|Kart Steering, Physical Forces and Setup - Theory and Practice||Kart Scaling & Weight Distribution|
|Understanding & adjusting front-end geometry||Getting the most from your Chassis|
|SETTING UP FOR SL KART TYRES||Lead weights|
by James Hughes.
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.
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. 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.
Camber. 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.
Caster. 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.
Toe In/Out. This is the angle at which the front wheels either point in towards each other, or away from each other. Zero degrees toe in/out means that the wheels are parallel. Toe in/out is set by changing the length of the tie rods.
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.
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. It is not usually possible (or necessary) to adjust the KPI although some camber adjust systems may let you do it.
Ackermann Steering. Ackermann steering uses the angle of the stub axle arms (and often an offset on the steering column) to make the inner wheel on a corner turn more than the outside wheel. With cars this is used to reduce tyre scrub on corner, but of more importance to karts is the greater wheel lift effect caused by increasing the inner wheels turn when compared to the outer.
To help explain how the front geometries affect the rear inside wheel, lets assume that the chassis is completely rigid - it is so stiff that it cannot bend in any direction. This assumption makes things a little easier to understand. Kart chassis are not actually this stiff - they flex in a number of areas. However, the differing effects caused by differing stiffness' in various parts of the chassis are beyond the scope of this article.
When we turn a corner, the steering geometry (but mainly the caster setting and scrub radius) causes the inside wheel to move down in relationship to the chassis, and the outside wheel to move up. As this happen, because our chassis is rigid, it pivots around a line from the inside front and outside rear, causing the inside rear to lift!
OK, so we have now explained how the front geometry is used to raise the inner rear wheel during cornering. There a quite a few other forces that come in to effect one a corner has been initiated, and that is what we will talk about next
These are the most obvious forces, and are a caused by the tyres exerting a force on the track, either forwards or backwards, with the result being to brake or accelerate.
Its is important to remember that this force is in the same plane as the track, that is, it is below the karts centre of inertia. For this reason, the force exerts a turning moment (or torque) on the entire kart. During acceleration this torque causes a weight transfer to the rear of the kart, and during braking it causes a weight transfer to the front of the kart. There is no actual movement of any mass, but the torque effectively forces the appropriate part of the kart 'harder' down on the track. It is possible to calculate the amount of weight transfer if we know the acceleration and the distance from the centre of inertia to the rear wheels, but that is beyond the scope of this article.
During cornering the driver feels like he is being pushed outwards from the kart. This is actually wrong, he is not being throw out but is simply trying to move in a straight line. The tyres of the kart are producing a grip which imparts an angular acceleration on the kart (and driver), forcing the kart to corner. It is this angular acceleration that the driver feels. The force which the ground imparts on the kart to make it corner is known as the Centripetal force, and it always acts at towards the centre of the imaginary circle we are cornering round. It important to remember that there is NO SUCH THING as centrifugal force.
The centripetal force is split into two components, a vertical and a horizontal. The horizontal force we have just described, but the vertical can be regarded as the cornering equivalent of the forward acceleration case. Because the centripetal force is acting on the kart, it imparts acceleration to it, and again, this acceleration is acting at ground level. Therefor a torque effect is again produced, but this time it is acting across the kart, and we get a weight shift to the outside of the kart. This weight shift also helps the inside wheel lift, as the weight shift reduces the weight on the inside wheel by an equivalent amount. Once cornering is initiated, this weight shift is more important to raising the inside wheel than the steering geometry.
The distance between the rear wheels affects how the centripetal force is distributed over the horizontal and vertical components. As track is increased, more centripetal force is distributed as a sideways force in relation to the weight shift. This means a wider track produces less weight shift to the outside rear, and more sideways force. A narrow track increases the weight shift and decreases the sideways force. Therefor a narrow track is less likely to exceed the grip of the tyres when cornering than a wide track. Consequently, the grippier the tyres used, the wider the stance can be before the grip is exceeded.
One final force to consider is a torque around the vertical axis experienced when accelerating during a corner. It is common knowledge that braking while cornering on a kart causes massive understeer (the kart attempts to continue in a straight line) while accelerating can improve cornering. This at first seems counterintuitive, since normally when accelerating there is a weight transfer to the rear, which you would expect to try to push the inside rear back onto the track. However, this weight transfer is dwarfed by the torque around this vertical axis caused by the fact that only one wheel rear wheel in on the track, and this wheel is offset from the centre of inertia.
The further this wheel is from the centreline of the kart (and therefor the centre of inertia), the greater the turning moment, and the more likely the kart is the overcome the grip of the tyre on the track . This causes the back to break away - oversteer when accelerating and understeer when braking.
