Measuring for diff width (or backspace)

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Idaho

Idaho

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I've made my brain hurt, sorting out the measurements for my differential width, along with wheel and tire combo. I'm mini-tubbing my 69 Dart so the 14.5" of space between the 1/4 panel lip and the frame is available.

I caught myself making an error that could cause a major headache and thought I might save others a potentially costly mistake in this process.

I'm planning to use 10" wheels and initially made a drawing using 10" as the wheel width. I had determined I needed 1.25" space between the tire and 1/4 lip in order to allow a low stance without having the tire contact the 1/4 panel up higher where it projects inward more than the 1/4 lip. So placing my chosen tire on the drawing at that distance gave the results on the lower drawing. The tire I chose happened to fit with 1.25" to the frame as well.

The error comes from not using actual outside measurements of the wheel. The result is a wheel mounting surface that is off by a half inch to the outside. That error would result in a wrong differential width. It could also result in ordering wheels with the wrong backspace.

Because little mistakes matter, I believe if one will be narrowing the differential, it is safest to get wheels and tires first, make a spacing device of some sort (like an adjustable axle between them) to accurately secure them in the wheelwells and take a measurement between the wheel mounting surfaces. However, the drawing is still needed in order to get the wheel, tire and backspacing correct.

I called the wheel manufacturer to get the outside measurement of the wheel since this is never specified.

Another potential error could happen in measuring backspace. I found conflicting sources online. One advised measuring from the wheel inside the outer lip (where the bead contacts). This requires cutting a straight edge to fit inside the outer lip. The proper way is to the outside of the wheel lip.
 

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Helpful, but does not point out that a 10" rim is not 10" outside to outside. When making a drawing it matters.
I could not get his drawing to load. If you can, could you post it here?
 
How To Measure For Wheels (from Bigblockdart.com)

Measuring for proper wheel sizes is a popular question. With wheels costing a bundle, and the fact that once you mount tires, you own them, it's a good idea to get it right the first time. So far, all the letters I've recieved have questioned the size of rears, so I'll cover this here.
To find the proper wheel and tire sizes for your car, you must first determine how much total room you have to work with. The easiest way to do this is to hang a plumb-bob from various points to the floor, mark the floor, and then you can measure these marks without any parts interfering. Before you measure, make sure the car's weight is on the axle, so it's in its ride-height position.
In the drawing, drop a line from the inner wheel lip to the floor and put a mark. Next drop a line from the frame-side, at the closest point in the well and put a mark. Dont forget about your fuel or brake lines if they are in the well. Last, drop a line from the wheel mounting flange and put a mark. These three points are all you need to find the wheel/tire sizes.

First, measure across the outer two points (C), to find the total clearance you have to work with. From this distance, you should subtract a fair amount of clearance on each side so the tire doesn't rub as the car leans. Around an inch of actual clearance on each side is about the bare minimum, due to sidewall flex in tires. The more air in a drag-type tire like the MT, the wider the sidewall gets, so dont skimp on this clearance measurement, or you'll be hammering your wheel lips to get them to work. Add some extra if you can get away with a smaller tire. 1.5" would be real nice. I understand that you want the biggest tire possible, but even I ended up hammering the wheel lips on my own car after it was painted because I "thought" I could get away with less. Choose a tire that has a side width measurement matching your new measurement. Assume C = 14.5. Take away 1.5" on each side for clearance and you have 11.5. You can assume that this tire will really be around 12.5 total width due to sidewalls bulging. That gives you an honest 1" on each side depending on tire pressure, which will most likely be enough for normal street driving.

Now, you want to center this tire in the wheelwell, to allow the most clearance on each side as possible. Assume here you're using a 10" wide rim. You want to center 10" in the wheelwell when its mounted. If measurement (C) was 14.5", then you need to divide the extra 4.5" between both sides to get the rim in the center. So, you can have 2.25" of clearance on each side of the rim to have it centered in the well. Now take the measurement (B) and subtract that 2.25" from it. Assuming here that (B) is 8", taking 2.25" away will give you 5.75" of backspacing. So, in this example, you can fit a 10" wide rim with 5.75" of backspacing, and it will be perfectly centered in the well. We all know that certain backspacing isn't available, so assume that you have to go with the closest offering, which might be 6". This will move the rim .25" closer to the inside of the car, leaving you with only 2" from the rim edge to the inner well. With our 11.5" tire, we figured it'd be 12.5" total, so centering 12.5" on the rim would give us an additional 1.25" on each side. Adding this to the rim, you'll find that you now only have .75" clearance on the inside, while the outside is now 1.25". You can see that when using the biggest possible tire, .25" is alot. Like-wise, if you go the other way, getting 5.5" of backspace, you'll only have .75" on the outside. When you get under 1" of actual airspace between the tire and the well, you're getting very close, and risk cutting the tire when the car leans.

