Mycarispurty Posted March 13, 2006 Share Posted March 13, 2006 Ok I was checking out these uber badass Rota D2 wheels for my 280Z. They come as low as 20 and 25 offset, 17x7.5 up front and 17x8.5 out back is what I'm interested in. Anyways I was converting inches to millimeters and calculating and crunching numbers. Here's what I've come up with. The 7.5" = 190.5mm, at 0 offset wheels that'd be 95.25mm which equals 3.76" of backspacing. However since they're +20, that'd make them 115.25mm at the +20 setup. Meaning they have a backspace of 4.54". The 8.5" = 215.9mm, at 0 offset wheels that'd be 107.95mm which equals 4.25" of backspacking. However since they're also +20, that'd make them 127.95mm at the +20 setup. Meaning they have a backspace of 5". Man my brain hurts now. According to Centerline saying their specs show 4" of backspacing optimal for the stock Z setup, and 5" backspacing if coilovers are used, these technically should fit my car unless I am totally missing something. The front has coilovers right now, while the rears don't yet. Quote Link to comment Share on other sites More sharing options...
2126 Posted March 13, 2006 Share Posted March 13, 2006 Well, you're doing your math correctly anyway! FYI I have 17 x 8's (f) and 17 x 9's ® on my 240Z, with coilovers. Both my front and rear wheels have -35mm offset. On the front I fabricated spacer that are roughly (don't have my notes with me) 7/8" (22.2mm)to 15/16" (23.8mm) wide and for the reat the spacers are roughly 1 1/8" (28.6mm) wide. With this setup I have zero rubbing issues. My tires are 215-40 (f) & 235-40 ®. The rear wheel spacers have positioned the wheel in the wheel well with roughly 1/8" clearance between the strut tube and the inside of the wheel. The outside of the rear wheel sits just about flush with the rear fender lip....close for sure! I sure if I went any bigger than the 235-40's in the rear, I would have a rubbing issue at the fender lip. If you want to be certain about your fit, make up a cardboard profile of the wheel and put it in place with the suspension loaded. This should give you a pretty darn good idea of how they fit. Quote Link to comment Share on other sites More sharing options...
Mycarispurty Posted March 13, 2006 Author Share Posted March 13, 2006 Upon more calculation/thinking, it appears from the centerline mounting points of the wheel, the fronts would have 4.5" of wheel going towards the suspension. And the rears would have 5" of wheel going towards the suspension. On the fronts that would leave 3" of wheel/lip going towards filling up the fenders and on the rear, 3.5" of wheel/lip going towards filling up the fenders. Those wheel/lip measurements of going towards filling up the fenderwell (all measurements of the wheel going towards the suspension aside) would line up with the fender just the same as a 6" wide wheel up front and a 7" wide wheel out back. Assuming I could get the measurement of the wheel going towards the suspension to fit, those wheels would fit pretty much like a stock 14x6 Datsun 6 star wheel or so as far as filling out the fenders and not sticking in or out. Maybe a 1/2" or 3/4" spacer between the wheel and the lugs and longer lugs just in case they stick in too far would remedy the situation. Now my brain hurts, and I doubt new wheels would even be worth the effort of having to crunch this many numbers. P.S. I suppose the easiest way to tell would be by jacking one side up, taking the wheel off, and taking 2 straight edges and laying one across the flat surface where the wheel mounts, and one T'ing off from that and measure the amount of space from there to the suspension. And then having someone putting the jack under the hub area and jacking it up to simulate suspension load, and taking another measurement. Then I'd know (if I were to get these wheels) what sorta spacer I'd have to run if any. Quote Link to comment Share on other sites More sharing options...
fastzcars Posted March 14, 2006 Share Posted March 14, 2006 Try this sight that grumpyvette posted.http://www.rsracing.com/tech-wheel.html#backspace. According to my math the 7.5 in rim has a backspace of 5 inches and the 8.5 rim has 5.5 inches. Quote Link to comment Share on other sites More sharing options...
Mycarispurty Posted March 14, 2006 Author Share Posted March 14, 2006 How'd you get that? If 7.5 x 25.4=190.5. Half of that (if wheels were 0 offset) would be 95.25. Add 20mm more makes it 115.25 (how far the mounting surface is from the inside lip). 115.25/25.4=4.54". I'm not trying to be an ***, just curious how you figured that when I got something completely different. Quote Link to comment Share on other sites More sharing options...
thehelix112 Posted March 14, 2006 Share Posted March 14, 2006 7" rim is physically 8" wide. Every rim is approx 1" wider than the rating, atleast thats how I always understood it? Dave Quote Link to comment Share on other sites More sharing options...
fastzcars Posted March 14, 2006 Share Posted March 14, 2006 thehelix112 7" rim is physically 8" wide. Every rim is approx 1" wider than the rating, atleast thats how I always understood it? Dave Yes ,that is why my math came out different than yours. The advertised rim width is measured from the inside mounting flandge, NOT the outside. Although when you talk about backspace and offset distances you do measure to the outside lip. As far as total physical rim width is conserned, it all depend's on the wheels contruction and material used. For example, forged wheels are usually thinner than cast wheels because the forging process squeezes the aluminum thus making a thinner, lighter, stronger wheel, the pores of the material is literally squeezed out. As compared to cast which the aluminum is poured into a sand mold, and the rim has to be thicker to compensate/add strength to the wheel. Look at the top of this section and read "wheel/tire questions" stickey that grumvette posted and you'll start to understand the terms and how the meassure out wheels. Quote Link to comment Share on other sites More sharing options...
Mycarispurty Posted March 14, 2006 Author Share Posted March 14, 2006 I know how to measure em, that's how I got all those #s...I just wasn't taking into account that the 7.5" measurement was from where the tire contacts on one side to the other. I suppose I could recalculate However if that one guy's chart I was linked to is correct, that'd put the front ones about 4 3/4" or so backspacing, and the rears a hair abot 5.5". I suppose that isn't so bad...who knows. Maybe I'll keep looking elsewhere for wheels. Quote Link to comment Share on other sites More sharing options...
fastzcars Posted March 15, 2006 Share Posted March 15, 2006 Mycarispurty, those wheels will fit great in a Z. you'll need 1/2inch spacers in the front ans none in the rear to get them to fit perfect. Here is a picture of my car with 17x7 front and 17x8's in the rear. The backspacing is 4 in front and 5 in the rear. I did have to roll the rear fenders to get the tires to not rub. the fronts are stock( no fender rolling needed). As you can see they fit pretty flush to the body. I hope that helps. Quote Link to comment Share on other sites More sharing options...
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