Postby davew » Fri Jan 16, 2009 9:56 pm
The image above isn't the easiest thing to follow in my view but the information is all there.
The first problem is that most people talk about "offset", I assume this is because it's the most visible thing when the wheel is on the vehicle. In terms of turning circle and track though the offset isn't important on it's own.
The important measurement is the one the wheel industry use - backspace. This is a measurement from the inside edge of the rim to the knave plate. You can more or less work out the backspace of a wheel if you know it's width and offset (measured from the outside edge of the rim to the knave plate).
The smaller the backspace is, the better steering you will have with any given set of tyres. Essentially the backspace measures the distance from the hub to the inner edge of the wheel and as a result the bigger the backspace the closer the tyre is to your radius arms - normally the limiting factor on a Land Rover.
From the picture above, the backspace for normal Land Rover wheels would be...
Standard Rim 5.5 inch - 4 inch backspace (offset is 1.5 inches)
Wolf Rim 6.5 inch - 4 inch backspace (offset is 2.5 inches)
Modular 7 inch - 3.8 inch backspace (offset is 3.2 inches)
Disco steel 7 inch - 4.8 inch backspace (offset is 2.2 inches)
Allied Rockathon 8.25 inch- 4 inch backspace (offset is 4.25 inches)
Matthew's new rims 10 inch - 2 inch backspace (offset is 8 inches)
The above shows straight away that Disco steels bring the tyre 1 inch closer to the radius arms than a modular of the same width. It doesn't sound like much but that's in the straight ahead position, as the wheel turns, the closer the wheel is already the quicker it moves towards the radius arm due to the offset pivot of the hub.
The disco steel is so badly off center (offset is much smaller than backspace) that it is not unusual to reverse the centers, turning them into a 2.2 inch backspace and turning them from a really awful trials wheel into one of the best. The problem is getting someone who can do it properly !
I don't have any info on 8 inch modulars but normally they come in at around 4 inch backspace, the same as the Allied ones.