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Emeraldlion
Samba Member
Joined: April 13, 2005
Posts: 743
Location: Texarkana, TX
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 Posted: Sat Oct 13, 2018 7:09 am Post subject: Feeling dumb on offsets? |
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| Ok guys, help a brother not feel dumb. I am going over offsets again to refresh and make sure I get right on my wheels. I am shooting for 5.5 inch sprinstars. I had always thought offset was from centerline of wheel so that a 0 offset 5.5 inch wheel would have 2.75 inch of backspace? All of the backspace charts I have seen online place a 5.5 inch wheel with 0 offset at 3.25 backspace. What am I missing? |
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Scott H
Samba Luddite
Joined: August 15, 2003
Posts: 3026
Location: NorthCarolina,USA
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| Posted: Sat Oct 13, 2018 10:08 am Post subject: Re: Feeling dumb on offsets? |
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The stated wheel width is bead to bead. The actual overall width of the wheel is greater than that. So, a "5.5" wide wheel may be 6.5" actual total width.
Offset is from the centerline of the wheel to the face of the mating surface between the wheel and drum/rotor.
Backspace is the distance from that mating surface to the outermost point of the wheel as it extends back toward the car.
I don't remember for sure but I believe most overall wheel widths are usually around 1" wider than their width designation. (A "4.5" wheel is actually 5.5", etc.)
So, your example of a 5.5" wheel with 0 offset having a 3.25" backspace sounds right.
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Emeraldlion
Samba Member
Joined: April 13, 2005
Posts: 743
Location: Texarkana, TX
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| Posted: Sat Oct 13, 2018 10:27 am Post subject: Re: Feeling dumb on offsets? |
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| That makes sense I wasn't thinking of the bead width. Thanks for helping clarify! |
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Enkiel
Samba Member
Joined: October 21, 2004
Posts: 2771
Location: Quebec, Canada
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| Posted: Fri Oct 19, 2018 8:38 am Post subject: Re: Feeling dumb on offsets? |
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So, if i need 3" to clean my springplate on 5 1/2", a 0 offset would work.
I'm looking at smoothies which are +15 offset and 98 backspacing.
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Mike Fisher
Samba Member
Joined: January 30, 2006
Posts: 17970
Location: Eugene, OR
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| Posted: Fri Oct 19, 2018 9:58 am Post subject: Re: Feeling dumb on offsets? |
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5.5" Sprintstars are pretty deep, so you won't be able to lower your car very much.
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EVfun
Samba Member
Joined: April 01, 2012
Posts: 5481
Location: Seattle
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| Posted: Sat Nov 03, 2018 7:21 pm Post subject: Re: Feeling dumb on offsets? |
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| Enkiel wrote: |
So, if i need 3" to clean my springplate on 5 1/2", a 0 offset would work.
I'm looking at smoothies which are +15 offset and 98 backspacing. |
No. That wheel would have about a 3.25" backspace. Offset is measured from the centerline of the wheel (also called ET.) It helps you understand where the tread face will land in your fender. Positive pushes the tire inward and negative pushes the tire outward. Backspace is measured from the lip of the wheel. A 5.5 inch wheel is 5.5 inches from the inside of the tire bead seat and is about 6.5 inches wide to the outside of the weight lip. So, an ET of zero will be a backspace of about 3.25 inches. It is commonly used to insure the wheel clears suspension parts.
Stock wide 5 wheel are 4 inches wide with a +33mm ET. They have a backspace of about 3.8 inches (half of 5 inches plus 33mm converted to inches.) My Bug wheels are 5.5 inches wide with a ET of +23mm. That puts the backspace at about 4.15 inches (half of 6.5 plus 23mm converted to inches.) That is pretty close to the spring plates with a short axle swingaxle Bug.
Here is another explanation with pictures and descriptive text.
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Enkiel
Samba Member
Joined: October 21, 2004
Posts: 2771
Location: Quebec, Canada
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| Posted: Sat Nov 03, 2018 7:43 pm Post subject: Re: Feeling dumb on offsets? |
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| EVfun wrote: |
| Enkiel wrote: |
So, if i need 3" to clean my springplate on 5 1/2", a 0 offset would work.
I'm looking at smoothies which are +15 offset and 98 backspacing. |
No. That wheel would have about a 3.25" backspace. Offset is measured from the centerline of the wheel (also called ET.) It helps you understand where the tread face will land in your fender. Positive pushes the tire inward and negative pushes the tire outward. Backspace is measured from the lip of the wheel. A 5.5 inch wheel is 5.5 inches from the inside of the tire bead seat and is about 6.5 inches wide to the outside of the weight lip. So, an ET of zero will be a backspace of about 3.25 inches. It is commonly used to insure the wheel clears suspension parts.
Stock wide 5 wheel are 4 inches wide with a +33mm ET. They have a backspace of about 3.8 inches (half of 5 inches plus 33mm converted to inches.) My Bug wheels are 5.5 inches wide with a ET of +23mm. That puts the backspace at about 4.15 inches (half of 6.5 plus 23mm converted to inches.) That is pretty close to the spring plates with a short axle swingaxle Bug.
Here is another explanation with pictures and descriptive text. |
Reading, and re-reading this post, i think my math are wrong then. I used strictly backspacing (since i think i do not need both to make my calculation?) :
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So going more into mathematics....
Rims are 5.5" (139mm)
room between suspension component and face of drum is 6" (152mm)
backspacing is 98mm
Tire aimed for are 195/65/15.
Theorically, tire is 28mm larger on each side.
So i'd take my backspacing of 98mm, add 28mm, and i'm at 126mm.
Theorically, i'd still have 26mm of room. Are my math correct? |
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EVfun
Samba Member
Joined: April 01, 2012
Posts: 5481
Location: Seattle
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| Posted: Sun Nov 04, 2018 8:32 am Post subject: Re: Feeling dumb on offsets? |
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Where did you measure the 5.5 inches? If that is the stated width then the overall wheel width is about 6.5 inches. That difference, wheel width for the tire vs. overall wheel width, is where most fit confusion comes from.
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Enkiel
Samba Member
Joined: October 21, 2004
Posts: 2771
Location: Quebec, Canada
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| Posted: Sun Nov 04, 2018 11:01 am Post subject: Re: Feeling dumb on offsets? |
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| EVfun wrote: |
| Where did you measure the 5.5 inches? If that is the stated width then the overall wheel width is about 6.5 inches. That difference, wheel width for the tire vs. overall wheel width, is where most fit confusion comes from. |
it's a 4.5 inch wheel
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EVfun
Samba Member
Joined: April 01, 2012
Posts: 5481
Location: Seattle
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| Posted: Sun Nov 04, 2018 2:20 pm Post subject: Re: Feeling dumb on offsets? |
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| Enkiel wrote: |
| EVfun wrote: |
| Where did you measure the 5.5 inches? If that is the stated width then the overall wheel width is about 6.5 inches. That difference, wheel width for the tire vs. overall wheel width, is where most fit confusion comes from. |
it's a 4.5 inch wheel |
Then the numbers you came up with look right to me too, you should have about 26mm of room left.
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| Wildthings wrote: |
| As a general rule, cheap parts are the most expensive parts you can buy. |
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