|Tuesday, July 30th 2013|
Advanced Racing Technologies .::. Products
Kart Chassis Setup
Understanding & Adjusting Front-end Geometry
ART recognizes this article on kart chassis setup basics for karting as being well written and very objective. Having written permission from John Learmonth we post this article for your reference. At this time ART has no affiliation with J.L.Products or any practical experience with the ZTB system.
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As with all racing vehicles, correctly and accurately set front-end geometry is vital to get the most out of any kart. Handling, grip, tyre wear and even acceleration can be adversely affected by poor settings caused by neglect, damaged / worn components, and most commonly by inaccurate or inadequate alignment methods or technique.
The same, but different!
Despite both having four wheels, kart chassis setups are quite different to car chassis (the kart chassis itself effectively forming the 'suspension' members and 'springs', having a very wide track to wheelbase ratio, and completely lacking a differential). Because of this, a kart's steering geometry settings also need to be different to make the kart steer and handle properly. Karts share all of the same steering geometries as cars, but by comparison some of the settings on a kart are somewhat exaggerated. A kart's combination of solid rear axle (no differential) and very short wheelbase relative to a relatively very wide track presents particular problems for a kart's front-end geometry.
A kart with similar geometry settings to cars (road or racing) would suffer from severe understeer at the moment of corner turn-in. This is because the combined grip of the rear tyres (which are locked together by the axle, and unable to rotate at different speeds) would simply push the front wheels straight ahead. This would happen because:
The rear tyres are substantially wider than the front tyres (more rubber on the road).
The rear track is usually significantly wider than the front track (increasing the directional stability of the rear end).
The kart will have a substantial rearward weight bias (more heavily loading the rear tyres).
The short wheelbase lessens the leverage the font tyres have over the rear tyres.
To overcome this problem, karts have steering geometry that's designed to lower the inside front wheel and raise the outside front wheel (relative to the rear axle) at corner turn-in. This change in front wheel height is much greater at the inside front than at the outside front (i.e. the inside front lowers much more than the outside front rises). Because of this the 'weight jacking' effect required to unload a kart's inside rear tyre at corner turn-in is mostly produced by the lowering of the inside front wheel.
Front wheel height change (with steered angle) causes a weight transfer (weight 'jacking') from the outside front and inside rear wheels to the outside rear and inside front wheels. This effect mechanically lifts (unloads) the inside rear wheel off (or nearly off) the track surface at the moment of turn-in. Once the kart is actually turned into the corner, this mechanical weight transfer becomes less important (it can even be counter productive in the rest of the corner) and is largely superseded by weight transfer due to 'G' forces (i.e. forward, lateral, rearward accelerations, and centripetal force). Because of this mechanical weight transfer to the inside front tyre (weight jacking), most of the front grip at the moment of turn-in comes from the inside front tyre, however at higher lateral 'G' forces (i.e. toward mid corner and exit) most front grip comes from the outside front tyre (or should do so).
On a kart, almost every steering geometry angle and setting is designed around this need to unload weight from the inside rear tyre at turn-in. A kart that does not do this enough, or does it too much will not handle well. From the moment of turn-in to mid corner the kart should effectively become a three wheeled vehicle. This condition needs to be maintained until the kart is exiting the corner, at which point the inside rear tyre needs to become progressively 're-loaded' as the corner opens out. If the inside rear wheel does not unload enough at turn-in the front will slide (literally 'pushed' by directionally stable rear grip). It may then just:
Continue to understeer through the rest of the corner, or
Suddenly gain front grip, causing the kart to change direction very abruptly and flick the rear end into a slide.
This can be one of the most difficult handling problems to drive with and is often mistaken for a rear grip problem since the rear slide can be the dominant sensation (the initial understeer often lasting only a moment or two). If the inside rear lifts too much the kart can become twitchy and difficult to drive smoothly. Getting this initial turn-in mechanical weight transfer just right is one of the most important factors in tuning the chassis and is mostly a function of correct front-end set-up and front to rear weight distribution. If the kart doesn't turn-in well, then you'll spend the rest of the corner catching up with this initial problem.
