Backspacing still doesn't make sense to me.

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'74 Sport

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I've been following several threads about how large a tire can be. I've also read from numerous sources on how to take measurements for determining backspacing. It still doesn't make sense to me. When reading about it, it all sounds so simple.

I pulled a rear tire off my son's car to begin the measuring process. What I found under there is not as simple as it sounds. The car is a stock '74 Dart Sport, but I swapped in a LBP rear axle from a '76 Duster. It fits in the exact same location as the original SBP from the Dart. The problem I have is determining where to take measurements along the rounded inner fender to determine the widest points. The edge of the wheel lip where the molding goes is not really the key to this, because the inner fender bulges inward immediately behind the lip. From there, it begins to follow a rounded arching shape up over the tire to the frame side of the wheel well. At one point on the face of the wheel well (towards the trunk), there is a small bulge protruding out to allow space inside the trunk for the deck lid hinge.

I really need to visit with someone who has first-hand knowledge of this Dart Sport / Duster setup. I am trying to determine how to order some 15x8 Cragar SS wheels, with the proper backspacing to fit wider tires than the skinny 215-70-R14 tires it currently has.

If you think you can help, please send a PM or e-mail.

Thanks,
Jerry jnl@vvm.com
 
I don't know about your car specifically, but here's some info on how to measure offset.

Offset
The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel. The offset can be one of three types.

Zero Offset
The hub mounting surface is even with the centerline of the wheel.

Positive
The hub mounting surface is toward the front or wheel side of the wheel. Positive offset wheels are generally found on front wheel drive cars and newer rear drive cars.

Negative
The hub mounting surface is toward the back or brake side of the wheels centerline. "Deep dish" wheels are typically a negative offset.

If the offset of the wheel is not correct for the car, the handling can be adversely affected. When the width of the wheel changes, the offset also changes numerically. If the offset were to stay the same while you added width, the additional width would be split evenly between the inside and outside. For most cars, this won't work correctly.

offset.gif
 
Thanks for the information. That stuff I do understand. What I need to know is where to take the proper measurements on the wheel well. It is not a nice simple case of square, vertical, and parallel surfaces. It is all curved, tapering, and convoluted under there. That's why I need to talk to someone who knows what it is I am dealing with.

Jerry
 
Jerry,
I can sympathize with you on this as I recently encountered similar issues on location of your measurements. I have a 72 dart swinger not a sport so I can only try to explain what I did. I mini-tubbed and did a spring relocation into my frame rails to gain space. I put in a new rear that had been cut down approx 3" per side or 6 overall. I jacked my car up (no wheels on) and dropped a plum bob from several places and marked those spots on the floor with a sharpie. Now I did this all over the place. First I did it from the center of the axle on the brake drum where the wheel would mount. Then on the other wheel the same thing and made a straight line connecting them so I had a good reference point. From the inside (dimple mark) just above the wheel lip, from the wheel lip itself, and from the outside edge of my leaf springs since this is the closest spot to rub on the inside. In my case I decided to hammer the dimples out of the outer fender well to make more room. So I dropped the plum bob from the closest contact point on the outside that I could find by dropping from several locations not just trusting by my eye. Mark all of these spots on your reference line and then using a scale you can get accurate measurements. Now here's the thing! Unless you trust someone to having or you are able to verify the exact measurements are the same to yours, your reference points will be different. Is your car using original leaf springs, super stock, or any other after markets? Mine is using super stock springs and I have a pinion snubber installed. So depending on how far your car will squat or wheel will go up into the wheel well will maybe let you get away with larger tires. Remember you want a little room for stretch when cornering etc. I compressed my rear until the pinion snubber would hit and then determined with my 28" tire how far up it would travel in the wheel well and then gave myself about 1/4 inch for flex. To give you an idea of my measurements. On the left side I needed 3.25" back spacing on an 8" rim and on the right I needed 3.5" bs. I went with 3.5" bs and then used a 1/4 spacer on the left side to even them out. I think using a plum bob is the most accurate and easiest way because I could see where exactly everything was at. If you want to fill your wheel wells and I mean stuff them to the biggest tire possible its the way to go. Now doing this, you should also know what tire you plan to use and take into consideration the sidewall because that is the widest area. It worked for me, hope it helps you out. If you have any questions let me know. The main purpose is to find the center of your wheel wells and then measure off where you need everything to fit. Is time consuming but is easy to get accurate.
This site should help with more detail and pictures...http://www.bigblockdart.com/techpages/wheelspacing.shtml
Adam
 
I think the easiest way is to mount a wheel and tire to the axle (that has approx the same overall diameter) and just measure how close it comes to the spring, inner wheelwell, quarterpanel opening, etc. From there you can get a very close approximation. Some tire/wheel stores also have a tool that can actually figure out what size tire/wheel/backspace will work. Or just borrow some different wheels and tires from friends and try them on. On my duster, I have the offset spring package and SS springs. I have 15x8.75 centerline wheels and 265-60-15 MT drag radials. Got about 1/2" to springs and front of wheel opening. Remember though, due to production tolerances both sides might not have the same amount of clearance. Measure,measure,measure,you get it..... good luck
 
Yeah, don't try to make it any more complicated than it needs to be. Special tools and measurements for tire sizes are prone to error. You won't really know for sure what it looks like until it's actually mounted.

Go from the known measurements of your existing wheels and reverse engineer it from there. For example, if you have 3in. backspace and 2 inches between the inside wall of the tire and leaf spring you can probably safely use one additional inch of backspace.

If the outside wall of the tire is already too close to the fenderwell you may have swapped in too wide an axle/rear combo and can't go much of anywhere with it unless you're talking about custom-made wheels with offsets that may not behave/handle properly.
 
Ok, well I have Weld Draglites. 15x8s with 235/60R15s on them, and I have lots of clearance. I know I can get 255/60R15s on with NO problem.

I'll get you the backspacing when I get home (Come to think of it, someone else was asking me about backspacing here).

I've got a 75 Dart Sport, so it should all be the same.
 
Maybe I can help Jerry. I switched over from the original Rallye wheels to Cragar SS on my 71 Demon last year. Firstly, the rear end is a stock 8 3/4 in the stock location along with the springs. I went with Cragar SS 15X7 with 255/60x15"s Bf Goodrich TA (banana skins) and the clearance between the leaf springs and tire side wall is 1/2". The clearance between the outer side wall and inner wheel well is 3/4". The reason I went with the 15x7's (4 1/8" backspace) and not the 15x8's (4 1/4" backspace) is the that tire sidewall would have been to close to the leaf springs. The section width (sidewall to sidewall) of these tires on a 7 inch rim is 10" whereas that section width changes to 10.5 with a 8 inch rim. Tire section width changes by approx. .25" for every .5" change in rim width according to Goodrich. The end all be all is that if I used the 8" SS rims the clearance between the tire inner sidewall and the leaf springs would have been 1/8 to 1/4 inch which is getting way to tight especially with flexible radial sidewalls.

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