69 Camaro with 70 12 Bolt - Backspacing/Fitment Questions

[American]

Keep in mind the wheels/tires are mounted on the car so my measurements aren't easy to make. Hang with me while I go over this... some of this is for my own clarity.

My 69 Camaro was purchased with the MB Old School wheels and Kumho Ecsta ASX tires.

Fronts:

17x8 (haven't measured but I'm sure that's what they are)
245/45/17

Rear:
18x8 with a 275/40/18

Here's the problem. I get a bit of rub on the outside edge of the tire, yes the rear suspension or at least the crappy air shocks need to go... but here's the problem, the rear is a 12 bolt out of a 70 model Camaro which if I understand correctly is about 1" wider. So figure 1/2" per side too wide.

Second problem: Discount Tire doesn't show backspacing but does show offset on a page I found. (Update at the bottom)

http://www.discounttiredirect.com/di...n=Old%20School



So, the fronts never rub, I'm happy there but the rears are obviously off and quite frankly I'd be happier with a 17" wheel and more sidewall.

The 18x8 they list with a 5x4.5 BP has a 0 offset.
The 17x9.5 has an offset of 6 and the 17x8 has options for an offset of 0 or 6.

So, the 18x8 listed on their site has no offset and a 4.5" BS. I'll gain a 6mm offset with the 17x9.5, this wheel has a 5.5" BS.

Assuming I'm starting 1/2" wider on each side because of the 70 model 12 bolt then I'm already at that disadvantage. Would the 17x9.5 with a 5.5"BS and a 6mm offset buy me any more clearance?

Found this on the Chevelle website:

How to Measure Wheel BackSpace:

Items required to measure wheel backspace:
Tape measure
Straight edge
Wheel w/o tire (preferred)


The easiest way to measure backspace is to lay the wheel face down onto the ground so the backside of the wheel is facing up. Take a straight edge and lay it diagonally across the inboard flange of the wheel. Take a tape measure and measure the distance from where the straight edge contacts the inboard flange to the hub mounting pad of the wheel. This measurement is backspace. The above photo shows three wheels with 2",3", & 4" backspace.


Off Set: Measuring Wheel Offset
To calculate offset you'll need the following measurements:

Wheel backspace
Wheel Width
Wheel Center line (outboard flange to inboard flange measurement / 2)

Subtract:

Wheel center line from Wheel backspace to get offset. If backspace is less than the wheel centerline the offset is negative
If backspace is greater than the wheel centerline the offset is positive


Tip:

To convert from inches to mm multiply by 25.4
To convert from mm to inches divide by 25.4
Backspace to Offset Conversion Chart
The table on the below is a quick reference for finding offset, pick the rim width and follow the row over to the backspace of your wheel.



If you look at the chart above then the offset of 5.5" agrees with the offset of 6mm.

So... using the chart above on my 18x8 having a 0 offset then it should have a 4.5" backspace. I should gain about 1" per side inset/backspacing going with a 17x9.5 and 5.5" BS... if my math is correct. But then add in a 1.5" wider wheel and I'm actually adding .5" outboard to what I have now?

UPDATE: Called Discount Tire direct:

17x8 with a 6mm offset has a 4.75" BS
17x9.5 with a 6mm offset has a 5.50" BS
18x8 with a 0mm offset has a 4.50" BS

What I think I need is:

17x9.5 with a 6" BS or more likely... a 17x8 with a 5.50" BS.

I'm also going to check the numbers on this rear end to verify what it is.

Who makes a decent wheel in 17 or 18x9.5 with a 6" BS?

Someone check me on this.

[JayR]

That's a ton of homework and fact checking but you're in the ballpark with a 6.0" backspace on a 9.5 assuming the 70 rearend is in fact 1.0" wider and it's centered. The easiest way to make sure is to find a stock 69 Camaro and take measurements from inside the wheelwell of the relationship of key points to parts of the body and wheelwells.

We can make you any backspace with 17x8, 9.5 or 10" which will fit your car because our wheels are 2 piece which is key in properly fitting wheels by being able to fine tune the backspace. We can also do 18s if you decide to go that way.

[American]

Here's the kicker.

It's well established that a 17x9.5 with 5.5" BS fits our cars quite well.

My 18x8 with 4.5" of BS is rubbing by around 3/4", keep that in mind.

Outside measurements on a 67-69 12 bolt are 60" and the 70 and up are 62.50" according to http://www.carnut.com/specs/rear.html

And this for a 69:
backing plate flange to backing plate flange is 54 1/2"
axle brake mounting flange to axle brake mounting flange is 60"
center perch to center perch is 45"

70 Model 12 Bolts are supposedly 55.5".

I've been paying attention to the inside measurements when really I probably should be looking at outside measurements, axle brake mounting flanges.... 2.5".

So, I don't know how much clearance everyone has between their tire and the outside fender lip with a 17x9.5" wheel with 5.5" of backspace but I bet it's about .5"...maybe less. My 18x8 with 4.5" of backspace should have about .5" more than that...but I don't. So, add the .5 I should have, then the .5" that's normal and my .75" of rub and I've got about 1.75" that I'm off. I should be at 1.25", so my math could be off or my assumptions or both.

What this means is that I may need a 6.5 to 7" backspace on a 9.5" wheel and that's too much.

Here's what has to happen. I need to narrow this rear end but when I look at axle lengths I'm a little alarmed.

67-69: 29.5" (x2=59")
70 and up: 30 1/16" (x2=60 1/8")

That kinda means that the brake mounting flanges and any gaps are .500" per side on the 67-69 and (hold on gotta count toes here)... the 70 and up is 1.175" so the 70 and up actually has an additional .675" width above and beyond the early 12 bolt.... if you follow me.

Anotherwords, let's say I narrow the 70 12 bolt to accept the early 29.5" axles, I still have 1.175" that I haven't accounted for or .675" per side and I'd still need 17x9.5 wheels with 6.175" of backspace instead of 5.5" as on a stock 69 12 bolt. I'd screw myself good if I bought wheels thinking I was good to go.

What am I missing here... replace the axle brake mounting flanges or???

I need a side by side comparison to know for sure OR I just need to sell this rear end and buy me a Moser 12 bolt.

[DriverzInc]

When you get a second, call the shop and I'm going to ask you to take some measurements. We can work it all out from there.
866-694-5940