Backspace & Offset ? Need Help

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barrist
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Backspace & Offset ? Need Help

Unread post by barrist » Sat Dec 09, 2017 1:14 pm

Will any of these work ? 65 Corsa

15x6 3.50 Backspace 0mm Offset with 205/60R15 841 revs per mile

15x6 3.54 Backspace 4mm Offset with 195/60R15 858 revs per mile

14x6 3.39 Backspace -2mm Offset with 205/70R14 821 revs per mile

Thanks for Any Help, I think I'm confusing myself, :dontknow:

Bill
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Bill
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66vairguy
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Re: Backspace & Offset ? Need Help

Unread post by 66vairguy » Sat Dec 09, 2017 6:57 pm

With a 6" wide rim (bead to bead 6" - approx 7" wide rim to rim) the ideal backspace is 4.5" (measured from the rim edge to drum contact surface of rim). The farther you get from stock geometry - the more pronounced bump steering and wandering get.

Speedometer reads with zero error at 825 RPM, but the original tires typically ran at 835 to 865 RPM (rolling speed as tires squish under weight of a car) causing the speedometer to read 2-4 MPH fast at 60MPH. Why did speedometers usually read fast - to allow for tolerance variations in speedometers and tires so folks didn't get a ticket because the speedometer was reading slow.

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barrist
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Re: Backspace & Offset ? Need Help

Unread post by barrist » Mon Dec 11, 2017 6:45 pm

Maybe I should have asked, Is 3 1/2 backspace enough for these 6 inch wide wheels ?

I'm hoping these will work. :pray: Merry Christmas to Me !

Thanks again,

Bill
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Bill
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bbodie52
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Re: Backspace & Offset ? Need Help

Unread post by bbodie52 » Tue Dec 12, 2017 6:49 am

barrist wrote:Sat Dec 09, 2017 3:14 pm

Will any of these work ? 65 Corsa

15x6 3.50 Backspace 0mm Offset with 205/60R15 841 revs per mile

15x6 3.54 Backspace 4mm Offset with 195/60R15 858 revs per mile

14x6 3.39 Backspace -2mm Offset with 205/70R14 821 revs per mile
:dontknow: MY GUESS...

The last size mentioned, P205/70 R14, work properly on my 1966 Corsa when fitted on 14" x 7" wheels that have a Backspace of 4-3/8”. 3.39" backspace is about 1" less outside clearance, so the same tire size may contact an unmodified body in the front as the inner wheel opening may cut the tire. The wheel specifications for a B.F. Goodrich Radial T/A P205/70 R14 specify a 5" - 7" rim for that tire size, so a 6" rim should be adequate to fit that tire, but the backspacing may be inadequate without wheel opening modifications.

A B.F. Goodrich P205/70R14 requires a rim width of 5.5" - 7". It has a section width of 8.2", a tread width of 6.2", and an outer diameter of 25.4".
A B.F. Goodrich P195/60R15 requires a rim width of 5.5" - 7". It has a section width of 7.9", a tread width of 6.7", and an outer diameter of 24.2".
A B.F. Goodrich P205/60R15 requires a rim width of 5.5" - 7.5". It has a section width of 8.2", a tread width of 6.7", and an outer diameter of 24.7".
A B.F. Goodrich P235/60R14 requires a rim width of 6.5" - 8.5". It has a section width of 9.5", a tread width of 7.9", and an outer diameter of 25.1".

In all cases with the tire sizes you mentioned, a 6" rim is adequate for the tire. Both of the 15" tires have a smaller outside diameter than the 14" tire, and a section width that is less than or equal to the 14" tire, but the 15" tires both have a 0.5" wider tread width. Since the rims you mentioned lack 1" of backspacing, all three tires might impact the front body opening. I would guess that the smaller tires you mentioned may clear OK on the rear, since the P235/60R14 tires I have on the rear are significantly wider and still clear OK (with 4-3/8” backspacing). The narrow tires may clear OK with 1" less backspacing. But you may have clearance problems in the front.
bbodie52 wrote:I've had some positive comments about the Chevrolet Rally Wheels on my 1966 Corsa. To answer the questions about sizing, here are the specs...

ImageImageImage


1966 Corvair Corsa Convertible SS Rally Wheels
RALLY RIM GM CODE: [9 2 11 YA] Production Date: 1969, February 11, Code: YA
"SS" Style, for Camaro/Chevelle with Disc Brakes YA 14x7 SS
YA — 14” x 7”, Backspace: 4-3/8” 1969 Chevelle SS Wheel
ORIGINAL GOODYEAR RWL TIRES:
Front: Goodyear Eagle ST P195/70 R14
Rear: Goodyear Eagle ST P235/60 R14
BF GOODRICH RWL REPLACEMENTS:
Front: BF Goodrich Radial T/A P205/70 R14 ($119.45 each)
Overall Diameter: 25.4" Revs per Mile: 821 Sect. Width: 8.2" Tread Width: 6.4"
Rear: BF Goodrich Radial T/A P235/60 R14 ($132.45 each)
Overall Diameter: 25.1" Revs per Mile: 829 Sect. Width 9.5" Tread Width 8.7"

The ideal tire diameter produces 825 revolutions per mile, but can go to 850 revolutions per mile and the speedometer only be 3% fast, or 2 MPH at 60.

