CCW offset help for solid axle car

[ElectricGrn99]

I am trying to order a new set of CCW’s for my mystichrome that has a solid rear axle in it. I can’t find what the offset should be for the rears which will be 18x11. From my Google searches I know that the 99-04 solid axle is slightly longer than the IRS but can’t find any measurements. Any help from someone who maybe has some CCW’s and a solid axle would be great.

 

[01yellercobra]

Why not use the measurements for a GT?

 

[apex svt]

I’ll try to keep this brief: I had a 99-04 SRA when I bought my wheels. Built a new SRA and found out it was a (94-98 woops). Now run a 3/4 spacer since that’s the difference between the two.

Anyways, I run a 17x11 at a 7.5in backspacing. Which would be correct for a 99-04 SRA, you could probably get away with running a 7.25in if you wanted. Would push the wheel out just a touch.

I can send you any pics you want so you can see the profile.

 

[DCguy]

The offset is more for determining the outside fitment ie the wheel lip with the fender. For clearance to your interior component ie shocks/springs/axle etc you need to determine your backspacing. Tire choice and how much you're lowered will also play a factor here. Lower the car the closer the inner fender will be to the tire, especially if you run a 40 series sidewall or ET street type setup.

99-04 axles is almost identical to an IRS so from an offset perspective you can run something very similar (+15 to +20). However, backspacing is very different since the IRS has more "stuff" you need to clear. That's why IRS guys usually run a 6.5in backspacing max whereas solid axle folks (99-04) you can run up to 7.5in comfortably and up to 8.5in with some minor clearancing. What this means is that we can run wider wheels.

I recently redid my rear axle and went through the measurement process to get the widest wheel/tire I can under there as I do intend to run the car pretty low.

Before:

After:

My rears are an 18x11.5in with a +22 and 7.1in of backspacing. I could easily have gone with 7.5in backspacing and had the wheels tucked in a little more but I wanted the option of running quad shocks. Here is how that clearance looks like. As you can see its pretty tight but with a 335/345 tire i'll have to check it again and adjust the quad or remove it. I have washers spacing out the quad body a bit since i've got rear coilovers and that makes things a bit more difficult. My rear spring rate of 275lb plus the added stiffness from the PHB should allow me to run the car quite low without worry of the tire contacting the fender.

Money shot from the rear. Even with a 7.1in backspacing, the lips of the wheels are completely flush with the fenders. With tires on the tire will likely stick out about a 0.5in outside the fender but i'm fine with that. If you want the tires flush, run more backspacing and remove the quad shocks.

 

[Bullitt1448]

The offset is more for determining the outside fitment ie the wheel lip with the fender. For clearance to your interior component ie shocks/springs/axle etc you need to determine your backspacing. Tire choice and how much you're lowered will also play a factor here. Lower the car the closer the inner fender will be to the tire, especially if you run a 40 series sidewall or ET street type setup.

99-04 axles is almost identical to an IRS so from an offset perspective you can run something very similar (+15 to +20). However, backspacing is very different since the IRS has more "stuff" you need to clear. That's why IRS guys usually run a 6.5in backspacing max whereas solid axle folks (99-04) you can run up to 7.5in comfortably and up to 8.5in with some minor clearancing. What this means is that we can run wider wheels.

I recently redid my rear axle and went through the measurement process to get the widest wheel/tire I can under there as I do intend to run the car pretty low.

Before:
View attachment 1805990

After:
View attachment 1805991

My rears are an 18x11.5in with a +22 and 7.1in of backspacing. I could easily have gone with 7.5in backspacing and had the wheels tucked in a little more but I wanted the option of running quad shocks. Here is how that clearance looks like. As you can see its pretty tight but with a 335/345 tire i'll have to check it again and adjust the quad or remove it. I have washers spacing out the quad body a bit since i've got rear coilovers and that makes things a bit more difficult. My rear spring rate of 275lb plus the added stiffness from the PHB should allow me to run the car quite low without worry of the tire contacting the fender.

