Mavic Engineers Talk: Testing Rolling Resistance
Improvement of the performance and the reliability of tires results from careful analysis of the various parameters and criteria characterizing a tire. Thus, it is necessary to develop specific tests to precisely evaluate these criteria. We are particularly interested here in the rolling resistance and the adhesion (grip) which are two key elements of the tire’s performance and the safety of a rider. Of course, improvement of these criteria does not have to be made to the detriment of the weight, comfort or durability.
1. Measuring tire resistance to rotation
A tire’s resistance to rotation (rolling resistance) corresponds to the loss of energy related to the deformation of the tire against the ground. As the tire deforms, it creates a force opposed to the progress of the cyclist and which is proportional to the applied vertical load (in other words, the weight of the cyclist and his bicycle) and to the coefficient of rolling resistance (Crr) of the tire. The Crr of a tire depends on the inflation pressure, the temperature, and above all, the composition and the construction of the tire.
The Mavic Research Team has developed a specific measuring bench to measure Crr in stable and repeatable lab conditions. Several mathematical corrections are applied after measuring to simulate the real rotation coefficient measured on the road. Comparative measures that establish a real classification of the tires are then obtained, but above all they enable us to improve the composition of the rubber of our tires to reduce their resistance to rotation.
The architecture of that testing bench is inspired by a method for measuring car tires defined in a specific ISO 28580 standard:
– A smooth surfaced, rotating drum with a diameter of 1m70 is turned by an engine at the desired speed. The size of the drum is important because if it is too small (like the kind used on a turbo trainer or stationary rollers) it can lead to exaggerated deformation of the tire on against the drum and therefore an erroneous measurement of the rotation coefficient.
– The drum is also set to a specific temperature to carry out tests in repeatable conditions. It is also an important parameter to control, in order to make consistent measurements at different times during the year.
– A standard test wheel fitted with its tire is placed on a triangular arm linked to the bodywork by a stress sensor with 0.03% precision and is placed against the drum.
– The wheel is loaded with additional mass to ensure the flattening (deformation) of the tire against the drum corresponds to realistic conditions. When the wheel is pulled by the rotation of the drum, the adhesion related to the rotation resistance is measured.
Furthermore, the tire pressure is carried out with a specific valve and a precision manometer because the control of that inflation pressure can represent a major source of measurement error. A deviation of +/- 0.5 bars (which is roughly the error margin of floor pumps available in retail shops) easily leads to errors of +/- 6.5 % on the values of the rotation coefficient.
To test a tire, 4 cycles are carried out on the machine: first, a heating cycle to soften the rubber and bring the tire up to temperature for a few minutes. Then the average of the 3 next cycles is made. Each cycle consists of 3 speed levels from 5 to 50 km/h. Ultimately the value of the rotation coefficient (Crr) is obtained.
All parameters (load on the wheel, inflation pressure, temperature) are maintained at constant level to carry out a true comparative study but can be adjusted independently for the sake of research studies.
2. Measuring the adhesion on wet ground
The tire is the only intermediary element between the bicycle and the road. It transmits the propulsion energy of the cyclist, guides his trajectory and enables braking. The adhesion of the tire to the road is therefore essential to the safety and performance of the cyclist. It is characterised by the adhesion coefficient.
The adhesion of the tire to the road involves 2 physical phenomena: the adhesion (in layman’s terms, these are the molecular links between the rubber and the road) and the indentation (which is linked to the deformation of the tire on the roughness of the road). If the road is wet, the molecular links called Van-der-Waals (adhesion) are significantly reduced and the adhesion coefficient decreases. Consequently, the tire will lose adhesion sooner, at shallower cornering angles compared to lean angles achievable in dry conditions. Furthermore, only indentation phenomena enable the connection of the tire with the road on a wet surface. In the rain in particular, the adhesion coefficient is therefore very much linked to the type and characteristics of the asphalt surface.
It is therefore essential for the safety of the cyclist to try and improve the adhesion coefficient of the tire on wet ground. That is why we have developed two complementary test protocols to qualify and quantify the tires in terms of adhesion on wet ground.
2.1. Adhesion measuring bench
This device developed by the Mavic Research and Development team accurately measures the stress caused by interaction between the ground and the tire on a continuously wet, sprayed test track. The test wheel equipped with the tire to be characterised is loaded with masses to simulate the deformation (or side loading) of the front wheel (most critical for the loss of adhesion). It is mounted on a very rigid, fully instrumented frame for measuring the stress applied on the wheel.
During the test, the wheel is driven at a specific speed by a vehicle trailing the test bench and the stresses are measured for various skidding (bend) and rolling angles. That test compares various tires and helps to characterise and develop new rubbers.
The adhesion coefficient deviations of the tire on wet ground can go from 0.80 for the least performing to 1.10 for the best tires.
2.2. In the field test on track
Mavic implemented a second test to evaluate the tire in the closest conditions to reality and to account for more subjective and qualitative criteria. This kind of test supplements the more quantitative tests which gather data in the lab, on test benches. During the road test, a tester carries out several passages in real conditions on a test track defined by two bends indicated with traffic cones. A sprinkler system waters the track. The ground temperature is strictly controlled because it affects the adhesion of the tires as well. The lower it is, the more limited the adhesion will be.
The same tire is blind tested at least three times by three different testers.
To push the tire to the limit each test is timed and the tester must make a passage at least 25 km/h minimum (or a time of 9.00s on our track). The average of the passage times is then made and above all the personal assessment of the tire tester is defined by the following criteria:
– Steady direction
– Shock absorption
In the end the tested tire is given marks out of 20 and a passage time. A reference tire is used as a standard at the start of each measuring campaign.
Finally, the results of tests in the field and the tests of the measuring bench (adhesion coefficient) are always compared and are always found to be in agreement.
3. Conclusions and use of data
After these various tests carried out on the tire, it can be very accurately characterised:
– Resistance to the rotation (measured numeral value of Crr)
– Adhesion coefficient on wet ground (measured numeral value)
– Behaviour experienced on dry and wet ground according to several criteria.
All the work of our dedicated Research team for the development of tires therefore consists of developing all the tire features to improve those various criteria, while maintaining excellent resistance to punctures and reasonable weight which remain the essential features of a bicycle tire. The next blog will be dedicated to the presentation of the tire features we can rely on to obtain a tire that completely fulfills the expected performance.
Mavic research engineer