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opelgt73 said:

For the wheels you need a 4x100 pattern with +20 offset
For any given wheel width there will be a range of acceptable offsets. This range of acceptable offsets will be different for different wheel widths. This has been discussed in great length both here and on classicopels.

-Travis
 

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crazy opeler
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It might be acceptable to change the offset on the rear of the car, but for your own safety I wouldent mess with it at all. You will greatly shorten the life of the wheel bearings and it will change the way that the suspension reacts.
 

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What Travis means is tha offset is relative to wheel width. Using a 20 mm offest on a 6" wheel and then on an 8" wheel will yield entirely different results, the backspacing will be different.

Bob
 

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But the wheel offset is calculated from the centerline of the wheel not the rim. So no matter how wide the wheels are you need to stick with the +20 offset. If you were to go with wider tires and you changed the offset to 0 the centerline would be in the wrong place in relation to the hub. at least that is my understanding.
 

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You are thinking backspacing, not offset. Backspacing is the physical measurment from the back edge of the wheel to the hub. Offset is how far off from the center you are.

Basically 1"=25.4mm. Take half the width then find the difference to the backspace and multiply by 25.4.

4" backspace on a 6in wheel would yeild a +25mm offset.
4" on a 8 in would be +0mm
on a 10 in would be -25mm
 

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man am i confused...i'll start searching for those rims though...but i still dont know exactly what to look for.

and also...your saying that if i build up my 1.9L i will be paying just as much as if i bought a 2.4?
 

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Wheels

For REAL information on wheel fitment, read the article at:

http://www.opelclub.com/public/blitz/articles/wheels.pdf

(and, apologies to the author, John Seaman, he has the stock offset slightly wrong, but the concept is spot on)

and go to the "wheel" heading in this forum at:

http://www.opelgt.com/forums/forumdisplay.php?s=&forumid=1130

But to further muddify the fuzzification, read on.

Chris,

You are talking about the wheel "track width", which is the centre-to-centre distance, and this does affect handling. But a wider wheel with the same offset will give you a wider total contact patch. More tire "outboard" of the track width. And it will have a greater "backspace". But generally, less offset is required to allow this wheel to fit. And too little off-set (or even a "negative" off-set) will cause the tire to rub on the outer fender, unless you have cut out the wheel arches and installed fender flares.

The issue here is the tire and rim clearance to the inner "stuff" that an excessive positive off-set, such as found on most FWD rims that otherwise fit Opels, will cause. The stock GT rim is a "5J x 13", with an offset of 25 mm (one inch), which should give you a 3.5 inch backspace. Except the "5 inch" rim doesn't include the rim "bead", and the actual rim width is closer to 5 1/2 inches. Which gives a stock backspace of 3 3/4 inches. My experience is that this can be increased up to 4 1/4 inches on a GT, and 4 3/4 inches on a Manta or Ascona, depending on the rim diameter and tire section, before the tire hits things, such as the panhard rod mount in the rear, and the tie-rods and inner fender at the front. And possibly the front spring, if you have installed a "Bob Legere Shackle" to lower the front and reduce spring binding.

And remember (AGAIN!) that a 7 inch wheel is actually a bit more than 7 inches wide (due to the rim bead). So the 8 1/2 inch "8J" wheel with a "zero" off-set" will give an actual backspace of 4 1/4 inches. And different manufacturers will measure off-set slightly differently, so ALWAYS measure the actual backspace, and don't just rely on the specified off-set to determine if a wheel will fit.

And this is one of my favorite pictures of a great wheel and fender package.
 

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Well, maybe this is nicer. I believe it belongs to Gil Wesson, of OGTS
 

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These are stock Manta wheels widened 2", all outside the center. That is, they are now 13x7.5, with stock back spacing but now with a 0 offset.
 

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Opeler
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Negative offset

After searching and reading a few of the available posts, it seems like the only problem with negative offset is that there may be clearance problems between the fender and wheel. Is this the case? Why wouldn't negative offset cause even more stress on the wheel bearings than positive offset?

I'm trying to figure out wheels and tires now, before sending my 69 GT off for Steinmetz flare installation (the same flares shown in Keith's post on Gil's GT). What I'm considering right now, is a set of Boyd Coddington Destructors https://www.newstalgiawheel.com/productdetails.asp?id=1990.

