Offset to Backspacing Conversion (245/40/16 and a 16x8 wheel)

[mr_han_solo]

I recently purchased a set of Hoosier 245/40/16 and I'm looking at purchasing Diamond Racing wheels in the 16x8 size.

 

I have combed through tons of threads and here is where I'm at. I've settled on +4 on the offset. I've converted to coilovers, and I've trimmed my front valence already, so I'm in the clear on both of those respects. I have done the Toyota caliper conversion on the front as well.  My problem comes in with the wheels themselves. Diamond Racing wheels sets theor ordering by backspacing, not offset. I was wondering how I can convert this +4 offset to backspacing. Or if someone running the 245/40/16 on 8" wheels (I know there are a ton of you) would just tell me what their backspacing is, I'll use that.

 

I'm currently running 235/60/14 on the "Iron Cross" wheels which have 102mm or 4" backspacing.

 

Thanks.

Edited January 7, 2016 by mr_han_solo

[seattlejester]

In theory

 

8 inch +0 offset = 4 inch of backspacing

 

8 inch +6 offset ~ 4.25 backspacing

 

In practice, since backspacing is measure from the hub and an 8 inch rim is actually closer to 9 inches wide lip to lip for a lot of manufacturers

 

8 inch +0 offset = 4.5 backspacing

 

8 inch +4 offset = 4.66 backspacing

 

8 inch + 6.35 offset = 4.75 backspacing

 

You can always call them and ask for the physical dimensions.

 

The iron cross wheels are  14x6 + 10 offset

 

In theory it should be roughly 3.5 inches of backspacing, but as you point out in reality it is closer to 4.

 

Just an FYI, diamond racing has had a few reports of wheels being out of round on occasion. Bart and bassett (sp?) also make similar wheels in similar price ranges.

[mr_han_solo]

Thanks!

[seattlejester]

Sure thing, there are some pretty good calculators online, but as mentioned, some manufacturers vary in what they consider width, I would call them and ask for a tech person and just ask what it is lip to lip and such to make sure it clears. With coilovers with smaller diameter springs and perches you should have some leeway, although that is a pretty big tire size to tuck under the car.

[RebekahsZ]

I will be over at my storage shed tomorrow and will try to measure my 16x8 Rotas. I hope you have really looked at those inexpensive stamped wheels-the hub mating surfaces look sketchy to me.

[RebekahsZ]

5" back spacing with same tire. VERY tight fit.

[seattlejester]

^Oops, RebekahsZ raises a good point I forgot, a 245 is on the wider side for an 8 inch rim and depending on manufacturer you are going to have a bit of bulge on top of rim width. 

[RebekahsZ]

I put my 16x8" Rota wheels with 245/45/16 Hoosier R6 tires on my new 75 280z in place of one of the flat tires that were on it. I just grabbed them out of storage to get the car on the trailer when I bought it out of a barn. The car had stock suspension, whereas I run coilovers in my race car. Here's the result. The wheel rubber metal on metal on the lower spring seat on the inside. And hung out the side of the fender on the outside. Since the car was at stock ride height, rubbing would only be a problem on the outside unless the suspension compressed about 3".

Edited February 27, 2016 by RebekahsZ