Offsets

[ant_tay]

Can anyone tell me how offset on alloys works please

Thanks Anthony

[Stoo]

have you tried searching?

ive explained a few times already

[The Simps]

http://www.topbuzz.co.uk/info/offset/b001.gif

[Stuart]

see sig link for all kinds of offset calculating fun

[The Simps]

If you take a wheel and cut it in half and draw a line down the centre of the width of a wheel, the offset would be the distance between the back face (mounting face) of the wheel and the centre line that you have just drawn (shown as a dotted line in Figure 1 (http://www.topbuzz.co.uk/info/offset/pic001.htm)).
It is very important not to deviate too far from the offset of the wheel originally fitted to the vehicle. In this case Vauxhall engineers developed the 'Negative Scrub Geometry". This gave a offset of -49mm (et49). A wheel of radically different offset can cause serious clearance and handling problems along with accelerated tyre and bearing wear.

http://www.topbuzz.co.uk/pics/info.gif The offset is commonly shown as a et number, i.e.et49. This means the offset is 49mm. The offset is always measured in millimetre's and can normally be found at the back of the wheel or stamped behind the centre cap (if fitted).

Negative scrub geometry had the intention that if you got a flat tyre you were still able to control the car (so long as you weren't going motorway speeds). So its best to keep as close to the et49 figure as possible.

However the bigger the alloy wheel goes, the more difficult it becomes to stay close to the et49. The reason being is that as wheel gets bigger in diameter, it also becomes wider, and so does the tyre that fits it. This is when other items in the wheel arch can begin to foul with the alloy wheel or the tyre. Namely the arch lip of the car and the suspension strut. When this happens the offset must change for the bigger wheel and tyre to fit in the arch. Plus if the car has been lowered, this will have to be considered too as the wheel is now tucked further up in the arch.

But some models of the Vauxhall/Opel/Holden/Bedford range can't change much. Namely vehicles that have arch lips very close to the wall of the tire or the arch shape itself is not curved. Namely the Vauxhall Nova and Astra models suffer from this. Otherwise it means body and arch modifications to accommodate the larger wheel. They will have to try and keep to the et49 offset as much as possible. Even the Nova GSi had standard alloy wheels at 47.5mm offset!

Typically on bigger Vauxhall models these do not have this restriction which means larger wheels can be fitted to the car without body or arch modifications. Typically a offset of between et32 to et38 fits larger models with 17", 18" and 19" alloys on (say) a Cavalier, Calibra, Vectra, Carlton etc.

[The Simps]

see sig link for all kinds of offset calculating fun


Thats very useful that!

[ant_tay]

so an offset of 38 on a 16" wheel is no good then for my nova

[Stuart]

so an offset of 38 on a 16" wheel is no good then for my nova


depends on the width etc

[The Simps]

so an offset of 38 on a 16" wheel is no good then for my nova

Depends if you're gunna flare your arches or run round arches.

[The Simps]

oh yes, and rim width, but I'm assuming it will be 6-7" wide.

[ant_tay]

they are 195/45/16s

[Stuart]

they are 195/45/16s


that dosent narrow it down very much

[The Simps]

You'll have plenty of inner arch/front strut clearance but will need big mods for the outer arches.

[ant_tay]

the whloe lip on each arch has been cut away

[Jay]

the whloe lip on each arch has been cut away

Still wont fit.

[The Simps]

You'll need a lot more than that mate.

[ant_tay]

at the moment i have 195/45/15's on there with an off set of 40 and the front wheels look like they stick out of the arch and on the back one wheel has 2mm clearance and scrubs a little and the other side has about 4mm and dont scrub at all

[ant_tay]

so what the fudge is wrong with my car

[Stuart]

standard nova tollerances

[The Simps]

nova tolerences are awful. I've got shat all clearance on my left rear but plenty on my right rear. Rears tend to be the worst for tolerences.

Tyre brands also make a big difference.

I was running 195/45 16 yoko a539s with plenty of clearance. I went to 195/45 16 yoko avs sports and they rubbed on the rears.

[ant_tay]

so what do i need to do to my arches to make them fit

[Jay]

so what do i need to do to my arches to make them fit

Get ET 49 Wheels.... Look is it really worth putting these wheels on your car, what are they any pics?