19" wheels offsets & tire sizes?

[Z51JEFF]

I'm having a set of wheels made as we speak. The fronts are 18-10 with 8 inches backspacing and the rears are 19-12. With 8.5 inches backspacing. The tires are 285-35-18 and 325-30-19.When I measured.The offset on both front,rear wheel I got 7.5 inches.

[WVZR-1]

Originally Posted by Z51JEFF

I'm having a set of wheels made as we speak. The fronts are 18-10 with 8 inches offset and the rears are 19-12. With 8.5 inches offset. The tires are 285-35-18 and 325-30-19.When I measured.The offset on both front,rear wheel I got 7.5 inches.

You used "back-spacing" measurements likely in your mentioned dimensions of being built. Depending upon the rim flange width, offsets would be something near 63 - 65 front and 50 - 52 for the rears. The constructor would likely quote you an advertised offset if you asked.

What would be an interesting project for the "Registry" I'd think is to develop/adopt a particular caliper/rotor measuring template that could be used when some are attempting wheel purchases. If you supply most constructors the brake specifics they can more easily discuss wheel lip possibilities for projects. Fitment first, aesthetics secondary. CCW uses two height dimensions and what they call over-hang, others use the same or similar but change the terminology. Those three dimensions should satisfy most any constructors needs.

[Z51JEFF]

Originally Posted by WVZR-1

You used "back-spacing" measurements likely in your mentioned dimensions of being built. Depending upon the rim flange width, offsets would be something near 63 - 65 front and 50 - 52 for the rears. The constructor would likely quote you an advertised offset if you asked.

Offset,back spacing I get the two mixed up. I've been having wheels built for my cars for years. This is what works for me. If you want to add some width to a wheel take the stock wheel and add the width, in the case of my I want to add a 1/2 in to the front but didn't want to move the centerline of the wheel so I added that 1/2 inch to the inside of the wheel,same for the rear. The wheels will still have the factory 56-36 whatever in relation to the mounting pad of the hub and the pad of the wheel.

[XfireZ51]

Originally Posted by WVZR-1

You used "back-spacing" measurements likely in your mentioned dimensions of being built. Depending upon the rim flange width, offsets would be something near 63 - 65 front and 50 - 52 for the rears. The constructor would likely quote you an advertised offset if you asked.

I was just looking at that. A rear offset of 36mm is 1.44". At 7.5" the car would look like something Big Daddy Ed Roth.

[Jagdpanzer]

Here is link to a cool wheel and tire comparison tool so you can play around with various tire and wheel size fitments

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

[WVZR-1]

Originally Posted by Z51JEFF

Offset,back spacing I get the two mixed up. I've been having wheels built for my cars for years. This is what works for me. If you want to add some width to a wheel take the stock wheel and add the width, in the case of my I want to add a 1/2 in to the front but didn't want to move the centerline of the wheel so I added that 1/2 inch to the inside of the wheel,same for the rear. The wheels will still have the factory 56-36 whatever in relation to the mounting pad of the hub and the pad of the wheel.

The problem when comparing "offset" and "back-spacing" some/most don't take into consideration that "back-spacing" is a measurement derived from "TOTAL WHEEL WIDTH" which is generally 1" or close to more than the advertised/specification width. 10 is actually 11, 12 is actually 13.

Here's a comparison of the 11@36 vs 12@38 (PhilW) vs yours 12@51. I used what was quoted in post #3

12@38 vs the "stock" - the inside dimension is 15mm apprx less and the wheel lip is pushed outboard 11mm approx and from all accounts PhilW's car is outstanding with that build.

Your choice 12@51 vs the "stock" - the inside dimension becomes 28mm LESS and the outboard wheel lip is "sucked in" 2mm. I can't imagine there being room for the 28mm clearance loss.

I didn't do the fronts.

[WVZR-1]

Originally Posted by Jagdpanzer

Here is link to a cool wheel and tire comparison tool so you can play around with various tire and wheel size fitments

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

Hello Phil - I generally do the math to establish the real deal but for quick comparisons of "wheels only" I sometimes use this. The advantage here is that you can +/- the offset without major changes to the information required. There's no back-spacing reference but if a person understands "Total Wheel Width" that's easy.

http://kgm.tiwing.com/calcs/offsetcalc.htm

I've done the math several times before relying on this calculator's results and aside from "rounding" I've found no issues. Is there? Could be. The rimsntires displays results sometimes differently and if a person doesn't understand the math can create issues. I could show you an example but it would confuse this thread I believe.

I believe I posted this calculator years ago.

[Jagdpanzer]

Dave,
I agree, these calculators are quite helpful but you should still verify by doing the math and physical measurement on the car to confirm the fit will end up where you want it and adequately clear the fender, brakes, wheel well liner, and inner suspension components. The dimensions of the actual tire selected should also be verified against the manufacturers published data as dimensions can differ somewhat from tire to tire of the same listed size.

[Z51JEFF]

Originally Posted by Jagdpanzer

Dave,
I agree, these calculators are quite helpful but you should still verify by doing the math and physical measurement on the car to confirm the fit will end up where you want it and adequately clear the fender, brakes, wheel well liner, and inner suspension components. The dimensions of the actual tire selected should also be verified against the manufacturers published data as dimensions can differ somewhat from tire to tire of the same listed size.

I've had 6 sets of wheels made over the years and I've never used a calculator to determine if a wheel will work on a car. It's very simple, put an OEM wheel on the car that you know fits and make any changes to that stock wheel. Push it in a 1/2 inch or what ever. In the past I've tried using measuring tools,even made my own but the easiest way is what I've described above. Now,if the company making the wheels screws up the dimensions,that's another story. In getting the wheels made for my Z I want the least amount of dish in the rear wheel as possible which involves leaving as much material on the back pad of the wheel as possible,right around 2 inches. It gets a little confusing but for what I'm paying for these wheels I'll leave the calculation the the wheel company.

[USAZR1]

Originally Posted by Z51JEFF

I'm having a set of wheels made as we speak. The fronts are 18-10 with 8 inches offset and the rears are 19-12. With 8.5 inches offset. The tires are 285-35-18 and 325-30-19.When I measured.The offset on both front,rear wheel I got 7.5 inches.

I can pretty much guarantee that a 19x12 rear wheel with 8.5" of backspace is going to have clearance issues on the inside,Jeff. Anything over 8.25" is a big gamble and especially if you lower the car.
Been there,done that and got the t-shirt.

+44 offset on a 12" wide wheel = 8.25" backspace
+51 offset on a 12" wide wheel = 8.5" backspace