Sizing Corvair Wheels and Tires wrote:...I surfed over to The B.F. Goodrich site to come up with some examples of tires that would work well on Corvairs. They have data sheets for all of their tires there, I used the specs from the Radial T/A here, other models will be a bit different. You should be aware that the sizes are a range, so it's a good idea to get the data sheet if you can, especially if you have chosen a size near the limit of what will fit. Unfortunately, BFG doesn't offer a 13" Radial T/A that will work well, these are 14, 15 and 16 inch:
FRONT...
P185/70R14 (all)
P195/70R14 (all, good replacement for 1960 - 65 6.50-13)
P205/70R14 (all, good replacement for 1966 - 69 7.00-13)
P205/55R15 (all)
P195/60R15 (all, good replacement for 1960 - 65 6.50-13)
P195/65R15 (all, good replacement for 1966 - 69 7.00-13)
P205/60R15 (all, good replacement for 1966 - 69 7.00-13)
P205/55R16 (a little tall, may rub on an early, fits lates)
REAR
Late models have a little more room in the rear, you could use these sizes on the rear of a late if you want the "big and little" look:
P225/60R14
P235/60R14
P215/60R15
P225/50R16
:link: http://autoxer.skiblack.com/tires.html
Backspacing - The distance from the inside of the rim to the point where the wheel contacts the brake drum. All car 5 1/2" wide wheels have a backspacing of 4 1/4". FC wheels have a backspace of 3 1/2". For reference, more backspace moves the wheel further in the wheelwell, less backspace brings it out. Maximum backspace for an early with 15" wheels is 5", late is 5 1/4". For FCs it's a little more complicated, 14" wheels are limited to the stock backspace because a deeper set wheel will hit the tie rod. I have not yet tested larger diameter wheels on an FC.

Image

Offset - If the wheel is centered in and out relative to the face that contacts the drum, then it's zero offset. If the wheel is in or out relative to the mounting face, then it has offset. The stock Corvair car wheel (early or late) is offset 1" inboard (source: Chevrolet Engineering Features, courtesy Larry Claypool).

Note: I prefer using backspace to offset, since there is some difference of opinion as to which way is "negative" or "positive" offset - the current catalogs show "positive" as the mounting surface of the wheel being closer to the outside of the rim than the inside. I will generally use backspace.
Brad Bodie
Lake Chatuge, North Carolina
Image 1966 Corvair Corsa Convertible

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barrist
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Re: Backspace & Offset ? Need Help

Unread post by barrist » Wed Dec 13, 2017 12:01 am

Thank You Guys for the answers. I'm still researching, The price is right at tire rack, not just a lot of choices or Help there.

Thanks Again,

Bill
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Bill
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barrist
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Re: Backspace & Offset ? Need Help

Unread post by barrist » Wed Dec 13, 2017 3:28 pm

Using http://www.rimsntires.com/specspro.jsp

Comparing 185/80/13 5.5x13 to 205/60/15 6.0x15

Suspension clearance:
100% clear (leaves even more room than OEM)

Fenders clearance:
Package 2 will stick out 31 mm (1.2'') farther. Make sure that you have enough room under the fender. If not, consider narrower tires or higher offset.

Wheelwells clearance:
The same.

SPEEDOMETER:
When speedometer reads 60mph (96.6km/h) actual speed will be 60.1mph (96.8km/h): 0.2% faster

Hmm :think: What do You Think ?

Bill
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Bill
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barrist
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Re: Backspace & Offset ? Need Help

Unread post by barrist » Thu Dec 21, 2017 9:44 pm

Ordered, I'll let Ya Know how it works. :tu:

Merry Christmas,

Bill & Sherl
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Bill
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barrist
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Re: Backspace & Offset ? Need Help

Unread post by barrist » Mon Jan 08, 2018 5:36 am

Well Guys, tires and wheels made it here. Just "HAD" to try them ON.
9 degrees, and Corvair is stored in the trailer, but it had to be done.
New wheels 002 (800x600).jpg
Looks like plenty of clearance.
New wheels 003 (800x600).jpg
And I saved a few bucks, due to fingerprints on the wheels. So, I added the knock-offs.
New wheels 004 (800x600).jpg


All came out as I wished, Merry Christmas to Me. :chevy:

Those are 15x6 3.66 Backspace 4mm Offset with 205/60R15
Have a Great Day,

Bill
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notched
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Re: Backspace & Offset ? Need Help

Unread post by notched » Mon Jan 08, 2018 6:31 am

4.5" is ideal on a 7" wide rim.
3.5" is ideal on a 6" wide rim so it looks like you chose wisely.
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hugofant
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Re: Backspace & Offset ? Need Help

Unread post by hugofant » Mon Feb 05, 2018 3:19 am

4.5" is ideal on a 7" wide rim.
3.5" is ideal on a 6" wide rim so it looks like you chose wisely.
hi,
want to ask for clarification regarding this:

if i've caculated correctly a 206/60/15 tire on a 7" 4,5" backspacing rim means + 3,2cm on the outside and + 0,6cm on the inside (to the original 185/80/13 5,5" 4,25" backspacing)

same tire on a 6" 3,5" rim would be +3,1cm on the outside and -1,9cm on the inside...

so, why not a 6" 4,5" backspacing rim? will be +0,6 cm on inside and outside - very close to the original view (and on the inside also not more than the 7" version).....or have i calculated something wrong?


...which would mean that this rim should also fit well....

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