry and mechanics are precisely what you need to define how the wheels of your car must be turned.

If the prolongation of the wheel axis passes through the centre of rotationthen the wheel, while turning, leave a clear tread. If the prolongation of the wheel axis is not directed toward the centre of rotation, then the wheel skids as it turns.

The tread will be erased from the skidding, and, above all, the control of a vehicle with wheels like that will be increasingly difficult as the speed increases.

So, for a good control of the car the prolongations of the axes of all the wheels must be directed toward the centre of rotation. But what this means for a car with four wheels? Since the axle of rear wheel in most cars is fixed, the prolongation of this axle should be directed toward the centre of this circle.

The front wheels are then turned so that the axis of each is directed toward the same centre. This means that for a good control one should be able to turn the front wheels by different angles, so that these wheels will be non parallel! You will say that the curves are not always arcs of circles, and in addition that the car does not stop, before steering. This, of course, is true, but it happens that along any curve we can consider that at every time the car moves along an arc of a circle whose radius and centre depend on that moment of time.

Consider an ordinary road. In order to be viable by car, it must have no sharp angles, namely, its middle line must be, as mathematicians say, a smooth curve. Take any point on this line and mark it in red, and another, a little far from the first, marked in blue. These two points define a unique straight line connecting them, and we draw that line. Now move the red point along the curve toward the blue point. At a time when the points coincide, the line defined by them will become the tangent line.

It is the linear approximation of the curve in a small neighbourhood of that point. We observe, however, zooming in, that the curve and the straight line are close only for a small stretch. Let us now take two red points on the curve, one on the right and one on the left of the blue point. Three points, which do not lie on a straight line, define a unique circle, that we draw. Now we move the two red points towards the blue point.

When the three points coincide we get a circle, which is called osculating. It is the second order approximation of the curve, and zooming in we may see that this approximation is better. We observe that in a stretch of the curve, along which the radius of curvature increases or decreases as in the ascending and descending parts of the curve in the figure the osculating circle always intersects the curvedifferently from the tangent, which is situated in the same part of the plane cut off by the curve, in a neighbourhood of the point of tangency.

Since the osculating circle in our case well approximates the curve and can be constructed at each point, the motion along a road with curves can be considered at every instant of time as the motion along an arc of a circle.

The radius and the centre of this circle depend, obviously, from where the car is. In this way, while moving along a certain curve, one can imagine that at every time the car moves on a small arc of a circle. And our first case, where the road is exactly an arc of a circle, is therefore essential to study the motion. But how to get that for any position of the steering wheels the prolongation of the axes are directed toward the instantaneous centre of curvature i.Steering is the collection of components, linkages, etc.

An exception is the case of rail transport by which rail tracks combined together with railroad switches and also known as 'points' in British English provide the steering function. The primary purpose of the steering system is to allow the driver to guide the vehicle. The most conventional steering arrangement is to turn the front wheels using a hand—operated steering wheel which is positioned in front of the driver, via the steering columnwhich may contain universal joints which may also be part of the collapsible steering column designto allow it to deviate somewhat from a straight line.

Other arrangements are sometimes found on different types of vehicles, for example, a tiller or rear—wheel steering. Tracked vehicles such as bulldozers and tanks usually employ differential steering —that is, the tracks are made to move at different speeds or even in opposite directions, using clutches and brakesto bring about a change of course or direction.

The basic aim of steering is to ensure that the wheels are pointing in the desired directions. This is typically achieved by a series of linkages, rods, pivots and gears. One of the fundamental concepts is that of caster angle — each wheel is steered with a pivot point ahead of the wheel; this makes the steering tend to be self-centering towards the direction of travel.

The steering linkages connecting the steering box and the wheels usually conform to a variation of Ackermann steering geometryto account for the fact that in a turn, the inner wheel is actually travelling a path of smaller radius than the outer wheel, so that the degree of toe suitable for driving in a straight path is not suitable for turns. The angle the wheels make with the vertical plane also influences steering dynamics see camber angle as do the tires.

Many modern cars use rack and pinion steering mechanisms, where the steering wheel turns the pinion gear; the pinion moves the rack, which is a linear gear that meshes with the pinion, converting circular motion into linear motion along the transverse axis of the car side to side motion. This motion applies steering torque to the swivel pin ball joints that replaced previously used kingpins of the stub axle of the steered wheels via tie rods and a short lever arm called the steering arm.

The rack and pinion design has the advantages of a large degree of feedback and direct steering "feel". A disadvantage is that it is not adjustable, so that when it does wear and develop lashthe only cure is replacement. BMW began to use rack and pinion steering systems in the s, and many other European manufacturers adopted the technology. American automakers adopted rack and pinion steering beginning with the Ford Pinto. Older designs use two main principles: the worm and sector design and the screw and nut.

