Wheel offset and backspace

CMF_Polarbear · Feb 22, 2010

Am I doing this maths right?
15"X6.5" rims
127mm backspace

Is ~32 offset?

Also that should fit the Micra fine? With spacers?

CMF_frank2 · Feb 22, 2010

this is handy polarbear

http://www.willtheyfit.com/
and my stock ally,s are -38 and stock steely is -45

frank

CMF_Polarbear · Feb 22, 2010

Only helpful if I knew stuff about my old alloys that are already on the car... Aside from 13X5.5 I can't see anything else written on them.

CMF_deNs · Feb 22, 2010

Total Width: 6.5" or 165.1mm

Total Width - Backspace = Space from centre line of hub to face of wheel
Therefore, 165.1 - 127 = 38.1mm

If the space from the hub face to the back of the rim is 127mm then all you need to do is see how far forward it is from the centre line of the 6.5" rim. In this case, the centre line is 82.55mm. The distance between the hub face and the centre line would be the difference between 38.1mm and 82.55mm. That would give it an offset of +44.45.

That is of course, unless I'm misunderstanding your idea of 'backspace'. I'm very tired and the above may well be incorrect, but if you draw it all out on paper it should start to make sense. Just figure out widths and where the centre line is and go from there.

Also frank, unless you're running huge flares, I think you mean +38 and +45.

---dens

CMF_frank2 · Feb 23, 2010

deNs WROTE:

Also frank, unless you're running huge flares, I think you mean +38 and +45.

---dens

i stand corrected o wise one

frank

CMF_Polarbear · Feb 23, 2010

Thanks Dens, I went looking for instructions on calculating backspace and offset so I could work backwards and for some reason it wanted me to add an extra inch to the 6.5 for like nominal width or something? I dunno it didn't make much sense to me.

CMF_chargeR · Feb 23, 2010

deNs WROTE:

"Total Width: 6.5" or 165.1mm

Total Width - Backspace = Space from centre line of hub to face of wheel

Therefore, 165.1 - 127 = 38.1mm

If the space from the hub face to the back of the rim is 127mm then all you need to do is see how far forward it is from the centre line of the 6.5" rim. In this case, the centre line is 82.55mm. The distance between the hub face and the centre line would be the difference between 38.1mm and 82.55mm. That would give it an offset of +44.45.

That is of course, unless I'm misunderstanding your idea of 'backspace'. I'm very tired and the above may well be incorrect, but if you draw it all out on paper it should start to make sense. Just figure out widths and where the centre line is and go from there.

Also frank, unless you're running huge flares, I think you mean +38 and +45.

---dens

Polarbear's original calculation is correct mate

. Backspace is not measured to where the tyre bead sits like wheel width is, it is measured to the extreme inner edge of the wheel which is generally 1/2" further in than the inner lip of the wheel. So a 8" wide wheel with zero offset will actually have 4.5" of backspace, not 4" as you might think, and the 8" wheel will actually be 9" wide from outer extremity to outer extremity (the nominal width).

Of course converting to offset from backspace is not extremely accurate becauuse not all wheels have an outer lip 1/2" thick.

I use this wheel calculator to compare between the two measurements: http://www.bigcustomwheels.com/rt_specs.jsp

CMF_deNs · Feb 23, 2010

chargeR WROTE:

"Of course converting to offset from backspace is not extremely accurate becauuse not all wheels have an outer lip 1/2" thick.

That's what I couldn't understand - I was sure that not all wheels would have such a standard but that's handy to know about how the proper width is calculated. I guess that means that the set I originally thought was 7" wide is actually 6" wide as I measured it from edge to edge as you've described. They never did seem like they were how 7" wide wheels should be.

So Polarbear whichever calculator you used was right in at least assuming that bit extra it had to calculate for the 'backspace' and since my post above didn't take that into account for the offset, I can safely say that both you and chargeR are correct here

You learn something new every day!

---dens

CMF_Polarbear · Feb 23, 2010

Haha I didn't find a calculator, I just found the info and equations and worked it out.

Ok guys, good to know I didn't stuff it up.

And that should be fine for a micra? I recall reading that the offset should be between 35 and 42?
Closer to 0 moves the rim... inwards?

CMF_deNs · Feb 23, 2010

Outwards (again, if I'm interpreting it right). Moving closer to 0 will increase the track of the car and bring the edge of the rim closer to the guard. This is a good thing

For 6.5" wheels you can go to about +15 to +20 without much worry, though for +15 and below you may need a slight stretch and/or negative camber on the tyres. I really can't remember. I know chargeR's running 13x6 +24's which bring it relatively flush with the guards, as an example.

The more + offset you have, the more inset the rim will be, the worse it'll look and the closer it'll get to the strut.

---dens

CMF_Polarbear · Feb 23, 2010

Ah right of course. I dunno what I was typing about this morning... Ahh well that's terrible logic early in the morning for you

Wheel offset and backspace

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