Having said that, although it's very important to drive smoothly it's also very possible to be too smooth when turning a kart into a corner. If the steering wheel is turned too gently at turn-in, the kart may behave in a similar manner as if it won't transfer weight from the inside rear wheel, but this article isn't about driving skills!
So let's have a look at what these various geometries are and how they work on a kart:
Toe is the degree to which the front wheels point toward or away from each other in the forward direction. Toe-in is present when the front wheels point toward each other, toe-out is present when they point away from each other. Toe-in tends to make the kart more directionally stable, but because of this can contribute to poor turn-in. Toe-out can cause the kart to be directionally unstable (not necessarily a bad thing!), and can assist the kart to turn-in to corners well. With toe-out, the inside front wheel moves down in relation to the chassis more than it will with zero toe or toe-in. This is because the toe setting has a slight influence on the Ackerman geometry, toe-out effectively increasing Ackerman and toe-in effectively reducing it (changing the angle of the steering arms; see below).
Toe (either in or out) creates friction at the contact patch of the tyre. This friction generates heat in the tyres (contributing to overheating in some conditions) and can contribute to excessive wear. This heat energy comes from engine power that is being wasted (i.e. not being used to accelerate the kart). Toe is adjusted by lengthening or shortening the tie-rods.
Camber is the degree to which the front wheels lean toward or away from each other. If the tyre treads are closer together at the top than at the bottom then camber is negative, and positive camber is of course the opposite. To maximise grip when cornering (particularly around mid corner), it's highly desirable to have as much of the rubber of the two outside tyres as possible on the track. Camber is the setting mostly responsible for maintaining the maximum outside front tyre rubber on the road in corners.
Camber can also introduce a rolling resistance, which is caused by a phenomenon called 'camber thrust'. This is the tendency of a cambered wheel to roll in the direction of the camber lean (like a bicycle wheel). E.g. assuming two equally cambered wheels, each 'wanting' to roll in the direction of it's camber thrust (but prevented from doing so by the steering linkage), then each wheel is forced to roll in a straight line, causing friction at the contact patches. Camber thrust doesn't really increase grip in corners, but camber can and does effectively decrease the size of the contact patch of the tyre by loading one side more heavily than the other. This can and does lessen grip, increase temperature, increase wear, and shorten the competitive and outright life of the tyre.
On most karts camber angle is adjusted by rotating the eccentric cam 'camber / caster adjusters' at the stub axle (spindle) mounting. If the king-pin bearings are housed in the stub axle itself, caster and king-pin inclination (see below) can also change when you adjust camber settings (karts with the king-pin bearings housed in the chassis have no adjustment for caster or king-pin inclination). Not all karts have adjustable camber, caster or king-pin inclination, but it's not usually difficult to fit adjusters if you need them (and you almost certainly do!).
Caster angle is the fore / aft lean of the king-pins (the bolts that the stub-axles pivot around), and is one of the geometries that needs to be significantly greater on a kart than on a car. All karts have a 'positive' caster angle, which is when the top of the king pin is closer to the rear of the kart than the bottom of the pin is. Caster angle is responsible for most of the 'self-centring' action of the steering, and is one of the two geometries that cause the front wheels to change height when the steering wheel is turned. The greater the caster angle, the greater the front wheel height changes.
An unfortunate side effect of caster is that it also causes a change of camber when the steering is turned, resulting in negative camber gain at the outside front wheel and positive camber gain at the inside front wheel. With most karts, caster is adjusted using the 'camber adjusters' (which can also change camber and KPI), but caster can only be adjusted if the king pin bearings are housed in the stub axles themselves (i.e. karts with the king pin bearings housed in the chassis will only change camber when the adjusters are rotated).
