Wheel Offset Confusion

[Steviewevie]

I'm confused by some aspects of wheel offset :duh:

I understand the concept, and I assume that it's a positive offset that the IS200 has. I understand that offset is important in terms of whether the wheel is going to foul on the inside of the car (e.g. suspension components) or stick out too far (past the arches).

However, what I don't understand specifically is what a valid offset range would be for different wheel widths. I've looked at Mat's posting as quoted below :

OFFSETS

FRONT

width | offset | tires range (width)

7 = 40 and up 215-225

7.5 = 42 and up 215-235

8 = 43 and up 215-235

8.5 = 45 and up 215-225

REAR

7 = 40 and up 215-225

7.5 = 40 and up 215-235

8 = 42 and up 215-245

8.5 = 45 and up 225-265 (255-265 requires rolling or having lip)

9 = 49 and up 235-275 (MAY require rolling or shaving lip)

9.5 = 52 and up 245-285 (MAY require rolling or shaving lip, 285 may require more modification)

10 = 45 and up 255-315(requires pulling fender)

As an example, with 7.5 inch wheels, the correct offset for the front seems to say "42 and up". To me, that means an offset of 42 or greater (numerically, e.g. 43, 44, 45, 50, etc). However, using the offset calculator here, if I start with a 7" wheels with 50 offset (I think that's right for the IS200 OEM alloys ?), and put in that I'm going to a 7.5" wheel with 42 offset, then it tells me I've got 2mm more inner clearance and the wheels stick out an extra 14mm, so I presume that's ok.

However, what's confusing me is when I start playing with the numbers. Given that the table above says "42 and up", as I start increasing the offset, e.g. 43, 44 etc, I start reducing inner clearance (and making them stick out less). If I get to 45 offset then I've got 1mm less inner clearance, and going further at ET48 I've got 4mm less inner clearance.

So am I understanding the wording "and up" correctly in the above table ? Surely it can't mean that anything that is 42 and higher will fit fine, because if you go up more than a few mm you're reducing inner clearance and at some point will presumably hit the inside of the car.

Similarly at ET41, the calculator shows that I've got even more inner clearance, but the wheels start sticking out even more.

So is the table above in fact showing the minimum offsets required to stop your wheels poking out past the arches, and saying nothing about coming into contact with the inner parts of the car ?

I get the feeling that I'm totally missing something here !

[Gord]

Its only a guide mate to what wheels will fit.

From what i've seen over the years of changing alloy wheels is, that it depends on the wheel itself :o

One member on here is using ET35 on his is200, when i tryed ET35 on my is200 it was way past the arch and no way would even pass the MOT test, Anees has some wheels with ET38 and he had the same problems :o

And then it also depends on what tyre size you fitting, say you have 225/40/18 with wheel ET 38, it might clip your arch, but if you used tyre size 215/40/18 you just might get away with not clipping your arch :D

Hope this helps you mate.

[Bazza]

offset is the measurement from the hub face to the centre of the wheel

if you take it the hubface is static, then increasing or decreasing the offset will move the centre of the wheel

the offset is a measurable measurement ! so in theory should apply to all wheels, but as Gord points out manufacturers seem to differ ??

basically the higher the offset the further in towards the body the centre of the wheel will be

the lower the offset, the further out the centre of the wheel will be

if the standard wheel is 7" and 50 offset

then from the hub face it will be 7"(177.8mm) /2 - 50mm = 38.9mm to the inner edge and 177.8/2+50= 138.9mm to the outer edge of the wheel

by decreasing the offset to 40mm

you end up with 48.9mm and 128.9mm respectively

therefore the inner edge will be 10mm further away from the body and the outer edge will be 10mm further away from the body

have a look at the inner clearance with the standard wheels, see if you can measure it, it will then let you know how much closer you can get

if there is 10mm gap it then seems feasible to allow up to a 60mm offset

you will find a limit to where manufacturers go up to in offsets unless you go specially made ££££'s

[Steviewevie]

Thanks guys. Bazza, your example seems to show that the IS200 has a negative offset though. This is one of the things that's been confusing me, trying to figure out if the offset is negative or positive. I've been assuming it's a positive offset, but if it's negative that will turn my examples on their head.

If it's a negative offset then that will start to make the "42 and up" example, for 7.5" wheels, work for me, in that with a 42 offset there's 8mm less clearance but going up from 42 there will be more clearance, so I can see that it makes sense that way. There's still presumably a limit on how high you can go before the wheel starts sticking out though, so these figures are just referring to the inside clearance, it seems. I guess it's easier to solve the problem of a wheel that's sticking out though, e.g. through wider arches.

