Wheel Specifications

[Alfadog]

Now, there are a whole lot of numbers on a wheel which mean different things - most of which I know already but there is a certain part of the specification that I don't understand. Hopefully one of you can help explaining this to me, and possibly some others here too.

Each wheel has the following dimensions (please say if there are others too!):

Stud number and pattern (e.g. 4/114.3)

Diameter (e.g. 15")

Width (e.g. 6")

Offset (e.g. 25 degrees)

What exactly is the "offset"? Why do different cars need different offsets? Do you need a different offset if you go for a wider or larger wheel? I'm just a little confused...

Thank you

[Graeme Suckling]

Good day Lachlan,

First I apologise, there is no way I can get to the 2003 Nationals in Brisbane this Easter.

Instead, the info. you require is thusly: -

the offset is in mm.

i.e. this dimension is the number of mm the centre line of the rim differs from the wheel mount flange surface.

A ZERO mm offset means that the rim centre line co-incides with the wheel mount flange.

From there things can go NEGATIVE (wheel flange towards the centre of the car) or POSITIVE (wheel flange towards the outside of the car).

Look at the average front wheel drive which has a 40 - 50 mm POSITIVE offset.

Used on an older vehicle that sort of offset will cause the tyre to rub on the suspension strut.

On a 240/260 the max. offset (positive) for the front is in the order of 11mm.

Any more than that and you start to muck up the Scrub Radius and other technical stuff.

The Zs I think came out with close to zero offset originally and yes if you fit a wider rim the offset must change to keep the tyre to strut dimension under control.

[Alfadog]

I'm very sorry to hear about you not coming this easter... I'll take pictures for you. How about 2004?

OK so offset doesn't really need to be exactly the same, in fact if it meant to have a positive offset, if a wider wheel is wanted the offset should be ... more?

My 240K has specified a 28 mm positive offset. How varied can the offset of new wheels be before some problems occur? Is it hard to say exactly?

Are front and rear offset values different usually?

Lachlan

P.S. would love to see some pictures of your cars some time Graeme... you should upload them to the gallery!

EDIT: Whoops, looks like you had the same idea as me and put URLs to your images in your signature! Good one.

[Graeme Suckling]

The front and rear offset values are the same because the rear wheels are not steerable. It is the front wheels that have a "scrub radius" i.e. where the steering radius circle differs in respect to the wheel offset radius.

You really don't want the wheel to move over an area of road surface instead of pivoting on the road surface.

You can alter the offset to a degree, but sooner or later you will introduce a bad factor of scrub radius.