Aez contro le riparazioni dei cerchi

[Clio16v]

Fortunatamente io non ne ho mai avuto bisogno, ma leggendo in giro parecchia gente in caso di cerchi rotti od ovalizzati per colpa - anche - delle belle buche che si incontrano sulle nostre strade si rivolge a ditte specializzate nella riparazione (so che ce ne sono alcune in Italia, fanno dal graffio alla rigenerazione completa). Ora, siccome stavo cercando dei cerchi nuovi per un amico che usarebbe come treno invernale, e di case non è che ce ne siano più come un tempo, sono capitato sul sito della Aez (uno dei produttori che ci hanno consigliato per avere buona qualità; è tra i pochi che oltre agli standard mi dicono non abbia neppure ancora delocalizzato): lì, tra le prime cose che fanno, è sconsigliare la pratica della riparazione, sostenendo - da quanto ho capito - che un cerchio riparato non è sicuro. Ho capito giusto? Voi cosa ne pensate?

Repair of light-alloy wheels

Unfortunately, repair of light-alloy wheels is still very common. This information explains why it would be better to steer clear of the repair of light-alloy wheels.

The following information was obtained from various Internet pages addressing the repair of one-piece light-alloy wheels:

1.) Damaged rims are heated across their entire area and then repaired by means of a rolling procedure.

2.) Subsequent curing ensures that any differences in strength remain within the tolerance range.

3.) The metal alloy on the damaged wheel is analysed, and we can assure customers that the material used in repairs in which parts are welded on or into the wheel will be identical to the original material (alloy, strength, material quality).

We will refute the above statements one by one:

To start with, here is an excerpt from an information leaflet published in March 1999 by the German Association of Tyre Retailers and Vulcanizers (Bundesverband Reifenhandel und Vulkaniseur-Handwerk e.V.). In this case, a badly damaged wheel (localized deformation of inner bead seat caused by shock load, no incipient cracking) was repaired at a "tyre clinic". The wheel was then intensively inspected with the following result:

1.) Machining of the rim beads caused dimensional changes exceeding the tolerance limits, in particular regarding the following functional dimensions: circumference of rim bead, hump circumference, rim flange radius and outer diameter.

2.) At the same level of local load, local heating and deformation reduces energy absorption by approx. 25 % . In practice, this change in material properties results in increased deformations caused by driving over obstacles or over grooved road surfaces such as rail crossings, transversal grooves, etc.

From our point of view, we therefore advise against such repairs

Source: BRV repair of aluminium rims)

On its website, the EUWA (Association of European Wheel Manufacturers) also strongly advises against rim repair in the following words:

"Repair of a damaged rim or disk by heating, welding or addition or removal of material is absolutely forbidden.

No guarantee on repaired parts is acceptable since such changes may introduce additional stresses in critical high-stress areas.

Wear on rim flanges (top of the flanges) can be tolerated up to a maximum of 10 per cent of the initial thickness of the rim material." (Source: www.euwa.org)

After in-depth tests and inspections performed in December 2004, wheel certification experts at TÜV SÜD Automotive agreed with this statement. Further information: www.tuev-sued.de/automotive

Why is "heating" and "welding" or "addition of material" so dangerous?

As a matter of principle, a deformed rim cannot be restored to its original state. Every material changes its properties once the yield point, i.e. the point prior to which a material deforms elastically (referred to as ReH in the diagram), is exceeded, so that the wheel is permanently deformed (e.g. impact). The higher the deformation, the less force is required to cause further deformation (exceeding of yield point (ReH) or ultimate strength (Rm). Material strength thus decreases! The material properties cannot be restored to those of a new wheel (not even by annealing and recrystallization).

On wheels, welding has several adverse impacts on the material. Firstly, localized heating causes thermal stresses (i.e. the material is subjected to internal stresses in addition to the stresses occurring during operation). Additionally, material composition is changed by separation and crystallization processes (brittling caused by quick cooling, inhomogenity caused by diffusion and the addition of additives). The strength of a welded wheel thus differs significantly from that of a new wheel. Even repeated thermal treatment cannot restore the original properties of the wheel.

With respect to metal alloys, the following can be said:

Determining the alloy composition of cast rims within the scope of repair work is completely impossible. To identify alloy materials and their proportions, material would have to be sampled from several locations of the rim. This would result in complete destruction of the wheel, which would no longer comply with the inspected original state (material sampling).

