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Discussion Starter · #1 ·
again, letting my ignorance show, but I am ignorant when it comes to some of the terms and dimensions listed in some of the threads concerning backspace and offset in regards to wheels for Opel GTs. I am watching a set of wheels on e-bay that say they have a 35mm offset, 5 3/8" backspacing, and bolt pattern is 4 x 100mm. The only part I am sure will work for the GT is the bolt patter, but I have no clue to the rest. Can some please help me and maybe even educate me along the way?
TIA,
Brett
 

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I was going to send you off to do a search for an excellent article by the late John Seaman (former Editor-For-Life of the OMC Blitz) but the link to his article (http://www.opelclub.com/public/blitz/articles/wheels.pdf), which USED to be on the OMC site, seem to be broken. And the article doesn't show up on the OMC site at the moment. And I hadn't saved it to my personal collection. Hmm...

OK, a quick primer on Opel GT wheel fitment:

First, you need the correct bolt pattern, and 4 X 100 mm is right. Same as a very common car, the older Honda Civic and Prelude (pre-'92 in the Prelude, somewhat later in the Civic I believe). Opel wheels are "stud centric", versus "hub-centric". In other words, the wheels are centred by the studs, not the hole in the middle.

Second, you need to make sure you have the correct back-space so the wheel doesn't rub on the rods and suspension pieces, and this is approximately determined by the wheel width and the offset. In simple terms, a "zero" offset (ET 0) would have the mounting hub face right at the centre of the wheel. A 25 mm positive offset (ET 25) would have the hub OUTBOARD of the centreline by an inch (25.4 mm to an inch) so the wheel centre is INSIDE the hub face by an inch. A 35 mm offset (ET 35) would have the hub face 35 mm, or about 1 3/8 " towards the outside. The bigger the positive offset (and almost ALL wheels are a POSITIVE offset) the more the wheel sits inwards, so it has a narrower track for a given width. And the bigger the ET (a standard term, that you will often see stamped on a wheel), the bigger the backspace.

OK, backspace. A 7 inch wheel is actually about 8 inches wide from the outer edges of the rim. The 7 inch refers to the TIRE mounting width. So a wheel that is advertised as being 7 inches wide is actually 8 inches wide at the rim edge, and an ET 35 will give a backspace of 5 3/8' (8 divided by two is 4, plus 1 3/8" equals 5 3/8"). So the wheel that you are looking at is probably 7 inches wide. And 5 3/8 " is TOO big for a GT. The stock wheel is 13 x 5, with a ET25, so it has a backspace of (add an inch to five gives you six, divided by two is three, plus 1 is) 4 inches. Depending on wheel diameter and tire sidewall bulge, a GT with a 15 inch wheel (a common "plus two" fitment) can tolerate a 4 1/2 inch backspace. So the math is seven plus one is eight, divided by two is four, plus 1/2 inch to get 4 1/2". A 1/2 inch is about 13 mm (ET 13, pretty rare), but experience has shown that an ET20 in a 15X7 wheel will fit, so long as the tire isn't too wide.

HTH
 

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Discussion Starter · #3 ·
Thank you Kieth. As always, a tremendous help, as well as educational. Am glad I checked before buying! Very well written as well... will save for future reference in case my aged memory fails to store it properly!
Brett
 

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Yeah............ I'm gonna have to go and say, yeah, what keith said. I, in no way, could of done better.


Only thing to add is remember... You have a rear wheel drive car and most of these wheels are made for front wheel drive. Pretty much anything in the 25 - 45 mm offset is going to be for a FWD car.

Also, if you dont mind doing it, you can always convert your hubs to 5 x 114 mm. That opens up a the door for a whole lot more to choose from. Thats what Im doing.
 

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That's right Thom. I meant to say that the Honda, being FWD, usually has wheels of about ET38 (or at least my old '88 Prelude with 14X6 wheels were that offset) and as much as ET45. Interestingly, those old Prelude wheels did fit my GT (six plus 1 is seven, half is 3 1/2, plus 38 mm or 1 1/2) with a 5 inch backspace. Or at least I didn't detect any interference. But since the wheel centre was inboard of the stock wheel centre-line, the handling left a bit to be desired. Not too bad, just not good enough to justify the swap, as they just weren't very attractive wheels.

A common wheel (well, not THAT common) that fits is for the earlier BMW 3 series (the E21, 320i from '77 to '83) or the older BMW 2002. They are usually ET20 or 24, but seldom come bigger than 14 inch (unless they are aftermarket wheels). Later 3 series (the E30, the older 323 and 325, with the 4X100 bolt pattern) might also work (I bought a set last month) but check the offset.

