New Offset After Adding 2" to outer lip on 19x9.5 +34 ?

[CLKFAN]

Like the title says:

What is the new offset after adding 2" to the outer lip on 19x9.5 +34 ?

These wheels are 1pc that will be getting widened 2" on the upper step lip like this example on a GT40 race car in the link provided below

link

http://www.gt40s.com/forum/gt40-tech-chassis-brakes-tires-wheels/24859-wheel-widening-2.html

Besides ending up with a 19x11.5 wheel I'm assuming that since the center mounting point never changes that there is no change but some input from you guys would be great. Thanks.

I can't find anything on this topic after searching here.

[wagonist]

Et 9

[CLKFAN]

Et 9

How did you come up with that number?

I know that 2" is 50.8mm but how is the calculation done and how does it go to et 9?

[macleanshaun]

Offset is calculated from center of wheel to hub mounting point.
Take your original offset, add 2" of lip (subtract from offset), and that's your new offset.
eg: original wheel is 16x7 et35. Add a 1" lip. New wheel is 16x8 et10.
If you add lip, it doesn't affect backspacing. Just offset.

[CLKFAN]

Offset is calculated from center of wheel to hub mounting point.
Take your original offset, add 2" of lip (subtract from offset), and that's your new offset.
eg: original wheel is 16x7 et35. Add a 1" lip. New wheel is 16x8 et10.
If you add lip, it doesn't affect backspacing. Just offset.

If I do that I get et -16 and not et 9

[180roman]

nope, when you add lip to the outside of the rim you subtract half the width you add from the offset

eg, add 2" of lip to the front = old offset - half the added width(1")

thats how you get the Et 9

if you widen the inner barrel then you do the opposite and add to the offset
eg. add 2" to the inside = old offset + half the added width(1")

[macleanshaun]

If I do that I get et -16 and not et 9

That would be correct. Don't know where that other guy got 9?

[macleanshaun]

nope, when you add lip to the outside of the rim you subtract half the width you add from the offset

Why half? This makes no sense.
You're adding lip to the outside only.

[180roman]

That would be correct. Don't know where that other guy got 9?

because here is the maths using OP's specs

Offset = backspacing - center line

original width 9.5"
original center line 4.75"
original backspacing 4.75" +34

because you are adding lip to the front you are not effecting the back spacing, but here is how it affects offset

New Width 11.5"
New Center Line 5.75" (here we can see that even though we did not change the backspacing the center line of the wheel has changed)

So we can now calculate offset using the formula at the start
Offset = 4.75" +34 - 5.75"
Here we can clearly see that the offset is reduced by 1"

Knowing that 1" is roughly 25mm the new offset = 34 - 25 = 9

[wagonist]

thank you from saving me from a bunch of typing.

OP, if your unsure how to calculat offset or just dont get the logic, use an offset calculator like this one:

http://www.1010tires.com/WheelOffsetCalculator.asp

if you stare at the picture of the rim long enough you'll get it

[180roman]

if you stare at the picture in the sig long enough you'll get it

fixed

[wagonist]

that too

[CLKFAN]

Thanks guys, I'm glad I asked

[macleanshaun]

You're forgetting that wheel width is calculated to tire bead, not outside of the lip.
Backspace is calculated to edge of wheel.
an 8" wide wheel with ET 0 will have 4.5" of backspace. 1/2" of bead on either side. 9" wide edge to edge.

[180roman]

You're forgetting that wheel width is calculated to tire bead, not outside of the lip.
Backspace is calculated to edge of wheel.
an 8" wide wheel with ET 0 will have 4.5" of backspace. 1/2" of bead on either side. 9" wide edge to edge.

i simplified the calculations by ignoring that since it has no effect on the result what so ever

the "true" width of the wheel is usually 1" wider then the bead section which is measured

so here is the calculation re done for you to prove the point

"true" width = 10.5"
Centerline calculated from "true" width = 10.5"/2 = 5.25"
Backspacing is 5.25" +34

so with the 2" added its now
"true" width = 12.5"
Centerline calculated from "true" width = 12.5"/2 = 6.25"
Backspacing stay the same being 5.25" +34

If you don't see a pattern here yet

Offest = 5.25" +34 - 6.25"

Which still gives the exact same result being ET9

you only need to use the "true" width of the rim if you are using a ruler to measure the offset