Wheel offsets confuse me. What should I choose?

[Alanpandew]

My 911.50 has the following OEM wheels;

Front: 20"x9" ET51
Rear: 20"x11.5 ET48

I can pick up a set of wheels from a WB 997 with the following specs

Front: 19"x8" ET57
Rear: 19"x11" ET51

I would use these exclusively for AX and maybe an ocassional DE. The difference in offsets has me confused - do I need to add spacers (and is that advisable for my application?), or am I looking at this backwards?

 

[BrianPetersen]

For a difference of a few mm in offset (which will translate to a few mm difference in scrub radius) I'd just run them as is, unless there is a clearance issue, and your proposed wheels are narrower than the originals so presumably so will be the tires, so there is unlikely to be a clearance issue. Brake calipers clear the smaller wheels?

 

[CapriRacer]

Go to Tire Rack and see what offsets they specify for wheels for your vehicle. They measure the clearances and determine what will and will not fit.

You can also go to Discount Tire's website, but the offsets are a little harder to pull out.

 

[NormPeterson]

The math is simple enough once you either convert wheel width to mm or offset to inches and compute the backspacing (distance from the wheel's mounting surface to the inboard flange face). Don't forget to add about half an inch for the flange thickness, as wheel width is measured between the bead seats.

Draw pictures so you can see what's going on. +offsets move the mounting surface outward from the midpoint of the wheel width, increasing the backspacing number. Something like this (for illustration only, not your setups). For only one or two cases, nothing this fancy would be necessary, and reasonably to-scale freehand sketches should be adequate.

If you don't need spacers for clearance reasons, don't bother with them. This gets into what tire sizes you plan on putting on these wheels, what their actual measurements are, and on differences between your wheel widths and the "measuring widths" for those tire sizes. Tire section width varies with changes in wheel width, by about 0.2" per half inch of wheel width change, which could affect clearances and whether small spacers would be required.

Tire Rack and similar tire and wheel sources tend to be rather conservative in their fitment recommendations, especially for front wheels & tires.


I am curious why you'd run wheels that are narrower than OE. You'd be handicapping yourself at either autocross or DE, and in autocross you'd be moved up in preparation class while downgrading your car's actual preparation level.


Norm