This is usually explained by insufficient lifting on the inside wheel, causing the initial understeer. As the car starts to turn, weight transfer through centripetally caused torque on the chassis lifts the inside rear. Unfortunately, you now have so much steering lock on trying to initiate the turn, that once the inside rear lifts, the fronts are turned so far that massive oversteer usually results.
This can often be mistaken for a lack of rear end grip, since the final sensation is one of oversteer, but it in fact almost the opposite, since it's too much grip on the inside rear which is the main culprit.
We can use the steering geometry to cure this problem. As we showed above, increasing caster causes the inside front to move down further, thereby increasing inside rear lift. Also, moving the front wheels out on their stub axle (increasing the scrub radius) gives a greater effect, with the same result. Also, increasing the Ackerman effect can have an influence on this - making the inside wheel turn further and therefor move further down.
This is where on turning the wheel, the kart immediately and rapidly changes direction, the rear end breaks away, which results in a spin, or rear end slide.
This is down to insufficient rear end grip - as the turn is started, the inside wheel lifts, but the outside rear is unable to cope with the extra cornering forces involved, and breaks away. So we either have approached the corner too fast - and hence corner forces have overcome the grip of the tyre, or the tyre isn't producing the required grip level.
If the inside wheel is lifting a long way, this can causes grip problem since the tyre is at a larger angle to the track. Kart tyres do not react well to large angles of attack (unlike road or car race tyres which are able to distort to a greater extent because of a lower profile ratio), and this reduces grip. Reducing caster may reduce initial lift, but may also detrimentally affect initial turn in. Since the centripetal force acting on the kart overrides the caster settings while corning, there is probably a problem with the chassis more than the front geometry. It is flexing too far and allowing the rear too far off the track. Moving the rear hubs outwards can improve this situation, since a wider stance makes it harder for the centripetal forces to lift the inside rear, thereby decreasing the amount it will lift in any given corner.
If the back breaks away under braking or acceleration, then its possible that our rears are too far apart, which increases the rotational torque under changes of speed. Since this is most noticeable in wet weather, its is more fully described in the next section.
It could be that our tyre is running at the wrong pressure - and is therefor not at the right temperature to produce the required grip level. Tyre pressures are an arcane science that also won't be explored here, so the best option is to try different pressures during testing, once initial handling has been sorted out.
This is where things get (even more) complicated. The ultimate aim is the same, but because of various changed factors, there are some alterations to make
In the wet, we are cornering much more slowly, and we cannot accelerate as fast, or brake as heavily because of the lower grip levels available.
Lower corner speed means we do not get the same level of centripetal force during cornering, so the inside rear may not lift correctly. Moving the front wheels outwards emphasises the twisting effect induced by caster, improving initial turn in. In effect the front geometry has more of an effect throughout the corner in wet weather than in dry, where it is overshadowed by the centripetal forces.
Because we still have the same amount of power available, in low grip conditions the rotational torque caused on power application can exceed the grip levels more easily, causing rapid spin out. Moving the rear wheels in reduces this torque, so more power can be applied without breaking rear end grip. Some people refer to moving the rears inwards as increasing grip. This is not strictly true - the grip level remains the same, but the power can be applied more efficiently along the chassis, giving forward acceleration, rather than as a rotation torque which can cause spin out.
You are sure to encounter many other problems with handling, but there isn't the space to go into them all here. However, hopefully the information presented should now be enough to make an educated guess as to the causes of any problems.
You will have guessed by now that we have described an optimal set of parameters for the kart geometry and wheel setup. Of course this never happens - tyre grip level change according to the circuit, weather, and the whim of the manufacturer, some circuits are mostly large radius corners while other are very twisty. What we need is to get to a setup that sits in the mid range of acceptable parameters, and adjust along this range for a particular circuit. For example, on a low grip circuit (or in cold weather) we may want to increase the caster to improve initial turn in. In very cold weather we may want to increase toe in, forcing the tyres to scrub, and therefor warm up faster to the required grip level. However, this can cause other handling problems, which may need to be over come.
The most important thing is to ensure that the inner rear contributes little or no grip in order to improve handling and reduce scrub and hence power loss. This is so important to keeping up speed through the corner, and makes handling so much more manageable.
However, we can also see from the explanations above that changing some settings can affect more that one area. For example, changing the distance between the rear wheels affects both weight transfer across the kart, and the rotation torque caused by accelerating or braking while cornering. Setting the kart up, even with of good knowledge of why a particular change works, still requires a certain amount of track time, although hopefully with the additional knowledge presented here, this track time can be greatly reduced.
Reprinted with the permission of Karting Magazine.
Thanks to James Hughes for supplying the article!
