Method for determining the aerodynamic moment of resistance of a wheel

Abstract

A method for determining the aerodynamic moment of resistance M.sub.aero-EM of a wheel arranged on an axis, by measuring the value of the mechanical power P.sub.m to be applied to the wheel in order to maintain it in rotation at a constant speed ω, the said wheel being equipped with a rotational-drive means and with a device for picking off and/or recording the numerical values of the said mechanical power and those of the rotational speed. The wheel is protected by a removable cap and is subjected to a flow of air.

Claims

1. A method for determining the aerodynamic moment of resistance M.sub.aero-EM of a wheel arranged on an axis, by measuring the value of the mechanical power P.sub.m to be applied to the wheel in order to maintain it in rotation at a constant speed ω, wherein the wheel is equipped with a rotational-drive means and with a device for picking off and/or recording the numerical values of the mechanical power and of the rotational speed, comprising: protecting the wheel by a removable cap, constantly subjecting the wheel to a flow of air, picking off the mechanical power P.sub.m applied to the wheel and the rotational speed ω of the wheel, inputting the picked-off measurements of rotational speed co and of mechanical power P.sub.m of the wheel into the following mathematical formula:
P.sub.m=ω(M.sub.aero-EM+M.sub.f)  (I) where P.sub.m represents the mechanical power needed to keep the wheel in rotation at a constant speed, ω represents the rotational speed of the said wheel, M.sub.f represents the value of the moment of friction of the hub of the wheel, and M.sub.aero-EM represents the aerodynamic moment of resistance of the said wheel.

2. The method according to claim 1, wherein the flow of air has a main direction substantially parallel to that of the wheel.

3. The method according to claim 1, wherein the flow of air has a main direction at an angle of between −40° and +40° with respect to that of the wheel.

4. The method according to claim 1, wherein the axis of the wheel remains fixed relative to the ground.

5. The method according to claim 1, wherein the flow of air has a speed identical to that of the rotational-drive means.

6. The method according to claim 1, wherein the flow of air has a speed different from that of the rotational-drive means.

7. The method according to claim 1, wherein the wheel comprises at least one means of holding the wheel suspension.

Description

BRIEF DESCRIPTION OF DRAWING

(1) The embodiments of the invention will now be described with the aid of the examples and of the single FIGURE which follow, which are not in any way limiting and in which:

(2) the single FIGURE depicts the value of the aerodynamic moment of resistance as a function of the external surface of various tires.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

(3) In order to implement this method, a wheel is placed in an aerodynamic wind tunnel, and is connected to a mechanical means of inducing rotation so as to cause it to rotate constantly at a speed co. As it rotates, the wheel is not in contact with the ground. The distance between the wheel and the ground is preferably small. The wheel is also connected to a means intended to stabilize its rotation so that the measurements can be taken correctly.

(4) The wheel is equipped with a device intended to pick off and/or record the numerical values of its rotational speed co and of the mechanical power that has to be applied in order to keep it rotating.

(5) According to an alternative form of embodiment of the method according to the invention, the wheel may be mounted on a vehicle which will then be fixed to the ground using pylons. A rolling road drives the rotation of at least one of the wheels. A means of raising the vehicle allows the vehicle to be taken away from the floor of the wind tunnel. Fixing the vehicle to the ground makes it possible firstly to dictate the attitude of the vehicle and, secondly, to stabilize it as it is raised. The wind generator subjects the vehicle to a flow of air with a speed V.sub.0 identical to the rotational speed ω of at least one wheel.

(6) Applying mathematical formula (I) below makes it possible to obtain the value of the aerodynamic moment of resistance M.sub.aero-EM of the wheel using the following mathematical formula (I):
P.sub.m=ω(M.sub.aéro-EM+M.sub.f)  (I)
where P.sub.m represents the mechanical power needed to keep the wheel in rotation at a constant speed, ω represents the rotational speed of the said wheel, M.sub.f represents the value of the moment of friction of the hub of the wheel, and M.sub.aero-EM represents the aerodynamic moment of resistance of the said wheel.

(7) M.sub.f, which represents the value of the moment of friction of the hub of the wheel, can be calculated, for example, from the technical data supplied by the bearing manufacturer.

(8) The rotational speed ω is obtained from the means of recording the rotational speed of the wheel.

(9) FIG. 1 gives, for seven different references of tire of the same make and the same model but of different sizes, the calculated value of the aerodynamic moment of resistance M.sub.aero-em (N.Math.m) as a function of the external surface area of each tire (m.sup.2). The external surface area of the tire is defined, according to the invention, as being the surface area of the tread and that of the sidewalls.

(10) The calculations, according to the single FIGURE, were performed with a speed of the flow of air and a rotational speed of the wheel that were identical, equal to 120 km/h. The flow of air has a main direction substantially parallel to that of the wheel.

(11) From FIG. 1, it may be seen that the value of the aerodynamic moment of resistance increases in proportion with the external surface area of the tire.