In the previous post we talked about tire noise, we have deepened the causes and some solutions to reduce it.
Today we talk about the second mark in the European tire label, in particular how tires affect fuel consumption; this parameter is related with tire rolling resistance.
What is the origin of this resistance? Which parameters affect it? Let’s have a closer look.
Let’s go immediately to the central point: when the tire touches the ground (enters the contact patch), it is deformed by the reaction force of the ground. When it leaves the contact patch not all the energy received is released. This behavior is named as hysteresis of the material.
How can we imagine this behavior? What does this concept, maybe still conceptually empty, means? The secret is the viscoelastic origin of the material. Let’s imagine the tire compound as a dish of spaghetti; these are glued each other in some contact points. When an external force is applied to spaghetti, these tend to stretch, but being glued each other a total stretch is not allowed, so the external energy is stored as elastic one.
If gluing points were perfect, when external force is released all the elastic energy would be released too; but in the reality this in not like that because every spaghetto slides respect others and a part of energy is dissipated as heat. This is the reason why the material is called visco-elastic.
So when this dish of spaghetti touches the ground, part of energy is stored and then released, another part is lost as heat. Due to this effect the pressure distribution on the contact patch is not a symmetrical parabola, but the peak is slightly translated onward respect to the wheel rotation axis. The resultant vertical force generates a moment opposite to the driving torque.
The main parts of the tire where energy dissipation is concentrated are:
– Thread: about 70%;
– Sidewalls: about 15%;
– Beads: about 15%.
In order to give some additional information, compared the global vehicle motion resistance, the rolling one has the following influence:
– 20% on highway;
– 25% on backroad;
– 30% on city road;
Parameters that influence fuel consumption
We can distinguish two categories: intrinsic parameters and the ones that can be controlled by the driver.
In the first group of course we can find the compound “recipe”, that with its ingredients represented by natural rubber, styrene, butadiene, carbon black, silica, sulfur and so on, defines the hysteresis level for the finished product, also function of temperature and frequency of road unevenness.
Another important parameter is the thread thickness and void ratio that is related to tire grooves dimensions: the bigger are the channels, the greater are tread blocks deformations, so the more energy is dissipated as heat.
The tire diameter is another influent parameter because the bigger is it, the smaller is the tire bending on the contact patch at leading edge, having as consequence less deformation.
It is not possible work with these parameters, the only thing that we can do is look carefully the European tire label. In order to reduce fuel consumption we can buy “green” tires; working with some data we can consider that if we use a green tire instead “black” ones, we can reduce up to 30% of rolling resistance with a real fuel consumption up to 6%. These numbers are not the truth, but are useful to understand better the phenomena.
In this group we can find tire pressure, because if it is constantly maintained in the optimal range allows reducing fuel consumption.
Vertical load increase also the rolling resistance, at the same tire pressure. So if for example we take our car for a holiday with our family, and we put on it 2 bicycles, a canoe and the car is full of baggage the fuel consumption is higher also because the tire rolling resistance is higher.
High speed increases rolling resistance because tire is affected by strong waves and vibrations. In addition to being a low efficiency condition, it is also dangerous for the integrity of the tire.
How to reduce fuel consumption
Going directly to the target, in order to reduce fuel consumption we should read carefully the European tire label when we buy a new tire set and take care of its state, having a periodical inspection in order to check tire pressure and wear, also for safety reasons.
A higher class on the label may mean a tire with a lower hysteresis, but if fuel consumption is reduced, on the other side also grip is reduced, because it depends on the hysteresis too.
The higher class may mean a compound with less percentage of fillers as carbon black, but also in this case in the other side tire wear gets worse.
As usual the optimum is a compromise between what we need.