Why Do Wheels Scrub? :S

[2005vzute]

Ok, so this might sound like a very dumb question and i have a feeling i've missed something obvious...the thing is, you often hear of people saying their 20's are scrubbing right? but why are they scrubbing?...the law states that a rim/tyre combo is + or - 15mm (i think) diameter to be legal...so when we upgrade our 15 inch wheels with fat tyres for 20 inch wheels with skinny tyres, there will only be an additional 15mm diameter MAX, if the combo is legal...so, with that in mind, why do 20 inch wheels scrub?

is it because people are going over that 15mm + or - legal diameter limit or what?

thanks in advance cheers everyone

[Verynice]

It's more so how wide the tyre is and also camber. When you jump up from 15" to 20" obviously you have a wider tyre, but can still keep within the 15mm difference.

If the tyre is TOO wide at the front, it will scrub on the struts and maybe also the plastic trim when turning.

Others may find they have a fair amount of negative camber at the front as well, which again, is putting the tyre closer to the strut.