Front End Geometry - Terms explained http://www.muller.net/karting/docs/technical/frontgeo.html
Understanding & Adjusting Front-end Geometry. John Learmonth. J.L. Racing Products http://www.muller.net/karting/docs/technical/frontend.doc
The Physics of Racing Brian Beckman http://www.getfaster.com/gftechtips.html
Thanks also to the following people who helped (usually via the UK Karting website, www.karting.co.uk)
Neil Dodson, Brian Kavanagh, Brian Pollard, Martin Capenhurst, Peter Holroyd
Copyright © 2000 by James Hughes.
In another article contributed by Century Performance Products, the importance of scaling the kart and achieving proper weight distribution are discussed.
Scaling the kart is perhaps the most important thing a driver
or team can do to ensure proper handling of their machine. When
the kart is scaled properly, the ideal weight distribution is
achieved. As a result, the kart will have the potential to perform
at its optimum level. If the kart is not scaled properly, the
opposite will be true. The machine will never perform at its optimum
level, nor will it respond positively to chassis adjustments.
Some common problems of an improperly scaled kart include under-steer,
excessive or insufficient load on any one tire, chassis binding,
and lack of side bite in cornering among many other problems.
An improper weight distribution can also lead to incorrect diagnosis
of handling problems at the track. For most karts, the following
weight distribution is recommended:
43% Front Weight
57% Rear Weight
50%/50% Left/Right Weight
These are just recommended starting points. Weight can be moved around at the track to fine tune the handling characteristics of the chassis. Moving weight to the front of the kart will provide more front-end grip. If weight is moved to the rear of the kart, the effect will be more rear-end grip. Weight can also be moved vertically up or down. Moving the weight upwards will provide more grip wherever the weight is located. For example, if weight is placed high on the seat, we could expect more grip in the rear of the kart. If weight is placed lower on the seat, we would expect the kart to lose rear-end grip.
The seat placement is the single most important weight adjustment on the kart and is done before the scaling process. Proper seat placement may result in almost perfect weight distribution before the weight is added to the kart. Each manufacturer has different instructions for doing this, and it is wise to follow those instructions as carefully as possible. If you have bought a used kart and do not have instructions, call your local dealer and they should be cooperative.
The following steps are very important to the scaling accuracy of your kart.
Ensure that the floor is level. To compensate for a floor that is not level, place shims under the appropriate corner scales.
Set castor and camber that is consistent on both sides of the kart.
Set spindle heights consistently on both sides of the kart.
Set toe (always remember to set toe after setting caster and camber) and center the steering wheel. If the wheels are not centered during weighing, the geometry of the kart will cause each corner to be loaded incorrectly. As a result, the readings on your scales will be false
Check the tyres to make sure they are at a race setting.
Add weight in the appropriate place to approximate a typical race fuel load. Remember that fuel weights are dynamic. They will change during the race.
Have the driver sit in his/her normal driving position before the scale reading is taken. Try to avoid unnecessary movements of the head or arms, as these actions will result in a false reading.
As well, the driver should wear full race gear during weighing to have the highest level of accuracy possible.
Zero all scales and take the reading.
Adjusting Kart Weight
If the weight distribution is off after the scaling process is complete, adding weight can aid in perfecting the distribution. As is the case with many drivers, weight usually has to be added anyway. Many manufacturers give detailed instructions where to place weight in order to improve the distribution. However, a good rule is to center the weight somewhere on the seat, given that the mass of the kart needs to be centered as best as possible. Adding weight to the seat aids in this. While adding weight to the seat is the best method, filling the frame with lead shot is not. The lead will shift during cornering, cause handling problems, and will deaden the chassis responsiveness.
If the driver does not need to add weight, they are probably at the weight limit of their class. Drivers in this case are always reluctant to add weight just to improve the weight distribution. However, there is evidence that adding the weight to perfect the weight distribution is more beneficial than leaving the ten pounds off. It is ultimately up to the driver to test both methods, then choose the quickest alternative.
After the correct distribution is achieved, the driver should make one more observation. The front wheels should weigh within five pounds of each other. The same situation applies to the rear wheels too. If this is not the case, re-check the factors effecting weight distribution given above and re-scale the kart. If the problem still exists, you may have to readjust the seat again and start the process all over again. Only then will the proper distribution be achieved.
Article courtesy Century Performance Products
This article was kindly contributed to our site by John Learmonth, J.L. Racing Products.
Does this seem familiar to you . . . you reach the turn-in point of the corner, turn the wheel, and the kart seems to abruptly go into oversteer. It then misses the apex and slides wide into the middle of the corner, scrubbing speed and bogging down through the corner exit? If it is, then the problem may have little or nothing to do with the rear end.
