Offset
A guide to why running the correct offset is so very important.
Offset may be defined as the distance from the mating face of the hub to the centreline of the wheel along the axis of travel when viewed from above. This can be seen in the diagram below.
A wheel which has it's centreline in direct alignment with it's mating face is said to have a zero offset. This can be seen below along with examples of positive and negative offset.
Most cars are designed to accommodate a specific offset and hence, using wheels with an incorrect offset can have an adverse effect on handling. The reason for this is best illustrated in the diagram below.
Ideally, a "steerable" wheel should pivot around the centreline of it's fore-aft axis. In this instance, under braking and acceleration, the tyre surface on both the left and right hand sides of the centreline have the same amount of leverage. Figure 1 shows a wheel which achieves this ideal.
It is important to understand that this neutral setup does not constitute a neutral offset. If the mating face of the hub is not in alignment with the steering pivot then it is necessary to introduce a degree of offset to ensure that the wheel pivots around the wheel's fore-aft centreline. This is the purpose of offset.
Figure 2 shows the resultant forces applied around the wheels steering pivot when the pivot is not centred due to an incorrect offset. In this case the wheel will turn to the right under acceleration in an attempt to establish equilibrium and resolve the forces. Under braking the wheel will turn to the left.
Now, in practice, a car with 2 "steerable" wheels will have a steering rack connecting both the left and right front wheels. As both left and right wheels are on opposite sides of the car, one will be trying to turn right whilst the other will be trying to turn left. This inevitably puts strain on the steering mechanism as the two wheels engage in a "tug-of-war", increasing wear and tear on the steering pivots and linkages. As long as everything remains equal this is the only downside to running wheels with an incorrect offset. However, on the road, things don't remain equal for long. Uneven roads, surface condition and other factors mean that one wheel will almost always have more traction than another. Whichever wheel has the dominant traction will also have the dominant say in which direction the car is going to turn. Hit a bump, a pothole or a patch of oil or mud and the car will show just how unstable this setup is.
Here's a little story for you...
The Vauxhall Calibra has an ideal offset of ET49 (ET from the German, "Einpresstiefe", meaning "insertion depth". I bought a second Calibra Turbo which came fitted with a lovely set of OZ Polaris 17" alloys. It was obvious from the way they protruded from the arches that the offset was not quite right. In fact, they had an offset of ET35 which meant that the steering pivot point was 14mm away from the fore-aft centreline of the wheel. Given that the tyre width was 215mm there was effectively 93.5mm of tyre width to the inside of the pivot and 121.5mm to the outside, a difference of 28mm. All was fine whilst taking it easy but I noticed that whenever I accelerated or braked hard the car would squirm and twitch all over the place. Had I not already owned another Calibra Turbo for some years I might have dismissed this as normal. The final straw came when driving quickly over a humped back bridge the car tried to throw itself violently into the wall. It was something that I had done in my other Calibra a hundred times or more without incident. Shortly after I sold the alloys, ironically to another Vauxhall owner who didn't care for my advice.
Obviously, the closer to the correct offset a wheel is, the less the handling will be affected. In addition, on a 4WD car the acceleration forces are divided more equally across 4 wheels instead of just 2 and hence the effect of incorrect offset is not as dramatic. I realise that a shiny new set of cheap alloys from eBay might enhance your cars aesthetic appeal but will it still look so cool wrapped around a tree...!!!?