Both types were enhanced by reducing the friction; for screw and nut it is the recirculating ball mechanism, which is still found on trucks and utility vehicles. The steering column turns a large screw which meshes with nut by recirculating balls. The nut moves a sector of a gear, causing it to rotate about its axis as the screw is turned; an arm attached to the axis of the sector moves the Pitman armwhich is connected to the steering linkage and thus steers the wheels.

The recirculating ball version of this apparatus reduces the considerable friction by placing large ball bearings between the screw and the nut; at either end of the apparatus the balls exit from between the two pieces into a channel internal to the box which connects them with the other end of the apparatus, thus they are "recirculated". The recirculating ball mechanism has the advantage of a much greater mechanical advantageso that it was found on larger, heavier vehicles while the rack and pinion was originally limited to smaller and lighter ones; due to the almost universal adoption of power steeringhowever, this is no longer an important advantage, leading to the increasing use of rack and pinion on newer cars.

The recirculating ball design also has a perceptible lash, or "dead spot" on center, where a minute turn of the steering wheel in either direction does not move the steering apparatus; this is easily adjustable via a screw on the end of the steering box to account for wear, but it cannot be entirely eliminated because it will create excessive internal forces at other positions and the mechanism will wear very rapidly.

This design is still in use in trucks and other large vehicles, where rapidity of steering and direct feel are less important than robustness, maintainability, and mechanical advantage.

The worm and sector was an older design, used for example in Willys and Chrysler vehicles, and the Ford Falcon s. To reduce friction the sector is replaced by a roller or rotating pins on the rocker shaft arm. Generally, older vehicles use the recirculating ball mechanism, and only newer vehicles use rack-and-pinion steering. This division is not very strict, however, and rack-and-pinion steering systems can be found on British sports cars of the mids, and some German carmakers did not give up recirculating ball technology until the early s.

Other systems for steering exist, but are uncommon on road vehicles. Children's toys and go-karts often use a very direct linkage in the form of a bellcrank also commonly known as a Pitman arm attached directly between the steering column and the steering arms, and the use of cable-operated steering linkages e.

Power steering helps the driver of a vehicle to steer by directing some of its power to assist in swiveling the steered road wheels about their steering axes. As vehicles have become heavier and switched to front wheel driveparticularly using negative offset geometry, along with increases in tire width and diameter, the effort needed to turn the wheels about their steering axis has increased, often to the point where major physical exertion would be needed were it not for power assistance.

The steering geometry

To alleviate this auto makers have developed power steering systems, or more correctly power-assisted steering, since on road-going vehicles there has to be a mechanical linkage as a fail-safe.Ackermann steering geometry is a geometric arrangement of linkages in the steering of a car or other vehicle designed to solve the problem of wheels on the inside and outside of a turn needing to trace out circles of different radii. It was invented by the German carriage builder Georg Lankensperger in Munich inthen patented by his agent in England, Rudolph Ackermann — in for horse-drawn carriages.

Erasmus Darwin may have a prior claim as the inventor dating from The intention of Ackermann geometry is to avoid the need for tyres to slip sideways when following the path around a curve. As the rear wheels are fixed, this centre point must be on a line extended from the rear axle.

Intersecting the axes of the front wheels on this line as well requires that the inside front wheel be turned, when steering, through a greater angle than the outside wheel. Rather than the preceding "turntable" steering, where both front wheels turned around a common pivot, each wheel gained its own pivot, close to its own hub.

While more complex, this arrangement enhances controllability by avoiding large inputs from road surface variations being applied to the end of a long lever arm, as well as greatly reducing the fore-and-aft travel of the steered wheels. A linkage between these hubs pivots the two wheels together, and by careful arrangement of the linkage dimensions the Ackermann geometry could be approximated.

This was achieved by making the linkage not a simple parallelogram, but by making the length of the track rod the moving link between the hubs shorter than that of the axle, so that the steering arms of the hubs appeared to " toe out". As the steering moved, the wheels turned according to Ackermann, with the inner wheel turning further. A simple approximation to perfect Ackermann steering geometry may be generated by moving the steering pivot points inward so as to lie on a line drawn between the steering kingpins and the centre of the rear axle.

With perfect Ackermann, at any angle of steering, the centre point of all of the circles traced by all wheels will lie at a common point.

steering geometry

Note that this may be difficult to arrange in practice with simple linkages, and designers are advised to draw or analyse their steering systems over the full range of steering angles.

Modern cars do not use pure Ackermann steering, partly because it ignores important dynamic and compliant effects, but the principle is sound for low-speed maneuvers.