King-pin inclination (KPI) is greater on most karts than on most cars, and is the inward lean of the king-pins (up, towards the centreline of the kart). KPI causes some of the self-centring action of the steering, and modifies the amount of camber change caused by the caster angle when the steering is turned (lessening negative camber gain at the outside front wheel and increasing positive camber gain at the inside front wheel). KPI also causes the wheels to rise and fall in an arc rather than in a straight (but angled) line as would be the case if no KPI were present. It would be unusual to deliberately alter KPI itself, but it can be adjusted using the 'camber' adjusters. Changing KPI will also change camber and can change caster. This does depend on the exact nature of the adjustment, some KPI adjustments producing no change in caster angle, e.g. from maximum KPI to minimum KPI, so long as the caster is at the central adjustment setting. This is also true for caster adjustments that don't affect KPI. KPI cannot be adjusted on karts with the king-pin bearings fitted in the chassis. On karts with the king-pin bearings housed in the stub axle, adjusting camber is actually a by-product of altering the KPI.
Scrub radius (also called 'king-pin offset') is another 'exaggerated' front geometry setting on a kart, and is the distance from the centre of the tread (at ground level) to the point where a line drawn through the king-pin axis intersects the ground. Scrub radius works hand in hand with the caster angle to alter front wheel heights as the steering is turned. The greater the scrub radius, the greater the front wheel height change.
Increasing scrub radius will also widen the front track. This track increase also effectively softens the front end of the chassis, lessens 'dynamic' weight transfer (transfer due to 'G' forces) to the outside front, and usually increases front grip (by keeping more weight on the inside front after turn-in). Scrub radius is adjusted using the track spacers on the stub axles.
"True" Ackermann geometry is provided by the relationship between the position of the king-pins and the position of the outer tie-rod ends (effectively, the inward angle of the steering arms). An additional Ackerman 'effect' is also provided by the use of two separate inner tie-rod end mounting positions. Ackerman effect (from both geometries) causes the inside front wheel to turn substantially more than the outside front wheel, enhancing the lowering of the inside front wheel.
Karts employ far more Ackerman effect than almost any other type of vehicle and it's primarily used for somewhat different purposes than on a car. Cars use Ackerman geometry to minimise tyre scrub when the vehicle is turning a corner (the outside front wheel needing to turn in a larger radius than the inside front). A kart (mostly) uses Ackerman effect (in conjunction with caster angle and scrub radius) to increase the inside front wheel's downward movement (by making it turn substantially more than the outside front), in order to unload the inside rear wheel at turn-in. Some karts have adjustable Ackerman, involving the use of different length tie-rods, and mounting them in different holes on the steering arms and / or Pitman arm (the arm to which the tie-rods attach on the steering column).
Monkey see, monkey do?
Look around the pits and you'll probably see quite a few karts with noticeably negative camber settings. Some of these karts will be this way through neglect, but many will have been deliberately set-up this way. Some kart racers see many full-sized racing cars using pronounced negative camber settings and conclude; "if it works for them, it should work for me too". The only problem with this idea is that most racing car tyres use radial construction, which have very flexible sidewalls and a fairly rigid tread. Kart tyres are made with cross-ply (or bias-belted) construction, and have stiffer sidewalls and less rigid treads than radial tyres. A cross-ply racing tyre doesn't work well at large camber settings for this reason.
The more rigid sidewall won't deform as easily as a softer sidewall, and the less rigid tread won't sit as flat (evenly) on the track as a more rigid tread. A radial tyre has the rigidities of it's sidewalls and tread 'tailored' to suit the different requirements in these sections of the tyre case, but a kart tyre casing is a compromise between these different requirements. Why? Cheaper of course!
Many racers will spend a lot of money having their engines blueprinted and getting the latest new pipe etc., in the belief that the only way to go any faster is to get more out of the motor. Yet many of these racers are simply wasting some of the engine power they already have, and every little bit counts!
