Latimea jantei si offset-ul (ET)
Este important sa utilizati combinatia recomandata de latime a jantei si dimensiune a anvelopei – latimea jantei indica ce dimensiune trebuie sa aiba anvelopa montata. La alegerea dimensiunii jantei trebuie sa luati in calcul latimea anvelopei la punctul de contact cu janta.
Latimea jantei este de obicei putin mai mica decat latimea anvelopei pentru asigurarea unei montari sigure a anvelopei. De aceea o anvelopa de 185 mm se monteaza de obicei pe o janta cu 6 inch (6J), desi este mult mai apropiata de 7 inch (7J).
Ce reprezinta offset-ul (factorul de echilibru al) jantei?
Offset-ul reprezinta distanta masurata intre suprafata de montare a jantei pe butuc si linia centrala imaginara a jantei. Acesta indica (in inch sau milimetri) cu cat iese in exterior janta fata de punctul de montare (butucul central). Offset-ul poate fi pozitiv, negativ sau zero, relativ la pozitia locului de prindere a jantei, fata de linia linia centrala a anvelopei. Un offset pozitiv inseamna ca suprafata de montare a jantei pe butuc este mai aproape de partea exterioara a jantei in timp ce unul negativ descrie o suprafata de montare mai inspre interiorul caroseriei. Offset zero este atunci cand suprafata de montare este aliniata cu linia centrala imaginara a jantei. Este foarte importanta utilizarea unor jante cu offset corect deoarece prea mult pozitiv sau prea mult negativ poate cauza deteriorari ale partilor autovehiculuiui in special la viteze mari, putand cauza chiar si explozii ale anvelopelor.
Cum aflam ce offset au jantele?
Offsetul este stantat pe janta ca valoare ET, ce provine din germanul Einpress Tiefe. ET indica distanta dintre suprafata de montare si centrul geometric al jantei (axa de simetrie) si este exprimat de obicei in milimetri.
Cand ET-ul descreste, jantele de aliaj vor iesi in afara, iar cand acesta creste rezultatul este o deplasare a rotii spre interiorul caroseriei.
Totusi, offset-ul nu este singurul factor responsabil de locatia jantei fata de pasajul rotii din caroserie. De exemplu desi au acelasi offset de ET30, o janta cu o latime de 9 inch va iesi mai in exterior decat o janta cu o latime de 6 inch. De ce? Priviti diagrama de mai jos:
Fig. Diferenta dintre o janta cu 9J si una cu 6J
Valorile latimii jantelor J si ale ET-ului – mituri si fapte
Exista cateva mituri despre instalarea jantelor ce sunt adesea confundate cu faptele. Vom incerca sa explicam punctele cele mai importante ce apar in cele mai frecvente intrebari atunci cand se inlocuiesc jantele autovehiculelor.
“Este posibil sa inlocuim offset-ul unei jante cu o toleranta de +/- 15 ?”
Aici nu trebuie sa generalizam. Fiecare modificare de offset al jantei trebuie analizata individual. Car manufacturers actually allow a change of the wheelbase to a certain degree, which is usually around 2%. For example, when using a wheelbase of 160 cm, an acceptable change will be 3.2 cm. This results in a maximum allowed offset change of 16 mm per side provided that the width of the wheel and tyre size will not be changed.
Individual cases should also consider how much space the car manufacturer has assumed between the wheel arches. Any offset change, or change to the wheel width, could cause the wheels to rub against this arch, especially when under a heavy load.
Increasing the offset then, may also cause the risk of the wheel rubbing with the car brake calipers. Therefore, when planning any ET changes, the standard distance between the wheel and any crucial points of the vehicle should be carefully investigated. This includes the edges of the wheel arches, brake calipers, MacPherson struts and other nearby components.
While aftermarket wheels are made in multiple offset modifications, most OEM wheels come in the offset appropriate for a given vehicle model. Most wheels in today’s cars are installed with positive offset.
“Is lower ET acceptable when it comes to alloy wheels?”
It does not matter whether you have alloy or steel wheels. The wheel must be located in the very same place, so the material does not often come into consideration.
However, when changing from steel to alloy models, it is often noted that steel wheels are narrower than alloy ones. This usually implies making fitting adjustments for wider wheels, which also means modifying other values.
Fortunately, there is a very simple formula to calculate how far the new wheel will protrude:
The formula to determine the new protrusion length of a car wheel
“Can I fit 7.5Jx16 ET35 wheels to a Renault Laguna?”
Renault recommends standard wheels with a 15-inch diameter, 6-inch width and ET45. Here the question is whether wheels with a 16-inch diameter, 7.5-inch width and ET35 can be used instead.
If we use these values in the formula presented above, the causes an increase in width by 28.75mm. The front wheelbase is 1480 mm, which means an extension of 14.8 mm is acceptable on either side. From these calculations, these wheels cannot be recommended for this particular vehicle.
“I would like to use wider wheels, what ET should I use?”
An appropriate offset value can be easily calculated to keep the wheelbase unchanged. To do this, simply use the formula presented below. Enter the parameters of the factory wheel and the width of the new wheel. The resulting value shows which ET is appropriate in this case.
The formula to determine the ET value for wider wheel changes
The following table shows acceptable wheel and tyre widths according to the German TÜV organisation. When analysing them, consider any potential changes between your summer tyres and winter tyres as well.
Fig. Latimile jantelor si anvelopelor acceptate conform TÜV
Test jante originale vs. jante replica
