Chassis dynamics during cornering

When trying to understand how a kart reacts through a corner, one needs to appreciate that there are a number of factors that influence how this happens, and inter-alia these include the centre of gravity, weight transfer, caster angle, chassis stiffness, as well as a few other aspects that we won’t delve into.

Centre of gravity

From basic physics we know that the mass of any object can be taken to act through its centre of gravity, or more commonly expressed as CG.The CG is an imaginary point located in 3-D space that if one could suspend the object from, it would be perfectly balanced in all directions.This 3-D space is commonly visualized along 3 orthogonal axes viz. X, Y and Z.If one then applies this to a kart, then the X axis would be in the fore-aft direction, the Y-axis would be vertical i.e. perpendicular to the track surface, and the Z-axis would be across the width of the kart.

For a kart, the CG’s location in the X direction is usually just in front of where the driver sits.  The Y location can be calculated mathematically, but it isn’t necessary to know where it is with any degree of accuracy for this discussion.  Suffice to say that it is slightly higher for taller drivers, and can be altered marginally by adjusting the seat height, sitting more upright, and also adjusting the rear ride height via the locating bolts in the rear bearing carriers.  On the other hand, the Z location of the CG can be influenced to a certain degree by the driver through leaning as the kart goes around a corner.  This, along with the kart speed and radius of the corner will affect the amount of lateral weight transfer to the outermost side of the kart.

Weight transfer

As we see from the above, it’s fairly difficult to move the X and Y locations of the CG without moving the seat via re-drilling of the bolt holes, or else rearranging any ballast weights that might be present on the kart.  From earlier chapters we know that the kart needs to be ‘jacked’ i.e. the inside rear wheel needs to be lifted off the track, to go around a corner as fast as possible.  This is aided to some degree by any weight transfer we can make in the Z-direction from the inner rear wheel to the outer rear wheel.  OK, so apart from the driver leaning we’ve seen that moving the CG might be difficult, but for weight transfer to take place what we are more concerned about is the angle between the CG and the tyre contact patch.  But here’s the good part - we can move the tyre contact patch fairly easily by moving the hubs in or out on the axle.

Moving the rear wheels inboard will increase the angle off the horizontal between the CG and the contact patch and thus increase the jacking effect, whilst moving them outboard will have the opposite effect.  Playing around with these settings, one will finally arrive at the best position to achieve the proper amount of jacking without the kart suffering from hopping as a result of too much lift being present.  Remember that this setting is correct for the track conditions at that particular time and will need to be modified as more rubber is laid onto the track by the other karts or as weather conditions change.

At the rear end we want to get complete load transfer across the kart but this is not the case at the front end, and the reason is fairly simple.  The less load a tyre has on it, the less it is able to grip the surface of the track.  The more evenly balanced the load is on the front tyres, the more grip they have.  To minimize the amount of load transfer at the front end we can only really play with the CG location in relation to the front tyres.  Recalling the earlier remarks about the angle between the tyre contact patch and the CG needing to be increased to maximize weight transfer, we can now use the opposite effect at the front of the kart.  What this means is that the lower the CG and the further the tyres are apart, the more grip we will have and this aids the cornering process.  It’s also one of the reasons why you try to set up the stance of the kart with a ‘wedge shape’ i.e. lower in the front than at the rear.

Caster angle

Looking at the picture on the right you will see that the steering caster angle is angled rearwards off the vertical, and adjusters on the yoke of the kart allow one to alter this inclination angle.  When you turn the steering wheel, the front end steering geometry additionally causes the front tires to go up or down in relation to the chassis.  As the steering wheel is turned clockwise (i.e. you are entering a RH corner), the end of the stub axle on the LHS will rotate forwards on the kingpin bolt, and in doing so the wheel doesn’t remain flat but will want to lift off the track.  At the same time the stub axle on the RHS of the kart will rotate rearwards around its kingpin and will want to press further into the track.  You can visibly notice this phenomenon when you turn the steering wheel as the kart is stationary – the same thing happens on your road car, the only difference being that the springs absorb some of the displacement and it becomes more difficult to see.

