Rim offset / ET / "backset"

[cgfen]

I'm considering buying a set of Kinesis wheels and i'm trying to determine if offset spacing is correct.

If i measure from the back of the wheel hub mounting surface to a straight edge placed on the top of the rear-most wheel lip, is that the same as what Porsche considers ET?

or is ET what i've described minus / or plus the thickness of the wheel hub?


I'd like to be able determine a method to directly compare say ET = X and measurement as described above to that same = X.

determining the thickness of the hub might be challenging.

thanks

Craig

[kurquhart]

What you are describing is backspace, not offset (ET). Offset is from the rear center surface to the middle of the wheel. Bill Verburg has some nice illustrations here that show the difference:
http://members.rennlist.com/1976c38/index.html

[cgfen]

Originally Posted by kurquhart

What you are describing is backspace, not offset (ET). Offset is from the rear center surface to the middle of the wheel. Bill Verburg has some nice illustrations here that show the difference:
http://members.rennlist.com/1976c38/index.html

many thanks


craig

[Jeff Alton]

"ET" equals offset. The offset is the distance the hub mounting surface of the wheel is from the centerline of the wheel. However, technically the centerline, wheel width and offset are meseaured from the edge of the bead seat of the wheel. Measuring from the lip can induce errors if both lips (inner and outer) are not the same size, which is common and even varies from part to part.

Cheers

[Bill Verburg]

Originally Posted by Jeff Alton

"ET" equals offset. The offset is the distance the hub mounting surface of the wheel is from the centerline of the wheel. However, technically the centerline, wheel width and offset are meseaured from the edge of the bead seat of the wheel. Measuring from the lip can induce errors if both lips (inner and outer) are not the same size, which is common and even varies from part to part.

Cheers

Jeff, is correct as to the definition of ET as the distance from the wheel center-line to the wheel mounting face but is in an extremely small minority choosing to measure the wheel center line from the beads. As virtually all modern safety wheels have symmetric beads almost every one uses the edge to edge measure rather than the bead to bead,

Why?, you may ask. Simple, It's far easier to determine where the edges are and to measure between them. The bead is will defined but nowhere near as easy to measure w/ the accuracy that is needed for meaningful results.

Both ways of measuring, IFF properly done will yield the same result for center-line.

The real issue is the amount of backspace and front-space which Jeff's preferred method provides.

When one is concerned about backspace and front space one wants to know the total extent of the wheel, either inboard or outboard from the hub mounting face so as to determine clearances(this necessarily includes the rim flanges outboard of the beads). The only way to measure that is using the wheel edge planes as illustrated here

[Jeff Alton]

Bill and I agree (I think ),

Just remember where the tire will sit is based on the bead, not the lip. The lip measurement is important if you are concerened about the wheel rubbing on the car.

I wanted to explain how it all works reference different parts of the wheel so that someone does not just measure a backspace from the lip and then attempt to calculate and offset from that. I have seen that too many times and then guess what? the wheel does not fit.

If you measure the the wheel with the tire off from lip to lip to get a total width, then measure the backspace from the inner lip, you can get accurate offset. Just don't try to short cut it.

Cheers

[cgfen]

Jeff, Bill,
Thanks for taking the time to clarify this for me.

cheers

Craig

[cgfen]

Originally Posted by Bill Verburg

Jeff, is correct as to the definition of ET as the distance from the wheel center-line to the wheel mounting face but is in an extremely small minority choosing to measure the wheel center line from the beads. As virtually all modern safety wheels have symmetric beads almost every one uses the edge to edge measure rather than the bead to bead,

Why?, you may ask. Simple, It's far easier to determine where the edges are and to measure between them. The bead is will defined but nowhere near as easy to measure w/ the accuracy that is needed for meaningful results.

Both ways of measuring, IFF properly done will yield the same result for center-line.

Originally Posted by Jeff Alton

Bill and I agree (I think ),

Just remember where the tire will sit is based on the bead, not the lip. The lip measurement is important if you are concerened about the wheel rubbing on the car.

I wanted to explain how it all works reference different parts of the wheel so that someone does not just measure a backspace from the lip and then attempt to calculate and offset from that. I have seen that too many times and then guess what? the wheel does not fit.

If you measure the the wheel with the tire off from lip to lip to get a total width, then measure the backspace from the inner lip, you can get accurate offset. Just don't try to short cut it.

Cheers

Raising this thread from the dead, because I am ready to buy yet another set of Kinesis wheels, this time for our 09 911 2S.
Wheels are not in my hands so I can't do actual measurements, but seller provided following info.
Overall width = 12.0 and 9.5
Bead width = 11.0 and 8.5

Backspace measured as 206mm rear and 173mm front.

Plugging that into excel shows


Top data set = bead to bead width
Lower data set = overall bead width

Note that ET changes between the two.
I assume that this is what Jeff was referring to when he said
"I wanted to explain how it all works reference different parts of the wheel so that someone does not just measure a backspace from the lip and then attempt to calculate and offset from that."

