There are 2 blocks of mass in a car: sprung and unsprung. The first characterizes the totality of parts located above the suspension, and the second is the wheels and all the parts adjacent to them. Both parameters play an important role in the dynamics of the car, but usually the emphasis is on the sprung mass, which is many times greater than the unsprung mass. This approach is very wrong, because the wheel part has a strong influence on the operation of the car.
Unsprung weight: what is it?
In a more detailed definition, this term refers to the combined mass of the following parts of the car:
- wheels;
- tire;
- brake discs;
- wheel hubs;
- drive shafts;
- wheel bearings;
- shock absorbers;
- suspension arms;
- springs;
- springs.
Torsion shafts, although adjacent to the wheels, but, according to the standard, refer to the sprung mass. The anti-roll bar is in an intermediate position.
Literally, unsprung mass means everything that is not supported by springs - that is, damping elements. The latter are also included in this block.
In other words, the unsprung mass is the carrying part of the car. A similar term in English is a more understandable phrase - unsprung mass. Translated, it means "non-spring mass", which explains the meaning of the term very clearly.
Unsprung to sprung mass ratio
Normally unsprung weight is 15 times less than sprung weight to compensate for wheel shocks. The higher this ratio, the smoother and more stable the move will be.
This property reflects the laws of physics, where a relatively light body can communicate with a heavier one the greater momentum, the smaller the difference in their masses. Therefore, in the absence of sufficient compensation from the sprung part, the car will lose traction. This drawback will manifest itself especially strongly when driving through pits and potholes, while high-amplitude vibrations are transmitted to the passenger compartment.
Thus, the less unsprung weight compared to sprung weight,the more stable the car behaves on the road.
Unsprung weight: what does it affect?
In order to correctly assess the value of the mass of the supporting structure of the car, it must be remembered that, first of all, it is due to it that the movement is carried out. In this case, the unsprung elements are not a monolithic body, but parts dynamically connected to each other, which, during operation, exert a mechanical effect on the sprung part. As a result, the driving characteristics of the car change.
The strength of these impacts is certainly related to the unsprung mass, which affects:
- smoothness;
- steadiness and stability.
In addition, there are two parameters that depend directly on the mass of the wheels: dynamics and gas mileage. Such a connection is no longer due to the impulse interaction of the sprung and unsprung parts, but to a change in the rotation speed. The more the wheel weighs, the harder it is to spin, slow down or turn in the other direction, which increases the energy cost and lengthens the time between the action of the driver sitting behind the wheel and the result.
Methods of regulation
There are 2 theoretical ways to increase the ratio between sprung and unsprung mass:
- weighting over the suspension part of the car;
- lightening unsprung components.
The first method is inexpedient to apply in practice, since an increase in the sprung mass greatly worsens the dynamics (acceleration, braking time, etc.). Secondthe method, on the contrary, allows you to achieve the desired effect without making the car heavier.
The reduction in unsprung weight is carried out mainly due to the wheels. Modern manufacturing methods such as forging and casting make these parts much lighter. According to experts, the positive effect of reducing the unsprung weight by only 1 kg is equal to lightening the body by 20-30 kg.
Cast and forged wheels
As noted above, the mass of the bearing part of the car is facilitated mainly due to the wheels. In this area, there are 2 technologies to reduce unsprung weight: casting and forging.
The first method involves pouring metal into a wheel mold, followed by turning and drilling holes. The material of manufacture is pure aluminum or its alloy. In comparison with a steel counterpart, a wheel made using this technology is 15-30% lighter. In addition, this method is quite fast.
Forging is a name borrowed from foreign literature for the technology developed in Russia for volumetric hot stamping of wheels. This method is much more difficult and longer than casting, but allows for a greater degree of lightness and strength.
Reducing the unsprung weight is also achieved by reducing the number of suspension parts (beams, axles, universal joints are excluded) and replacing steel construction materials with aluminum.