So the IS200 has negative offset, yes ? Are pretty much all the wheels on alloy wheel websites negative offset too then ? I only see offset being mentioned, not whether they're positive or negative.

[Bazza]

Thanks guys. Bazza, your example seems to show that the IS200 has a negative offset though. This is one of the things that's been confusing me, trying to figure out if the offset is negative or positive. I've been assuming it's a positive offset, but if it's negative that will turn my examples on their head.

If it's a negative offset then that will start to make the "42 and up" example, for 7.5" wheels, work for me, in that with a 42 offset there's 8mm less clearance but going up from 42 there will be more clearance, so I can see that it makes sense that way. There's still presumably a limit on how high you can go before the wheel starts sticking out though, so these figures are just referring to the inside clearance, it seems. I guess it's easier to solve the problem of a wheel that's sticking out though, e.g. through wider arches.

So the IS200 has negative offset, yes ? Are pretty much all the wheels on alloy wheel websites negative offset too then ? I only see offset being mentioned, not whether they're positive or negative.

Lexus and majority of cars are positive offset

if they were negative it will be written -45mm and not 45mm or +45mm

zero offset is when the centre of wheel lines up with the hub

positive is when the centre line is behind the hub ie towards the car

negative is when the centre line is in front of the hub

quite rare to see negative offset and is associated with deep dishes

simple idea is to imagine looking at your standard wheel from the rear of the car

less offset and the wheel moves out

more and it moves in

[Rory]

[Steviewevie]

simple idea is to imagine looking at your standard wheel from the rear of the car

less offset and the wheel moves out

more and it moves in

Thanks. But how then, can you use information like "42 and up" when choosing wheels, because as the offset gets bigger then as you say the wheel moves in so surely it's going to be coming into contact with the inside of the car, which is what I was saying in my initial examples using the offset calculator.

Where does this "42 and up" figure come from ? Am I right then that it's a guide to how to avoid the wheels sticking out past your arches (but taking on Gord's point about tyres affecting this too) but doesn't say anything about if the wheels might rub the inside ?

[Gord]

simple idea is to imagine looking at your standard wheel from the rear of the car

less offset and the wheel moves out

more and it moves in

Thanks. But how then, can you use information like "42 and up" when choosing wheels, because as the offset gets bigger then as you say the wheel moves in so surely it's going to be coming into contact with the inside of the car, which is what I was saying in my initial examples using the offset calculator.

Where does this "42 and up" figure come from ? Am I right then that it's a guide to how to avoid the wheels sticking out past your arches (but taking on Gord's point about tyres affecting this too) but doesn't say anything about if the wheels might rub the inside ?

Yes mate, 42 will mean, don't go any less or you may have problems clipping the arch. as i've said its just a guide mate, best bet if you not sure if wheels will fit, is go and try them or ask if anyone else running the same ET and tyres :D

I once got some wheels of the net, told them what ET i wanted, but they sent ET35,i would not fit them, as they might not have taken them back, the wheel place was only in Bolton so took them back, he did put the wheels on my car, to try and prove his point (his listing in the book said would fit is200) but when fitted it came very clear they would not :o

[Parthiban]

As Gord says it's just a guide, and I think the "and up" is purely arbitrary as it's rare to find alloys that have very high offsets.........look on eBay and most of them are in the 30s and 40s, so the guide is to prevent you from going too low rather than too high.

[Steviewevie]

Thanks all. It's pretty clear to me now what I wasn't "getting". I did seem to be understanding the offset calculation etc properly. I did think that Mat's table that I quoted was to avoid scraping the inside, but I now see that it's the opposite, it's so you don't go too far outwards.

I see what you mean now, Parthiban, about the "and up" being pretty arbitrary, I think I was taking that a bit too literally.

The reason that Bazza's example confused me was because I was mixing up the issues of the centre of the wheel itself, and the position of the mounting surface of the wheel.

I think I understand it now ! :D

[ormi]

its meeeeeee thats got the off set of et35 and to be honest lowered boot full 5 up blasting down the road no arch to tyre contact at all they were also on r34liners is200 as well and his was on the floor.running 215 40 18s though.

pic below.

[Boxbrownie]

I have no idea when it changed but 35 years ago when we used to race Minis and FF it was always referred to as Offset (hub face outside of wheel centre) and Inset (hub face inside of wheel centre)....no confusion.

Best regards David