20.5" 442M

Gavalarrr · 11-05-2018, 03:14 AM

Who'd have thought? My 19" 442m alloy wheel is actually 20.5" in diameter. Seems obvious now but I have never previously had cause to think that the measurement is taken from where the tyre bead seats behind the rim

Just sharing a bit of Monday morning trivia in the hope that at least one of you didn't know this!

I'm prepared for a plethora of comments to the contrary!

HighlandPete · 11-05-2018, 03:56 AM

Quote:

Originally Posted by Gavalarrr

Who'd have thought? My 19" 442m alloy wheel is actually 20.5" in diameter. Seems obvious now but I have never previously had cause to think that the measurement is taken from where the tyre bead seats behind the rim

Just sharing a bit of Monday morning trivia in the hope that at least one of you didn't know this!

I'm prepared for a plethora of comments to the contrary!

I was simply intrigued by the topic title.

Next we will be learning that bigger rims/wheels aren't really any bigger, don't fill the wheel arches any more than the smallest size.

APJ · 11-05-2018, 06:02 AM

I checked my 20" the other week and wondered the same, they measure just over 21.5". No doubt the measure is done from a different place or where the tyres sit behind the rim edge, in other words toward the centre of the outer part of the wheel, if that makes sense

Gavalarrr · 11-05-2018, 06:31 AM

Yes, it's where the tyre bead sits I think. For the same reason, an 8J wheel is 9" wide in total!

HighlandPete · 11-05-2018, 06:44 AM

Quote:

Originally Posted by Gavalarrr

Yes, it's where the tyre bead sits I think. For the same reason, an 8J wheel is 9" wide in total!

Yes you are correct, rim width/diameter dimension codes are for the tyre bead. All standards work around those codes. Acceptable tyre fitment is also governed by the rim dimensions, such as in the ETRTO & ISO standards.

teaston · 11-05-2018, 06:54 AM

It's all about tyre sizes, so they're to fit a tyre with a 19" internal diameter and the beads will seat 8 inches apart.

goodbyalfa · 11-05-2018, 09:10 AM

And next in the news, water is wet....

Lorcan · 11-05-2018, 10:23 AM

It amuses me greatly that tyre dimensions are a combination of inches, millimeters and percentages. When I used to buy plastics it often came in 8' x 4' x 5mm. No wonder the aliens won't talk to us

11-05-2018, 03:14 AM	#1
Gavalarrr Enlisted Member 11 Rep 45 Posts Drives: F32 Join Date: Sep 2015 Location: Lancashire iTrader: (0)	20.5" 442M Who'd have thought? My 19" 442m alloy wheel is actually 20.5" in diameter. Seems obvious now but I have never previously had cause to think that the measurement is taken from where the tyre bead seats behind the rim Just sharing a bit of Monday morning trivia in the hope that at least one of you didn't know this! I'm prepared for a plethora of comments to the contrary!
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11-05-2018, 06:02 AM	#3
APJ Captain 245 Rep 724 Posts Drives: F36 435d M Sport Plus Join Date: Oct 2018 Location: Greater Manchester iTrader: (0)	I checked my 20" the other week and wondered the same, they measure just over 21.5". No doubt the measure is done from a different place or where the tyres sit behind the rim edge, in other words toward the centre of the outer part of the wheel, if that makes sense
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11-05-2018, 06:31 AM	#4
Gavalarrr Enlisted Member 11 Rep 45 Posts Drives: F32 Join Date: Sep 2015 Location: Lancashire iTrader: (0)	Yes, it's where the tyre bead sits I think. For the same reason, an 8J wheel is 9" wide in total!
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11-05-2018, 06:54 AM	#6
teaston Banned 10995 Rep 32,881 Posts Drives: X3 M40d Join Date: May 2012 Location: The High Seas iTrader: (1)	It's all about tyre sizes, so they're to fit a tyre with a 19" internal diameter and the beads will seat 8 inches apart.
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11-05-2018, 09:10 AM	#7
goodbyalfa King of the North (living in the east...) 1056 Rep 2,154 Posts Drives: M340i mineral white Join Date: Apr 2015 Location: Suffolk iTrader: (0)	And next in the news, water is wet.... __________________ 2021 M340i in mineral white, options galore Previously 2018 440i GC in MG 2015 330D F30 MSport EBII 2012 Alfa Guilietta 2.0d Veloce 2009 Fiat bravo sport 2000 ST Mondeo
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11-05-2018, 10:23 AM	#8
Lorcan The artist formerly known as AC Schnitzer UK 3609 Rep 4,518 Posts Drives: The Yellow Peril Join Date: Apr 2013 Location: Norfolk UK iTrader: (0)	It amuses me greatly that tyre dimensions are a combination of inches, millimeters and percentages. When I used to buy plastics it often came in 8' x 4' x 5mm. No wonder the aliens won't talk to us
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