View attachment 1805992

Money shot from the rear. Even with a 7.1in backspacing, the lips of the wheels are completely flush with the fenders. With tires on the tire will likely stick out about a 0.5in outside the fender but i'm fine with that. If you want the tires flush, run more backspacing and remove the quad shocks.

View attachment 1805993

What tire/size are you going to run in the rear?

 

[Steve@TF]

The offset is more for determining the outside fitment ie the wheel lip with the fender. For clearance to your interior component ie shocks/springs/axle etc you need to determine your backspacing. Tire choice and how much you're lowered will also play a factor here. Lower the car the closer the inner fender will be to the tire, especially if you run a 40 series sidewall or ET street type setup.

99-04 axles is almost identical to an IRS so from an offset perspective you can run something very similar (+15 to +20). However, backspacing is very different since the IRS has more "stuff" you need to clear. That's why IRS guys usually run a 6.5in backspacing max whereas solid axle folks (99-04) you can run up to 7.5in comfortably and up to 8.5in with some minor clearancing. What this means is that we can run wider wheels.

I recently redid my rear axle and went through the measurement process to get the widest wheel/tire I can under there as I do intend to run the car pretty low.

Before:
View attachment 1805990

After:
View attachment 1805991

My rears are an 18x11.5in with a +22 and 7.1in of backspacing. I could easily have gone with 7.5in backspacing and had the wheels tucked in a little more but I wanted the option of running quad shocks. Here is how that clearance looks like. As you can see its pretty tight but with a 335/345 tire i'll have to check it again and adjust the quad or remove it. I have washers spacing out the quad body a bit since i've got rear coilovers and that makes things a bit more difficult. My rear spring rate of 275lb plus the added stiffness from the PHB should allow me to run the car quite low without worry of the tire contacting the fender.

View attachment 1805992

Money shot from the rear. Even with a 7.1in backspacing, the lips of the wheels are completely flush with the fenders. With tires on the tire will likely stick out about a 0.5in outside the fender but i'm fine with that. If you want the tires flush, run more backspacing and remove the quad shocks.

View attachment 1805993

are you sure you dont have a 94-98 SRA? according to my notes, the 94-98 SRA is slightly narrower than the IRS (nearly the same width). i would use the same specs for both (on a new edge). the 99-04 SRA sits out .5" more on each side. that's why IRS cars get deeper lips than new edge GTs. ive dont this hundres of times. believe me, i would have heard about it if it were wrong lol. i could also just be misunderstanding what im reading, im having a drink lol.

OP, if you are POSITIVE you have the SRA from a 99-04 GT than i would recommend you go with 18x11 with 7.25" backspacing if you want fender flush. i have put 11.5 on the back of a GT with 7.5" bs but they poke out some. but on the right tire it can work.
you could also just tell them that you have an 04 GT and see what they say. that's essentially what you have now in the rear. they used the same specs we did for IRS cars so im sure theyre SRA is the same as well or super close.

when i got started with these cars i was just handed wheels for a new edge GT. i had to figure out how to go from there for the IRS cars (which i owned at the time).

 

[DCguy]

are you sure you dont have a 94-98 SRA? according to my notes, the 94-98 SRA is slightly narrower than the IRS (nearly the same width). i would use the same specs for both (on a new edge). the 99-04 SRA sits out .5" more on each side. that's why IRS cars get deeper lips than new edge GTs. ive dont this hundres of times. believe me, i would have heard about it if it were wrong lol. i could also just be misunderstanding what im reading, im having a drink lol.

OP, if you are POSITIVE you have the SRA from a 99-04 GT than i would recommend you go with 18x11 with 7.25" backspacing if you want fender flush. i have put 11.5 on the back of a GT with 7.5" bs but they poke out some. but on the right tire it can work.
you could also just tell them that you have an 04 GT and see what they say. that's essentially what you have now in the rear. they used the same specs we did for IRS cars so im sure theyre SRA is the same as well or super close.

when i got started with these cars i was just handed wheels for a new edge GT. i had to figure out how to go from there for the IRS cars (which i owned at the time).