I'll obviously have to have a set of spacers, due to the bolt pattern change, but I don't think that it would be much of a problem to get those. Anyways, I don't have a copy of the OGTS catalog here at home, but if I remember correctly, the wheels on Gil's car are 15x11, and the tires are Yokohama AVS, which are no longer made in that size. With the aforementioned choice of wheels, I can have the backspacing set to whatever I choose.

So to sum it all up:
I want to make sure that the wheelwells are filled (11" seem to do it on the rear of Gil's car)
I need to do it with currently available wheels and tires.
I don't mind changing the bolt pattern, but I know that I must order the wheels with an adjustment to the backspacing to compensate for the width of a pattern changing spacer.
I really dislike the available wheel styles for the 4x100 bolt pattern.

Anyone have a clear idea of what backspacing to order, considering all that?

Secondly, as I mentioned earlier, the AVS tires aren't available any longer in the size that Gil has on his car. On my 72 GT, I have 17" rims that were installed by one of the previous owners. I like the look of a lower profile tire, and I'm comfortable with the slight harshness of ride quality, due to the shorter tire sidewall. That said, if I purchase a 17x11 wheel, would a 315/35-17 work out clearance wise? According to the tire size calculator that I use (http://www.miata.net/garage/tirecalc.html), it puts the radius of the tire at 1.1" taller than the stock GT size of 165/80-13, as mentioned by the John Seaman PDF file. Is that too much?

Hope that wasn't too long, I'm just boggled myself. Once I get this figured out, then it's on to figuring the front wheel and tire combination. :rolleyes:

-Dave
 

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AHHHH My good man,,
I have been well veresed in the topic of wheels, here is what you need for offset on a gt : 9" wheel ask for a +3 offset, 8" = +12 and 7"=+16.
those offset will fit under the stock sheet metal. the 9" you will need to roll you fender lips. on the 9" you also can only run a 40 or less series tire on the front. the rear you have a bit more room unless you are slaming the car. On the rear you can run up to a 50 seriers tire.
So for instance if you were asking for wheel you would ask for a 17x8+12 with a 4x100 bolt pattern. Now since we are in the age or ricers most ofsets are +40 and up that can be fixed by using spacers.
Hope this all helps
 

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Opeler
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Well, as I stated in my long winded post, I'm having more severe body modifications done than rolling the fenders. Now that I'm at work, I can accurately state that the wheel / tire combination on Gil's car is:

Rear: 15x11 wheels with 285/40-15 tires (no longer available)
Front: 15x9 wheels with 225/50-15 tires

Unless of course, that he's changed the configuration in the past 4 months. That said, I need to find the same sort of dimensions, width wise, in a wheel-tire combination that's available today. I know that the rear will have a deep dish look, which will end up being a negative offset number. As I mentioned in my previous post, it seems that a negative offset would have more stress on the bearings than a positive offset, due to effectively increasing the axle's length beyond the support structure of the bearing itself. Kind of the difference between holding a pumpkin up by it's stem, or taping a yardstick to the stem and holding it up by that.. Silly, but hopefully that's more clear?

-Dave
 

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bear said:
AHHHH My good man,,
I have been well veresed in the topic of wheels, here is what you need for offset on a gt : 9" wheel ask for a +3 offset, 8" = +12 and 7"=+16.
those offset will fit under the stock sheet metal. the 9" you will need to roll you fender lips. on the 9" you also can only run a 40 or less series tire on the front. the rear you have a bit more room unless you are slaming the car. On the rear you can run up to a 50 seriers tire.
So for instance if you were asking for wheel you would ask for a 17x8+12 with a 4x100 bolt pattern. Now since we are in the age or ricers most ofsets are +40 and up that can be fixed by using spacers.
Hope this all helps
Yo Bear,

9 inch wide wheels in the front???!!! What is your turning radius? I'm running 7.5 inches in the front and I have a terrible turning radius because my tires hit the footwell. And I've already taken a sledge hammer to the footwell.

Manny
 

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Opeler
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Ok, so I'm sitting here about one click away from ordering wheels and tires from http://www.newstalgiawheel.com. Well, not literally, but close enough. :D

I'm looking at:
17x11 rears with 275/40-17 BF Goodrich's
17x8 fronts with 215/45-17 BF Goodrich's.

The wheels are completely custom, so I can have 4x100 bolt pattern drilled, and from what I've read, I can go with 4" backspacing on both. On the rears, obviously I'll end up with a negative offset, but the fronts should be around zero offset, right? 8" wheel, 4" backspacing? Seems like that should be zero offset.