And of course you can use spacers, although that comes with it's own issues (have a look around and search this Forum for "spacers").

And as Thom mentioned, you can get your hubs re-drilled to a different bolt pattern, although I don't know what is involved in that.

HTH
 

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Keith,

You obviously know what the heck you're talking about. I have a question for you. The car in my avatar has the following:

This car has:
- 15 x 8 inch wheels up front with a offset of +12 mm
205/50/15 Yokahamas
- 15 x 9 inch wheels in the rear with a offset of +3 mm
225/50/15 Yokahamas
- Full lowering front Sport Spring part # 3001 with 3023 bushings (approx. 1 1/2 to 2 inches lowered)
- 3 mm Lowered rear Sport Progressive Springs, pair part # 3004

I am basically emulating the look of this car with a few minor changes. I am getting custom boyd coddington wheels and the smallest they can make them is in a 16" Im going to get a 16 x 8 all around because the next size up they would do for the rear is 16 x 9.5 and that is too wide. So, based on that, what off-set do I get to have the same "look" of a wider wheel in the rear? Im assumming I can get the same +12 for the front, but since the wheel is 1/2 inch narrower in the rear than my model car do I get say... a +5 or 6mm offset??? Or do I go te other way and get a 15 x 6 with a 0 - -1 mm offset?

Thanks
 

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Thom71GT said:
Keith, You obviously know what the heck you're talking about. I have a question for you. What off-set do I get to have the same "look" of a wider wheel in the rear? I'm assuming I can get the same +12 for the front, but since the wheel is 1/2 inch narrower in the rear than my model car do I get say... a +5 or 6mm offset??? Or do I go the other way and get a 15 x 6 with a 0 - -1 mm offset?
Thanks
Um, no, but I learned to talk like I do after the third year in grade 3. OK, maybe it was the sixth year of Engineering...

Based on the information on the wheels above, both have a 5 inch back space. I have been told to be very careful when the backspace exceeds 4 3/4 inches, as some wheels and tires will fit, while others will not. The only way to know for sure is to trial-fit them, which really isn't an option for custom wheels. I would be inclined to go with a bit less positive offset on the front wheels (say 6, no more than 10 mm) which would give you the required 4 3/4 inch back space. But with the wider 8 inch wheel/tire, you might have to watch the wheel-opening clearance to the outside of the tire. Either the opening's lips will have to be "rolled" (to get the lip away from the tire) or you might have to cut out the wheel wells and install flares.

Then to make the wheels stick out more and look wider (to give a bigger rim edge, if they actually change the outer dimensions of the hub to the rim when they alter the offset, versus just machining the back of the hubs), go to 1/2" less offset (13 mm less, so from minus 7 to minus 3 mm ET, depending on your front offset) to get the rear wheel out further. But then you will DEFINITELY have tire to wheel opening issues, so rolling the lip might not even be enough.

Andrew Gorman had similar sized wheels at the OMC Picnic this year. Andrew, if you are reading this, what offset are your wheels?
 

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Uh, I'M so confused!!! The guys at OGTS told me that they had to roll the fenders on this car. So, if I'm reading this right Im looking at

Front: 16 x 8 and a +3 to +5mm offset and

Rear: 16 x 8 and a -3 to - 7mm offset?

If I get this size, am I safe on the clearance? If I can get away from having to roll my fenders, then all the better.


Dallas, the gentleman that owns the beautiful corvette yellow Opel Spyder has 16 x 8 front and 17 x 8 rear and I think they both have a -11mm off-set. I do not now if he rolled his fenders or not. Dallas, if you read this, did you have to roll them?
 
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yikes!

hey guys, on the front there is a more pressing issue than rolling the fender lips, in fact you'll never get to that point. If the tires stick out that far on the front then when the wheels turn they will hit the foot well inside the fender. Not even a maybe, it is for sure.
I ran 16x7's on the front with 35mm of backspace and they cleared fine, just seemed a bit set in too far. With 1" spacers, for 12mm or so of offset they still fit fine, but with 1.5" spacers, for 0mm offset, they hit the foot wells. That would equate to a 16x8 with an offset in the 10-20mm range for the front. Closer to 10 would fit better in the rear, but even 20 would be close. You might have to pound the weld seam in the back of the rear wheel well flat for 20mm, or run into clearance with the trailing arms, but it would be close.
 

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Dang!! So what do I do? Just get all four with a -8 to -11mm off-set and have deep dishes?
 

