Okay say one has a rim, and the offset of the thing is unknown, how do you work out what it is?

It's stamped on it. Inside on mags, usually outside on steel. Look for a number with a 'P' (for positve) beside it. If it's a standard gen3 wheel it will say'46P'.

thanks for solving the mistery.

now what are the limits to be considered when buying new wheels.

for example: if standard is P46 - maximum should be P50 and minimum P40?

My 20"x8.5" Rims have a 35mm offset..
At the rear I also have a 5mm spacer .. to be on the safe side.. to stop the rubber from rubbing.
The rears WILL and DOOOO hit the wheel arches on a STD Magna.. A flared guard like yours is worse as the guard is lowered and also the flare and the brackets stick in even more ..

The Fronts are Flush with the guards... and I cannot lower over them until I flare the actual metel out more.. (Mind you Im about to add a 20mm spacer to the fronts :( so this will make them protude 20mm past the guards..... so I now HAVE TO Wide body the bastard :D)

Haydn

It's stamped on it. Inside on mags, usually outside on steel. Look for a number with a 'P' (for positve) beside it. If it's a standard gen3 wheel it will say '46P'.

Yes okay - I have some TJ Sports rims here which are 16 X 7 and indeed have a moulded '46P' inside them. But the query is regarding some Simmons style 3 piecers which don't seem to be carrying any marks that I recognise, so I guess I'm wondering if there is any way to measure or calculate the offset from the rim itself?

I think this is how its done.... correct me if I am wrong anybody.

Measure the distance from the face that sits against the hub the the inner side of the rim. Then measure the width of the rim and divide it by 2. Subtract the second measurement from the first and that is your offset, if it is above sero it is positive offset.

i.e x - (y/2)

x = hub mounting face to inner edge of rim
y = rim width

Normally offset is expressed in millimetres.

Umm arent spacers illegal ? :redface:

Umm arent spacers illegal ? :redface:

So? Spacers for 20s :D Who wouldn't...now both spacers and rims are illegal :badgrin:

As for measuring offset, wouldn't it be difficult to measure it yourself, because the slightest error, could easily lead to you getting the wrong figure in the end. :confused:

now what are the limits to be considered when buying new wheels.

for example: if standard is P46 - maximum should be P50 and minimum P40?

Depends on where you are. From a legal point of view you are not usually allowed to narrow the track of a vehicle. Especially FWD. This means you cannot go above the original offset ie no greater than 46P (also referred to as ET +46). This is also a good idea if fitting wider rims as they would only foul on the suspension struts.

You are allowed to widen the track ie bring the offset closer to zero. In QLD this is limited to 26mm which equals a difference in offset of 13 (because it applies to each wheel) so the lowest you can go is ET+33. In NSW the allowed increase in track is 25mm so ET+34 is the legal limit.

From a practicle point of view you want to get the closest to the original offset that doesn't cause fouling. The reason is as you widen the track you put extra strain on some steering and driveline components which will shorten their life span. Also FWD cars have a deliberate front suspension setup where by a line drawn down the centre of the strut will hit the ground somewhere outside the centre of the road wheel. This is for stability reasons. Widening the track too far will push the wheel out and mess up this setup and can make steering difficult or dangerous.

As for measuring the offset, there is a way of doing it but I wouldn't buy a wheel that doesn't have it stamped on it. There is no industry standard applicable to wheels imported to this country which means there is some dodgy crap being sold legally out there. Obviously you would be OK with BBS or other well known brand but you will find the price to go with that confidence is VERY high. "LIFETIME WARRANTY" is another one you want to look at closely. What exactly do they mean?

Yes spacers are illegal everywhere for on road use.

Yes okay - I have some TJ Sports rims here which are 16 X 7 and indeed have a moulded '46P' inside them. But the query is regarding some Simmons style 3 piecers which don't seem to be carrying any marks that I recognise, so I guess I'm wondering if there is any way to measure or calculate the offset from the rim itself?


I think (now that I think about it) the reason why the 3 piece wheels don't have an offset stamped on them is because they can be built up to make any offset you want just by changing the size of the outer and inner rims. I think that might have been part of the reason why they were used in racing initially - just a guess.

If you know how wide each piece is you could work it out as above.

Check this RTA NSW site
http://www.rta.nsw.gov.au/registration/downloads/vsi/vsi09_rev4.pdf

I think (now that I think about it) the reason why the 3 piece wheels don't have an offset stamped on them is because they can be built up to make any offset you want just by changing the size of the outer and inner rims. I think that might have been part of the reason why they were used in racing initially - just a guess.

If you know how wide each piece is you could work it out as above.

Yes I think you're onto something there - the offset would by nature vary with 3 piecers but the vendor of these is claiming they're 35P so okay I'll believe that for the moment. The other part of the problem is that I'm trying to compare them to a early 80's Italian made wheel which either pre-dates the current offset measurement system or else is using some other EU specification altogether.

These Italian wheels are off a Ferrari Mondial (the boss' 'other' car), and we only have one original wheel, the spare. The current on-car rims are a mishmash of sizes front and back and spacers are used at the back as well so its not very satisfactory. The Mondial is rear engine east-west rear wheel drive so the plot is similar to a Magna, only back the front as such.

I'm going to try the calculation above but I think I'll also take the thing to the local tyre guy and get his opinion on them. Thanks for the pointers everyone - I'm learning something here I'm sure!