Even though many parameters determine the grip on each tire, everything is finally led to the vertical force acting on each wheel. The greater such downward force is and the more the grip between the tire and the asphalt.
Centre of gravity
The centre of gravity is the point of a system (chassis plus engine plus driver) where one can consider all the weight concentrated. Once we define this in a kart, and generally such point is positioned in the stomach of the driver, we can calculate all the forces acting on the kart concentrated in the center of gravity. For example the weight of the kart can be concentrated in the centre of gravity as the sum of all vertical forces when the chassis is not moving (weights multiplied by the gravity acceleration). We will call “a” the longitudinal distance between the centre of gravity and the front carriage, and “b” the distance from the centre of gravity and the rear carriage. So we will be able to say, following the laws of the physical equilibrium, that the vertical forces acting on respectively on the rear tires (Fp) and the front tires (Ff) will be equal to:
Fp = Pvehicle * (a + b)/b,
Ff = Pvehicle * (a + b)/a,
where Pvehicle is the total weight of the kart (chassis, engine and driver) and:
Ff + Fp = Pvehicle.
We are making an approximation considering the weight of the kart a force.
Effects of weight distribution variation
So the variation of the position of the seat of the chassis along the longitudinal length of the kart varies the parameters “a” and “b”, which means that also weight distribution of the kart varies between front and rear tires. Generally the longitudinal movement of the seat has a maximum value of around 4-5 cm, but a few centimetres determine great differences in weight distribution. For example the distance between front and rear carriage is generally in karts around 104 cm. With front weight equal to 40% and rear equal to 60% the centre of gravity is 41.6 cm from the rear axle and 61.4 cm from front carriage. If we move the seat 2 cm to the front of the chassis distribution will be 42% on front tires and 58% on rear tires. Such variation of 4% of the weight could appear a small quantity, but can really determine great difference in kart performance. If we move the seat of the chassis towards the front carriage the vertical force on front tires will increase and the force on rear tires will decrease. This will automatically determine an increase of front tire grip and a reduction of rear tire grip. So moving the seat to the front increases oversteer and moving it to the back increases understeer. The variation of the two parameters “a” and “b” will not vary directly shifting the seat. In fact the movement of the seat will determine a movement “c” of the centre of gravity as follows:
c = Pdriver/Pvehicle*x,
where “x” is the movement of the seat.
So it is extremely simple to setup the basic grip on the four wheels just by the right longitudinal positioning of the seat of the chassis. After such positioning the other parameters of the chassis will vary the forces on the four wheels for fine tuning.
Generally weight distribution must be regulated to have 60% of the weight on the rear tires and 40% on front tires. It is also true that every chassis has its own particular regulations for what concerns weight distribution. Ask your chassis builder or shop for the right values of weight distribution between front and rear tires. Such parameter is really too important to be mistaken. In addition to this we must consider that weight distribution should be equal between right and left tires on the front and on the rear. Because of the engine positioned on the right hand side, the seat will be slightly shifted to the left hand side of the chassis. This is of great importance especially when braking before a curve, since the balance of the chassis will be perfect, and the kart more stable in such phase, only if left and right wheels will have the same grip acting on them and the centre of gravity will be in a central position respect to the wheels.
Also the driver, especially in wet track conditions, can move inside the seat and vary weight distribution. For example moving your body forward when entering a corner helps front grip, and moving the body backwards when exiting a corner helps traction. This, also thanks to lower speed of the kart, has great effect on wet track conditions.
Next issue we will proceed consider weight distribution on the four wheels varying centre of gravity height, which will also determine great effects when running along a curve.