Factory Reproduction Wheels offset/backspacing

[npritchard777]

I posted this up on *************.com but thought I should check in here too.

So I think I might not have the right rear wheels. I've been trying to figure out this offset / backspacing thing and found this website; http://www.joby.se/corvette/div/corvette_wheels/

It has this for the ZR-1
1990 (zr1)
front 17"x9.5" 56mm (7.4" backspacing)
Goodyear Gatorbacks, P275/40ZR-17
rear 17"x11" 36mm (7.93" backspacing)

And this for the Grand Sport
1996 Grand Sport (coupé)
front 17"x9.5" 56mm (7.4" backspacing)
Goodyear Eagle GSC, P275/40ZR-17
rear 17"x11" 50mm (7.93" backspacing)

1996 Grand Sport (convert.)
front 17"x8.5" 56mm
Goodyear Eagle GSC, P255/40ZR-17
rear 17"x9.5" 56mm (7.4" backspacing)


So anyway, I went to measure the new wheels but couldn't find my tape measure anywhere (just moved and can't find anything). But I noticed the label on the box said these wheels were a 7.37" backspacing. I put a picture below.

http://i148.photobucket.com/albums/s5/stroked_z/IMG_0598b.jpg

I had specifically confirmed with the Factory Reproductions guy before I bought them that they were a 36mm offset. What I can't figure out is how the offset of 36mm works with a 7.37" backspacing. Of course, I'm a chump when it comes to wheel fitment.

I love the look of the wheel and would hate to send them back. So my question is, are these going to fit right or are they going to be sticking out?

http://i148.photobucket.com/albums/s5/stroked_z/IMG_0595b.jpg
http://i148.photobucket.com/albums/s5/stroked_z/IMG_0597b.jpg

[GOLDCYLON]

In case you missed it over at CF. It will be a little off

http://members.cox.net/goldcylon/17x11b.jpg

[Jeffvette]

Based on my rough calcs, it's a GS offset wheel.

7.37 (backspace) minus 5.45 (10.9 Inside lip of wheel/2) = 1.92 * 25.4 = 48.76

Those wheels will be sunk in.

[npritchard777]

Thanks for the replies. I had a feeling it was the GS offset when I read the backspacing. The ad on eBay stated "True ZR-1 Wheels" so I specifically asked if this was the correct wheel for an actual ZR-1 and was assured it was.

I left the guy an email on eBay and he called me back and left a voice mail to call him. I'm hoping they inadvertently sent the wrong 17x11's but I got a feeling when he assured the fit he wasn't assuring that they would be flush with the panels.

Oh well, I'll keep y'all posted. I really want to refinish my stock wheels anyway and was hoping to have these on until I get around to that some day. lol :-D :icon_scra :neutral:

[npritchard777]

By the way, this is from the eBay ad;

True ZR1 1988-1996 Corvettes
Bolt Pattern 5x4.75
17x9.5 Offset +54mm & 17x11 Offset +36mm

[tomtom72]

Sorry I'm late but you might wanna put this as a bookmark. It's over at our mothership.

http://www.zr1netregistry.com/ZR1_suspension.htm

I keep it as it has a host of info. I went thru the same thing on looking for wheels when I got my Z. "Yes, these are real ZR-1 wheels, sir!" ....No that's not what I asked, I asked you for the offset on the wheel you're selling in this ad........"Oh, well, err, umm, I'll have to transfer you, please hold." .....after awhile I got tired of listening the musack!

FWIW, if you want I can look up the phone number. I talked to a lady at OEConcepts I think....been 5 yrs now & i can't remember, but I have all the invoices. It was a strugle to get someone who knew that there was a difference among the off sets by application. I don't really have anything bad to say about the OEConcept wheels, not much good to say either...well they're round. Cheap chrome, hard to balance one of'em...but they were less than a grand shipped. Ya get what ya pay for.

Good luck in your search!:thumbsup:

:cheers:
tom

[WVZR-1]

By the way, this is from the eBay ad;

True ZR1 1988-1996 Corvettes
Bolt Pattern 5x4.75
17x9.5 Offset +54mm & 17x11 Offset +36mm


If they measure something close to 7 7/16" for a "backspacing" you will be fine. There's certainly a good bit of confusion "still"! The "ONLY" confirmation is do the "measurement" your self! To hell with labels and ads- get out the "measuring device" BEFORE- you mount tires! It's really that easy!!!!

I believe this to be the correct math: 36 /25.4 = 1.4173+ 6.0(12.0/2.0) = 7.417 +/-

[npritchard777]

Ok, so I'm thinking my original reference was incorrect. I got a link sent to me from the other forum to check out (http://www.bigcustomwheels.com/info_specs.jsp). I plugged in the ZR-1 vs. GS dimensions and it popped out this;

http://i148.photobucket.com/albums/s5/stroked_z/wheels.jpg

I still need to find my tape measure and check for myself, but I'm feeling a little better about it now. I'll be going to the hardware store for a new tape measure (since I'm tired of going through moving boxes) after work and check for myself.

