Offset question for rims

[pipeliner]

I just went through the entire wheel size thread. Im looking into buying a set of US Mags Bandit 109. I think my back spacing is good but I'm not for sure about the offset in the back. Here's what I'm what I'm wanting to run. Nitto 5552 in the front and back. Tire size up front is 255x45. Rims up front are 18x8 4.50 backspacing and 1 offset. Tires for the back are 295x45 Rim size is 18x9 with 5.25 backspacing and 8 offset. Any help would be appreciated.

[pipeliner]

Nobody has a clue???

[Nacho-RT74]

I'm not sure about the question, basically because some concept mistakes ( unless I am who is wrong )

backspacing is an absolute measurement from the rim center seat up to the inner wheel. Doesn't really matter the total width, the BS is an absolute reading

Offset is a RELATIVE measurement, being +/- from the center of the wheel width

so is not the same BS a -1 offset on a 7" or on a 9" wheel. First one gets 2.5" BS and second one gets -3.5" BS

let me know somebody if I'm wrong.

[Nacho-RT74]

[randy73]

I looked on there website and the offset is a little less than I would want, but they will work. They seem to be a 1/2" - 3/4" closer inboard than stock. So your tire will sit more under the car and I would worry about rubbing with that big of tire on the rear.

Nacho,
Offset matters because if you have to much positive offset, the tire will protrude and not fit in the wheel well correctly and the opposite is true for negative offset, plus the possability of tires rubbing against the car. Backspace centers the rim over the hub, offset centers the rim to the wheel well.

[Nacho-RT74]

I'm not saying offset doesn't matter... just that is a relative value, not an absolute value.

on these statements:

"Rims up front are 18x8 4.50 backspacing and 1 offset"....  "back...  Rim size is 18x9 with 5.25 backspacing and 8 offset"

something doesn't match to me on these maths

[pipeliner]

I just found a chart a guy uses for b and a bodys and he says it's pretty much been on the money. It says for an 18x9 rim with 5.25 backspacing a +6 offset would be idea and on a 18x8 with a 4.50 backspacing a 0 offset would be idea. Does that sound close. I guess I could run a 285 40 18. That would get me right at a 27" tire or maybe a spacer if I had to.

[Nacho-RT74]

once again... just school me If I'm wrong.

+6 offset makes no sense on what I understand about offset

5.25" BS on 9" rims makes ( on my maths ) +0.75 offset. Center line of the wheel is a 4.5"... Offset is a wheel center line relationship value.

please school me if I'm wrong

[pipeliner]

once again... just school me If I'm wrong.

+6 offset makes no sense on what I understand about offset

5.25" BS on 9" rims makes ( on my maths ) +0.75 offset. Center line of the wheel is a 4.5"... Offset is a wheel center line relationship value.

please school me if I'm wrong

Well the way I'm takeing it to get the rim dead center from where you're mounting it on a 5.25 backspacing you would have to use a +6 offset to get it dead center. Since the rim I'm using has a +8 off set that would have my rim off center by 2mm making it 2mm closer to the leave springs. Does this sound correct or am I way off lol

[randy73]

I would call them, as a 1mm offset does not sound right. Check out Keystone Klassic wheels, this is what I have on my 73.

https://www.cragarwheel.com/wheels/wheelsdetails/series-32-keystone-klassic

[pipeliner]

So I guess backspacing and offset is the same. One in inches and the other in MM. Figuring out rims is a pain in the ass

[pipeliner]

I guess I could the 9" rim up front which is +8 offset also

[randy73]

you are missing the point, the offset for the wheels you want do not match the offset needed. It is not just the offset you have to take into consideration, but the back spacing, offset and rim width, they all have to line up or there will be issues.

