~Wheel reply~

[Bill Verburg]

This post is a reply to a PM, the mail system can't handle this size message, my apologies, ignore unless you find it of interest


The above shows the relation ship between an 11" 275/40 tire mounted on a 9.5x17" rim.

The backspace(distance from the back of the wheel mounting face) is ~152.4mm, the oil line and alignment bolts need to be modified for this much backspace. You can go up to ~ 138mm of backspace w/o those mods. The frontspace is ~114.3mm which brings the tire right out to the inner edge of the fender well, the tire barely clears w/ SC/Carrera flares only because of camber change on compression.

In your message you are confusing backspace and o/s


So for an 11" rim which is ~12" overall use say ~150mm(~5.9") of backspace, ~155mm(~6.") frontspace, ~-2.4mm o/s.

The unknown thing(to me) is where that places the tire relative to a 930 flare, if you can provide me w/ the measurement from the inner fender well wall to the inner lip these #s can be further refined.

[RarlyL8]

This may be of little help Bill, but the wheels on the rear of my widebody use a 6" backspace. Overall rim width is 12.5" with a tire section width of 13.5". The oil lines must be moved and the nuts shaved (ouch!) as well as the fenders rolled or, in my case, trimmed. In other words, this is a maxed out fitment that does not rub using street alignment settings.

[rattlsnak]

Here is the intial PM to Bill,.. (condensed)


>> I am getting ready to buy some custom offset 17" wheels and dont want to use spacers for my 72 widebody. I tried a pair of FUCH 9x16s on the front and it was real close to hitting the outer fender and very close to the strut, so I am going to go with an 8.5. I know the 9 has a 130mm backspace, so I want to use a 120mm backspace in the front to make sure I have room.
In the rear, I want to use an 11" wheel, but not sure what to use for an backspace. I dont want to use a spacer, and also it needs to be far enough away on the inside that it wont hit, but far enough away from the fender as well. Ive read some threads that a 9 is very close with a 245 or such, but i will be running a 275 or 315 on the back, so Im thinking of going with the 8 backspace of 112/110, which will leave about 6.75 towards the outside of the car. Considering a 9 has @ 3.75/4 towards the outside, I would think that it would fit. Thoughts?<<

Bill I reread it and noticed I used the word 'offset' instead of 'backspace' by accident. I rewrote above.
If I understand your post, I can go as much as 138 Backspace w/o mods. (rear) So by using 110 or so, I will be fine there. I will measure the distance from the flange (hub) to the inner lip of the outer rear flare, and the inner fender well as well,and let you know.

Thanks,
Marc

[rattlsnak]

OK my hub to the fender is just over 155mm. So if I go 130mm B/S in the rear, that leaves 152mm left for the outer lip to make 11". I'm worried this may be a little close. Of course by going the full 138 inside, they should fit. WOw, thats going to be close. I think I may go with a 10.5 rim instead!

[Bill Verburg]

Marc, 130mm b/s is ultra conservative. 9x16 Fuchs have ~138mm the closer to 153mm you get the more interference at the trailing arm and oil line. It's not that big a deal to grind the trailing arm adjusters but moving theoil line can be problematic on the earlier cars w/o the shelf in the inner wall.

I think that up to ~143mm would have almost no trouble on the inside.

That works out to ~160mm front space , -9.4mm o/s w/ an 11" bead width rim.


Rarly's #s jibe well with the #'s I have.

Again the distance from the inner wall to the fender lip would help to refine these figures.

The other issue is that the o/s, f/s and b/s #'s are not infinitly variable, they are generally available only in .5" increments.

[RarlyL8]

Bill - backspacing IS infinitely variable.
The wheel fitment on the passengers side of my car is 1/8" offset to the outside by using a thin spacer. If you have turbo studs, or just longer than stock studs, you can fine tune the backspacing with a custom thin spacer. I use racing lug nuts with no ends so I can see how many threads protrude. Works great on those diffucult to fit situations.

