1 reply to this topic

[Klein91]

so I'm looking at replacing my steel wheels and trims with alloys, I looked at some focus rs alloys, I've found a 'genuine' set of white 18"alloys but I thought the rs alloys where 19" does anyone know? Also if I got the 18's what tyre size and other stuff would I need to make them fit right and what do you think about white alloys on a silver car? Cheers

[BOF]

Also if I got the 18's what tyre size...

Usually, what happens is that the rolling radius of the wheel/tyre combination stays the same as you move throughout the range (that is, wheels may go from 16 -> 17-> 18 -> 19 inches, the profile of the tyres gets lower so that the total diameter stays more-or-less constant, otherwise they'd have to fit different speedo gearing as the tyre size changes and the gearing would change when people opted for different sizes).

There are sites scattered around the internet that will will calculate rolling radius for you; choose one that you like, and check that your proposed combo is within a sensible range.

Additional factors to take into account are that the width of the tyre ought to be sensible for the rim pan dimension. Unfortunately, the tyre width is in mm, while the the tyre pan dimension is in inches, but tyre manufacturers publish a range of dimensions for which their tyres are recommended. Checking this out is a lot of fiddling work, because, as far as I know, there isn't a mathematical rule that you can apply, but you have to check the data. As a general-ish sort of rule, you don't want the tyre width to be too much wider than the pan, or you will lose handling precision and gain vulnerability to sidewall injuries (unfixable!), and you don't want the tyre width to be too much narrower than the pan, because you will lose ride comfort and gain road noise. Of course, your definition of 'bad ride comfort' or 'lack of handling precision' may be set at different levels than Ford allowed for, in the beginning, but that's part of the game.

In addition, you don't want to change the ET by much, otherwise you start to run into unstable handling if one of the tyres does somewhat deflate or you are on a split mu surface.