In the car, 2 mass blocks are distinguished: sprung and unsprung. The first characterizes the totality of the parts located above the suspension, and the second is made up of wheels and all 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 more than the unsprung mass. This approach is very wrong, since the wheel part has a strong influence on the operation of the car.
Unsprung mass: what is it?
In a more detailed definition, this term refers to the total mass of the following parts of the car:
- wheels
- tires;
- 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, belong to the sprung mass. The anti-roll bar takes an intermediate position.
In a literal sense, unsprung mass means everything that is not supported by springs - that is, shock absorbing elements. The latter are also included among the components of this block.
In other words, unsprung mass is the bearing part of the machine. A similar term in the English language is a more understandable phrase - unsprung mass. Translated, it means "springless mass", which very clearly explains the essence of the term.
The ratio of unsprung and sprung masses
Usually the unsprung mass is 15 times less than the sprung mass, which allows you to compensate for tremors from the wheels. The higher this ratio, the more smooth and steady the stroke will be.
This property reflects the laws of physics, where a relatively light body can give a heavier impulse in a collision, the larger the momentum, the smaller the difference in mass. Therefore, in the absence of sufficient compensation from the sprung part, the car will lose traction. This drawback is especially pronounced when driving through pits and potholes, while high-amplitude vibrations are transmitted to the cabin.
Thus, the smaller the unsprung mass compared to the sprung mass, the more stable the car behaves on the road.
Unsprung mass: 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 dynamically interconnected parts, which during operation have a mechanical effect on the sprung part. As a result, the vehicle’s motion characteristics change.
The strength of these effects, of course, is associated with an unsprung mass, which affects:
- smoothness of movement;
- sustainability 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 pulsed 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 energy costs and lengthens the time between the action of the driver sitting at the wheel and the result.
Regulation Methods
There are 2 theoretical ways to increase the ratio between sprung and unsprung masses:
- weighting of the suspension part of the car;
- relief of unsprung components.
The first method is not practical in practice, since an increase in the sprung mass greatly worsens the dynamics (acceleration, braking time, etc.). The second method, on the contrary, allows you to achieve the desired effect without weighting the car.
The reduction of unsprung mass is carried out mainly due to the wheels. Modern manufacturing methods, such as forging and casting, can greatly facilitate these details. According to experts, the positive effect of reducing unsprung mass of only 1 kg is equal to the relief of the body by 20-30 kg.
Cast and forged wheels
As already noted above, the mass of the bearing part of the car is facilitated mainly by the wheels. In this area, there are 2 technologies for reducing unsprung mass: casting and forging.
The first method involves pouring metal into a wheel form, followed by turning and drilling holes. The manufacturing material is pure aluminum or its alloy. Compared with a steel counterpart, a wheel made using this technology is 15-30% lighter. In addition, this method is quite fast.
Forging is the name borrowed from foreign literature for the technology developed in Russia for volumetric hot stamping of wheels. This method is much more complicated and longer than casting, but allows to achieve a greater degree of lightness and strength.
Reduction of unsprung mass is also achieved by reducing the number of suspension parts (beams, bridges, cardans are excluded) and replacing steel materials of structures with aluminum.