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 mailto:jnl@vvm.com

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There's a difference between ignorance and stupidity.
Ignorance is not knowing any better.
Stupidity is knowing, yet doing it anyway.

I understand it enough to do it, but not quite good enough to explain it.........


One thing though, measure clearances against the existing tire...

Then measure the tire, so you can see what you can change.


Also, a better way (for me) is to think in terms of wheel offset instead of backspacing.......

Just some info

Overall rim width is usually about 1 inch more than stated rim width (i.e. 6" rim is 7" wide)

A tire increases/decreases in width .2" per every .5" change in rim width (i.e. a tire thats 8" wide on a 6" rim with be about 8.4" wide on a 7" rim)

Tire manufacturers generally give you dimensions of there tires, and the rim width the measurement was done on (see www.tirerack.com for specs)

See

http://www.tirerack.com/wheels/tech/tec ... techid=101

there's more at tirerack, hopefully it's understandable........

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

hey jerry

to understand this think as your wheel placed inwards or outwards in the wheel well. Say for a moment you keep your tires. If your backspacing is 2" smaller, all the tire and rim would be 2" discplaced to the outter side of the well. If you're considering on ordering diffrent backspacing wheels, try to think of it this way: of you use a 7" wider rim and the backspacing is the same, your wheel would protrude 1" more (if you had 6" rims). So if you wanna get 8" wheels, and you keep your backspacing, your wheel will be 2" discplaced outwards.

One more advice I can give you is keeping 6" rims on your front end, otherwise you'll end up putting more stress in the front end and steering would be heavier especially with manual boxes.

I have 14x6" rims on the front, with 195/70 14 tires. Backspacing has been reduced 1" so I have 2" extra between the centers of my front wheels. On the back I have 14x8" rims and the backspacing has been reduced one inch, so there I gained 6" extra on the rear axle. This helps handing tremendously, especially hen cornering at high speeds or in the highway. car sticks to the road way better. rear tires are 235/65/14.

15" tires are a lot more expensive than 14", so there you are wondering between looks and the possibility to replace your tires on early stages of wear, wich will result in better handling and safety.

Hope this helps.
Juan

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Juan Ignacio Caino

Please use e-mail button istead of PM'ing. I do log in sometimes but I'll be answering quicker thru e-mail.

Sorry to say that was confusing............. And I know enough to not have things rub........... (still don't know enough to expain it)


All I've got to say is convert to offset and don't use backspacing.........
(I use a spreadsheet)


Here's another one from tirerack
http://www.tirerack.com/wheels/tech/tec ... ntpage=101

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Let's slow down for just a second. I believe I have a decent handle on the theory and math to determine offset (and backspacing).

This is backspacing:
(A) Find the total overall available clear width.
(B) Subtract wheel width from (A)
(C) Divide remainder by two
(D) Measure from hub mounting surface to inner fender surface (on frame side of wheel well).
(E) Subtract (C) from (D) and there ya go

This is offset:
(A) Measure from hub mounting surface to inner fender surface (on frame side of wheel well).
(B) Divide wheel width by two
(C) Subtract (B) from (A) and there ya go - negative if wheel centerline is inside (frame side) hub mounting surface OR positive if wheel centerline is
outside (fender lip) hub mounting surface

Do I have these correct?

Now, here is the real problem... I don't know where to take the actual measurements. Read my first post again, and you will see what causes my uncertainty. The shape of the wheel well surface, with all it curves, bulges, and tapering. The total available clear width is dependent entirely upon where you take the measurement. I try to envision where the tire could be up high in the wheel well when the springs are compressed going over a bump or through a pothole -- a measurement up there is where the curving wheel well gets narrower. And the actual lip of the wheel well is not at issue, because the inner surface (undercoated) is curved inward toward the tire.

I know this sounds more complicated than it should, but for a rookie like me it's important to understand it completely. I need to get this right before I place an order for some new rims.

Jerry

_________________
There's a difference between ignorance and stupidity.
Ignorance is not knowing any better.
Stupidity is knowing, yet doing it anyway.

Nope:

Backspacing:
Distance from inside of rim flange to mounting surface

Offset:
distance (positive or negative) of mounting surface to centerline of wheel.


Some examples:

Wheel 1: 14 x 6 rim
7" distance flange to flange
0" offset
it would have 3.5" backspacing.

now lets change it a little,
we increase width to 14 x 7, but all on outside
8" distance flange to flange
still have 3.5" backspacing (it grew out),
but now offset is negative 1/2" (negative offset wheels have more spacing on outside, positive offset on inside (fwd cars have positive offset)

if we wanted to keep 0" offset, the wheel would have to have been increased 1/2" on inside and outside, in this case
14 x 7" rim, 8" flange to flange, 0" offset, 4" backspacing.