Power can be wasted in three ways, brake pad drag, friction in the wheel bearings, and incorrect wheel alignment. These problems cause an increase in rolling resistance, and since the engine doesn't magically gain power because the kart has more rolling resistance, the kart goes just that bit slower. If the increased rolling resistance is due to bad alignment, the kart will probably handle poorly as well.
Try thinking about it this way, if you're losing only 1Ú2% per lap to the kart in front due to poor alignment (or any other reason) then in ten laps on a 700 metre track you'll lose 35 metres. It doesn't even take a kart length to lose a race!
So, what is good alignment?
A 'well aligned' kart will handle better, accelerate better, have a higher top speed, and wear it's tyres less. In a loaded state (i.e. with the driver sitting in the kart), a well aligned kart will have the toe and camber settings at, or very close to zero (i.e. the front wheels longitudinally and vertically parallel with the rear wheels). This will ensure that the tyres are being used as they were designed, not slowing the kart on the straights and also maintaining a wide patch of rubber on the track in corners.
It should also have enough scrub radius and caster to adequately 'jack' weight from the inside rear tyre at turn-in (a kart should effectively be a three wheeled vehicle in corners). It should be kept in mind that excessive caster will cause excessive camber change when the steering is turned, and that if caster is lessened then a compensating increase in scrub radius will probably be required. All wheels should also be in the same plane as each other with the steering at the exact straight ahead position with no load in the kart.
Most karts, most of the time, will handle and accelerate better with toe set to absolute zero in the condition in which it is raced. It should be understood that toe (usually), and camber (always) don't remain the same when the driver's weight is placed in the kart. Sometimes slight toe-out will help turn-in to corners, but rarely more than two millimetres (except in wet conditions, when larger toe-out settings can be helpful). Setting camber to zero will nearly always be the best starting point, and can be fine tuned using tyre wear as a guide, or tyre temperatures across the tread.
how do I achieve accurate alignment?
There are several wheel alignment methods and tools available for this purpose:
Methods not requiring special equipment:
The cheapest way (free!) and most commonly used to set toe, is the old scribe a line around the tyre method. This involves (oddly enough) scribing a line around the circumference of each tyre, setting the steering straight ahead (or as straight as you can guess), then measuring between the scribed lines at the front and the back of the tyres with a tape measure. Any difference in these measurements (in mm's) is the toe setting (more or less). The downsides of this method are that it doesn't work for measuring camber and can be somewhat time consuming and awkward. It doesn't set the front wheels equally with the kart's centreline or parallel to the rear axle, and the effect of Ackermann geometry can throw the alignment off if the steering is not exactly at the straight ahead position.
Also free, and somewhat more accurate (if done carefully and correctly), is stringlining the kart. This involves accurately arranging parallel stringlines along each side of the kart and measuring in from the stringlines to the appropriate points on the wheel rims. The downsides of this method are that it requires careful preparation, a flat floor and a great deal of care and patience. If the wheels are not perfectly straight (i.e. not damaged) then accuracy will also be adversely affected. A distinct advantage of this method is the ability to align the kart with the driver seated in it (* 'dynamic' alignment). The biggest downside of stringlining is that it is almost impossible do at the track.
'Dynamic' alignment is definitely an advantage because toe and camber settings will significantly (and often substantially) alter when the driver's weight is placed in the kart. Some karts (very few) won't change toe with driver weight, but most will suffer some amount of toe change, up to three millimetres is not uncommon (equivalent to about 8 or 10mm of inaccuracy on full sized car tyres). Karts will gain toe-in (never toe-out) and negative camber with driver weight. Just about every kart ever made will change camber with driver weight, more so than toe change. To achieve 'dynamic' zero toe and camber, most karts will need some amount of toe-out and all karts will need some positive camber without the driver seated.
© Copyright. John Learmonth. 1999. All rights reserved.
© Copyright. John Learmonth. 1999. All rights reserved.
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