Wheels are more and more creating the so-called “car fashion”; they are longed and requested .

They support the vehicle and together with the tyres, following the forces brought into play by the dynamics of motion,  provide a safe driving. Nowadays it is more and more necessary not only to be able to recognise  sizes and specifications of  a wheel  but also its manufacture and origin, cause safety, while driving, must be assured at 100 %.

We deal with rims every day,  as much as with the tyres that cover them.  They are a real supporting part of  the vehicle -  together with tyres they form the so-called “sprung mass” – and they can rightly be called the soul of the wheel; the earliest human invention whose evolution fromBiblical granite to wood carriages marked the passing of times. Nowadays the use of alloy – hardy and  light as well as malleable and conformable in various styles -  has gradually be preferred to the use of simple plate.

If you look at them from afar they may look very similar but if you look at them closely alloy wheels are the result of a complex engeneering technology, that assures their perfect resistance

to the stresses they undergo during the driving.

KEY  WHEEL INDICATOR

It may seem obvious , but not all understand what is hidden behind a wheel indicator: a mix of abbreviation, codes and measures  in inches and millimetres,  often unknown or not comprehensible to the outsiders.

Let’s analyse, for example, the following Key Wheel Indicator:   7J x 16 – ET 30- 5/120

The first number, that on modern cars ranges from 5 to 10 - higher in some cases-, is the widht of the rim well, that is  the distance between the two flanges, where the tyre will be fitted.

If the tyre size changes,  the wheel width should  be adapted to it, even if there is always a little allowance: the same tyre fitted on a narrower channel will grant a higher bead adherence, also in case of  a low inflation pressure, on the contrary,  fitted on  a larger channel, it will permit a wider projection of the wheel allowing the tread  to work at the maximum width.

This ranging in  dimension is allowed  only for  little variations – about 0,5 inches- in comparison to what the producer fixed cause a too narrow channel  could  belly out the tyre causing an irregular wear and tear, and on the other hand a too much large one would inevitably  roll-off the rim.

The letter J - that follows - settles the physical conformation of the well profile by the tyre collar, and can  vary in JJ in case  its   height from the end is higher of about 1 mm than the previous one or in B  to identify the steel wheels flanges.

The second number indicates the wheel external diameter in inches. Useless to say that it has to match the tyre size  that otherwise would be too small or too large to allow the proper pitch.

Missing those 12 inches tyres fiiting the old city cars – like for example the 125 R12 of the old Fiat 500 – it is difficult nowadays to find a city car fitting tyres smaller than 14 inches.  The trend is towards bigger and bigger,

wheels, up to real paradoxes like the wheels in 19”,20 and 22” of some maxisuv, sizes almost unexpected only 10 years ago.

This trend includes also some tuners extreme  exercises of styles, providing wheels even in 32 inches where tyres show such non-existant profiles that they appear suitable only to stay on the velvet parade of an Exhibition.

A very important parameter for the vehicle structure is the ET (from German Einpresstiefe that means  camber)

or off set  or compensation that is the value, in millimetres, indicating the distance between the

centerline of the wheel and the plane of the hub-mounting surface of the it. It is positive if it is more than 0 and negative if it is less than 0; in case of low ET the wheel is more projecting and vice versa in case of high ET.

Wheels with low ET  widen the car track, providing an higher dynamical stability, but always keeping within certains limits.

On the other hand, lowing the ET too much  can cause mechanical wear and tear, stressing extremely the bearings and suspensions  and causing their too early wear and tear.

A very important element to choose correctly a  wheel is the PCD (Pitch Circle Diameter)  it indicates the rim number of holes and the diameter of the circumference that cuts the center of each hole.

The  CB (Center Bore) indicates the diameter of the rim center hole, optimum for the right wheel centering

during the wheel mounting.

In addition, it is necessary to consider some other parameters. to match correctly wheel and tyre.

First of all the the well profile or  Hump that can be more or less articulated, according to the mechanical retention of the tyre in its housing, and that prevents the rim roll-off in case of inflation pressure leak in consequence of tyre puncture.

The differrent well profiles, encoded according to the ETRTO -European Tires and Rims Technical Organization- regulation are scheduled by the following abbreviations: H (single hump) H2 (double Hump) FH (Single Flat Hump) FH2 (Double Flat Hump) CH (Combination Hump), EH2 (Double Extended Hump) ed EH2+ (Double Extended Hump Plus).

Clear evidence of the importance of the well profil is given by the more and more increasing usage of deflated tyres, critical situation where comes into play a higher mechanical contrast against longitudinal and transverse forces occurring during acceleration, braking and cornering.

HAVE A LOOK AT THE ACCESSORIES

At the end let’s have a look at the valve slot for tubeless,  a real opening in the product integrity, essential for the tyre inflation. Its position is basilar to the protection of the valve, that can have different lenghts out standing from 25 to 54 mm and consequently exposed to the risk of crashing against obstacles on the way, for example against sidewalks.

Usually inclined 20 ° to the rim axis , the hole may have as many different sizes as many are the types of valves, that have to adhere perfectly to guarantee the airtight  whose lack would cause an unconvenient loss of the internal pressure. The most popular ones, present on almost all current production, are 11.3 mm holes, 4 mm narrowed in comparison with the  rubber valve diameter  that they will house.