Kart weights and rotating masses - TKART - News, tips, tech about karting

WEIGHTS AND ROTATING MASSES IN KARTS

TKART Staff

13 October 2017

A kart consists of several elements of a certain weight. This results in so-called motion inertia, i.e. the force opposing acceleration. It is an important factor and deserves further analysis

WHAT IS IT?

HOW DOES IT WORK?

IN PRACTICE

SIMPLE MASSES

driver; engine; chassis; tank + fuel

ROTATING MASSES

tyres; wheel rims; disc brake; chain; plate

DISC BRAKE

1 di 2

TYRES

Focussing on the specific components of a kart, we can say that inertia is determined by simple masses (such as the driver’s weight, the weight of the engine and the chassis, with tank and fuel), which determine the parameter " m ", and rotating masses (tyres, wheel rims, disc brake, chain, plate, etc.) that determine the instant inertia "I". Regarding the masses "m", the situation is simple, since they are determined by the overall weight of the kart (and engineered) plus the driver. The moment of inertia, on the other hand, does not depend solely on the weight of the rotating objects, but also by the square of the distance of the masses from the centre of rotation (radius r).

WHAT IS IT?

HOW DOES IT WORK?

IN PRACTICE

For example, let’s take a disc that rotates: its moment of inertia is given by the formula:
I = m * R²R is the radius of the disc and m is the mass.
However, if we consider the brake disc, which is actually a plate, the moment of inertia formula becomes:
I = m * (R_e² - R_i²)
where R_e is the outer radius and R_i the inner radius.

As a result, the greater the outer radius of the disc or plate, the greater the moment of inertia I, i.e. the resistance to acceleration.
Clearly, the greater the inner radius (i.e. the thinner the disc plate), the less it will be I.
Added to this is that the dependence of the moment of inertia on the radii, both inner and outer, is square and, therefore, their measurement will have a greater

effect compared to the total weight of the disc.
Let's look at an example to gain a better understanding.
On the one hand, a cast iron disc weighing 1 kg, with a 9 cm outer radius and a 7 cm inner radius. Reasonable values for a traditional rear brake disc.
On the other hand, a disc with a mass weighing about half of the former, i.e. 0.6 kg, perhaps made of Ergal (aluminium alloy), with a 7 cm inner diameter (i.e. with a disc plate of the same width as the former), but with an 11 cm outer diameter. The respective moments of inertia will be:
I_ghisa = 1 * (92² – 7²) = 32 kg m²I_ergal = 0,6 * (11² – 7²) = 43,2 kg m²Despite the disc made of Ergal being almost half the weight, its moment of inertia is considerably higher than the brake disc made of cast iron due to an outer diameter that is only 2 cm greater.

THE HEAVIER THE KART AND THE MORE ROTATING MASSES IT HAS, THE GREATER THE INERTIA OPPOSED TO ACCELERATION AND, THEREFORE, THE GREATER THE POWER REQUIRED FROM THE ENGINE

The brake discs Tony Kart, CRG, IPK, Parolin