Poor corner turn-in is a very common handling problem. If the kart understeers at the instant you turn the wheel, the front tyres are momentarily sliding. Often they suddenly regain grip, causing a violent change of direction, which upsets the rear end giving the rear tyres little chance to produce grip, so the kart slides into the corner in an oversteering condition.
As this initial understeer may only last a fraction of a second it's often not noticed, and can lead the driver to think the problem is in the rear end of the kart. This condition (turn-in understeer/oversteer) is usually caused by not having enough scrub radius (front track width) and / or caster angle in the front end. This can easily be made much worse by incorrect toe and camber settings.
If the kart just understeers at corner turn-in, the problem may still be not enough scrub radius and / or caster, but may also be that you are still braking when turning-in. Too much rear weight bias can also cause both turn-in understeer and understeer/oversteer.
For effective chassis tuning, certain basic things have to be at least close to correct to begin with, or you are likely to only be getting seat time. When trying to diagnose handling problems, it is very important to make sure the problem is not the tyres or the toe and camber alignment.
Tyres are the point of contact between the kart and the track, and are the single most important part of the chassis, as this is where the grip happens. The best chassis in the world is only able to grip and handle to the capacity of the tyres. If the tyres are old and hard, any chassis adjustments you make will only result in different kinds of bad handling (with poor grip).
So tyres must be in good condition (the newer the better, and usually no more than 4 meetings old for YEQ's). They should also be at least close to the correct pressures, (tyre pressures should be set for maximum consistent grip, and only adjusted to fine tune the handling, not to make large changes). I personally prefer about 20 to 23 psi (1.4 to 1.6 bar with factor 14.5) in YEQ's, but this will vary with kart make / model, track conditions, total kart weight etc. As a general rule you should use more pressure in hard tyres and less in soft ones (YEQ tyres are a fairly hard compound).
Even if you manage to find a half way reasonable set-up with old (hard) tyres, the settings are likely to be useless as soon as you put new tyres on the kart, resulting in the waste of the new rubber and uncompetitive times.
Toe and camber, at least as the basic starting point should always be set to zero settings (ie. the front wheels parallel to each other both horizontally and vertically). Zero toe and camber may not always be the absolutely perfect settings, but are going to be very close.
A zero toe setting will lessen tyre scrub and friction on the straights, which will lessen the rolling resistance of the kart. Rolling resistance lowers the acceleration and top speed the kart can produce, resulting in wasted engine power (which you've probably paid your engine builder lots of money for).
Zero (or very close to zero) camber settings will help to keep the full width of the front tyre's tread in contact with the track surface when cornering, particularly at mid corner and corner exit.
As it's very common to see full sized racing cars using obvious negative camber settings, some karters conclude that if this is OK for F1 and Indycar teams, it must be the right thing to do. Unfortunately for those karters thinking this way, the tyres used in most other forms of motorsport are radically different in their construction being radials, while kart tyres use cross-ply (bias-belted) (diagonal) construction. Radial tyres have much more flexible sidewalls than cross-plies, and because of this can work well at larger camber settings. The stiffer sidewall of a cross-ply tyre means it has to be kept very close to vertical to work correctly.
Inaccurate camber incorrectly loads the tyre and lessens the size of the contact patch. As a result this smaller tyre 'footprint' will have a tendency to overheat. Especially in hot conditions, this contributes strongly to premature tyre wear and inconsistent handling (ie. the kart will handle differently as the race progresses, probably tending to increased understeer).
At most race meetings you will see plenty of front tyres with substantial wear on the inside edge, yet virtually unworn on the outside. You won't see many, if any tyres looking like this at the front of the grid for the final.
It's really very simple, if the rubber's not in firm contact with the track, it's not providing grip, and the rubber which is contacting the track is being asked to do the work of the entire tread width, which it wasn't designed to do. Poor camber settings can have a similar (but not identical) effect as fitting undersized (narrow) tyres. I'm sure nobody would deliberately put narrow tyres on their kart!
In racing, it takes only a tiny deficit to lose large amounts of track distance. For instance, if you're only losing 1% to the kart in front due to poor alignment (or any other reason), then in ten laps on a 700-metre track you will lose 70 metres.
Looking at this another way; on the same track, assuming a 'hot lap' time of 30 seconds, then a 1% deficit is equal to losing 3 tenths of a second per lap. An expensive engine blueprint might gain you 3 tenths. So why waste this costly and valuable advantage with poor alignment?
Toe and camber are among the most important settings on the kart, but they are also the two settings most likely to significantly alter when the driver's weight is placed in the seat. This shouldn't be surprising, since driver weight can easily be over half the on-track kart weight.