[Bill Verburg]

The wheel fitment on the passengers side of my car is 1/8" offset to the outside by using a thin spacer

Yes, for those that are willing to use and machine spacers that would be true. And play w/ the lug lengths and nuts.

The wheels themselves are only available in usually .5" increments.

[Kevin Stewart]

I understand the way you guys are measuring, but today we measure positive or negative from center line, we no longer use the rear lip, example plus 55 the center of the wheel would be a little over two inches closer to the front wheel lip, as i recall Porcshe has been using it for a long time as a negative 15 or 23, when you call a wheel company they will convert your measurements to the later style, same results for what its worth, Kevin

[rattlsnak]

]--------------------------------[
]----------------[
........................]---------------[
..]----------------------------[

Ok, heres my crude drawings
#1 ] and [ =equals outer fender and inner fender. Each dash is 10mm. 330 total.
#2 the center ],[ is the hub. 170 outer, 160 inner
#3 this is the tire. 11" equals @ 280mm.
(ignore the ... dots)
So if I use a 140mm b/s, I should have @20mm leeway on the inside before hitting the bolts, and @ 140 f/s and @ 30mm to the fender. (or @ 1.1") which is fine. I would rather have a little leeway there also, as I can correct this by spacing if I had too.

[Bill Verburg]

Ok i'll see what I can come up w/ this evening

330mm from inner rear wall to inner edge of the flair

170mm from the wall to the mounting face of the wheel

160mm from the mounting face of the wheel to the inner edge of the flair

11"tire

[Bill Verburg]

Marc, there is something wrong w/ the #s you gave.

An SC/Carrera has ~305mm between the inner wall and lip. You car w/ 930 flares has only has 330mm?( ~1" more)

Rarely's #s and my suspicions support more like 345mm

[rattlsnak]

sorry mistyped, its 350mm. And its 160 from the mounting face to the inner fender well, and 190 from the mounting flange to the flair.

[rattlsnak]

Rarly, are you running 315s or 335s on the back?

[RarlyL8]

I'm running 345/35-15s out back.
Fenders are trimmed and nuts are shaved 1/2 off.
Alignment is street trim flat. Tires have rubbed on rare occasion.
350mm is about right. My drivers side and passengers side are not symetrical. Drivers side has about 1/3 inch more room. I found this to be true on my last car as well. It was a '73 widebody with 930 flares and the same wheels and tiires.

[widebody911]

So did you get the backspacing/offset #'s worked out? I'm going thru the same wheel shopping process.

[Bill Verburg]

Ok here we go - a hypothetical wheel for 930 wheel well



Wheel 11" bead, 12" (305mm)overall cross section
backspace 140mm(5.5")
frontspace 165mm(6.5")
tire 315/30(320mm cross section)
o/s -14.5


tire clearance on inside 14.5mm
tire clearance on outside 15.5mm

each mm of additional negative o/s adds 1mm of clearance on the inside and subtrats 1mm of clearance from the out side.

[widebody911]

Cool diagram Bill - that's a keeper!

[rattlsnak]

Thanks Bill, i went with 5.5 B/S in the back and 5 in the front.
The company actully figures out there own offsets based on rim halve sizes. And they can go in .25 increments. You just tell them the most B/S you can have. In my case, 5.5" and they figure out the rim halves. (i'm probably going to get a 6.5 inner shell, because the centers are @1 offset) Kinda hard to write and explain, but I told him what i had to have so if they are not right he will fix them.

[Bill Verburg]

Measure them up carefully when you get them and let me know, I'd like to add to my collection of known and verified fitments.

[Lukesportsman]

As many people here start with standard non flared cars before adding RS/SC or RSR/Turbo, can you give us some more numbers? I went out and measured about 265mm on a 72 non flared car. Does this sound about right for a non flared car?

If this is accurate, than I'm set as the outer tire wall comes in right at 300 mm from the inner wall as the car sits and I haven't added any camber yet (with rough hand unit its about .5 degrees positive as I've just bolted it together). Just want to make sure I get "enough" flare when deciding.

Rattlsnak: Any updates, yet?