I measured the rear end housing when I rebuilt it to make sure....its a 99-04. It has the 14mm lower control arm bolts as well.

I don't have the measurements on the IRS, but perhaps someone can add that here for context. I've always heard the 99-04 was closer to the IRS width than the 94-98 housing but your statement says otherwise.

These are per Ford manual:

Overall housing width:
94-98................56.094in to 56.198in
99-04................57.512in to 57.616in

Brake Flange Mounting Surface:
94-98...............54.10in
99-04...............55.51in

So there's about a 1.4in difference between the mounting flanges which equates to 0.7in wider per side.

In terms of wheel choice for the rears this means you'd have to run less backspacing and a lower positive offset (like a +10) for a 94-98 rear end vs. more backspacing and a higher positive offset (like +20) for a 99-04 rear to account for the increased track width.

Your business is wheels so you probably have more insight but my point is that folks should get the car on stands and determine their backspacing if they want to avoid the guess work. Everyone's setup will be different depending on suspension setup, tire choice, ride height, etc etc so a few min with a tape measurer is time well spent IMO.

I do also have C-clip eliminators without ABS rings and did have to shave my axles down a tad in order to get them properly seated so my measurements/setup is just a data point....not for everyone.

 

[Blkkbgt]

The offset is more for determining the outside fitment ie the wheel lip with the fender. For clearance to your interior component ie shocks/springs/axle etc you need to determine your backspacing. Tire choice and how much you're lowered will also play a factor here. Lower the car the closer the inner fender will be to the tire, especially if you run a 40 series sidewall or ET street type setup.

99-04 axles is almost identical to an IRS so from an offset perspective you can run something very similar (+15 to +20). However, backspacing is very different since the IRS has more "stuff" you need to clear. That's why IRS guys usually run a 6.5in backspacing max whereas solid axle folks (99-04) you can run up to 7.5in comfortably and up to 8.5in with some minor clearancing. What this means is that we can run wider wheels.

I recently redid my rear axle and went through the measurement process to get the widest wheel/tire I can under there as I do intend to run the car pretty low.

Before:
View attachment 1805990

After:
View attachment 1805991

My rears are an 18x11.5in with a +22 and 7.1in of backspacing. I could easily have gone with 7.5in backspacing and had the wheels tucked in a little more but I wanted the option of running quad shocks. Here is how that clearance looks like. As you can see its pretty tight but with a 335/345 tire i'll have to check it again and adjust the quad or remove it. I have washers spacing out the quad body a bit since i've got rear coilovers and that makes things a bit more difficult. My rear spring rate of 275lb plus the added stiffness from the PHB should allow me to run the car quite low without worry of the tire contacting the fender.

View attachment 1805992

Money shot from the rear. Even with a 7.1in backspacing, the lips of the wheels are completely flush with the fenders. With tires on the tire will likely stick out about a 0.5in outside the fender but i'm fine with that. If you want the tires flush, run more backspacing and remove the quad shocks.

View attachment 1805993

Why did you want the option of running quad shocks?

 

[DCguy]

Why did you want the option of running quad shocks?

Im building the car to be able to take a corner and do the occasional HPDE event......not so much drag oriented other than digs on street maybe. I want the extra dampening for that.

 

[01yellercobra]

I thought the quad shocks were just to help with wheel hop? Which is supposed to be eliminated with good control arms. I never had a problem with it in my GT with it's screaming 200rwhp.

 

[DCguy]

I thought the quad shocks were just to help with wheel hop? Which is supposed to be eliminated with good control arms. I never had a problem with it in my GT with it's screaming 200rwhp.

Well that's one reason Ford put them on there sure....its still an additional damper on the axle which on an SRA with coilovers is going to be a pretty rough ride. Anything I can do to mitigate that is welcomed in my book. I don't have all the answers lol.....i'll likely test it out with and without them on there.....I just want the OPTION of running them hence why i set my backspacing to allow for it.

 

[01yellercobra]

I was just curious. I do remember thinking my car rode better without them. But I removed them the same time I swapped out the control arms so who knows.