Thanks all,

-Nate

[Jagdpanzer]

Went through the same drill last summer.
With digital calipers, machinist square, tape measure and strait edge in hand I measured up one of the GM original stock rear A-moulds off my 94 Z.
Here is what I found:

Rim Width 11.0"
Rim Diameter 17"
Overall Wheel Width 12.0" (outboard flange to inboard flange)
Overall Wheel Diameter 18.5"
Offset +36mm (from wheel centerline)
BackSpacing 7.375" (from hub mounting face to inboard flange edge)
Center Bore Diameter 2.75"
Emblem Hole Diameter 2.5"
Emblem Diameter 3.0"

(reasonable tolerances to be considered)

[npritchard777]

Hey thanks, now I have a control to compare it to when I get to some measuring! :razz:

-Nate

[Aurora40]

Based on my rough calcs, it's a GS offset wheel.

7.37 (backspace) minus 5.45 (10.9 Inside lip of wheel/2) = 1.92 * 25.4 = 48.76

Those wheels will be sunk in.
Yeah, that's what I was thinking. Offset is the distance from center. Backspacing is the distance from the edge.

Presumably the wheel is 11" wide, so the center is 5.5" from the edge. 7.37 - 5.5 = 1.87". 1.87" * 25.4 mm/in = 47.5mm offset.

But hell, I don't know. The real test might be just toss it on the car and see if it looks right.

[WVZR-1]

Yeah, that's what I was thinking. Offset is the distance from center. Backspacing is the distance from the edge.

Presumably the wheel is 11" wide, so the center is 5.5" from the edge. 7.37 - 5.5 = 1.87". 1.87" * 25.4 mm/in = 47.5mm offset.

But hell, I don't know. The real test might be just toss it on the car and see if it looks right.

Bob,

I'm quite "confident" of my math and the calculating "scheme"! You need to use the "total" width of the wheel which is 12" to calculate "backspacing"!

I was rather dumbfounded when I stumbled on the calculator later this morning. You need to use the calculator yourself because it has some interesting results. It will +1 and +2 wheel diameters and give "mathematical" tire equivalents for you if you like. "Mathematical" possibilities some/most of the results aren't available in the "real world"!!

It's worth a trip to the calculator!!

http://www.bigcustomwheels.com/info_specs.jsp

-Dave

[Aurora40]

Bob,

I'm quite "confident" of my math and the calculating "scheme"! You need to use the "total" width of the wheel which is 12" to calculate "backspacing"!

I was rather dumbfounded when I stumbled on the calculator later this morning. You need to use the calculator yourself because it has some interesting results. It will +1 and +2 wheel diameters and give "mathematical" tire equivalents for you if you like. "Mathematical" possibilities some/most of the results aren't available in the "real world"!!

It's worth a trip to the calculator!!

http://www.bigcustomwheels.com/info_specs.jsp

-Dave
Yeah, that's a pretty cool site. So what does the 11" width refer to, the width between the start of the flanges/seats?

[WVZR-1]

Yeah, that's a pretty cool site. So what does the 11" width refer to, the width between the start of the flanges/seats?

That's pretty much it and you'll see it displayed on the top of the wheel/tire image of the calculator. These calculators are quite interesting, it used to be "paper/pencil/the mind!

Most people that have problems with "wheel fitment" actually try to work with "offsets" and if you deal with primarily "backspacing" the confirmation of dimensions is much easier. Fitment is the "entire wheel/tire" combination not just the mounting flange width which is used primarily to supply the "section width" the tire manufacture desires for the tire size.

[npritchard777]

Found my tape measure! The backspacing is approx 7 7/16 or 7.4375. They should fit.

http://i148.photobucket.com/albums/s5/stroked_z/ZR-1/IMG_0600a.jpg

Thanks all!

-Nate

[Jagdpanzer]

Found my tape measure! The backspacing is approx 7 7/16 or 7.4375. They should fit.

http://i148.photobucket.com/albums/s5/stroked_z/ZR-1/IMG_0600a.jpg

Thanks all!

-Nate

Close enough for government work!

[WVZR-1]

If they measure something close to 7 7/16" for a "backspacing" you will be fine. There's certainly a good bit of confusion "still"! The "ONLY" confirmation is do the "measurement" your self! To hell with labels and ads- get out the "measuring device" BEFORE- you mount tires! It's really that easy!!!!

I believe this to be the correct math: 36 /25.4 = 1.4173+ 6.0(12.0/2.0) = 7.417 +/-

It's good your "tape measure" confirmed!