I am finally getting around to some cosmetic upgrades on my GTA. I have decided to get rid of the C5 Y2K 18x9.5 wheels. I have decided to go with the Forgeline MD3P wheels. I need to run a minimum 18x10.5 or 18x11 wheel on the back so I can cure my traction issues. I will run a MT ET Street Radial II in a 305/35/18 to replace the current MT Sportsman SR (18x12x26). I am running a Moser 12 bolt that is set up for a 4th gen fbody.

I know the rear end for a 4th gen is not as long as the 3rd gen stuff. This is where I need some expert advice. Can anyone calculate the proper backspace so that I can fit this size of wheel and tire on the back of my car. The car is currently lowered 1.5 inches and has adjustable LCA's and panhard.

I am not sure what to run up front but would likely go with something like an 18x9 with a 245/40/18 unless anyone has any reccomendations on what may be better and why.

And lastly I have lots of options for the center portion of the MD3P. I am going with a polished lip but they can paint the center to my instructions. I am not sure if a polished center is too much or if I should go with a polished center with paint on the sides of the mesh web???

Here are the wheels I want to purchase...

http://www.forgeline.com/products/pr...ries/md3p.html

And here is a pic of my car as it currently sits with the C5 wheel setup. This combo will be for sale CHEAP and will include spacers.....

http://img.villagephotos.com/p/2003-...1/HPIM3761.JPG

 

The 4th gen rear in wider (longer). If you try 11's on a 3rd gen rear, they'll either be sticking out, or you'll hafta cut some sheetmetal to get the to fit & stay under the fenders.

With a 4th gen rear, it'll make it a lot easier.

 

Do you know exactly how much longer??? I have a couple of 4th gen 10 bolts but no longer have any 3rd gen diffs to reference to.

You also mention it wil be easier with the 4th gen differential because of length. I assumed that because my 3 piece forged wheels were bieng built to my spec that I wouldnt have to be concerned with if one rear end is longer but rather just have the correct offset to place the wheel in the opening properly based on measurements. What did I overlook now hahaha????

 

The 93-97 4th gen rears are 3.8" wider from hub to hub so the stock thirdgen wheels will set out some, but still be under the fenders. The 98+ rears are abit shorter, at about 3.5 inches wide, but measurements are showing more like a 3.125 inch difference. LS1 rear is about 64 3/4inch long and 9 bolt was 61 5/8.

Wheels built to YOUR spec, will make it tons easier to fit them.

My 17" x 9" with 5" bs fit the rears with PLENTY of room to spare. Push it out further on a 4th gen rear & that only makes it that much easier, just can't have a deep outer dish on your wheels.

7" bs on a wheel on a 4th gen rear, under a 3rd gen, will be a direct bolt on.

 

Oh yeah, you asked about front tires to.....

I'm running 275s up front on my 9" wheels & wouldn't think of running 245s again. Why run a wider than stock custom & only use stock width tires?

 

So are you saying that a 7" backspace will allow me to run my 18x11 on a 305/35/18 tire??? Again this is why I am requesting some help. I expect to spend 4K on the wheels alone and want to make there are no suprises.

 

As shown in my sig above.....I have 315/35R17s on my 17x11s, so yeah....

In fact, 305s would be a bit skinny on 11s, unless that brand runs wide.

 

http://www.mickeythompsontires.com/s...StreetRadialII

10.5 to 11 should be fine. A lot of people run these tires on 11.5 and they still look good. Cosmetics are important but I need my tires to to grab and as of right now they are useless up to 70mph which is why I am going to try these out. I mainly use the car on the street so I want something that handles somewhat okay but gives me the option to still go WOT in the lower gears.

 

I find that my car tracks enough as it is with 245's on the front. How are the 275's versus the 245's??? We have some crappy raods up here and they are often quite pounded down in the main driving lanes which causes my car to search a lot from left to right.

 

Stay with 245s then....The county I live in, is the same way. Front wheels dart left & right bad. But I'm only like 5 miles in the county & everything I do is in the next 2 counties, where the roads are good & the front wheels behaves perfectly fine.

 

If you want width for hook, go ahead and step up to 315s/335s or.....345s!

Sometimes I wish I had went ahead & got 335s instead of 315s.

 

I need more width to run that exact tire. Its been a toss up between some Toyo drag radials and also the Nitto's but I really want to try the MT's out. My MT Sportsman SR's were a decent tire and allowed my 10 bolt to live behind 500+ HP. Now that I have a 12 bolt I want to hook a lot better without modifying much more in my wheel well as the BFH has already been in there as well as bump stops, ect being removed.

 

That is what I did to fit my 7" bs/11" wheels, but had to do quite a bit more cutting in the forward area. With a 4th gen rear end, it woulda been a lot less, if not none extra.