If a kart is aligned to zero toe and camber without the driver seated, it is certain to have some unpredictable amount of negative camber with the driver. Most karts (but not all) will gain some unpredictable amount of toe-in, up to three millimetres is quite possible (this is equal to about 8 to 10 mm's on a full size car tyre). Toe-in can easily contribute to poor turn-in as it makes the kart more resistant to change of direction and lessens turn-in weight transfer (see below).
As the kart will always be raced with the driver in it, it is strongly recommended both the toe and camber be adjusted to zero settings with the driver seated in the kart.
This is by far the best starting point for setting camber, and the exact setting can be fine tuned using tyre wear as a guide. If you're lucky enough to have a tyre temperature gauge, adjust from tyre temperatures across the tread. A kart set up to zero toe and camber with the driver will have some positive camber and probably some toe-out without the driver.
Worn steering components can cause substantial increased toe and camber change with driver weight, and can also contribute to unstable alignment settings in transitional stages of the corner. It doesn't take very much wear in the tie-rod ends, king-pin bearings or wheel bearings to affect alignment.
To check for worn tie rod ends, attempt to move the rod end up and down vertically. Any up and down movement indicates wear is present (some cheaper rod ends have some movement even when new).
Scrub radius (also called 'kingpin offset') and caster angle work together to produce a diagonal mechanical weight transfer from the inside rear tyre and the outside front tyre, to the outside rear tyre and the inside front tyre. This weight transfer causes the inside rear tyre to be physically lifted from the track surface at turn in. If this weight transfer is not great enough, the combined grip of the rear tyres can simply push the front wheels straight ahead.
This mechanical weight transfer means the inside front tyre is much more heavily loaded than the outside front tyre at turn-in. As a result, the inside front tyre provides most of the front-end grip at turn-in. Once into the middle part of the corner most of the kart weight is transferred to the outside tyres (due to cornering force). The mechanical weight transfer then becomes far less important (and can even be counter productive) and is largely superseded by weight transfer due to cornering forces (lateral 'g' forces, causing frame flex).
A wider rear track width will also make lifting the inside rear wheel off the track more difficult. As it's often necessary to use a wide rear track, this makes it all the more important to have the front-end settings properly sorted out.
Many people will advise you to add toe-out if the kart won't turn-in to the corner properly. This may help turn-in, but introduces it's own problems. I would recommend instead, increasing the scrub radius, and / or the caster angle. The only conditions in which I would recommend using toe settings other than zero would be in the rain, or if all other means of improving the turn in have failed (in which case some extra toe out may be an acceptable compromise).
Increasing either caster angle or scrub radius will increase the inside rear wheel lift at corner turn-in, which is really what you are after (if the kart is turning in badly). Increased caster may require using more positive camber to keep the tread flat on the track during cornering. Be aware that increased scrub radius and / or caster can contribute to front tyre overheating in some conditions.
A very flexible chassis may need more scrub radius and / or caster than a more rigid chassis, as much of the mechanical lift can be taken up with excessive frame flex, especially if the kart is a bit of an old floppy noodle!
Too little weight over the front wheels (and too much over the rear wheels) could also be the problem. If you suspect this, try moving the seat forward. Because moving the seat is such a pain and can result in lots of holes in your nice seat, many people don't experiment with moving it. When moving the seat, don't move it more than 2 cm's at a time. Trying to tune a chassis with bad weight distribution is often impossible.
Incorrect driving style can easily cause poor turn-in as well. When turning into a corner, many racers (in an effort to be smooth) turn the steering wheel too gently. This causes an indecisive lifting of the inside rear wheel, when what you want is for the wheel to lift AT the point of turn-in.
Poor turn-in tends to get worse during the course of a race. When the tyres are sliding they are subject to more friction which increases the amount of heat build up in the tread. This sets up a downward spiral of grip loss and exaggerated tyre wear.
One of the keys to a good handling chassis, is balanced grip front to rear. This balance can be greatly enhanced if the tyre footprints can be kept as large and consistent as possible, for as much of the corner as possible.
Many racers will try to balance the handling by reducing the grip at the end of the kart they perceive has too much. A better approach is to increase the grip at the end of the kart that has less grip. Grip is only too great if the chassis cannot use it, usually manifested by chassis hop. If this happens, do whatever is necessary to stiffen the chassis and / or lower the centre of gravity until the hop goes away. Only if the hop can't be eliminated, or the other end of the kart just can't be adjusted to give equal grip front to rear, should you consider deliberately reducing grip at either end of the kart. Always go for the more grip option if possible.
