Offset finally explained?
08-31-2010, 02:24 PM
#1
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Offset finally explained?
Is the following post from the email list correct? If so it might explain why I could never understand wheel fitment. I even tried drawing it out once. I always thought that the offset was the distance from the hub to the inner edge of the wheel.
Thanks, Neil
Subject: Re: Tires
From: "jerry feather" <jfeather@bresnan.net>
Date: Mon, 30 Aug 2010 06:44:15 -0600
X-Message-Number: 3
Hi Joe. Yes, the "ET" numbers in Porsche wheels stands for the offset in
millimeters. However, you need to understand that by offset it means the
distance in millimeters measuring from the center of the wheel width outward
to the face of the wheel that mates to the hub upon mounting. It is not the
distance from the inner edge of the wheel as is frequently thought.
Jerry Feather
From: "jerry feather" <jfeather@bresnan.net>
Date: Mon, 30 Aug 2010 06:44:15 -0600
X-Message-Number: 3
Hi Joe. Yes, the "ET" numbers in Porsche wheels stands for the offset in
millimeters. However, you need to understand that by offset it means the
distance in millimeters measuring from the center of the wheel width outward
to the face of the wheel that mates to the hub upon mounting. It is not the
distance from the inner edge of the wheel as is frequently thought.
Jerry Feather
08-31-2010, 02:29 PM
#2
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From wikipedia:
The offset of a vehicle's wheel is the distance between the centerline of the wheel and the plane of the hub-mounting surface of the wheel. It can thus be either positive or negative, and is typically measured in millimeters. Offset has a significant effect on many elements of a vehicle's suspension, including suspension geometry, clearance between the tire and suspension elements, the scrub radius of the steering system, and visually, the width of the wheel faces relative to the car's bodywork.
Zero Offset - The plane of the hub mounting surface is even with the centerline of the wheel.
Positive Offset - The plane of the hub mounting surface is shifted from the centerline toward the front or outside of the wheel. Positive offset wheels are generally found on front wheel drive cars and newer rear drive cars.
Negative Offset - The plane of the hub mounting surface is toward the back or brake side of the wheel's centerline.
"Deep dish" wheels typically have negative offset or a very low positive offset.
To maintain handling characteristics and avoid undue loads on bushings and ball joints, the car manufacturer's original offset should be maintained when choosing new wheels unless there are overriding clearance issues.
Wheels are usually stamped with their offset using the German prefix "ET", meaning "Einpresstiefe" or, literally, "insertion depth". An example would be "ET45" for a 45mm offset.
Zero Offset - The plane of the hub mounting surface is even with the centerline of the wheel.
Positive Offset - The plane of the hub mounting surface is shifted from the centerline toward the front or outside of the wheel. Positive offset wheels are generally found on front wheel drive cars and newer rear drive cars.
Negative Offset - The plane of the hub mounting surface is toward the back or brake side of the wheel's centerline.
"Deep dish" wheels typically have negative offset or a very low positive offset.
To maintain handling characteristics and avoid undue loads on bushings and ball joints, the car manufacturer's original offset should be maintained when choosing new wheels unless there are overriding clearance issues.
Wheels are usually stamped with their offset using the German prefix "ET", meaning "Einpresstiefe" or, literally, "insertion depth". An example would be "ET45" for a 45mm offset.
08-31-2010, 04:55 PM
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I like how the centerline of that wheel in that diagram (in post #2) isn't drawn in the center. Geometry FAIL. 
But, yeah, what the text says is correct regarding offset.
Here's another diagram that's rather nice, IMHO.
But, yeah, what the text says is correct regarding offset.Here's another diagram that's rather nice, IMHO.
08-31-2010, 06:19 PM
#4
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this is why i like using "backspacing" it cant be misunderstood or miscalculated like offset. if you calculate offset, you need to know the real width of the rim, not the published width. that is usually the most common calculation error. (e.g. 11" rim is usually really about an 11.5" rim)
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08-31-2010, 07:37 PM
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not really. backspacing is that edge of the wheel, and offset is to the inner lip of the wheel which is used to correspond to the half way point. Although, sure, it could be confused, but not as common.
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08-31-2010, 07:46 PM
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08-31-2010, 09:07 PM
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Whew!!! At least no one said I was wrong.
A thought I have had it that I wonder that if the offset is negative, is it called "ET?"
Jerry Feather
A thought I have had it that I wonder that if the offset is negative, is it called "ET?"
Jerry Feather
08-31-2010, 09:16 PM
#11
negative offset of course is possible. Having a negative offset would imply a HUGE wheel lip/dish configuration, more likely seen on an old racecar than a modern street car. However, more often than not, ET is a positive value.. some are small (like Porsches 8x16 911 fuch has a 10.5ET (10.5mm is less than 1/2" ) while modern Porsches tend to have 50-60mm offsets (2-2.5")
Due to ABS, and the subsequent suspension/scrub radius needs, modern cars have a "high", positive, ET.
Due to ABS, and the subsequent suspension/scrub radius needs, modern cars have a "high", positive, ET.
08-31-2010, 10:51 PM
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08-31-2010, 11:10 PM
#15
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So, is it just me (I can hardly wait to see who jumps on this) or is it really stupid to list the offset in millimeters and the width in inches, on a rim?
There is absolutely no way to figure out the where the wheel mounting surface is, in relationship to the edges of the rim without converting one or the other. I'm sure that there is probably a "master" chart somewhere that has all the sizes with the mounting flange to edge dimensions.
Seems retarded.
There is absolutely no way to figure out the where the wheel mounting surface is, in relationship to the edges of the rim without converting one or the other. I'm sure that there is probably a "master" chart somewhere that has all the sizes with the mounting flange to edge dimensions.
Seems retarded.