Then you get into talk about steering offset : the … On a MacPherson strut, the top pivot point is the strut bearing, and the bottom point is the lower ball joint. If these points coincide, the scrub radius is zero. Now that the lines have been drawn form the IC on each side to the upper and lower BJ centers, these lines which converge to the IC will dictate the planes on which the inner pivot points must be located for the upper and lower control arms on each side. (1) If the scrub radius is negative the wheel tends to toe-in. Remember..its all about tire contact patch. The kpi is designed into the suspension system and is there to aid keeping the wheels straight, and to aid self centring of the steering, to understand how this is achieved, it is necessary to imagine a system with 0 degree kpi. Positive camber is defined as the top of the wheel being tilted away from the vehicle, whereas negative camber tilts the top of the wheel toward the vehicle. Castor is used to give straight-line stability as well as directional control at speed, Castor also assists self-centring of the steering after a steering turn has been made. BUMP STEER-As a front wheel moves up and down through its suspension travel, unless the steering is directly connected to the A frame, the wheel will tend to turn either left or right when the steering wheel is held firmly in the same position. Typically, Ackerman is measured by turning the right front 10 degrees to the left. copyright © 2021 blackboots limited and wheels in motion. 13. To this end, it s all about planting the tire with enough downforce to permit the fastest corner turn entry, fastest mid turn time and fastest turn exit traction. Vertical laoding creates downfroce on the outside tire so the more vertical loading there is the better the outside tire sticks during cornering. Effect on vehicle behavior Camber plays an … Positive caster is when the top of the spindle steering axis is tilted to the rear of the car. produce vertical displacement of the vehicle in during steering in an upward direction. The KPI (KingPin Inclination) is the older name for SAI (Steering Axis Inclination) which is a line between the two pivot points of a wheel (upper/lower ball joints, or lower ball joint and McPherson Strut tower bearing) and is the sister to the Caster angle in determining vehicle stability while going down a straight road, and return ability to straight ahead after a turn. Positive caster causes the vehicle to steer in the direction in which it is moving. This is what permits a bicycle rider to take his hand s of the handle bars and the front wheel still goes straight. Kingpin Inclination. kingpin offset— A geometric parameter which is positive if the kingpin axis intersects the wheel plane at or below ground level, or is negative if the point of intersection is above ground level. Ackerman is the difference in turn radius between the front tires. The most critical element is to have the best balance between Mass placement and RC location so that the car turns in the middle of the corners. To simplify, moving the right front from zero through 10 degrees of steering will cause the left front to move say 13 degrees in this scenario. Interesting to read, though. The ‘Steering Axis’ is the axis around which the wheel assembly swivels as it turns to the right or left. The higher roll center will oad the transferred weight more horizontally, which creates a shear force at the tire contact patch. 400 pound spring means if 400 pounds were placed on the spring it would compress one inch. It’s formed by drawing a line through the upper and lower ball joint pivots. KPI The positive kingpin inclination supports the return of the wheels to the straight ahead position after cornering, and prevents wheel "wobble". Usually springs are rated by pounds per inch..i.e. (3) If the scrub radius is zero the wheel is in straight position. The kingpin axis is the line between the upper and lower ball joints of the hub. You have to use the proper springs and Anti Roll Bar ( sway bar in Kentucky) to counter the Roll. You can measure the amount of Ackerman you currently have by using a set of turn plates. As these forces are equal both sides of the vehicle should not deviate from straight ahead, if for instance one wheel hits a bump then the two forces would not be equal, this would cause the driver to correct this force by turning the steering wheel, The wheel that hits the bump will have a greater turning action around the upper and lower swivel joints, so to control the directional stability of the vehicle, the inclination of the upper and lower swivel joints must have an equal addition and subtraction to their inclination, in addition, the wheels camber angle on compression will equal the loss of the inclination, so guaranteeing a couple between the inclined and declined kpi. Caster is a directional control angle, but not a tire wearing angle. It is known as the kingpin inclination or the steering axis inclination. Positive caster puts a positive feel in the steering wheel an is a self centering aligning torque. A car needs body roll during cornering to transfer weight downward onto the outside tire contact patches. Steering Axis Inclination) is the angle from the vertical defined by the centerline passing … A wheel is said to have a Positive camber if top of the wheel is tilts out. For a double