Tyre question for dummies (like me)
01-04-2012, 03:30 PM
#1
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Tyre question for dummies (like me)
I have 245/40 Continental DWs on the front, on 8" wheels. C3's.
The section width on those is 9.8". I get just barely some rub at lock on a hard downhill turn.
Im looking to replace those with 255/45 Bridgestone RE050A's, that are acceptable on an 8" wheel, but with a section width of only 9".
My assumption...is that this will give me a slightly softer more compliant ride, and pull about 1/4" of clearance back into the wheel well with the .8" narrower SW..which may be more like a full inch, being on the narrow end of the wheel size spectrum allowable for that tyre.
Just runnin this past the peanut gallery.
The section width on those is 9.8". I get just barely some rub at lock on a hard downhill turn.
Im looking to replace those with 255/45 Bridgestone RE050A's, that are acceptable on an 8" wheel, but with a section width of only 9".
My assumption...is that this will give me a slightly softer more compliant ride, and pull about 1/4" of clearance back into the wheel well with the .8" narrower SW..which may be more like a full inch, being on the narrow end of the wheel size spectrum allowable for that tyre.
Just runnin this past the peanut gallery.
01-04-2012, 03:42 PM
#2
Chronic Tool Dropper
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Jeff--
Is the diameter the same? That will probably affect the rubbing at the front guard at least as much as the difference in width.
Is the diameter the same? That will probably affect the rubbing at the front guard at least as much as the difference in width.
01-04-2012, 04:51 PM
#5
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255 mm/ 25.4 = 10 inch wide tire (section width).
Am I missing something? (very possible since I'm sitting in the peanut gallery
)
01-04-2012, 05:04 PM
#6
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There's section width, rim width, tread width...all different measurements in different places.
Section width is the WIDEST point from the outside edges of the tyre given a specific sized wheel. It will be wider if you use a wider wheel than spec, or narrower of you use a narrower sized wheel than spec.
01-04-2012, 05:24 PM
#7
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Going to a 255/45 from the 245/40 will be wider and taller. It will likely make your rub problem worse, and introduce more sidewall flex in your steering. I would thing you would want to go down a size to 235/40 18 . Closer to the stock circumference and width however you may have other reasons for wanting that much tire on an 8 inch wheel.
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01-04-2012, 05:28 PM
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01-04-2012, 05:40 PM
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I run a 255/40 17 on a 9.5" wide 60 mm offset wheel at the front of my 80 Euro S. I had a rub on the inside of the fenderwell with the tire contacting at full lock. I solved the problem by adding a second rack bumper to each end of the rack (where the tierod screws in to the rack. I gained a little turning radius but it eliminated the rub. Several individuals runing wide rims and wide tires on the front of thier car have done this and eliminated the rub
Paul
1980 Euro S Red/Blk Lea
Paul
1980 Euro S Red/Blk Lea
01-04-2012, 05:47 PM
#10
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I did not know that tires give a wheel spec. I am used to looking at it the other way around. I have a wheel and it has a given width - the "rim width" you mentioned, I believe - which is measured from inside lip to inside lip of the wheel. Then, from that "wheel width" measurment, acceptable tire widths are from 90% to 110% of it.
I believe there is an adjustment to these numbers at the extremes (super thin wheels or super wide wheels), but I don't know what that is.
01-04-2012, 06:40 PM
#13
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Hmmm, learning something in the peanut gallery...
What I have used (above) are rules of thumb that have worked well and I don't even remember where I got them from. However, from TireRack.com (my underlining):
What I have used (above) are rules of thumb that have worked well and I don't even remember where I got them from. However, from TireRack.com (my underlining):
Tire Specs Explained: Measuring Rim Width
The measuring rim width is the industry standardized rim width upon which the tire must be mounted in order to confirm it meets its dimensional targets. Because the width of the rim will influence the width of the tire, a standard rim width for every tire size is assigned and must be used. This standardized measuring rim width allows all of the tires produced around the world to meet the same dimensional standards and therefore, be equivalent with regards to their physical size. The measuring rim width is sometimes referred to as the tire's "design rim width."
The assigned measuring rim width changes with the tire size's section width and with the tire size's aspect ratio. As tire section width increases, the measuring rim width increases proportionately in 1/2" increments. Therefore, relatively narrow wheel widths are assigned for smaller tires while wider wheel widths are assigned for larger tires.
The skeptic in me can also see the practicality and legal grounds for this. They cannot possibly test the tires on all wheels!
The measuring rim width is the industry standardized rim width upon which the tire must be mounted in order to confirm it meets its dimensional targets. Because the width of the rim will influence the width of the tire, a standard rim width for every tire size is assigned and must be used. This standardized measuring rim width allows all of the tires produced around the world to meet the same dimensional standards and therefore, be equivalent with regards to their physical size. The measuring rim width is sometimes referred to as the tire's "design rim width."
The assigned measuring rim width changes with the tire size's section width and with the tire size's aspect ratio. As tire section width increases, the measuring rim width increases proportionately in 1/2" increments. Therefore, relatively narrow wheel widths are assigned for smaller tires while wider wheel widths are assigned for larger tires.
01-04-2012, 08:56 PM
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As DRRASTA says, it sounds like worstening your problem.
245 to 255 is increasing the tread width by 10mm and uping the profile from 40% to 45% of that tread width means that the wider tread will also be raised from [40% of 245 =] 98mm in tyre wall height to [45% of 255 =] 114.75mm.
ie the outer edge of the read will be, theoretically, out 5mm each side and up nearly 17mm each side.
If it rubs on the outer edge, it's more likely rim offset that's an issue as the 928 is one of the few cars that run 65mm front rim offset, so that spec is hard to find.
245 to 255 is increasing the tread width by 10mm and uping the profile from 40% to 45% of that tread width means that the wider tread will also be raised from [40% of 245 =] 98mm in tyre wall height to [45% of 255 =] 114.75mm.
ie the outer edge of the read will be, theoretically, out 5mm each side and up nearly 17mm each side.
If it rubs on the outer edge, it's more likely rim offset that's an issue as the 928 is one of the few cars that run 65mm front rim offset, so that spec is hard to find.
Going to a 255/45 from the 245/40 will be wider and taller. It will likely make your rub problem worse, and introduce more sidewall flex in your steering. I would thing you would want to go down a size to 235/40 18 . Closer to the stock circumference and width however you may have other reasons for wanting that much tire on an 8 inch wheel.
