Offsets hurt my brain

[19782plus2]

First things first, hello, this is my first time in the forums as I recently picked up a '78 280z. 

 

Second, while I am sure you've all heard this a million times, but I need help with fitment. I am looking at 15x8, 0 offset up front, and 15x8 or 15x9 in the rear, but I'm unsure about the offset in back. I'm looking at either Rota RKRs or Enkei Compes, and I plan on getting Techno Toy Tuning micro big brake kit front and rear. I'm on stock fenders and don't plan on changing that, and I'm looking for flush fitment. Would all of this work? 

[Michael]

Welcome!  

 

Just a brief note on backspacing and offset… Add 1” to the nominal wheel width.  So, a 15”x8” has 9” lip-to-lip width.  Half of that, is 4.5”.  If the inner mounting-face of the bolt circle is 4.5” outboard of the inner lip edge, that is 0 offset.  It’s also 4.5” backspacing.  Confusingly, often we see offset in millimeters but backspacing and width in inches.  And unfortunately, most wheels these days are designed for FWD cars, resulting in large positive offset.  So, an offset of +25mm means a backspacing of 5.5”.  For wide wheels on a Z, we are limited on backspacing, because the wheel rim or the tire sidewall will contact the McPherson strut spring perch.  We can roll or cut the fenders, but other than a coilover setup with smaller diameter spring perch, we’re stuck on the inner-side.

 

On my 1978 280Z, with 14x7 wheels with zero offset (thus, 4” backspacing), on the rear, I measure 0.75” between the wheel inner lip and the spring perch.  A 15x8 with zero offset (4.5” backspacing) would presumably move the wheel inner lip 0.5” further inboard, meaning that is just barely clears at the wheel.  I can not vouch for how it would clear at the tire.

 

Thus on the inner side.  On the outer side, it all depends on spring stiffness and ride height.  My car is “jacked up”, and there is ample room between the top of the tire and the fender lip.  I could probably run “infinite” width tires and not risk contact with the fender lip.  Others, especially on lowered cars, may not have such convenience.

 

As a trial-fit, I just installed 15x7s with 225/40-15 tires.  The result is a bit… gappy. 

Edited March 19, 2022 by Michael

[Dat73z]

I am running a similar setup to what you describe. 

 

Specifically on the rears is the T3 BBK, T3 LCAs, stock tophats, T3 coilovers (non-evolved so using OEM cores) with ground control springs and sleeves, 280z hubs. 15x8 0 offset rota RKR with 225 50 Toyo R888. 

 

Not sure what you mean by "flush" but everything will fit. If you want to go low you will need to roll your fenders. I think if you run 205s for a bit of stretch and dial in more negative camber you can get away with no roll. I wanted a square sidewall, maximum tire, the ability to go wider in the future so I opted to roll. I may go back and trim/weld for more clearance. 

 

Here are some pics, hope it helps

 

[jhm]

First off...if you haven't yet read this thread, I would start here.  109 pages of different wheel & tire combos.  Many of the early photos are NLA, but anything in the last few years should still be viewable and entirely useful.

https://forums.hybridz.org/topic/58528-wheel-show-post-your-pics-of-you-wheels/#comments

 

Also, as @Michael pointed out, your biggest LIMFAC is often the strut spring perches interfering with the inner side of your wheel/tire.  Just because a particular wheel/tire combo fits within the Z's outer sheetmetal doesn't mean that it can be used without other modifications to your brakes or suspension.  This is one reason that so many people convert to adjustable coilovers with smaller diameter springs (2.5" ID is very common, and much smaller than the OEM strut springs.) 

 

I'd be surprised if 15x8 fit inside the stock sheetmetal without interference with the strut spring perch; and there's no way a 15x9 would fit.  As @Dat73z illustrates, you can fit a 15x8 inside the stock sheetmetal...provided that you have swapped the stock springs for an adjustable coilover setup.

 

Hope this helps.

[19782plus2]

Thank you all for your help! I understand better the type of setups I’m looking at now.

[Dat73z]

Here's a picture of the inboard clearance at ride height (~2 figer gap) with coilover sleeves. Theres room to go further inboard. I think I measured at one time you could fit 245s under the stock rear arches rolled maybe trimmed and welded if you have the proper offset. I believe some other members here have done that for maximum tire. I think someone here has actually gone wider but I don't recall their username. 

[19782plus2]

Do you have any more pictures of your setup? I think that’s the way I’m going to go.

[calZ]

You can actually get 275s in the rear without flares as long as you have adjustable suspension

 

 

[Dat73z]

Thanks @calZ that was the thread I was thinking of

[Dat73z]

3 hours ago, 19782plus2 said:

Do you have any more pictures of your setup? I think that’s the way I’m going to go.

What pictures are you looking for? It's a fairly basic setup. Nothing custom or fabricated except the q45 diff mounting which doesn't affect the track width or anything

[19782plus2]

On 3/20/2022 at 5:19 PM, Dat73z said:

What pictures are you looking for? It's a fairly basic setup. Nothing custom or fabricated except the q45 diff mounting which doesn't affect the track width or anything

Just some pictures of the whole car if you have so I can see how it looks. Thanks.

[Dat73z]

@19782plus2 

 

[19782plus2]

19 hours ago, Dat73z said:

@19782plus2 

 

That’s a really nice setup. Think I’m going to go with that. 

[Michael]

The trouble with 225/45 or even 225/50 on 15" wheels is total tire diameter (23" or 23.8", respectively).  I suppose that in the front it doesn't much matter, but in the rear, it results in artificially deep gearing, meaning high highway rpms... and reduced grip for drag racing purposes.  Something close in diameter to factory-stock would be the M&H 245/55-15 drag radial (https://www.mandhtires.com/index.php?route=product/product&product_id=159), but I'm not sure that this is appropriate for a 7"-wide wheel.  Eventually I'd love to get 325/50-15 tires, but that's more of a pipe-dream than a "plan". 

[Dat73z]

@Michael in the link @calZ posted above 17x8.5's were widened via welding to 17x9.5 and fit into the factory arches. There's a lot of good detail in there.