[Troy]

Offset and backspacing are related. Ignoring anything to do with wheel size names, offset is how far the wheel mounting surface is from the actual center line of the wheel. So you can imagine that something 8" wide has a center line at 4". If you move (offset) this mounting surface +12 mm it means the mounting surface moves about 1/2" toward the outside which means the back edge of the wheel moves closer to the car when mounted. Your 2 halves will be 4.5" inside and 3.5" outside. HOWEVER... an 8" wheel is only 8" at the inner edge of the tire mounting flange while backspacing is measured from the back edge of the wheel (as if it were laying on the ground). That flange to hold the tire is roughly 1/2" wide so it makes an 8" wheel actually 9" in total width so a perfectly centered (0 offset) mounting surface will actually be 4.5" from each outer edge of the wheel. Stock Mopar (and Ford) muscle car wheels all were 0 offset so a 6" rim width had 3.5" BS and 7" had 4" BS. To keep things in that same position an 8" wheel needs 4.5" BS, a 9" wheel needs 5" BS, and a 10" wheel needs 5.5" BS. As tires get taller and wider, they tend to run into parts of the body. In back, this is usually a low spot in the inner wheelhouse near the outside (just inside the trim) just above the axle. Solutions are wheel tubs or moving the tire closer to the springs. To move the tire inwards, you can increase the BS (which will increase the offset by the same amount but converted to mm). In the front, wide rims start to hit steering/suspension components so you can shift the tire away from the car - but too far and it starts hitting the wheel lip when turning. A tall tire in front hits the fender brace up top.

Real world example: some popular American Racing wheels are actually designed for Jeeps/trucks but have the same bolt pattern as our Mopars. This also includes a lot of old popular wheels like slots and "daisy" mags. These wheels, unfortunately, only come in 3.75" BS in the 8" size. I thought: no big deal, it's only 3/4"! Well, for the 235 witch tires on front it was no big deal but the 275 rear tires wouldn't even clear the wheel lip trim to mount the wheel on my Cuda. I had to buy 255 tires (about 1" skinnier total and about 1/2" each side) to gain the space back from moving the wheels out. On a Charger it would have still fit because the '68 Charger has probably 3" of space to play with from the factory. The same wheels and tires on my 70 Mach 1 hit the edge of the fender in turns when the suspension is compressed. Had I bought a 0 offset wheel the tires sizes I picked would have fit fine in all those circumstances (I have 8", 0 offset wheels on all my other cars).

Troy

[pipeliner]

you are missing the point, the offset for the wheels you want do not match the offset needed. It is not just the offset you have to take into consideration, but the back spacing, offset and rim width, they all have to line up or there will be issues.

Yeah I guess I still don't get it lol. So what backspacing and offset do I need on the front and back to run my 295x45x18 and 255x45x18

[metallicareload99]

you are missing the point, the offset for the wheels you want do not match the offset needed. It is not just the offset you have to take into consideration, but the back spacing, offset and rim width, they all have to line up or there will be issues.

Yeah I guess I still don't get it lol. So what backspacing and offset do I need on the front and back to run my 295x45x18 and 255x45x18

First off, I don't know what car you have, sorry I might have missed it somewhere. I'm going to assume it's a second generation Charger tho.

- I have a 68 Charger, iron head 440, A-833, Dana 60, MoPar Performance XHD replacement springs in back, tubular upper control arms up front.
- My front rims are 18" X 8" with I believe a 4.25" back spacing and 255/45r18 tires. These rims clear everything fine except the caliper, so I had to run a 1/4" spacer. The top edge of the tire on the other hand rubs the outer lip of the fender on hard cornering (auto cross) or going over big bumps, backing out of the drive way. More backspacing might fix these issues or create more by having the tire rub on the frame rail at full lock, but I can live with it as it is now
- In the back I used to run 295/45r18, but now I have 305/45r18 on 18 X 9.5 inch rim with 5.25" backspace. This setup seems to only hit on the top edge of the tire where the wheel housing has that bulge and it might rub occasionally on the inner wall of the wheel well. At any rate the annoying problem with this set up is the rubbing on the outer edge. I think more than 5.25" backspace would have helped, as would leaf springs with more arch, or my plan hopefully in the near future is reshaping that low spot.

I wish I went with more than 5 1/4" backspacing on the rear. 5 1/2" might have been perfect. I'll take Troy's word on the difference between backspacing and offset, but my understanding of it has agreed 100% with that diagram that was posted earlier. Whatever you want to call those measurements, it might be worth your while to take some more with your particular car before spending money. I mentioned what is working on my mine and what I would do different. Yrmv, Good luck!

[pipeliner]

you are missing the point, the offset for the wheels you want do not match the offset needed. It is not just the offset you have to take into consideration, but the back spacing, offset and rim width, they all have to line up or there will be issues.

