I had a long discussion on the phone tonight with Dave Farish, who is also building an R1 Striker, and one of the topics was wheel offset. The summary of the question we were discussing is: is a wheel with the offset ET35 closer to the bodywork than a wheel with an offset ET15?
Offset (ET) is the distance from the hub-mounting surface to the centerline of the wheel. A high-numbered offset brings the wheel closer to the car, whereas a lower offset pushes it out to the fender.
With adapters, you need to subtract the width of the adapter from the offset of the wheel to calculate the wheel’s correct offset.
For example, a 20mm adapter on an 18″ Porsche wheel with ET50 (everything is measured in millimeters) means the final wheel offset is ET30.
I spoke to a very helpful chap on the Team Dynamics stand at Autosport, who explained it all to me in words of one syllable – to summarise, the offset number is somewhat counter intuitive – the higher the offset (ET) number the closer the wheel is to the car, and if you add a spacer, it lowers the offset number, and moves the wheel away from the car.
All of this means my plan to buy ET35 wheels and spacers still flies. I can then adjust my wheel offset from between ET35 and ET15, which seems to be the range of suggested offsets applicable to the Striker.