Huh? - The Technical Jargon ABC

Those dealing with alloy wheels are confronted with various abbreviations, lots of cryptic technical terms, and loads of designations that need explaining. In order to help you find your way through this tangled mass of foreign words, we’ve compiled an overview of the most important special terms and explanations associated with the topic of wheels.

 

1-part cast or forged rim construction
1-part cast or forged rims, also known as monobloc, consist of one piece, and are manufactured either by means of aluminium counter-pressure casting or by forging. The weight and strength of the rims depends primarily on the type of construction, but also the alloy used and the type of processing  play an important role.
2-part bolted rim construction
2-part bolted rims just like 2-part welded rims consist of a rolled or pressed rim ring and a forged or counter-pressure cast star hub. As opposed to the various welded constructions, however, the rim ring and star hub bolted together and the joint is sealed with a sealing compound or a gasket – so that the air doesn’t escape. If the rim ring or star hub should ever become damaged, they can be replaced individually.
2-part welded rim construction
A two-part welded rim consists of a rolled or pressed rim ring and a forged or counter-pressure cast star hub. The two parts are welded permanently together. Therefore, if the rim ring or spider is removed by accident, the entire wheel has to be replaced.
3-part bolted and welded rim construction
Just like the 3-part bolted construction, the 3-part bolted and welded construction consists of inner and outer rim ring and the star hub. Since the parts are also welded, the construction offers greater stability and more meticulous designs. Another advantage: Thanks to the welded connection, not as many screws are needed, which in turn reduces the weight of the wheel.
3-part bolted rim construction
3-part rims consist of an inner and outer rim ring and the star hub. The parts are bolted together and sealed with a special sealing compound or gasket to prevent the loss of air. If one part of the construction becomes damaged, it usually can be replaced individually.
ABE
ABE is the abbreviation for  Allgemeine Betriebs Erlaubnis (Vehicle Type Approval). What an ABE means effectively, is that you don’t have to make an extra trip to MOT and pay 50 € for a certificate, but just  can put a copy of the approval in the glove compartment and drive off, provided no other requirements have to be fulfilled.
Body-to-tyre clearance
Body-to-tyre clearance denotes the space required between the wheel housing and the tyre. This must be large enough so that the wheel can move freely inside the wheel housing – even when the vehicle deflects quite a bit when going over bumps. The smaller the distance between the wheel and wheel housing, the sportier  the car looks.
Double hump
The hump or double hump is the bulge on the bead which prevents the tyre from slipping off into the rim well as a result of lateral force, such as when speeding around corners.
Hub bore
The hub bore is the centre bore of the rim on which the wheel is mounted. If the hub diameter of the vehicle is less than the hub bore, you can usually compensate for it with plastic centring rings.
Hub diameter
The hub diameter is dependent on the particular vehicle type and model and is needed for centring the wheel. The hub diameter of the rim can be greater than the vehicle – it can be adjusted using centring rings – which of course isn’t possible the other way around.
Hump
See double hump
Load index
The load index of the tyre (for aluminium rims = wheel load) refers to the axle load entered in the vehicle’s registration papers. An axle load of 1200 kg requires a wheel load of at least 600 kg. According to the Load Index Table, this corresponds to a tyre load index of 90.
Measuring rim
The measuring rim, also referred to as recommended rim or theoretic rim, is a rim which stands in a specified proportion to the tyre nominal width. The associated tyre sizes are gauged based on the measuring rim.
Number of holes
The number of holes indicates the number of mounting holes on the wheel. In other words: The greater the number of holes, the more bolts you’ll have to remove and retighten when changing a tyre.
Offset
The offset is the distance between the centre line of the rim and the attachment face of the wheel disc. The offset determines how far the wheel stands inside or outside compared to standard wheels. For instance, if the standard wheel has an offset of 40mm, but the new wheel has an offset of 30mm, the new wheel will stand out 10mm more.
P.C.D.
See pitch circle diameter
Pitch circle diameter
The pitch circle diameter, known as the P.C.D., is the diameter of the circle on which the centres of the mounting holes lie. The pitch circle is given in mm.
Rim construction
Rims have various constructions: 1-part cast or forged, 2-part welded or 2-part bolted, as well as 3-part bolted or 3-part bolted and welded.
Rim diameter
The rim diameter, sometimes called rim nominal diameter, is a uniform value used in the rim designation which indicates the diameter of the rim in inches.
Rim nominal diameter
See rim diameter
Rim size
The rim size is made up of the rim width and the rim diameter. See rim designation for details.
Rim width
The rim width is the inner distance between the rim flanges.
Rolling circumference
The rolling circumference of  a tyre is the distance the tyre covers in one revolution.
Run flat
Run flat tyres are indestructible tyres. Of course they’re not really indestructible, but when they’re flat, you can continue to drive with them a short distance without damaging the rim. However, the rim has to have a special shape for this, otherwise the emergency function doesn’t work.
Speed index
The speed index, also referred to as Vmax release, is more or less the maximum permissible speed for a tyre. Logically, it should be equal to or exceed the maximum speed of the vehicle entered in the registration document. If it falls short, you’ll have a real problem with MOT and probably also during driving. That’s why the speed index is one of the most important values when selecting the right wheel/tyre combination.
TÜV acceptance certificate
A TÜV acceptance certificate – or parts acceptance certificate – certifies that certain parts installed in or on the vehicle comply with regulations. It’s required for all conversions that are not specified values in the vehicle registration document. After mounting or installing the parts, you have to drive to TÜV/MOT to have the change approved. If the part is accompanied by an ABE, according to which no conditions, such as wheel covers or reworking exist for the particular vehicle, the TÜV/MOT registration isn’t required.
Track width
The track width is the distance between the centre of the left tyre to the centre of the right tyre – but measured on an axis, not diagonally.
Tyre designation
The tyre designation gives you all of the important data on the dimensions and structure of the tyre. An example for clarification: The designation 7.5 J x18 ET38 LK 4x100 means that the tyre has a width of 7.5 inches and a diameter of 18 inches. ET 38 stands for the offset of 38 mm. The tyre designation also contains various abbreviations. The letters following the tyre size, for instance J, H, P, K, JK, indicate the type of rim flange, the abbreviation H2 stands for double hump. And LK 4x100 means that the bolt circle diameter has 4 bolt holes and a diameter of 100 mm.
Tyre designation
A tyre designation contains numerous technical data about the tyre: The designation 195/65R15 91H means for example, that the tyre has a width of 195mm, a eight/width proportion of 65%, the R stands for tyre type, radial in this case, the next number is the internal size of the tyre or the rim size in inches, and the 91 stands for a load index of 91. The last letter denotes the speed index of the tyre.
Vmax release
See speed index
Wheel diameter
See rolling circumference
Wheel spacing
Wheel spacing is the widening of the track on the one hand, and on the other hand it refers to the use of spacer/adapter discs  for widening the track. There are two ways to widen the track: Either with the previously mentioned spacer/adapter discs, and/or with wheels that have a lower offset as the standard tyres.