Yeah I guess I still don't get it lol. So what backspacing and offset do I need on the front and back to run my 295x45x18 and 255x45x18

First off, I don't know what car you have, sorry I might have missed it somewhere. I'm going to assume it's a second generation Charger tho.

- I have a 68 Charger, iron head 440, A-833, Dana 60, MoPar Performance XHD replacement springs in back, tubular upper control arms up front.
- My front rims are 18" X 8" with I believe a 4.25" back spacing and 255/45r18 tires. These rims clear everything fine except the caliper, so I had to run a 1/4" spacer. The top edge of the tire on the other hand rubs the outer lip of the fender on hard cornering (auto cross) or going over big bumps, backing out of the drive way. More backspacing might fix these issues or create more by having the tire rub on the frame rail at full lock, but I can live with it as it is now
- In the back I used to run 295/45r18, but now I have 305/45r18 on 18 X 9.5 inch rim with 5.25" backspace. This setup seems to only hit on the top edge of the tire where the wheel housing has that bulge and it might rub occasionally on the inner wall of the wheel well. At any rate the annoying problem with this set up is the rubbing on the outer edge. I think more than 5.25" backspace would have helped, as would leaf springs with more arch, or my plan hopefully in the near future is reshaping that low spot.

I wish I went with more than 5 1/4" backspacing on the rear. 5 1/2" might have been perfect. I'll take Troy's word on the difference between backspacing and offset, but my understanding of it has agreed 100% with that diagram that was posted earlier. Whatever you want to call those measurements, it might be worth your while to take some more with your particular car before spending money. I mentioned what is working on my mine and what I would do different. Yrmv, Good luck!

Thanks so much for the info. Yes I have a 70 Charger RT. I'm running the same springs but an 8 3/4 rear.

[HeavyFuel]

Offset and backspacing are related. Ignoring anything to do with wheel size names, offset is how far the wheel mounting surface is from the actual center line of the wheel. So you can imagine that something 8" wide has a center line at 4". If you move (offset) this mounting surface +12 mm it means the mounting surface moves about 1/2" toward the outside which means the back edge of the wheel moves closer to the car when mounted. Your 2 halves will be 4.5" inside and 3.5" outside. HOWEVER... an 8" wheel is only 8" at the inner edge of the tire mounting flange while backspacing is measured from the back edge of the wheel (as if it were laying on the ground). That flange to hold the tire is roughly 1/2" wide so it makes an 8" wheel actually 9" in total width so a perfectly centered (0 offset) mounting surface will actually be 4.5" from each outer edge of the wheel. Stock Mopar (and Ford) muscle car wheels all were 0 offset so a 6" rim width had 3.5" BS and 7" had 4" BS. To keep things in that same position an 8" wheel needs 4.5" BS, a 9" wheel needs 5" BS, and a 10" wheel needs 5.5" BS. As tires get taller and wider, they tend to run into parts of the body. In back, this is usually a low spot in the inner wheelhouse near the outside (just inside the trim) just above the axle. Solutions are wheel tubs or moving the tire closer to the springs. To move the tire inwards, you can increase the BS (which will increase the offset by the same amount but converted to mm). In the front, wide rims start to hit steering/suspension components so you can shift the tire away from the car - but too far and it starts hitting the wheel lip when turning. A tall tire in front hits the fender brace up top.

Real world example: some popular American Racing wheels are actually designed for Jeeps/trucks but have the same bolt pattern as our Mopars. This also includes a lot of old popular wheels like slots and "daisy" mags. These wheels, unfortunately, only come in 3.75" BS in the 8" size. I thought: no big deal, it's only 3/4"! Well, for the 235 witch tires on front it was no big deal but the 275 rear tires wouldn't even clear the wheel lip trim to mount the wheel on my Cuda. I had to buy 255 tires (about 1" skinnier total and about 1/2" each side) to gain the space back from moving the wheels out. On a Charger it would have still fit because the '68 Charger has probably 3" of space to play with from the factory. The same wheels and tires on my 70 Mach 1 hit the edge of the fender in turns when the suspension is compressed. Had I bought a 0 offset wheel the tires sizes I picked would have fit fine in all those circumstances (I have 8", 0 offset wheels on all my other cars).

Troy

Great explanation!