PLEASE HELP! 15x8 wheels

[Nimble]

That's incorrect, a 225 sitting on a 15x8 ET38 will not sit in the same place as a 225 sitting on a a 15x7 ET38, what tire shop did you work at again? A 1" wider wheel will put roughly 13mm's more on either side of the tire, both on the shock side and the fender side.

[eTiMaGo]

Originally Posted by Black Yaris

I am relaxed.... and you are taking what I am sayin about 7 and 8 inch wide wheels and there offsets out of context... because if you were to run a 225/50R15 on a 15x7 with a +38, the position of that tire will be unchanged going to a 15x8 with a +38.... and that is all I have been saying all along..
and that is 100% true, no matter what tire, or offset calulator you run that though

Originally Posted by Nimble

That's incorrect, a 225 sitting on a 15x8 ET38 will not sit in the same place as a 225 sitting on a a 15x7 ET38, what tire shop did you work at again? A 1" wider wheel will put roughly 13mm's more on either side of the tire, both on the shock side and the fender side.

Dammit you guys are BOTH right.

I hope this picture can help clear things up a bit:



Black Yaris is saying that between 7" and 8" widths, the same width tire will remain in the same position if the offset is unchanged. Take a good look and you'll see this is true, just as nimble pointed out, the RIM will extend 13mm in BOTH directions, and the tire will remain centered. You're both agreeing on this part

The tire will only be moved in or out if the offset is changed. So, the offset comes into play when calculating the distance between the outer edge of the tire, and the inner wheel well and outer fender. This is where things get tricky, as it is really a function of tire width and not rim width.

AND REMEMBER! This is confusing, a smaller positive offset means that your wheel sticks OUT more than a bigger positive offset. It's hard to visualize this in your head, so look at this picture, it should clear things up:



On my car I have 215 width tires, and the rim's offset is 42mm. Now, the front wheels stick out a little more than the back due to my disc brake's hub being a little thicker than stock and acting as a spacer. So, the effective offset for the front wheels is more like 40 or 38mm, and you can see this best in this picture:



You can see, as far as 215 tires go, that's a perfect fit. If I had wider tires on it (235 or 245mm), they would not stretch inward so much relative to the rim, and I would definitely have some rubbing on the outer fender, in which case a bigger offset would be prefered, to tuck the wheel back in a little and achieve a flush fit on the outside.

On the inside, though, the extra tire width and increased offset may cause some rubbing as a result, and that is where fender rolling comes into play, physically extending the volume in which the tire and wheel can move when the suspension is under load, or when the steering wheel is being turned.

PS I hate this stuff, it confuses the hell out of me and gives me massive headaches, but that's the best way to learn

[PetersRedYaris]

The reality is, were bickering over 7mm of offset when the rim is 25.4mm (1 inch) wider than the recommended 7 inches. Like Nimble said, that's adding about 13mm to both sides. 38mm, 45mm, and everything in-between WILL rub on both the inside and outside. You want to modify your car to squeeze 8 inch wheels under it, pick your poison.

[Black Yaris]

Originally Posted by Nimble

That's incorrect, a 225 sitting on a 15x8 ET38 will not sit in the same place as a 225 sitting on a a 15x7 ET38, what tire shop did you work at again? A 1" wider wheel will put roughly 13mm's more on either side of the tire, both on the shock side and the fender side.

smoke another one because the tire will be sitting in the same spot, and that is what I said... THE TIRE WILL BE SITTING IN THE SAME SPOT

since the tire is 225 mm wide (8.9in) wich is wider than either wheel the tire is going to sit in the same position regaurdless... you need to figure out TIRE clearance, not wheel clearance in this case and that is what I did with the +38 offset

[largeorangefont]

Originally Posted by Black Yaris

smoke another one because the tire will be sitting in the same spot, and that is what I said... THE TIRE WILL BE SITTING IN THE SAME SPOT

No, not quite. The mounting pad of the wheel will be in the same place relative to the wheel centerline on a 8 inch. vs a 7 inch rim with the same offset. But the mounting pad would be in a different location relative to the inner and outer wheel lips, and this change the position of the wheel in the wheelwell. The only time this relationship would not change is if the wheel was 0 offset, meaning the mounting pad was in the dead center of the wheel. Then you would increase equal distances on the outside and inside, and relative position would stay the same.

The +38 is a fixed number. That means 38 MM from the center of the width wheel to the mounting pad. That distance has to stay the same, regardless of rim width, but the width still grows. Because the offset is favored to one side of the wheel (not 0 offset), one side protrudes furthur than the other as width grows.

That is why the wider wheel you go the less offset you have, because you have to account and compensate for uneven growth on both sides of the wheel centerline.

[Nimble]

Originally Posted by Nimble

That's incorrect, a 225 sitting on a 15x8 ET38 will not sit in the same place as a 225 sitting on a a 15x7 ET38, what tire shop did you work at again? A 1" wider wheel will put roughly 13mm's more on either side of the tire, both on the shock side and the fender side.

I'm quoting myself again....well, because I'm right. Think about it. You cannot, I repeat CANNOT have a wider wheel with the same offset as a smaller wheel sit in the same place in the wheel well. Why have offsets at all then? So a 15x5 ET 38 would sit in the same place as said 15x8 ET 38? That's what your reasoning suggests.