The wheel of a vehicle has several technical functions.
First of all it’s the connection between the vehicle hub and the tyre, of which assures the inflation pressure too.
The wheel undergoes to several stresses: weight stress, , torsional stress , tensile stress and also stress due to the brakes application. In addition, it has also an important function of brakes ventilation when brakes are located on the vehicle hubs (that is in modern vehicles and for requirements of limitation of the hanging parts, actually almost on all of them).
Therefore the planning stage and the test of the vehicle wheel is tightly connected to the kind of vehicle they have to fit out.
There is a preliminary dimensional aspect linked up both to tyre dimension and to the specific vehicle fitments ( number and type of bolts/nuts, ET, hole pitch).
In addition there is a structural aspect connected with the vehicle weight and with its performances.
The stresses due to the weight of the vehicle itself – that is resistance to loads resulting from dynamics, ( so more important as higher are the vehicle performances) power to transfer and braking to support are specific features of each vehicle.
The last is a fluid-dynamic aspect connected with dimension and with the braking system power (that in its turn is sized according to the performances of the specific vehicle) that has to be dissipated.
The alloy wheel , as for the features of its material, assures a more favourable ratio between weight and resistance than the steel wheel. Its features permit to have wheels more resistant but less heavy and with a lower moment of inertia. It results from this an improvement of the performances (as much higher as much big is the wheel size). In addition its thermal conductivity higher than the one of steel wheel assures a more efficent heat transfer and dissipation of thermal energy resulting from brakes application.
It’s very important to perceive the significance of the technical features of a wheel. Some explanatory pictures will help us to explain such arguments like the well width, ET and PCD
A = Well of the rim in inches
B = hole pitch in inches
C = ET in millimetres
D = PCD; that is number position and distance in millimetres od the wheel holes.
The Rim Well
See picture n.1, the rim well is that part of the wheel where the tyre is fixed. The well profile is scheduled in each of its parts according to the ETRTO (European Tires and Rims Technical Organization) regulation.
Profile types are H (hump) FH (Flat Hump) EH2 (extemdemd Hump) EH2+ (extemdemd Hump for Flat Tyres)
Evidently the widht of the tyre depends on the rim width. The ETRTO technical rules fix also for each well width which are the tyres sizes that are allowed to be used.
Using tyres larger or narrower than the suggested ones can cause a not sufficient adherence between the tyre bead and the corresponding area of the rim and the consequent leak of air and not sufficient inflation pressure.
The ET or offset is the distance between the centerline of the wheel and the plane of the hub-mounting surface of the wheel. It can thus be either Zero Offset, positive or negative. See the picture:
A= Rim well width
C= distance in mm between B and the wheel connection (offset)
Zero Offset - The plane of the hub mounting surface is even with the centerline of the wheel
Positive Offset - The plane of the hub mounting surface is shifted from the centerline toward the front or outside of the wheel.
Negative Offset - The plane of the hub mounting surface is toward the back or brake side of the wheel's centerline.
Changing the ET measure, considering the same well width, you can decide to make the wheels stick out more or less than the original wheel. For example, considering the same well width, if you lower the offset of a wheel from ET 45 to 20, the rim will stick out of 25 mm more.
However it’s important to remember that lowing the ET too much ( as well as using the spacers) can cause a too early wear and tear of the bearings and suspensions and be counter-productive of the roadholding. In addition there is a danger that the wheel hits against the mudguard or fender.
The PCD ( number, Position and distance between the holes)
Each wheel is marked by the number of holes and by the centre to centre distance. The center to center distance is the diameter of the circle that cuts the center of each hole (see picture)
PCD is a very important data to permit the right fixing of the wheel to the hub. If number of holes and centre to centre distance are not correct, the wheels could not be fixed to the hub.
Usually the wheels code is quoted on the wheel itself, behind one of the spoke or behind the hiding bolts cap.
Here you find an example about how to read the data concerning a wheel. If behind the wheels spoke you read the following words:
7.0J x 17 EH2 – ET 35 – 5/100
7.0 indicates the well width measured in inches (1 inch = 2,54 cm) on which fix a tyre with the suggested dimensions.
J is the letter determining the structure of the rim well by the tyre bead.
X indicates that the rim is one piece
17 is the wheel diameter (where it will be fixed a tyre equally sized)
EH2 is the structure of the profile of the rim well
ET 35 is the offset of the wheel, that is the side shifting (camber) of the wheel towards the hub axis, in millimeters
5x100 is the wheel PCD, that is the number of holes (5) and the centre to centre distance in millimetres (100)
Usually it is quoted also load index, production lot and producer brand.