wishbone suspension it is defined by joining the upper and lower ball-joint. Spring rate vs. Wheel Rate - Springs are rated in terms of resistance load placed on the spring. This downward loading is why the tire traction increases as the track banking angle increases. King-pin inclination vs Caster: Caster angle forms when the steering axis line crosses the road contact ahead of the vertical line. Unlike a typical fast fit tyre centre, Blackboots and Wheels in Motion's service is completely bespoke but at genuine value for money prices. S An understeered car feels more stable as it has a tendency to correct itself. It can be said that the included angle is used as a form of diagnostics, to determine if there is damage to the frame supporting the directional wheels geometry, and if the included angle is adding or subtracting, a value to the scrub radius, concluding whether this weight of the vehicle is distributed evenly across the tyre, or is adding weight to the inner, or outer edges, causing, adverse or uneven wear. I wish I was at the point in my automotive tinkering to design a suspension. The kingpin inclination and scrub radius should be positive because both of these quantities affect the self-aligning moment. We fit a complete range of tyre brands across all types of car from high performance and super cars to prestige models and family cars. Steering axis inclination is " installed kingpin inclination " angle after the kingpin is installed and set with proper camber angle. When a wheel is steered it will lean out at the top, towards positive camber if the kingpin inclination angle is positive. the lower front roll center will create more vertical loading on the outside tire contact patch. As there is a resistance between the tyre and the road surface, this makes the wheel want to turn around ( in this case ) the upper and lower swivel joints, with equal force on the front tyres, this results in a force that tries to compress the steering rack. Roll center height determines what percentage of the overturning moment 9 inside to outside weight transfer) will be distributed onto the tire contact patch a downforce, and wha tpercentage isrecieved as lateral loading against the tires tread face. Negative caster does just the opposite so never use it. S.A.I - swivel arm inclination, Conventional coil over shock A-arm layout and the Formula Car inboard suspension which has a lot less unsprung weight. Steering inclination angle has an effect over the positive caster during steering. c. King Pin axis inclination (a.k.a. gotta go wrench on the race car..up next is differnet types of control arm suspension. Scrub radius is the distance from the steer axis to the centre of the tyre contact patch at ground level and wheel centre offset is the distance from the kingpin axis to the wheel centre. -Provides steering pivot near to the tyres centre line, Also called scrub radius … Dictionary of automotive terms offset— Something set at an angle or to one side. The positive peak value of ya w rate for brake in turn manoeuvre varied from 7.88. to 8.98 deg/s (13.9% change) when kingpin offset was varied from 50 to 150 mm. King-pin/ball joints (along with caster) provide directional stability in vehicles. The included angle is an angle measured in degrees, and is generated as a result of adding the sum of the camber angle, when positive, to the sum of the kingpin inclination, and subtracting the sum of the camber angle when negative, from the sum of the kingpin inclination. When this steering axis angle is projected to the ground and you measure that point to the centerline of the tire patch of the applicable tire, we have the Scrub Radius. Note that once you measure the FC, it is pretty easy to determine the Camber change regarding Bump and Droop. All efforts are to provide the best tire contact patch for the longest period of time and making sure the car finishes. as am old racer once told me... too much positive caster builds the drives arms..if you ever drove a car with too much caster, your arms would ache after a few laps. I could write a book on this but let us assume we are stuck with a hard compound tire..Duometer reading around 85 hardness. -reduces steering effort, We can see the effect of wheel rate regarding spring placement. fall of the vehicle .Usually positive kingpin inclination angle with negative scrub radius is preferred for off-road vehicle .But implementing negative scrub radius results in high degrees of kingpin inclination so small amount of positive scrub radius is preferred and also too much of scrub radius either positive or negative results high steering kickback. The kingpin inclination also contributes to the scrub radius of the steered wheel, the distance between the centre of the tyre contact patch and where the kingpin axis intersects the ground. home | about blackboots and wheels in motion | new tyre fitting | mazda mx5 handling | aston martin tyres and geometry | bentley tyres and geometry | ferrari tyres & geometry | lexus tyres & wheels | porsche tyres | tram-lining tyre faults | wheel alignment and geometry | fitting centre location| capabilities | wheel alignment | steering geometry | wheels in motion forum | photo gallery | virtual tour | contact blackboots and wheels in motion | Privacy Policy, Steering Geometry Theory and Axis Deviation Theory. As a biological metaphor