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off-the-shelf wheels

Ok, I'm going nuts searching these forums (which by the way have great info and I continue to get sidetracked) for wheel information. Kwilford provided a great explanation for wheel sizing on a GT. Can anyone tell me what wheel manufacturer produces a 15" in the appropriate size for a stock GT without having them custom built? Also, what size tires would should considered?

Thanks, I know this subject has been beaten to death but I just can't find the answer!!!
 

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Well, I just got off the phone with Gil and OGTS and learned some NEW things.

If I want to get a 15" wheel and NOT Have to roll the fenders, I found another place http://www.newstalgiawheel.com that will make a 15 x 6 in the off-set that I need. The best they can do is a 4 1/2" backspace (+12mm off-set) and that works. AND, they are only $225 a piece, vs the $500 a piece I was looking at earlier, plus having to roll the fenders.

You need an offset of +12 to +15mm and use a 205/50-15 tire. You could get away with a 215 or 225 in the back with a +12mm offset.

The problem that you are going to have finding one that is 4 x 100mm bolt pattern is most cars that have 4 bolt patterns these days are FWD and will have an offset of around +40mm. That would put the tire so deep into the wheel well that it would rub badly on the inside. Modern cars that have 4 x 100 mm bolt pattern with rear wheel drive.... well, I dont know of any...

Ok, so modern as in mid 80's... Older BMWs and such. Many wheel companies still that WOULD make a 4 x 100mm bolt pattern for a rear wheel drive car make them HUGE.. 18, 19 or 20 inches. TOO BIG for the Opel.

If you are pickey, like me, and you want a cool looking modern wheel, you are going to have to bend a little. Meaning, perhaps convert your car to a 5 lug bolt pattern. Take the hubs off the front, remove the rear axles and take them to a machine shop, along with one of the wheels that you bought and have them drill the holes. Just make sure they center it correctly.

Also, just as you had to be somewhat flexible, so does the wheel company. They will need to make the right off-set, which brings you right back to a "custom made" wheel.

Of course if you don't mind hard work, dont want a larger wheel and wider tires, you could buy my old wheels and have the clear coat stripped and have them polished.
 

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Ok, sorry, I got kind of long winded and don know if I really answered your question. So... here it goes.

jmbinjax: "Can anyone tell me what wheel manufacturer produces a 15" in the appropriate size for a stock GT without having them custom built?"


No, there are none that I have found, and I've been looking for 2 years.


Let me add that I am pickey, but I havent even found any that make any that I DONT like.
 
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Thats pretty good looking wheel. I think I even may of looked at those at one time. Thing is, with a +20mm offset, you are still looking at rolling the fender so that when you turn the wheel, the tire doesnt rub up against it and slice it open.
 

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Thats great that you found some there that you like. But like I said. Im picky. REEEEALLL Picky and I havent found any that I like that will fit without having to go to a custom wheel.
 

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Thom71GT said:
Thing is, with a +20mm offset, you are still looking at rolling the fender so that when you turn the wheel, the tire doesn't rub up against it and slice it open.
Well, the thing is, the bigger the ET (offset), the more the wheel sits INSIDE the wheel well. So an ET13 on a 15x7 wheel will sit 7 mm (about 1/4") closer to the fender lip than the same wheel in an ET20. The ET20 wheel might cause the tire to rub against the inner fender at full-lock, but not the outer fender lip.
And I have it from several authorities (well, folks who have this size) that a 15x7 with an ET20 and a 205/50-15 tire works just fine in a GT. The speedo is perfect (1.1% high, or about than 1 mph high at 60 mph), and the feel isn't too "heavy", which a bigger wheel definitely causes.
 

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Something to keep in mind when you read my posts about sizes and rubbing, etc... I have to FULL SPORT SPRING and my car has been lowered front AND rear, so my wheel and tire size requirements are going to be totally different than someone with a stock spring.

So, my size postings are those that have the car lowered all around.
 

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Back from the Dead.

From what I can tell these would fit. I also want to note that my GT is lowered in front about 2 or so inches from stock. The back is about 1 and 3/4 inches lowered. If someone can take a look and see if I am on the right track. I believe if my math is right the front will have about 4.5 inch backspace and the rear the same?? I don't know exactly. These aren't the exact wheel I want but I think this will keep the look similar to what I have now.
15 8 XXR 513 Black Rims Staggered 89 E30 AE86 240sx S13: eBay Motors (item 220653726415 end time Sep-12-10 08:04:28 PDT)
 
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