As an example of this, I have SS springs on my 69 Dart, which are very stiff. The car doesn't lean at all around turns, but in some intersections when the intersecting road is at a weird angle, the rear has no choice but to lean one way, and my MT Pro 12.5's rub. I had 14.5" of clearance and assumed 1" on each side from the tire size was enough. I had to hammer the wheel lips, and slightly hammer in the tack welds on the inner wheelwells. They now just barely touch during the worst conditions, but it was much closer than I had thought, and if I had a car that I couldn't "hammer" on, those tires would've been wasted money.

Obviously, radials will have different sidewall characteristics than bias ply's, but whatever tire you want, call the manufacturer or supplier and ask about total widths.
How many of you would return or sell the tires if they didnt fit? I bet every one of us would try to hammer or trim something to get them to work, and if they didnt work, you'd now have a dented car and still no tires. It's worth waiting on hold to get the tire info.
A last point to keep in mind is tire diameter. This is merely a matter of measuring from the center of the axle to the closest point in the well. It's good to first decide on maximum width so you know how far out the tire will be. Sometimes an inch of height is more important than width, so you may have to use a narrower tire to get inside an obstruction for the height. Another note here is that usually, the front lower edge of the wheel opening is the limiting factor. Its good to keep this in mind when moving your springs...Moving the rear back a little can give you more clearance for a taller tire.
 
Well here is what I did! I did my mini tub. I placed the wheels and tires where i want them. Then had the rear cut to fit. Depending on tire size even the space you gain from a mini tub fills up quick.
 
What back space are you using on your 10" Wheel? I have the Axle Measurements you need flange to flange. I have done this twice already.
 
I have ordered a 4" backspace 18" wheel. Any info you have would be helpful to me and I'm sure others.

What space are you allowing from the outer wheel to your 1/4 panel lip? The necessary amount will vary of course depending on the car model, some manufacturing slop, what tires are planned, and how low the car will sit.

On my 69 Dart, I figure I need about 1.5" in order to allow a stance with the wheel center 7.5" below the 1/4 panel lip. This allows 3.5" for suspension travel before the diff contacts the frame.

This is cutting it close with a 28" tall tire. Anything taller I believe will require more distance from the quarter due to the inner 1/4 panel closing in.
 
Idaho said:
I have ordered a 4" backspace 18" wheel. Any info you have would be helpful to me and I'm sure others.

What space are you allowing from the outer wheel to your 1/4 panel lip? The necessary amount will vary of course depending on the car model, some manufacturing slop, what tires are planned, and how low the car will sit.

On my 69 Dart, I figure I need about 1.5" in order to allow a stance with the wheel center 7.5" below the 1/4 panel lip. This allows 3.5" for suspension travel before the diff contacts the frame.

This is cutting it close with a 28" tall tire. Anything taller I believe will require more distance from the quarter due to the inner 1/4 panel closing in.
Click to expand...

I am running 325/50/15's and my car sits low. Down at least 1" past the lip on the wheel.

My wheels have a 5.5" back space and with the axle measuring 47.5" axle flange to axle flange, the tire fits dead nutzz center of the wheel opening with 1.25" space on each side of the tire. My wheel opening lip has not been modified at all.

Sooo if you are using a 4" back space your axle width flange to flange needs to be 3" over all shorter. Sooo flange to flange your finished axle measurement should be 44.5".

This is all based on a 2.5" brake off set that works for the 8.75" or Ford 9" rears.