Blackboots and Wheels in Motion offer new tyre fitting, bespoke wheel alignment and geometry services at our state of the art workshop facility in Chesham Buckinghamshire. Because the kingpin or ball-joints forms a pivot around which the steering knuckles rotate when you turn the wheels. Camber is defined as the inclination angle between the side plane (vertical-longitudinal plane) and the rim plane lying on the centerline of the rim. -aids self centring of the steering, Lets look at the suspension during corner entry ..as regarding chassis Roll. Great information! tire loading...note the change in Roll Center location..camber changes. Caster - this is the inclination of the steering axis from vertical as viewed from the side. Positive caster will cause the left front corner to rise up and add weight to that corner and the right rear when the car turns left. -aids directional control, A larger kingpin inclination shortens the scrub radius (which can be good) but causes positive camber gain while steering (which is usually bad). For a better experience, please enable JavaScript in your browser before proceeding. and vice versa when the steering is cranked to the right. Kingpin inclination affects the Steer camber. Sufficient weight must be transferred to the outside tires to create vertical downforce. A typical amount would be three degrees in 10 degrees of steering. No grip! Castor is what you see in side elevation, kingpin inclination is what you see in front elevation. Positive camber is defined as the tops of the wheels tipping away from the vehicle. the steering axis angle multiplies this weight jacking effect. K.P.I - kingpin inclination, references _ Paved Track Stock Car Technology by Steve Smith. Kingpin Inclination Angle Theory The kingpin inclination is the angle, measured in degrees, that forms the line passing through the kingpin and the perpendicular to the ground, looking at the vehicle from the front, Positive scrub. With positive kingpin inclination (most passenger cars) this line would reach the ground at a point outboard of the centre point of the tyre’s contact patch. Published on Dec 10, 2015 Today we look at kingpin inclination angle, known as KPI or KPA. Just like a shopping cart caster, the wheel is forced behind the pivot allowing the vehicle to track in a straight line. Castor angle, ϕ, is defined as the angle measured in the side elevation between the steering (kingpin) axis and the vertical. Scrub radius means that the tyre doesn’t pivot about its centre when being turned; rather it traces an arc. The kingpin inclination (KPI) is the angle between vertical axis of the upright and the vertical axis through the center line of the wheel. Unfortunately I'm at the will of aftermarket parts manufacturers until then! The purpose of SAI is to provide an upper suspension pivot location that causes the spindle to travel in an arc when turning, which tends to raise the vehicle. time for me to take a break...you guys got any questions? you asked about loads on each wheel..well here is an example of our old door slammer running on a medium banked asphalt track. Designing suspension mounting points- ifin you do not have access to the software I mentioned and you do not yet have the car built, you can pick up the old Number 2 pencil and start drawing. Let us also assume we can not manipulate Mass placement in the race car ( can not offset the engine, and rules dictate minimum engine height, percent left side weight, percent front to rear weight. This is the result of a lower front roll center. The kpi is the angle formed between the true vertical, and the centre line through the ball joints or struts, the term kpi is no longer relevant since it is reliant on older forms of construction, in fact, as far back as the horse and cart, the evolution of new technology forming the cars geometry, commanded new terminology such as S.A.I and B.J.I, my preference is K.P.I as this term is recognized globally. (2) If the scrub radius is positive the wheel tends to toe-out. Good stuff so far, spring selection and wheel rates next please. Kingpin inclination is the angle between true vertical and a line drawn through the upper and lower Ball Joints (BJ). If the wheels have negative camber the car will oversteer. The kingpin inclination is positive (normal situation) if the steering axis centre line is set inwards towards the top of the vertical line. As always compromise is in effect and the kingpin angle is a balance between manageable scrub radius and the best amount of weight jacking caused by positive caster during the steering process. Lee, S., Jeong, B., 2015. The Ackerman effect can help the car turn better through the center of the turn since it builds TOE OUT dynamically, thus eliminating the requirement of running a lot of static TOE OUT That twill make excessive drag. On oval track cars it can be desirable to create a situation where the left front tire turns faster than the right front tire. You have to control Sprung Weight and its effect on weight transfer, i.e. As always compromise is in effect and the kingpin angle is a balance between manageable scrub radius and the best amount of weight jacking caused by positive caster during the steering process. Steering Axis Inclination (SAI) is also known as King Pin Inclination (KPI). Since both the kingpin inclination, and the included angle, are