Confirmation of offset calculation please

[Gus Berges]

I trust the experts here implicitly and just looking for validation of the calculation I did using the following calculator: 1010TIRES.COM - Wheel Offset Calculator My apologies for this not being Porsche specific (it's for a Maserati...), but just wanted to confirm with the wheel experts my results...

Original wheels are 7.5" wide x 15" high with an offset of 40. If I were to replace them with 8.5" wide rims x 17" high with an offset of 27, this would yield the same inner clearance, but the wheels would extend (by nearly an inch, 26mm to be exact) towards the outside. Assuming the clearance is there, or at worst case, would have to roll the fender, this would give me the look of a deeper dish wheel, correct?

If instead of an 8.5" width, I do a 8" (vs. the original 7.5"), then the offset would need to be 34 to yield the same inner clearance, correct?

Again, just want to confirm that the results of the calculator listed above are accurate as I don't know how to do the conversion. The other factor from reducing the offset that I need to take into account (besides rubbing against the fender or the inner portion) is the space required by the calipers, correct?

Thanks in advance for your help!

[Gus Berges]

Anyone?

[KTL]

Gus,

Going from a 7.5 width to 8.5 width, the inner clearance is lessened by the increase in width. Inboard width of the wheel increases by 1/2 inch (half of the one inch overall width increase) and decreases by the reduced offset. Offset of +40 minus +27 offset = +13 which is basically a 1/2 inch movement of the wheel outboard. So the rim width grows by 1/2 inch but the offset shifts the wheel outward 1/2 inch with a net effect of zero. So yes your inner clearance stays the same.

Outer clearance is also lessened by the 1/2 inch width increase and you also must add the offset difference. 1/2 inch width plus 13mm offset difference = approx. 1 inch outward movement of the rim edge.

Caliper clearance is often a function of the wheel face shape & not necessarily the wheel offset. Some wheels have curved spokes that allow for more brake clearance. But a flat face wheel (with a flat backside of the face) needs to have a sufficiently deep hub/mounting surface to push the backside of the face away from the caliper.

Caliper clearance to the wheel barrel is also sometimes an issue. But that is typically a concern when installing larger brakes with original wheels. In your case where you're using larger diameter wheels, the caliper clearance to the wheel inside diameter is irrelevant.

CCW gives a nice procedure on checking caliper clearance against the wheel. Look under the "Documentation" heading where it says Brake Caliper Measurements and McPherson Strut Clearance Diagram. CCW - Complete Custom Wheel // Technical Both of those are links to pdf documents

When you're cutting it really close its important to do some actual measuring on the wheels. Meaning an 8.5 in. wheel is not really 8.5 in. wide. It's actually around 9.5 in. wide from outside edge to outside edge, since the stated 8.5 width is from the area where the tire bead sits on the wheel. Bead seats are around 1/2 in. wide, so using an additional 1 inch is generally accepted practice. However if you're really pushing the limits of your available space, it's best to physically measure what wheels you've currently got on the car and get actual measurements of the wheels you're considering.

Don't forget tires also play a role in the game. All tires of a stated size do not actually measure out to be that size. Meaning, all 225/50-15 tires are not 225mm wide (some are wider, some are narrower) and 23.86 in. tall (theoretical diameter is 15 in. dia. plus 2 x (225 x .50) = 23.86 in.) Not to mention the width of the tire changes depending on what width wheel you place it on!

[Bill Verburg]

Play w/ this calculator

wheel fitment

[Gus Berges]

Kevin: thanks a million for the explanation/clarification. Since these would be custom made wheels, I have no actual wheel to try fitment width, hence my concern regarding accurate calculation of the offset necessary for proper fitment.

Bill: had not seen that calculator before. Thanks for the tip!