Good turn-in is usually not that difficult to tune into the chassis, if you know what to adjust. Be careful not to go too far though, as too much turn-in can make the kart twitchy and difficult to drive smoothly. Too much scrub radius and caster makes the kart overly sensitive to steering input, and can result in a chassis that needs constant steering corrections. Remember that every time the steering wheel is turned it causes substantial diagonal weight transfer between the front and rear wheels. If you're sawing away at the wheel, the tyres are being rapidly loaded and unloaded, which really upsets the stability of the chassis.
It's not unusual to spend a day testing and still be unsure if any improvement has been made. Lap times may well be better at the end of a test day, but this is often due to the driver improving with practice. It's very possible to be faster at the end of testing, with a worse set-up! This can be especially true if the number of practice days are limited.
All else being equal, a well set-up 'bad' chassis will always handle better than a poorly set-up 'good' chassis, and there is usually more gain to be had from a good chassis set-up than a good engine set-up. Keep in mind that a handling problem may not occur during practice, but can raise it's ugly head in racing. This is usually the result of the driver being able to find just that bit extra on race day!
Front-end settings have a huge effect on the overall handling of the kart. If your kart turns-in to a corner the way it should, then the rest of the corner will be easier and faster to negotiate as you are not having to catch up with the effects of poor turn-in. In addition, the rest of the chassis is likely to be easier and less confusing to tune if the front end is functioning properly.
Finally, many handling problems that may at first seem similar can easily stem from different causes. Any adjustment you make to your kart is only correct if it lowers your lap times, assuming that the basic alignment is at least close to accurate.
John Learmonth. J.L. Racing Products.
© Copyright: John Learmonth. 1999. All rights reserved.
(C) Copyright 1998-2000 Resonate Brendel Racing. Do not duplicate or redistribute. firstname.lastname@example.org
The karter invariably depends on his engine to put in good lap times. He spends hours and hours and many dollars preparing the engine so that it will deliver the maximum amount of horsepower. By the time he bolts it onto his nice shiny chassis, every detail of that engine is perfect. But what about the nice shiny chassis? Sure it has been washed and polished and at the track the wheels and tyres will be pumped up and moved in and out to suit the track. But is it perfect like the engine.
It is all too often that the karter who wants to improve the handling of his kart will go out and purchase the latest in wheels, tyres and other optional equipment and then after paying out a handsome sum of money, find that the thing in fact handles worse. The fact is that the first stage in achieving a good handling kart that will deliver the greatest percentage of power available to the ground is to go right back to basics. It is this area with which we will now attempt to deal with. What is written in this article will, with these basic requirements to good handling, apply equally to all karts regardless to make or class in which the kart is to be run.
Regardless as to whether you have just purchased a new chassis or you are trying to improve your old chassis, before going out on the track, you should strip the kart of it's seat and wheels and get the tape measure out.
The first dimension to check is the parallel between the front stub axles at dead a head steering and the rear axle. to do this, it is necessary to measure each side of the kart from the back of the rear axle to both the bottom and the top of the king pin bolt. Both sides should be even. If this is out it is then necessary to stretch the side of the chassis that is shorter. this can be achieved by jacking between the bearing hanger at the rear and the king pin post. It is usually necessary to jack the chassis a little further than the difference, as it will spring back. However, carry this operation out with care, otherwise you could possibly end up with the longest kart in the country. it is vital that the kart be the same length on both sides. If it is not, then it will not matter how much care you take with the front end alignment, as the kart will always want to steer better in one direction and will wear tyres.
It is important when equalising the chassis length that both sides are equal for both dimensions A and B as shown in the sketch in the book. To ensure this, it may be necessary to heat the front axle and twist the top of the king pin with the greatest lean back to a more upright position to match the other side.
Once the chassis is the same length on both sides, it is time to align the steering. Before proceeding to carry out this task, take your front wheels to a machine shop and have your rims machined so that the inner and outer diameters on both wheels are all exactly the same size. Once this has been done it is possible to use a straight edge to check the front wheel alignment.
The first thing to do when aligning the steering, is to centralise the steering. This is necessary so as to have the kart steering evenly in both directions, and tracking well in a straight line.
The steering shaft in most modern karts is offset to the brake side of the kart. This, with the wheels fitted, it is necessary to find the difference from the centre of the steering shaft at the steering yoke to the inside of each front wheel level with the steering arm on the king pin. This amount of offset should then be built into the tie rods when the steering yoke is at bottom dead centre. Once this has been achieved, the toe in, toe out desired can be adjusted by equally lengthening or shortening both tie rods. Before setting the toe in, it is wise to place your straight edge across your machined wheels and check that both are set on the same amount of camber. It may be necessary to heat and bend one or both stub axles to achieve this. Once the camber is equal and at the desired angle, proceed with the front wheel alignment.