Even renowned casting technology institutes cannot always accurately identify alloy composition in the melt, as local material composition is largely determined by the casting process. Basically, only a few institutes at universities and research facilities are able to undertake analyses of the melt (composition of cast).

The costs of such analysis considerably exceed the purchase price of a new wheel.

Depending on wheel design and dimensions, cooling rates differ across the wheel area. Mechanical and technological properties (including crystallization rate) are established during the casting process

Additionally, the production of "repair parts" whose material composition is identical to that of the area into which the part is to be welded, is simply impossible. Even if it were possible, it would result in the material changes described above

Paint damage:

Only specialist repair shops have the required technical know-how and can restore wheels after paint damage.

Why does AEZ not have any “paint markers” on offer?

In our spray painting facility, wheels are cleaned fully automatically, passivated, primed and then sprayed in silver using wet-in-wet technique and given a protective clear-coat finish. After priming, the paint is baked in an oven. After the wet-in-wet procedure (e.g. silver paint and clear-coat finish), the wheels are also automatically subjected to thermal treatment (baking oven). The silver coat and the clear-coat finish are again oven-baked.

This procedure ensures ideal paint protection against environmental influences (UV radiation, salt, moisture, dirt, etc.) at all points.

A paint marker would bring only "temporary" improvement as it does not offer the same adhesive properties as the baked powder paint, lacquer or clear coat finish. After a short period of using the wheel in standard weather conditions, the paint (paint marker) would gradually chip off or even corrode.

High gloss: This special paint finish cannot be produced with a paint marker. The reason lies in the technical procedure used to produce high gloss wheels. High-gloss wheels are first coated with a black powder, and then oven-baked. After this step, a special silver paint and the clear-coat finish are then applied by means of a special technique which is impossible to manage in a "paint marker".

Should you need any further information, please don't hesitate to contact the experts at AEZ Light Alloy Wheels and TÜV SÜD Automotive.

Contact:

AEZ, Mr. Ing. Pierre Czompo, Tel.: 0043-2256-801-513

TÜV SÜD Automotive, Mr. Jürgen Hübner, 0049-89-32950-693

AEZ Technical Department, 2008

AEZ Leichtmetallräder - Felgen alloy wheels Alufelgen Leichtmetallfelgen Aluräder tuning rims | Repair of light alloy wheels

P.S. se avete marche da consigliare per i cerchi invernali, sono benvenute. No, niente latte. è una misura piccola e la differenza di prezzo non credo sarebbe elevata. A meno che non mi convinciate che siano meglio di "ferro". Io penso alla fine siano praticamente uguali. Anzi, quelli di lega confido siano un pelo più resistenti.

[AndyMcnab]

"As a matter of principle, a deformed rim cannot be restored to its original state. Every material changes its properties once the yield point, i.e. the point prior to which a material deforms elastically (referred to as ReH in the diagram), is exceeded, so that the wheel is permanently deformed (e.g. impact). The higher the deformation, the less force is required to cause further deformation (exceeding of yield point (ReH) or ultimate strength (Rm). Material strength thus decreases! The material properties cannot be restored to those of a new wheel (not even by annealing and recrystallization). "

Io non sono esperto nell'ambito della riparazione dei cerchi, però credo che i miei studi di ingegneria civile mi possano aiutare un pelo.

Le leghe usate nella fabbricazione dei cerchi devono essere leggere per migliorare le performance rispetto ai cerchi in ferro e resistenti per far si che alla prima buca non si deformino.

Questo è un grafico che indica il comportamento di un materiale simile all'acciaio (ma direi che può andare bene anche per le leghe dei cerchi)

Nel primo tratto del grafico, nel segmento inclinato, i metalli danno il meglio di se. Finché non raggiungi il limite di snervamento il metallo, una volta libero dalle tensioni, torna alla forma e alle capacità originarie.

Questo vuol dire che su quel segmento si può fare avanti e indietro finchè si vuole.

Arrivati al limite di snervamento il metallo non si rompe e continua a assorbire le tensioni ricevute fino ad arrivare alla tensione ultima. Qui si deforma ancora un poco prima di rompersi.

Il problema è che superato il limite di snervamento, la struttura del metallo è compromessa. Cioè non tornerà più alla caratteristiche di fabbrica.

Questo vuol dire che se prendendo un dosso, il tuo cerchione si deforma, ha superato il limite di snervamento.

In un'officina possono anche farlo tornare alla forma di partenza, ma il tuo cerchione non avrà più le caratteristiche con le quali lo hai acquistato.