Now get out your Credit Card out and place the order!:D
 
Thank-you very much. I've been admiring your work. I know you've been very careful to center those 325s. A 13.5 sw tire in that 14.5" space is a tight fit. I've been using one for a test - here's a couple pics. One shows how low I figure is feasible leaving 3.5" for suspension travel (7.5" from wheel center). One shows how the inside 1/4 skin curls inward. Then some unintelligible scribbles on the floor from using the plumb bob :)

A couple that may be useful to someone:
frame to frame (wheel side) - 38 7/8"
quarter lip (inside) to lip - 67 3/4"
lip to frame - 14.5"

Yup I better put that card to use before my wife cuts it up :)
 

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72ScampTramp said:
Well here is what I did! I did my mini tub. I placed the wheels and tires where i want them. Then had the rear cut to fit. Depending on tire size even the space you gain from a mini tub fills up quick.
Click to expand...

I'll be looking through your thread.
 
72ScampTramp said:
Well here is what I did! I did my mini tub. I placed the wheels and tires where i want them. Then had the rear cut to fit.
Click to expand...

yes but people need to be aware of what brake system running also... disc or 2" drum or 3" drum will push the wheel back out and may cause your style wheel that you wan't backspacing not be available if not all figured out first. And on my 71 I had the lip inside the wheelwell I cut it open and beat it over and tack welded up and epoxied across the seam. I like the lowered look and that lips needs changed to get it with a fat wheel/tire combo. Also my lip is rolled and need to consider wheel stud length to get the wheel out. It fits with no sidewall rubs and can just get it out of the wheelwell.

15*10 with 3.5" backspace and 315 radial! tire measures 13.5" wide

carshow009.jpg
 
When we completed my mini-tub project, I took down all of the measurements and used the formula that I provided in the link above.

A mini-tubbed 67-69 Dart with a stock width rear end will require a 7 1/2 inch backspace to center a 10 inch wheel. It's been done and verified dozens of times all over the web.

I wanted to run a 3 1/2 inch back space wheel, so when I ordered my Strange 60 built, I spec'd that I wanted it to be 8 inches narrower than a stock A body rearend. After bolting it in place, my measurements paid off, it fit like a glove. I did trim back the outer lip of the quarter panel and grind back the seam along the frame rail, but I have NO clearance issues running a 325/50R15 tire.
 
-Sledgehammer- said:
yes but people need to be aware of what brake system running also... disc or 2" drum or 3" drum will push the wheel back out and may cause your style wheel that you wan't backspacing not be available if not all figured out first. And on my 71 I had the lip inside the wheelwell I cut it open and beat it over and tack welded up and epoxied across the seam. I like the lowered look and that lips needs changed to get it with a fat wheel/tire combo. Also my lip is rolled and need to consider wheel stud length to get the wheel out. It fits with no sidewall rubs and can just get it out of the wheelwell.

15*10 with 3.5" backspace and 315 radial! tire measures 13.5" wide

carshow009.jpg
Click to expand...

The size of your brake system "drum" has nothing to do with your wheel fitting. The measurement you are referring to would be "brake standout", Mopar 8.75, Dana 60's and ford 9" all use 2.5" as that number. What could affect it is the drum or rotor thickness. Also it is not necessary to trim the fender lip to make a 325/50/15 tire fit on a Dart. You can trim it if you want but if your measurements are dead on it most definitely is not necessary. As far as getting the tire out with leaf springs YES it can be a PITA especially when setting the car down low with 3" studs. Mine has a 4 link with 3" studs and I have to unbolt the coil over top bolts so the rear end will droop enough to clear the studs and get the tire out.

Very nice car you have there!
 
whatever you wanna call it but... if I changed nothing else and switched to 3" drums I bet my wheel wouldn't fit :D I don't have to unbolt the upper shock mount to get the wheel out but with leafs and studs it does come out angling the wheel out right :cheers: lowering it you run into the lip for sure and harder to get wheel out... thanks for the compliments on my car but I am gonna change the color. :D
 
I wound up making this mock up axle in order to have peace of mind before sending out axles for shortening. I set my mounting surface width at 49.5". This gave me 1 1/4" from tire to the frame and about an inch to the upper outer fender well with the axle contacting the bump stops.

Easy to get confused with measurement points.

Mounting surface width avoids the differences between brands. For the Ford 8.8 out of the Explorer this measurement is 59.5".

Flange to flange is used by the shop shortening the axles. This measurement is taken from the outside of the brake plate mounting flanges. For my 8.8 the measurement is 54".

There's 1 1/8" of tube projecting out from the flanges so that adds another 2 1/4" for a total housing width of 56 1/4".
 

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I have a spreadsheet that gets REALLY close to proper backspace using a desired axle length for both Duster and Dart style bodies.
 
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