Am I making a mistake somewhere?

-Dave
 

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Taavetti said:
8" wheel, 4" backspacing? Seems like that should be zero offset.

Am I making a mistake somewhere?

-Dave

In truth, an 8" wheel is 8" from bead to bead. Externally, the rim will be 8.75" to 9" typically. Assuming it's 9" wide overall, then the backspacing would have to be 4.5" to make it zero offset. Contact the manufacturer to find out the exact external wheel width.

It's screwy, I admit, they measure wheel width from the bead, but they measure backspace from the overall width!
 

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The particular wheels I've selected (Boyd Coddington Classics), since they're "made to order", can be set to whatever I specifiy at order time. I just had an email conversation with the dealer, and on the final note, he wants me to measure the actual space there. Bob, do you think that if I told him 4.5" of backspacing for both the rear and the front, that I'd have any problems? Would the fronts end up rubbing the foot well like Manny mentioned? I know that if I got less backspacing, I'd end up with the problem you described in a different post, regarding the inner wheel hitting the front suspension. Who'd have thought that figuring out wheels and tires would be more confusing than making video games. ;)
 

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Taavetti said:
Would the fronts end up rubbing the foot well like Manny mentioned?
Unfortunately it's not that simple. Many factors influence fender/suspension/tire clearance. Ride height, wheel offset naturally, alignment, tire width, tire height, .....

Often the only way to know is by test fitting. I know that is not practical, especially with custom wheels. I know from experience that 13" front wheels with 4" backspace is about the limit, you can't go much more or the inside of the rim actually will hit the upper a-arm. But as wheel diameter grows, you gain some clearance. I've gone as much as 5.5" backspace on a 16" wheel up front.

Out back, the limiting factor is the panhard bar mount in the right wheelwell. As I recall the limit there is 5.5" backspace, regardless of wheel diameter, but a tire that 'bulges' beyond the wheel may hit well before that, especially on a diagonal bump in the road (axle tilts on angle, tire will hit sooner than with a pure vertical bump). Then of course, the outward limits are the wheelwells, again compounded by the ride height, alignment specs, tire width and diameter, etc.
 

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Sure, but it seems easy enough thinking this way, using your examples:

Front:
You describe that you've done a 5.5" backspace on a 16" wheel, and having enough clearance. If I'm using a 17x8 front, and simply stick with 4" backspacing, that should (but probably not ;) ) clear suspension components, and leave me with a 0 offset. With the Steinmetz flares, there will probably be enough radius cut away for lock to lock turning without rubbing the body. The wheel/tire combo that I picked is not that far off from the original stock wheel/tire diameter.

Rear:
17x11 at 4" of backspacing, considering the tire I mentioned, should clear considering your max limit of 5.5", leaving that extra 1.5" of space for bulge of tire. I don't expect that there will be all that much bulge, considering the width of the wheel compared to the tire selection. Your concern of the outward direction shouldn't be a problem either, again due to the bigger radius of the flares.

I really think the combination might work. The key factors being that if I stick with a 4" backspacing, an overall diameter that doesn't massively exceed the diameter of the original tires, and count on the larger space under the flares to absorb the suspension travel space requirements..

Considering all that, does it seem feasible yet? It's sure been hard to find the right sizes. It's almost to the point where I'm ready to give up on the cars, just because I can't find the right place to start, overall.
 

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Taavetti said:
If I'm using a 17x8 front, and simply stick with 4" backspacing, that should (but probably not ;) ) clear suspension components, and leave me with a 0 offset. With the Steinmetz flares, there will probably be enough radius cut away for lock to lock turning without rubbing the body. The wheel/tire combo that I picked is not that far off from the original stock wheel/tire diameter.
That front wheel you describe will give a 13mm offset, as the 8" rim is roughly 9" overall. So measuring from the back flange, 4.5" in would be '0' offset, not 4".

Where's your ride height at? You haven't commented there.....
That will affect the tires rubbing.

The tires may still rub anyway (not suspension parts however), as at full lock the wider rims/wider track causes the tire to rub at the footwell area, just inward from the bottom of the front fender. Probably not the location you were concerned with, but it's the most common place for them to rub.

The rears should be okay though, but I'd probably go with a 5" backspace to be sure the tires don't hit the cut-out fenders at full bump. The flares may cover the tires themselves, but you need to cut away a lot of sheetmetal to allow for vertical movement.
 
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