See the pattern here?
offset = (flange to flange)/2- ((flange to flange) - backspacing )

Now for tires:
wider tires grow to each side of the center line of the wheel. So a 1" wider tire grows 1/2" out, and 1/2" in.


I'll assume the following, 14x6" rims, 3.5" backspacing, 0" offset, a 215/70 tire is about 8.5" wide. (I'm making all this up for an example)
Then you measure a far the existing tire is from hitting anything.
The rim inside comes out 1/2" from hitting front suspension, so for safety, your max backspacing will be 3.75".
The inside of the tire is 2" away from hitting anything (lets say it's rear leaf springs)
The outside of the tire is 2" away from hitting the front fender.

With a 15x8" rim and 3.75 backspacing, you'll have an offset of (8+1)/2-((8+1)-3.75) = 4.5 - 5.25 = negative .75", or the centerline of the tire as moved out .75".

So now you have a clearance of 2.75" on the inside, and 1.25" on the outside with a 215 tire.

But with a rim 2" wider than existing rim, the tire will be (2"/.5")*.2" = .8" wider. So now the tire that was 8.5" wide is 9.3" wide on the 8" rim.

Half the gain is inside, half outside. So you have a clearance of 2.35" on inside, and .85" on outside.

With .1" safety, the tire width can grow .75" out towards the fender. Or 1.5" total.

So you're looking for a tire that's 10.8" wide on an 8" rim.


Now that you're totally confused...............



look at the tirerack info, draw pictures, make a spreadsheet, eventually you'll figure it out...........

for me it's kinda hard to explain without pictures

_________________
Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

you're looking for worst case.

take into consideration suspension travel.

if suspension travel is 3" up , when tire moves up 3", what's closest thing to tire (at any point on outside of tire, and any point on inside of tire).

the tire is curved, and so is the fender, so the widest point of tire may not even come close to narrowest point of fender. move widest point up 3", how close is it now to fender...........

_________________
Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

try this

http://www.bmwcsregistry.org/Technical/ ... atives.htm

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

if with SNIP you mean I was being snippy, it ain't true-. I just explained out loud what's my way of figuring out things. Sorry if came up twisted.

_________________
Juan Ignacio Caino

Please use e-mail button istead of PM'ing. I do log in sometimes but I'll be answering quicker thru e-mail.

BTW, is there any negative effects on having positive offset? just wondering if my new great looking wheels are gonna gimme headaches somewhere down the road

_________________
Juan Ignacio Caino

Please use e-mail button istead of PM'ing. I do log in sometimes but I'll be answering quicker thru e-mail.

no you're not snippy.

snip as in 'cut out' . Just didn't want to repost your whole post......., but I wanted to reference it.........



I was waiting for you to say my post was confusing.......

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

How much positive? As long as you're not hitting any suspension components, I think you're fine.


If the offset is within 20mm or so of what it was originally, I wouldn't worry about it.

_________________
Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Now I'm really confused. I had my 3.5 backspacing original 14x6" rims. I had them offset a li'l bit more (1 inch in the front) so now I have a 14x6" rims with 2.5 backspacing. I believe original rims (at least our argentinian ones) had about zero offset so now I have 1" offset (positive? negative? sticking out) on the rear I had them rims replaced with 8" wide rims and I had them backspaced the same as my front wheels, so they're 3" offset.

_________________
Juan Ignacio Caino

Please use e-mail button istead of PM'ing. I do log in sometimes but I'll be answering quicker thru e-mail.

negative offset, wheels stick out; positive offset wheels wrap around brakes..........


14x6 3.5" backspacing =

0" offset [ backspacing - (rim width + 1)/2 = offset)]
[ 3.5 - (6 + 1) / 2 ]
[ 3.5 - 3.5 = 0]

14x6 2.5" backspacing =

negative 1" offset [ 2.5 - (6 + 1) / 2 ]
[ 2.5 - 3.5 = -1 ]

14x8" 2.5" backspacing =

negative 2" offset [ 2.5" - (8 + 1)/2 ]
[2.5 - 4.5 = -2 ]

14x8" 3.5" backspacing =

negative 1" offset [ 3.5 - 4.5 = -1 ]


Note: 1" is added to stated rim width, since overall width of the rim is about 1" more than stated width. Use overall width if you've got it.

_________________
Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes