This posting is about some of the research and information I've gathered whilst looking into this and some links to some sources of information to explain the concepts I've learned
Wider wheels scrub on the radius arm (or other aspects of the inside of the wheel arches) or you want to just make the tyres look bigger and have potentially more stability by having a wider axle width on the road or you need more turning circle (all of these would be nice on Katy)
What they are for, is to move the wheel further out from the hub. This has the positive of making the wheel look bigger, and it gives some additional space when turning the wheel which will give increased turning circle and stop the tyre rubbing on the radus arm (or anywhere else) which is why I was looking into them
The trouble is that they are heavy lumps of metal and the effort of moving them around causes more strain on the suspension and transmission, and can increase bump steer which is a steering effect you get when you change the suspension geometry of any car, and it's bad because Excessive bump steer increases tire wear and makes the vehicle more difficult to handle on rough roads which is where Land Rovers are meant to go.
I've also heard (especially from the really helpful people on Land Rover Scene ) about all of the problems you can have if they are not absolutely tightened and come loose behind the wheel when you are driving, which ase led to wobbling wheels and even wheels coming off on the motorway, and also that many insurance companies don't like them
The guys on Land Rover Owner explained more
"Quality is very important as you are putting load on them from the hub to the spacer then from the spacer to the wheel from acceleration, braking, bumps, cornering and centripetal [see comments] forces from the spinning of the wheels and spacers themselves.Spacers should not effect wheel balancing as most spacers are turned on a lathe. Spacers can cause slightly higher wear rates in brakes and wheel bearings due to the extra forces they have to deal with under breaking and turning due to larger moving mass that comprises of the wheel, tyre and spacer combination when compared to wheel and tyre only"
OK that's enough for me to explore the other option which is why I then asked the same experts about wheel options and got a lot of comments about wheel offset which was something else I had never learned about so here is what I now understand about those concepts
Wheel offset is the difference between the mounting plate that connects to the hub and the centre of the wheel. Fitting wheels with different offsets will move the tyre out like a spacer or inwards. Moving it outwards requires you to put on a tyre with a smaller (or negative) offset whereas making the tyre move inwards requires an increase (more positive offset)
To help you get your head around the concept this diagram may help
So to get more room for tyres to prevent scrubbing on the body or suspension which is the problem I've had I need to get wheels that have a more negative offset.
If I wanted to fit bigger wheels but had a problem with them sticking out of the wheel arches I would need to fit wheels with a more positive offset because that brings the wheels further back into the arches
It's been suggested that all four should be the same as the ones I have on the rear and I will certainly investigate getting some more like that
I've still got more thinking about this to do, and I need to get under Katy and on top to check out all the wheels I have and work out wat ofset they have, and see what I should have and make decisions