So far we have trued the kart so that it is now the same length on both sides and the front end alignment is correct. The next stage is to check if the kart is twisted. To achieve this place the wheels and tyres back on the kart and with the tyres correctly inflated, place the kart on a flat floor. Then using a set of scales lift each front wheel by hooking the scale hook around the king pin. Then spin the wheel you have lifted and slowly lower the kart until the wheel touches the floor and note the amount of lift needed at the point of contact. Each side of the kart should require the same amount of lift. If this is not the case, the chassis is twisted. To rectify the problem, place the rear wheel on the same side as the kart is light at the front and with someone standing on the opposite rear wheel twist the light front side of the kart down. This should be repeated until the both front wheels carry the same amount of weight. Once the front is even the back will also be even.
All that now remains to be checked is if the rear axle is located central to the chassis. This is best done by firstly centering off the chassis tubes and then checking the axle diagonally with the tops of the king pins. This diagonal check is important and will tell you if the chassis runs out of line in the centre. If the diagonal check shows up a fault in the chassis, it is best to leave the chassis alone and simply offset the axle slightly to overcome the problem. Once this is done, the ends of the rear axle can be used accurately for setting the position of the rear hubs.
All of the preceding information is useless unless you work accurately. It is also useless unless you follow the order of work as we have laid out. Thus to get the maximum results for your labours you:
* Check and correct the alignment of the rear axle with the alignment of the centre line of the front stub axles.
* Centralise the steering.
* Set the front camber equal on both sides.
* Adjust the front wheel alignment.
* Check the chassis for twist and correct and finally you correctly locate the rear axle.
Once all this is completed, you have a chassis that will give the best results it is capable of, regardless to the class in which you are going to run.
It is only after all this has been done, that you should even start to think about setting up for a particular class.
A few more small jobs to carry out before setting up. First pull every bearing and moving part off your chassis and thoroughly clean and oil it where necessary. Make sure that each moving part is in good condition and if it is doubtful, replace it. Make sure that all king pin bearings are a good fit and not sloppy. The same applies to steering shaft bearings, tie rod ends, wheel bearings and axle bearings. Check the kart for any cracks and repair where necessary. Then put the whole thing back together using new nyloc nuts throughout. When you have done all this take the kart out onto the track and you will be delighted just how much difference a correctly assembled, true and correct chassis is. Even without changing your original wheel and set arrangements.
Having gone this far, get some practice in and collect some lap times, before going on and learning how to set up for a particular class.
We have covered the act of putting your chassis right. The whole aim of this was to let you put your shop in order, before going out on the track and wasting a lot of time trying out different set ups on a chassis that had no hope of handling well. So if you have not done this, go back to the beginning and get things right before continuing.
For further information on this subject we recommend that you purchase the book "Racing The Go-Kart" (click for details)
Copyright AKR Publishing Pty Ltd.
(This should be read after the Chassis setup page)
The first basic thing to realise about SL tyres is that they give less grip. As a result the wide stance of a kart that runs on open tyres will not necessarily work. To start setting up on SL tyres, set the kart up fairly wide and then after several laps bring the rear track in 1.0cm (0.5cm per side). Keep on repeating this procedure until the back of the kart stops sliding and starts to lift when cornering. Each time out on the track it is necessary to travel about four laps before testing for grip as the SL tyres require considerable warming up.
If, with the rear wheels set in as far as they will go, the rear still wants to slide out from under you (oversteer), then it is almost certain that the seating position is too low. to correct this, simply lift the rear of the seat two centimetres and redrill and locate in this new position. Repeat if necessary. Sure you will end with a few additional holes in your best seat, but these can be filled after the correct seat position is found.
An additional cause of oversteer can be that the seat is actually located too far back in the chassis. If after all else is tried it may be necessary to relocate the seat in a more forward position. Understeer is the term used to describe the situation that occurs when you turn into a corner and the kart tends to travel straight ahead, ignoring your commands through the steering wheel.
Now understeer that occurs only when you turn in one direction and is replaced by oversteer when you turn in the other direction, is usually the result of a twisted chassis. If this is the case, go back to the beginning of setting up your chassis and start again. Once you have the kart handling consistently in both directions and you find that you have the rear end of the kart handling well, but the front is pushing or understeering, the first step is to widen the front track.
This can usually be achieved by spacing the front wheels out on the stub axles. However, if there is no room to adjust the front wheels out, it will be necessary, particularly on some earlier karts, to either further recess the outside wheel bearings or to modify the stub axles to provide for further widening of the front track.
The ideal toe in, toe out to run is zero. Or in other words, the front wheels should be parallel when the kart is travelling in a straight line. this set up provides the minimum amount of drag and thus the best straight line performance. However, some added toe out can reduce understeer and if the kart is suffering excessive understeer, a little toe out will improve lap times.
