Wheel width / offset calculations

[ledrocnoc]

I've been looking at upgrading my C43 wheel with some AMG monoblocks from an 2000 (or so) E55 (18x8 ET31 and 18x9 ET35). From what I have read so far on the forum, 225 front and 255 rear tires on these wheels will fit, but since my goal is also to fit wider tires (245 front and 275 rear), I've been trying to figure how wide I can go with these wheels.

I've found a shop that will do some fender work for me ("grow" them out, maybe even install flare if needed), so I am mostly concerned about rubbing on the inside of the wheel well (struts, steering knuckles, etc...).

I put together a small spreadsheet to try to estimate, for a wheel width/offset and tire width, how will the new tire width be distributed (inside vs, outside) compared with the stock setup.

Here is a rim drawing and the spreadsheet I put together:


http://home.comcast.net/~ledrocnoc/C43/wheel_offset_1.gif


http://home.comcast.net/~ledrocnoc/C43/wheel_offset_2.gif

Dimension A is the width of the tire measured from the wheel hub inside surface (where is contact the car hub) to the edge of the rim or tire on the inside of the wheel, while dimension B is the corresponding dimension to the outside of the rim or tire.

For any tire, in my model:

A = (tire_width)/2 + (rim_offset)
B = (tire_width)/2 - (rim_offset)

So from going to

FRONT C43 17x7.5 ET35 rims with 225 tires to FRONT E55 18x8 ET31 rims with 245 tires the total width grows by 20 mm (!) distributed as follow:

- 6 mm on the inside
- 14 mm on the outside

REAR C43 17x8.5 ET 30 rims with 245 tires to REAR E55 18x9 ET35 rims with 275 tires, the total width grows by 30 mm distributed as follow:

- 20 mm on the inside
- 10 mm on the outside


So, that means that I need

- Inside FRONT wheel well clearance of at least 6 mm
- Inside REAR wheel well clearance of at least 20 mm

- Outside FRONT fender clearance of at least 14 mm
- Outside REAR fender clearance of at least 10 mm

Besides having people validate my model :-) I have a few questions:

1 - Obviously, if I don't get the inside clearance needed I can always add some spacers. I've read mixed feedback on spacers on C43, with some people complaining about vibrations. What the general club202 wisdom on spacers?

2 - If my shop can widen the front fenders by the required 14 mm, will this still prevent rubbing at full (or partial) lock when the suspension is compressed?

Anything else I need to think about?

Thanks.

DC

[c55m8o]

Though your spreadsheet probably does the same, Click Me (http://err0neous.org/mbz/offset.html) for a wheel offset calculator. This is replicated on a lot of other sites all over the internet as well.

[alhacker]

i with those numbers u can install the wheels with a bit of fender flaring... i installed 18x8.5 et15 on my car 1997 c230 i had many issues but everything is resloved... i had to negative caber all the wheels to 8 front-11 rear degrees, and to massage all fender out then install fender flares for the look only...

[Denlasoul]

bump.

This Thread is a pretty good one in determining widths and offsets. The link further down is also a good reference tool.