Top data set leads me to believe that wheels will work, rears no problem, pretty much identical to factory rear wheel and that fronts will work but may need a 5 - 7mm spacer.

Lower data set confounds me.
If as Bill says, "Both ways of measuring, IFF properly done will yield the same result for center-line." What am I doing wrong when considering overall width?


As a reference, factory wheels;

Front = 8.0 ET 57
Rear = 11.0 ET 67
Thanks,

Craig

[Bill Verburg]

Originally Posted by cgfen

Raising this thread from the dead, because I am ready to buy yet another set of Kinesis wheels, this time for our 09 911 2S.
Wheels are not in my hands so I can't do actual measurements, but seller provided following info.
Overall width = 12.0 and 9.5
Bead width = 11.0 and 8.5

Backspace measured as 206mm rear and 173mm front.

Plugging that into excel shows


Top data set = bead to bead width
Lower data set = overall bead width

Note that ET changes between the two.
I assume that this is what Jeff was referring to when he said
"I wanted to explain how it all works reference different parts of the wheel so that someone does not just measure a backspace from the lip and then attempt to calculate and offset from that."

Top data set leads me to believe that wheels will work, rears no problem, pretty much identical to factory rear wheel and that fronts will work but may need a 5 - 7mm spacer.

Lower data set confounds me.
If as Bill says, "Both ways of measuring, IFF properly done will yield the same result for center-line." What am I doing wrong when considering overall width?


As a reference, factory wheels;

Front = 8.0 ET 57
Rear = 11.0 ET 67
Thanks,

Craig

When you measure wheel width from bead to bead, backspace must also be measured from the bead. You used the the backspace measured from the edge for both calculations.

This is another reason not to use the bead with for anything but tire discussions.
ET53 looks to be correct for the front and ET54 for the back

if you want 8.5" front to fit like factory 8ET57(that is have the outer edge of the lip to be in the same place you would need 8.5ET63 for 11

compared to factory 11ET67's your 11ET54's stick outboard 13mm more

[cgfen]

Originally Posted by Bill Verburg

When you measure wheel width from bead to bead, backspace must also be measured from the bead. You used the the backspace measured from the edge for both calculations.

This is another reason not to use the bead with for anything but tire discussions.
ET53 looks to be correct for the front and ET54 for the back

if you want 8.5" front to fit like factory 8ET57(that is have the outer edge of the lip to be in the same place you would need 8.5ET63 for 11

compared to factory 11ET67's your 11ET54's stick outboard 13mm more

Bill, thanks for the reply. I corrected my spreadsheet accordingly and now ET values correspond.

Sorry, but I'm have a bit of trouble parsing your post.

When you say, "ET53 looks to be correct for the front and ET54 for the back".
Do you just mean that the calculations are correct?

When you say, "if you want 8.5" front to fit like factory 8ET57(that is have the outer edge of the lip to be in the same place you would need 8.5ET63".
I'm assuming ET changes 5mm due to 0.5" wider wheel.

"For 11 compared to factory 11ET67's your 11ET54's stick outboard 13mm more". That is very clear. I wonder if the rear wheels were built for WB, not NB?

And finally, if front wheel should be ET63, I would think calculated ET53 would be an issue since as I understand it; as ET decreases, wheel moves away from car centerline. 10mm would probably put tire into and outside of fender lip.

TIA for your patience and answer.

Craig

[Bill Verburg]

Originally Posted by cgfen

Bill, thanks for the reply. I corrected my spreadsheet accordingly and now ET values correspond.

Sorry, but I'm have a bit of trouble parsing your post.

When you say, "ET53 looks to be correct for the front and ET54 for the back".
Do you just mean that the calculations are correct?

When you say, "if you want 8.5" front to fit like factory 8ET57(that is have the outer edge of the lip to be in the same place you would need 8.5ET63".
I'm assuming ET changes 5mm due to 0.5" wider wheel.

"For 11 compared to factory 11ET67's your 11ET54's stick outboard 13mm more". That is very clear. I wonder if the rear wheels were built for WB, not NB?

And finally, if front wheel should be ET63, I would think calculated ET53 would be an issue since as I understand it; as ET decreases, wheel moves away from car centerline. 10mm would probably put tire into and outside of fender lip.

TIA for your patience and answer.

Craig

your last 2 rows are done correctly, the first 2 are no due to the inconsistent measurements used in bead width and backspace


yes, if wheel width increases by .5" w/ no change in ET, the wheel projects .25" further inboard and .25" further outboard

997 Used
Front 8 x18 or 8 x19 ET57 or 8.5 x19 ET55
Rear 10 x18 ET58 11x18 ET51(4, 4S), 11 x19 ET67, 11 x19 ET51(2, 2S) 11.5 x19 ET67

it's not clearly labeled what the other wheels go to, I'd say that the low Et are widebody and the high Et narrowbody

[cgfen]

Thank you Bill.