zmetalmilitia

I am trying to figure out what exactly backspaing is and what offset is. Here's what I understand so far. Backspacing is the dimension from the inner wheel lip to the mounting surface of the rotor. Offset is the dimension of how far off the center of the wheel is off center of axis. If I understand this correctly, it doesn't matter what the offset is but what the backspacing is.

For example, I am looking at 17X8 wheels. I measured my backspacing (on my car, not any wheels), the area from the face of the rotor to where there would be any obsrtuction. The first area I ran into was the upper control arm and the measurement is about 5". So if I go with a 4.5" to 4.75"backspacing I should be good, right? How does the offset matter in this case? If the offset is anything but zero, then it will affect my backspacing. As long as the tire doesn't hang over the rim, 4.5"-4.75" backspacing should be ok. Correct me if I'm wrong. Yes, I have looked at dodgestangs chart, but my application is unique.

8" tire. 4" center of axis. 4.5" backspacing.0.5" negative offset, center of wheel moved in towards the car. I think I answered my question. Any clarification is helpful. Thanks.

dodgestang

Not mine...but available in many sites on the web
X is backspaing
y is rim width
Grid is offset that the two will produce

6 offset tends to be the magic number for 'most' widths

zmetalmilitia

The offset is in mm?

dodgestang

yes

JapanGT

My understanding........

Offset is the distance from the center of the wheel to the inner side ( where the tyre lip sits against the rim ).
You can have positive offset or negative. Front wheel drive cars tend to have positive offsets, with the wheel
flange toward the outside of the wheel.

In theory subtracting the wheel center line from the backspace gives youpositive or negative offset amount.
But using that formula on my wheels with the above offset chart does not work.
My rear wheels are 9.5" width. Backspacing is 5.25". Therefore the center line is 9.5"/2 = 4.75"
To find the offset we subtract (backspace) 5.25" - (center) 4.75" = 0.5" or 12mm approx.
This does not match the above table.

This page explains offset/backspace but uses the same tablehttps://www.rsracing.com/tech-wheel.html

Jav

zmetalmilitia

JapanGT- Do you have any issues running 9.5's in the rear? Could you have gone to 10" wide? How much space is there available running 9.5" wide? I am wanting 17X10 in the rear and I am trying to figure the correct backspacing. I am going to measure tonight, from the axle flange to the inner wheel well area. thanks for any info.

JapanGT

Hi there,

My wheels are supposed to ship this month ( its only been two months.............. ). I ordered American Racing Cobra 427P's
through StreetorTack. 17 * 8 with 4.5 BS up front and 17 * 9.5 with 5.25 BS on the rear. Before ordering, I made up a jig
out of wood and bolted it on with two wheel nuts so I could check clearance. So though I believe they will fit without
interference I cannot say for sure until they arrive. If you are not in a hurry I can take some pictures once they arrive
and you can possibly learn from my mistake...............

zmetalmilitia

Any pics would be great whenever you get them. I am ordering mine this week, and hopefully they will fit. I did the same as far as a jig goes, on the front I boltes on a peice of cardboard, 17" in diameter and measured the backspacing. Up front I have 5" but wil probably go 4.5 to avoid too close of a fitment. As for the back, Iam in the process of rebuilding my rear end, I have the housingsitting in there, not bolted in yes as I amwaiting on my 3rd member to arrive. But measuring the axel flange to the inner most part of the wheelhouse, I have about 5.5-5.75 of clearance. If Igo 17X10's in the back, then I should go with about 5.25-5.50 of backspacing.When I get everything together, I will post pics too and send my info to dodgestang to put in his chart. Happy days.