 

So back to the backspacing. You mentioned a 7 inch backspace would be adequate to run a 18x11 on a 305/30/18 . Would an 18x10.5 really give any needed extra room or am I best to stick it out with the 18x11?? Also what does that translate to in offset on the 11" rim with 7" backspace??? Is it a positive 38mm offset???

 

Correction.....

Offset does not SOLEY depend on the bs. Offset is calculated by the combination of the wheel width & the offset.

A 10.5" wheel does not have the same offset as a 11" wheel, even if the bs is the same. Offset gives me a headache every time.

Why they go & screw up a perfectly good measuring system like backspacing?

 

my long math for the nite....

11" rim is 5.5" to center

7" of backspacing minus 5.5 center is 1.5 inches

1.5 inches equals roughly +38mm

How badly did I miss my calculation??? This is why I am asking for some clarification before I attempt to order these damned things hahaha!

 

you failed!

wheel width is measured by the inside of the bead seat. So a 18x11 wheel actually measures about 12" in total width. Another monkey wrench in the equation, who made it like this, i dont know...

anyway

If i was putting a 18x11 on the car, i would want it to have 7.5" of backspacing, but it requires a lot more metal work. 7" will work, but the tires will be about flush with the outside of the car. Not ideal in my book, but most people here with 4th gen 17x11 wheels and spacers have the wheels in this approximate location.

What i always tell people is just take a wheel off the car, put a straight edge across the rotor/drum face, and measure back. See what kind of room there is and what sticks out and gets in the way, ie, bump stop area, the front of the wheel well, panhard bar at much wider widths, brake lines, control arms ect.

I managed to get a 18x12 with 8.5" of back spacing on the car, but it was a shat load of work. Totally worth it, but i dont ever look forward to doing it again.

Probably the best thing you can do is call the company you plan on buying from, and talk to them. They will be able to answer all these questions on measurement and you will have the right answer, no guessing.

 

Hahaha thanks for the honest answer. I could care less if I am right or wrong, I just want to know what the correct answer is

I assume you are basing your backspace advice off the fact I have a 12 bolt designed for a 4th gen??? I have talked to a rep at Forgeline already but he says that he can not give solid advice on this as the 4th gen rear in a 3rd gen firebird doesnt compute to him.

I already have the bumpstop off, front inner wheelwell massaged, brake lines tucked away and have Spohn LCA's from my current MT tires (18x12x26) so I have some room for the fudge factor on 11 inch rims I hope. Now the kicker....my car is in storage for the winter and my 12 bolt is not in the car at this time. I cant install it until I get the car out of storage. I was hoping to be able to have someone with a simular setup (4th gen rear and 11 inch rims) post up their offset so I could have the rims ready to install when the car came out of hybernation. It may be asking too much but I thought I would ask anyhow.

 

As as FYI, the 4th gen direct fit 17"x11" wheels are 50mm offset 7.93" BS
Never used the 4th gen rear in a 3rd gen though to see how everything fits.

 

Another thing to keep in mind is that the tire will extend past the edge of the rim by varying amounts, depending on just about everything you can think of... width, aspect ratio, and even the manufacturer.

Backspace is kind of a dangerous dimension in some respects, because the actual thickness or dimension of the rim from the inner bead (where the rim width is measured) to the outer edge (where 99% of the manufacturers measure backspace from) varies depending on the wheel material and the manufacturer. So even though you may have 7" BS between two wheels, the bead may not be in the same location on both either. In the end, you may have more clearance than you expected, or less than you wanted. Adjust with BFH accordingly.

Offset is simple enough. If the rims' width is centered in relation to the wheel hub mounting surface, offset is zero. If the rim is towards the inside (like most FWD rims are) then its positive offset. An 11" rim with 0"/0mm offset will have about 6" backspacing. So for 7" BS its going to be about 1" offset.

 

Wrong....Backspacing is the SIMPLEST way of measuring. It is NOT measured to the tire bead surface. It is measured from the outer rim edge to hub mounting surface. A 2nd grader could measure it. All it takes is a straight edge and a ruler.

Offset requires measuring the rim width. Not the TOTAL width, but the tire bead surfaces.....Measuring from the hub mounting surface to the centerline....Measuring from that to the hub mounting surface....Now for the "simplest" part...Is the result a Negative or Positive offset? (yes, negative vs positive are clear, but throw ANOTHER wrench into figuring it out) AND...Chances are you measured with a English ruler, which means you'll hafta convert your final number to metric!

Now....WHICH is easier??

 

Do you post just to argue with people? Sure seems like it.

Thats what I said. Now go away.

 

No.....Now, you'd made it my goal today to stalk your posts!