do you understand wheel offsets ?

.... thats great because sadly I dont at all 🙂 

Not for an F or TF this but i have another car where its standard wheels have an offset of  24mm.
Now im thinking of bolting on different wheels that have an offset of approx 48mm.   
But because the pcd  pattern is different i'l need converters/ spacers to match the pcd of the car. The spacers themselves are 25mm thick.

Would i be right in thinking the width of the spacer should be subtracted from the offset of the new wheels: 
if so then 48 - 25 would give a combined offset of 23mm which is only 1mm away  from the standard wheel = a perfect fit.

Or have i got this arse about face ? 



 

It is fairly straight forward.
You can have Zero offset, Positive offset and Negative offset.
Zero offset is when the face that mates with the hub is exactly in the centre line of the wheel.
So a 6inch rim that has Zero offset would have 3 inches equally spaced from the hub..3 ins out and 3ins in.

Positive offset of say 1inch with a 6inch rim would mean there is 4 inches Inward of the face and 2 inches outwards...resulting in a fairly flat faced looking wheel.

Negative offset of 1inch on a 6 inch rim would mean 4 inches Outward from the face and 2 inches inward. A deep dish looking wheel.
If your wheels are looking a bit too inset of the wheel arch by adding a negative offset wheel it would fill the arches better still keeping the same width rim.

 

Wheels can be positive offset, neutral offset or negative offset. 
Neutral or zero offset is where the mating face of the wheel is in the centre of the rim.
Negative is where the mating face is biased to the inside of the wheel, The old style deeply dished wheels on beam axle rear drive cars were usually neg offset. Rear drive cars with independent rear suspension often had positive or neutral  offset wheels.
Positive offset is where the mating face of the wheel is biased towards the outside of the wheel rim, as is the fashion these days, it is because front wheel drive requires it to maximize drive shaft length to reduce the flex angles of the joints. These days, even rear drive cars that have no such requirement use pos offset wheels (to look more modern as front drive is the seen as the norm to the average punter)
My ford ute is a case in point the wheels are made to look positive offset by use off excessive material of the outside, but are actually negative offset, this is archived by using ridiculously long wheelnuts, to reach the studs, that are buried deep within the wheel, utterly stupid as the wheel is much heavier than it needs to be......all in the name of fashion.

So by using spacers you are effectively pushing the offset more negative by the thickness of the spacer.
Which can effect wheel clearance and more critically it can have an undesirable effect on suspension and steering geometry, not to mention increasing wheel bearing loading.
To offset this offset you would need to use a wheel with a more positive offset (by the thickness of the spacer)

That should be as clear as mud!

To demonstrate the effect on the load on wheel bearings. Go outside and find a brick (everyone has a stray brick lying about) now hold the brick in your hand up close to your chest, you can feel the weight but it is manageable, now hold the brick with your arm out stretched away from you........the brick is getting heavy pretty quick now isn't it!
That's what excessive offset can be doing to your wheel bearings!

 

Well done Cobber..a picture paints a thousand words.
Easier to "see" what I was trying to explain in words!
Notanumber just needs to find out if his offsets are positive or negative to tell him how they will look or indeed work.

Chris, I can give you an example that I dealt with a couple of years ago, when I did the big brake conversion on the TF I took the opportunity to fit the much lighter MX5 wheels which had a negative offset of 48mm, simply by using 20mm wheel spacers that reduced the offset to 28mm which was perfect for the TF.  As I alluded on another thread the wheel bearings on the F/TF are much smaller than those on similar powered cars so it becomes more important than ever not to overload them by spacing them in or out by too much. 
 

Thanks guys. This would be for my B. Id read a few people were running 15 and 16" MX5 wheels on theirs with pcd converters without clearance issues. As there wasnt any detail i compared the offsets which seemed to check out but thought it best to check i was calculating it correctly rather than guess and get it wrong.

So to conclude. Is the consensus then that a (decent quality) 25mm thick converter, when used with a 48mm offset wheel would have a combined offset that is very close to that of the original wheel which has a 24mm offset.

Yes. If the wheels are positive offset 48mm. Adding a spacer/pcd converter will reduce the offset by the width of the spacer/pcd converter.

Another good website for an explanation is MWS in Langley.
The concentrate on wire wheels but the explanation for offset is clear.