A similar thing happens when you turn the steering anti-clockwise, and in summary, the net result is that the front outside tyre will go up, and the front inside tyre will go downwards.  Because the chassis is effectively rigid, this means that all four tyres are no longer all resting on the ground and it’s similar to having a wobbly bar stool with only three legs in contact with the floor.  This all sounds rather dramatic, but it’s essentially one of the major reasons why a kart can negotiate a corner.  Remembering that the kart has no differential at the rear, so the outside wheel needs to travel further than the inner one otherwise one of them will be dragged along.  To help understand what happens, let’s assume the chassis is completely rigid to the extent that it cannot bend in any direction.  That being the case, it pivots around a line joining the inside front and outside rear, causing the inside rear to lift as shown in the picture below.  

Note that although there is an amount of weight transfer taking place at the front end, both front wheels are still firmly in contact with the track, thus providing maximum grip.  The angle of the nosecone to the track one observes here is largely 
due to the jacking effect of the front wheels as the steering is being turned.

Increasing the caster angle will increase the jacking effect the more the steering wheel is turned.  This is of great importance in wet conditions because one is usually going a lot slower through the corner and therefore it is a bit harder to get the inside rear wheel to lift.  Increasing the caster angle also makes turning into the corner a lot easier, and has the additional benefit of adding more grip to the front, so it’s a very useful tool particularly on ‘green’ tracks.

Chassis stiffness

Motor vehicles of today employ either a monocoque or unibody chassis design which integrates the body and chassis together thus forming a composite structure which has better stiffness as well as weight advantage. This differs from the older design of a ladder chassis that you may be familiar with if you’ve watched TV programs such as Overhaulin’, Custom Garage, etc., and for this type of design the bodywork essentially bolts on top of the ladder structure.Although longitudinally fairly stiff, a ladder chassis leaves a bit to be desired in terms of torsional rigidity.A kart chassis is fairly similar to a ladder chassis, but it has a narrow ‘waist’ just behind the front axle and this reduces the torsional rigidity of the chassis even more because it softens up the front end compared to the rear.

To an extent, this waist makes the front and the rear appear to operate as though they were independent of each other even though it is not the case.  Although you have no control on the stiffness of the chassis itself without altering the materials of construction or its shape, you do have a fair amount of control over the stiffness of each end of the chassis.

Chassis stiffness is also sometimes referred to as roll resistance.  If the roll resistance is reduced at any given end, the chassis will be able to twist easier, and vice-versa.  For a kart, the roll resistance is governed by how much ‘stuff’ there is between the CG and the tyre contact patch at the end under consideration.  The roll resistance at the rear end of the kart is used to unload the inside rear tyre and transfer the load to the outer tyre.  So as an example, adding more seat supports, tightening a rear centre bearing, adding a rear torsion bar, tightening up the rear bumper, changing to a stiffer axle, etc., will all aid in stiffening up the rear end of the kart and assist in lifting the inside rear wheel.

The roll resistance at the front end of the chassis is a lot lower.  To increase the roll resistance one can add a torsion bar.  Widening the front track will create more of a jacking effect when the wheels are turned that results in more front end grip and a quicker turn in to the corner.  Similarly, narrowing the front track will have the opposite effect.

Summary

Examining the foregoing, logic tells you that whilst you are travelling along the ‘curve’ the best thing possible is to have the inside rear wheel off the track to prevent it being scrubbed along.  That way you get all the available power being transmitted to the outside rear wheel and get the best speed through the corner – remember it’s all these milliseconds that add up over the race distance to put you a second or two out in front.  This is easier said than done and you would do yourself a favour to check out the thousands of photos posted on the Club’s website only to find that it doesn’t happen as often as you might think.  It’s all good and well to stiffen things up to get better cornering but remember this - there are points of diminishing returns where the chassis will get too firm to perform properly.  As with all good things in life, it’s really all a question of balance.

Emile McGregor - MSA Technical Consultant