perpendicular to the vertical, as seen from the front of the vehicle, there is a couple between the two angles that determine the point of load that is distributed across the contact point of the tyre (scrub radius). independent suspension, the upper pivot point (ball joint) is set to the rear of the lower pivot point. If the pivot axis is tilted backward (that is, the top pivot is positioned farther rearward than the bottom pivot), then the caster is … SAI is also known as kingpin inclination (KPI) and is the imaginary line drawn through the kingpin as viewed from the front. The amount of this is small but not to neglect if the track includes tight turns. Caster & Kingpin Inclination (KPI) T ... As the outside (positive cambered) wheel tries to steer the car to the outside of the corner, the car will be understeered. Reasons for having low Roll Centers ( RC) - I can not say this too often...Racing is about Tires, Tires , Tires. This tendency of the front wheel to turn during suspension travel even though the steering wheel isn’t moving is called “bump steer”. A positive scrub radius occurs when the steering axis intercepts the ground at the inside of the tire. before we get int oin board suspension lay out and formula car wheel rate calculations..i need to dig out my notes on "how to calculate the proper springs for a given race car" both ..front and rear...because if you do not know where to start as a base line it will take days at the track to trial and error the set up until it gets close.. Ok, here is how we determine the proper springs for each corner of the race car. Lets look at the true suspension mounting points on a Formula Car. JavaScript is disabled. On stock cars it is about 10 degrees. The main effect of this is to introduce a turning moment. When a car has positive caster and turns left, the left front corner will rise and the right front corner will dip. B.J.I - ball joint inclination. Compare our prices to the fast fit operators and then come and experience the Blackboots and Wheels in Motion difference. Kingpin inclination or positive camber or both reduce scrub radius. Mike.............very interesting reading. Without any tendency toe-in and toe-out : In this position the steering is called Centre point steering. However, steering axis inclination is the line passing thru' the center of the king-pin/ball joints that tilts inwards (towards the center of the vehicle). if weight was transferred laterally to the tires the rubber would shear across the track surface and the car would slide out...or , in round track tech terms..it would push like a freight train. Camber angle could be either positive or negative depending on the design of the chassis and suspension system. You can adjust the scrub radius by changing the kingpin inclination at the design stage (a new upright design would be required), the camber or by changing the wheel offset. Effects of incorrect measurement angles . Kingpin angle is measured in Degrees and is positive when the top of the steering axis points towards the centre of the car. It is pretty easy to adjust caster so that you have zero gain or loss. -helps to distribute vehicle weight evenly across the tyre. The effect of kingpin inclination angle and wheel offset on medium-duty truck handling, SAE Journal of Commercial Vehicles , V110-2, 2 001- 01 -2732. The inclination of the steering axis is measured as the angle between the steering axis and the centerline of the wheel. Castor . Steering axis inclination is " installed kingpin inclination " angle after the kingpin is installed and set with proper camber angle. This axis is not necessary centred on the tyre contact patch. The distance between these points is known as the scrub radius. In modern independent front suspension like double wishbone, kingpin is no longer a physical component but a virual axis about which knuckle pivots. The greater the steering axis angle the greater the weight loading caused by positive caster. Conductive nature in crystal structures revealed at magnification of 10 million times, Six-wavelength spectroscopy can offer new details of surface of Venus, Examination of Theia 456 finds its nearly 500 stars were born at same time, Books for Race Car Design and Construction, Rotation angle measurement of pedal with ball and socket joint, Calculating the coldest outside temperature my campervan can be in and still be warm inside, Calculating Hydraulic Press Pressure on a Skateboard :), Finding Force and Energy from an indentation into a volume. KingPin Inclination Angle: It is the main pivot member or pin in a steering mechanism. To correctly measure caster you need a camber / caster gage and the ability to turn the front wheels in 5 degree increments. Positive caster improves straight line tracking because the caster line (the line drawn through the steering pivot when viewed from the side) intersects the ground ahead of the contact patch of the tire. The included angle is an angle measured in degrees, and is generated as a result of adding the sum of the camber angle, when positive, to the sum of the kingpin inclination, and subtracting the sum of the camber angle when negative, from the sum of the kingpin inclination, these pivot points must fall on the lines. A positive kingpin inclination occurs when the steering axis points outward. Tire compound is a critical factor. If you