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What relationship? Posted by sam96CS [Email]  (#852) [Profile/Gallery]
(more from sam96CS) on Sat, 6 Jul 2013 08:25:49
In Reply to: Ford Focus and Fiesta rims, Noel, Sat, 6 Jul 2013 06:40:21 Members do not see ads below this line. - Help Keep This Site Online - Signup |
"It seems that you can have more/less offset, and more/less backspacing and have it work out correctly, but I don't know how the relationship changes."
I'm not sure I understand what you mean. At the link you provided there is a backspace to offset conversion chart. Based on this chart, for every increase/decrease in offset there is an equal increase/decrease in backspace. This positive 1:1 relationship looks true for all wheel widths.
I don't know how to correct for a wheel that has less offset than is ideal for its width. If a wheel has too much offset, then you can put a 4x108 spacer between the wheel and hub that is equal in thickness to the excess amount of offset. That corrects the offset error, but spacers introduce new issues such as the need for longer lug bolts, vibration, and greater risk of the bolts working loose. In other words, not an ideal solution.
Are you asking how much the offset and backspacing should change for every incremental change in wheel width? Let's assume you want the location of the center of the tire's contact patch to remain unchanged as wheel width changes. This means the increase/decrease in width should be divided evenly between each half of the wheel's width. We know from Aero and Super Aero wheels that a 6-1/2" wide wheel should have a 27 mm offset. Suppose we go to a 7" wheel? In that case the width in millimeters has increased by 25.4/2 = 12.7 mm. We want to divide that 12.7 mm evenly between the inside and outside halves of the wheel. As the wheel gets wider, the offset must be decreased by half the increase in width. In this example, by half of 12.7 mm, which is 6.35 mm. So you want a 7" wide wheel to have a 27 - 6.35 = 20.65 mm offset, which you'd probably round to 21 mm. In plainer English, reduce the offset by 6.35 mm for every half inch increase in wheel width. The backspace will be reduced by the same amount.
So how do we KNOW this math works? It helps to look at what Saab has already done. The offset for stock 6" wide wheels is 33 mm, and the offset for stock 6-1/2" wheels is 27 mm, which is 6 mm less.
If you want 7-1/2" wide wheels the decrease in offset relative to a 6-1/2" wheel's offset should be 12.7 mm (6.35 mm x 2), which you would round to 13 mm. So the offset would be 27 - 13 = 14 mm. The backspace in this example would also be reduced by 14 mm. By giving this example I make no claim that there is in fact enough clearance for 7-1/2" wide wheels. I know there is enough for 7" in 9000s made with wider wheel wells, which started in '93 I think. I know because I had 15" x 7" super CS wheels with 205/60-15 tires for 50,000 miles with no rubbing.
Additional disclaimer: I'm just doing math here. I'm not a wheel/tire/suspension/handling guru. I've assumed that you would also adjust the tire's aspect ratio to keep the diameter as close to stock as possible.
I haven't sold my 15" x 7" wheels yet. If you'd like I can measure the offset and backspace to see if they agree with my math.
, Fri, 28 Jun 2013 18:07:13
, Sun, 7 Jul 2013 07:13:57, Sat, 6 Jul 2013 08:25:49 <-- Viewing This Message
, Sun, 7 Jul 2013 03:45:04, Sun, 7 Jul 2013 11:56:16, Sun, 7 Jul 2013 07:01:37, Thu, 4 Jul 2013 04:35:49
, Fri, 28 Jun 2013 21:36:27, Sat, 29 Jun 2013 03:10:20, Thu, 4 Jul 2013 02:19:12, Sun, 7 Jul 2013 03:37:37, Thu, 4 Jul 2013 05:27:37, Fri, 5 Jul 2013 05:33:24, Fri, 5 Jul 2013 06:21:23, Wed, 3 Jul 2013 15:20:11
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