A point of interest here is that the most stable set up is the one where the kart tends to understeer just a little into corners, while under brakes and then turns to neutral steering when the power is evened out and the kart is driven out of the corner. By this we do not mean total understeer, just enough to let you think that understeer may be present. A kart set up this way is very stable into corners and is easy to drive. A kart that oversteers is always difficult to drive, requires too much concentration and is a handful in an emergency.
When setting your kart up for handling it is a good idea to find the longest corner and set the kart up to bounce a little on this corner, this bounce should not be enough to put you off line. It should be kept within easily controllable limits. If the kart is set up this way, then it will be giving maximum grip through the longest corner and this is where the greatest saving in lap times is available.
The first thing that should by said about tyre pressures, is that they are very personal things. What is the correct pressures, will vary from kart to kart and driver to driver.
However, as a guide to a good starting point and assuming that you are running 4.5 fronts and 7.1 rears, the following will help. A good starting point for your tyre pressures are 13psi-14psi at the front and 15psi-17psi at the rear. However, the only way to correctly set your tyre pressures is with the help of a tyre temperature gauge or pyrometer. By increasing or decreasing the pressures until both front tyres give close to even temperatures across the tyre tread will provide both the best tyre performance and the longest tyre life. In other words, the temperature on the outside edge, the middle and the inside edge of the tread on any given tyre should be as close to equal as you can get it. It is usually impossible to get dead even temperature readings, but the closer the better. The same applies to the rear tyres. Do not try to obtain the same temperature readings on all tyres, just even readings across each tyre. The outside tyres on any given track will always register higher readings, as they do most of the work. However, the difference between outside tyres and inside tyres should be kept as low as possible. the closer this difference, the better all four wheels are sharing the work load.
As to the running in of new tyres, we are not too sure how much this improves tyre life. However, it does not hurt the tyres and can be stated as a wise practice.
Finally it must be said, that all of this is a waste of time if your chassis preparation does not include making sure that the rear axle, the brake disc and the front wheels all run free. Also important is the wheel balance.
For further information on this subject we recommend that you purchase the book "Racing The Go-Kart" (click for details)
Copyright AKR Publishing Pty Ltd.
:- should be fitted lower down if the driver is a bigger build and higher up for small or light drivers. The lighter or smaller driver may achieve better rear wheel grip if the lead is higher on the seat, the heavier driver should try securing the weight to the sides of the seat to keep the kart balanced. Also use the weights to balance the kart alround, more towards the front to achieve better front end grip, and back for better rear end grip. In the wet have the weight as high as possible to achieve better grip all round.
Rear track :- is very important, moving the rear wheels in will cause the kart to grip the track a bit harder don't go in to far otherwise this will cause the kart to handle badly, moving the wheels out will give a better ride it will also stop the rear of the kart from bouncing around the corners! when moving wheels in or out, only go about 10mm each side at a time and test it!
Front track :- if your kart is not turning into the corner properly try widening the front by 5-10mm each side this should reduce the front end slide (understeer) and cause the steering to be more direct. Narrowing the front track will make the steering less responsive, the front wheel alignment should be set to 0mm - 2mm toe out, check with your karts supplier for the recommended settings.
|The Kart does not want to turn into Corners.||
too much rear end grip (Balance is out)
-Move front track out,
-Move rear track out,
-Add rear seat stays,
-add rear torsion bar,
-move set forward,
-remove front torsion bar,
-increase rear tire pressures
-check toe out (start with 1mm out)
|The kart turns in too quickly.||
too much front end grip (balance is out)
-Move front track in,
-move front track in
-remove rear seat stays
-remove rear torsion bar
-move seat back
-add front torsion bar
-lower rear tire pressures
-check toe out
|Kart does not grip in corners/ other karts are faster in corners.||
Too little grip
speed to fast
too much slide, loss of time in corner
-move front track out
-move rear track in
-remove seat stays
-remove torsion bar(s)
-adjust tire pressures
Kart bogs down in Corners.
(Kart becomes unstable, hops and loss of time in corner)
Too much grip in setup
(when rectified it can provide better acceleration and in turn better top end speed)
-move front and rear track out
-add seat stays
-add torsion bar(s)
-adjust tire pressures
|Other Karts are much faster in the straights||Gear ratio too short/ Wrong driving line in previous corner||
-Take off a few teeth
-evaluate driving line
-shorten pipe length (flex flange)
-check carby settings
|Kart lacks acceleration over short distances||Gear ratio too tall/ Wrong driving line in previous corner||
-add a few teeth
-evaluate driving line in corners
-lengthen pipe length
-check carby settings
-Check kart setup
Remember tires are best when new and could play a key role in getting your correct setup.