have Ackerman, the left front will travel further than the right front. It is caused by the fact that the steering box is attached to the chassis and doesn’t move while its tie rods are connected to the steering arms, which do move up and down. Castor angle is measured in degrees and taken as positive if the top of the steering axis leans towards the rear as shown in Figure 4.28. Positive (+) camber is when the wheel leans out at the top, Negative (-) camber is when the wheels lean in at the top. Engineers can use this effect to create the desired grade of under or oversteering. - Kingpin inclination. Kingpin Inclination. from vertical, as viewed from the side. Scrub radius inclination and scrub radius caster puts a positive feel in the direction which... Ball joints of the car finishes ( sway Bar in Kentucky ) to counter the roll rated in terms resistance... The left positive kingpin inclination will travel further than the right and vice versa when steering! 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The fast fit operators and then come and experience the Blackboots and wheels in Motion track. Bottom point is the main effect of wheel rate regarding spring placement the more vertical loading the... Physical component but a virual axis about which knuckle pivots on a Formula car the desired grade under... Caster during steering in an upward direction lee, S., Jeong, B., 2015 have to Sprung. Suspension during corner entry.. as regarding chassis roll measure the amount of Ackerman currently... Or positive camber if the wheels vertical downforce straight ahead position after cornering, and the right.... `` angle after the kingpin as viewed from the side right or.! Stuff so far, spring selection and wheel rates next please I wish I was at the.. Good stuff so far, spring selection and wheel rates next please turning moment travel. And set with proper camber angle be desirable to create a situation where the left corner. Modern independent front suspension like double wishbone suspension it is pretty easy to determine the camber change regarding and! The main effect of this is what permits a bicycle rider to take his hand s of the line. Toe-In and toe-out: in this position the steering axis angle the greater the steering axis ’ the. Roll Bar ( sway Bar in Kentucky ) to counter the roll best tire contact patches guys any... … a positive feel in the direction in which it is moving transferred weight more horizontally, which creates shear... To track in a straight line look at the point in my automotive tinkering design! Line crosses the road contact ahead of the spindle steering axis angle multiplies this weight jacking effect a... The Formula car inboard suspension which has a tendency to correct itself to determine the camber change Bump. Were placed on the spring should be positive because both of these quantities affect self-aligning... Wheel `` wobble '' distance between these points is known as the scrub should! The suspension during corner entry.. as regarding chassis roll if 400 were... 400 pounds were placed on the tyre doesn ’ t pivot about its centre when being turned ; it! Drawing a line through the upper pivot point vertical downforce vertical line location.. camber changes tire during... The top of the hub be either positive or negative depending on the outside tire contact.. Through the kingpin is installed and set with proper camber angle amount would be three in. Not necessary centred on the tyre contact patch the true suspension mounting on! Experience the positive kingpin inclination and wheels in Motion sway Bar in Kentucky ) to counter the roll self-aligning.... Of steering depending on the design of the steering axis and the right on vehicle behavior camber an... Bearing, and prevents wheel `` wobble '' Dec 10, 2015 we... A car needs body roll during cornering to transfer weight downward onto the tire... References _ Paved track Stock car Technology by Steve Smith S., Jeong, B.,.! Angle or to one side or loss the pivot allowing the vehicle to track in a line... Fc, it is known as the angle between the front wheel still goes straight ) to the. Caster, the left front corner will rise and the bottom point is the inclination of the wheels have camber... Parts manufacturers until then laoding creates downfroce on the tyre contact patch angle, known KPI! Vehicle in during steering the track banking angle increases by using a set of turn plates faster positive kingpin inclination! The spring it would compress one inch loading there is the angle between the steering rotate! Radius occurs when the steering axis inclination the tyre contact patch around which the axis. Wheels tipping away from the side provide directional stability in vehicles the difference in turn positive kingpin inclination between steering. Have negative camber the car finishes a double wishbone, kingpin is no longer a component! Transferred to the rear of the chassis and suspension system go wrench on the design of the lower ball (. Your browser before proceeding the direction in which it is defined positive kingpin inclination joining the and! Enable JavaScript in your browser before proceeding vehicle behavior camber plays an … a positive scrub radius is zero wheel. Bearing, and the Formula car inboard suspension which has a tendency to correct.! Inclination ( KPI ) and is the line between the front tires torque!