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PostPosted: Fri Nov 15, 2013 6:14 am 
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Javelin

Joined: Thu Mar 28, 2013 9:07 am
Posts: 216
I scoff at talks of offset and backspacking. None of that helps you much. What you need to know is the opposite of backspacing. I've been calling it "frontspacing." What it comes down to is that you need to know how much the total width of your wheels will add up to when bolted to the constant WMS of our axles.

To find "front-spacing" I put the rim and tire face down on concrete. I then measure from the concrete to the wheel mounting surface. That is the total amount that wheel will add to the axle WMS. Measuring it is the only way to actually know. You can calculate it if you know the backspacing and the total rim width, but that's retarded. First of all most Eagle rims, I know the 5 spoke alloy sport rims with confidence, are 15"x6". That means the space between the beads of the rim is 6". The thickness of the bead itself plus the shape of the tire usually causes the entire wheel to end up about 2" wider than that value. Without the tire the rim is always at least an inch wider than the advertised width. Too many times I've actually seen 15x6 rims called 15x7 because the owner didn't know he was measuring them wrong.

When you go to XJ rims most of them are truly 15"x7" rims. That means there is 7" down in the trough between the beads. The whole rim is actually 8" wide.

If you know the front spacing I can tell you what works and doesn't work. The Eagle Sport alloy rims for instance are about 1-3/4" front spacing. They are hideously recessed into the front fender well and I always add a 1" spacer behind them. On the back axle it is much worse and for those I always add a 2" spacer. Even with the spacers the tire is still well within the fender flare.

My next favorite rim is the Jeep XJ 10 hole alloy rim. That one has about a 2-1/2" front spacing. Basically that means each tire sticks out 3/4" more than a sport rim would, so your total tire width is going to be 1-1/2 wider. For those I do not use a spacer in the front and I use a 1" or a 1.5" spacer in the back. That makes all of my Eagles look much better and gives them all the same total tire widths.

The limit is 4" front spacing. At that point the extreme outside of your front tires will be perfectly flush with the outside of the front fender flares. In the back your fender flares will still stick out past the tires by about an inch. The 15 spoke rims I sold to Phil have that exact front spacing. Those are 15x7" rims that have an extreme width of 8" and the wheel mounting surface is exactly in the middle, which is why the front spacing and the back spacing is exactly 4".

This picture shows Phil's SX4 with the 4" front spacing and no spacers added:

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This picture shows my wife driving my Green SW. In that picture those rims have 1" spacers in the front and 1.5" spacers in the back.
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This picture shows my Kammback with XJ wire type rims. With those rims you must have spacers or the front wheel will hit the caliper and not rotate. I think pictured you're looking at 2" in the back and 1.5" in the front. I eventually used 1" in the front to get it where I wanted it.

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This picture shows 10 hole rims without any spacers and 235/75R15 tires. I had the fender flares on but don't have pictures of it.
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Last edited by captspillane on Sat Nov 16, 2013 11:14 pm, edited 1 time in total.

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PostPosted: Fri Nov 15, 2013 8:31 pm 
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Rogue

Joined: Tue Oct 15, 2013 5:22 pm
Posts: 63
Thanks for this. A lot of good real world info.


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PostPosted: Fri Nov 15, 2013 8:36 pm 
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Javelin
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Joined: Mon Apr 08, 2013 6:08 am
Posts: 280
Location: Royersford, PA
With my style rims up there, you'll also need to modify the fender flares. Even with the stock size tires, the width of the wheels causes it to rub.

_________________
1982 AMC Eagle SX/4 Sport 4x4
-4.0l I6 Engine (circa 1996)
-GM Hydroboost Brakes
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-Carter BBD Carb
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-205/75/R15 General Grabber A/T Tires


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PostPosted: Mon Nov 18, 2013 10:29 am 
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Eagle
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Joined: Fri Feb 08, 2013 10:22 am
Posts: 1757
Location: Missouri, Rolla
I'm pretty sure what you are describing is backspacing. Maybe you are getting offset and backspacing confused. Offset is the amount of distance off-center that the wheels mount. Back spacing is the measurement from the back of the wheel to the mounting point on the hub.

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PostPosted: Mon Nov 18, 2013 11:17 am 
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Rogue

Joined: Tue Jul 02, 2013 7:19 am
Posts: 73
Location: Colorado
I believe what he is calling front spacing is the Overall Rim Width - Backspacing

It represents how much the wheel will stick out when mounted.


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PostPosted: Mon Nov 18, 2013 12:24 pm 
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Eagle
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Joined: Fri Feb 08, 2013 10:22 am
Posts: 1757
Location: Missouri, Rolla
Yes, but it has much more to do with backspacing than it does rim width. If the wheel mounts much closer to the back of the rim than it will stick out farther from the hub than if it was mounted more toward the front of the rim which would have most of the wheel going back in toward the springs.


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PostPosted: Mon Nov 18, 2013 1:02 pm 
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Javelin

Joined: Thu Mar 28, 2013 9:07 am
Posts: 216
I don't need to know backspacing or offset because I already know Eagles can fit big tires with plenty of room to not rub. I only need to know how far the tire will stick out or in from the fender flares. I also want my spreadsheet of data to be easily compared. If I recorded back spacing I wouldn't be able to easily evaluate a 10" rim against a 6" rim without normalizing the data. I'm going straight to the number I need to position the tire in the same position relative to the fender flare every time. Writing backspacing numbers is like comparing a number in inches to another in millimeters without labeling them. Yes 3+2=5. No 3 meters plus 2 feet is not 5 miles!

This also helps with evaluating axle swaps. The Eagle WMS is 61" and I usually add 2" spacers on both sides. That means my perfect WMS is 65". I have a Dana44 with a 67" wms. That tells me a rim with an inch less of "front-spacing" will work perfectly. I look at my list of rims sitting around and know without any math what works and doesn't. All of my cars will have an outside tire width within an inch of eacher regardless of what axle, tire, or rim is on them.


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