Some racing cars use reverse Ackermann geometry to compensate for the large difference in slip angle between the inner and outer front tyres while cornering at high speed. The use of such geometry helps reduce tyre temperatures during high-speed cornering but compromises performance in low-speed maneuvers.

The Ackermann condition of vehicle train is fulfilled when not only the pulling vehicle wheel axes, but also the trailer wheel axes are pointing in the theoretical turning center momentan centrum. From Wikipedia, the free encyclopedia. Accessed April Modern Steam Road Wagons. Acta Polytechnica Hungarica.

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Namespaces Article Talk. Views Read Edit View history. By using this site, you agree to the Terms of Use and Privacy Policy.Kart steering is far more complex than a simple system for turning the front wheels on a bend. Its operation uses the Ackermann system: a revolutionary solution invented and patented in to optimise the direction of the front tyres and, consequently, driving around a bend.

The starting point for explaining the Ackermann steering geometry is obvious: while on a bend, the outer front wheel has a wider trajectory that is, a wider curve than the inner wheel which has a narrower curve.

It should also be considered that a turning kart must have a rotation centre around which to do so, and around which the front wheels can rotate. That being so, it is evident that if the front wheels are perfectly parallel during the bend, no rotation centre would be created because the axles of the wheels would remain parallel and without a common rotation point which would be obtained from the intersection of the two axles of the wheels. In this way, the front wheels would slide, generating friction between tread and asphalt, tyre wear and loss of performance.

By causing the front wheels to rotate in a non-linear direction when turning the steering wheel. The grip on the front is accentuated and the wheel travel direction is even more decisive.

If the wheels remained parallel during a bend, there would be two rotation centres that would cause the kart to swerve, making it slip and rub the front wheels on the asphalt.

The Ackermann system allows the front wheels to swerve at different angles.

Steering Geometry Diagnostics

This creates a single centre of rotation, which coincides with the common point between the two axles of the front wheels and the axis of the rear axle. To ensure that the Ackermann steering geometry is more than zero and therefore the front wheels turn in a non-linear manner, the front stub axles are made with tie-rods directed towards the inside of the chassis, the ends of which have the holes the steering rods are hooked onto.

In this way, a system is created in which the distance between the rotation axis of the stub axles is greater than that between the points of attachment of the steering columns with the steering rods. However, this is not enough. In fact, if it stopped there, the variation of the two angles of the front wheels with respect to the axis of the front stub axle would not be in optimal motion. Therefore, in order to give greater progression to the variation of the two steering geometries of the front wheels i.

This determines the non-linear variation of the front wheel steering angle, which will adapt in the best possible way to any steering geometry. Unlimited articles. Home News Magazine. Its operation uses the Ackermann system: a revolutionary solution invented and patented in to optimise the direction of the front tyres and, consequently, driving around a bend read more. Already subscriber? Continue reading your article. Start your hour free tour No Credit Card required. They can interest you.

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The steering geometry

True or False 10 Jan. Champion Advice 27 Oct. Under Review 22 Oct. True or False 17 Oct. How To 06 Oct. How To 19 Jun. True or False 16 Feb. How To 14 Dec.Caster Caster fig. Caster is the backward or forward tilt of the steering axis that tends to stabilize steering in a straight direction by placing the weight of the vehicle either ahead or behind the area of tire-to-road contact. Caster controls where the tire touches the road in relation to an imaginary center line drawn through the spindle support.

It is NOT a tire wear angle. The basic purposes for caster are as follows:. To aid directional control of the vehicle To cause the wheels to return to the straight-ahead position. Manufacturers give specifications for caster as a specific number of degrees positive or negative. Typically, specifications list more positive caster for vehicles with power steering and more negative caster for vehicles with manual steering to ease steering effort.

Depending upon the vehicle manufacturer and type of suspension. Figure Negative caster fig. With negative caster, the wheels will be easier to turn. However, the wheels tend to swivel and follow imperfections in the road surface. Positive caster fig. Positive caster helps keep the wheels of the vehicle traveling in astraight line.

When you turn the wheels, it lifts the vehicle. Since this takes extra turning effort.

steering geometry

Camber Camber is the inward and outward tilt of the wheel and tire assembly when viewed from the front of the vehicle. It controls whether the tire tread touches the road surface evenly. Camber is a tire-wearing angle measured in degrees.

The purposes for camber are as follows:. To load the larger inner wheel bearing Positive and negative camber fig. If the wheel is aligned with the plumb line, camber is zero. With positive camber, the tops of the wheels tilt outward when viewed from the front, With negative camber, the tops of the wheels tilt inward when viewed from the front. Suspension wear and above normal curb weight caused by several passengers or heavy loads tend to increase negative camber.

Positive camber counteracts this. Toe Toe fig. Toe controls whether the wheels roll in the direction of travel. Of all the alignment factors, toe is the most critical.

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