wheel circumference
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From: Hachiroku ハチロク on 4 Nov 2009 15:58 On Wed, 04 Nov 2009 08:06:34 -0800, Jeff Strickland wrote: > > Since we're talking about Toyotas, and presumably passenger cars, it would > be difficult to put tires on that require a change of gears. I wrote a > calculator that tells me the size of a given tire, and I can then plug in > other tire specs to see what the size of an alternative might be. Hey! I did too! But I lost it. It's on a disk around here...somewhere... However, there is always one available here: http://www.miata.net/garage/tirecalc.html
From: C. E. White on 4 Nov 2009 22:25 ----- Original Message ----- From: "Tegger" <invalid(a)invalid.inv> Newsgroups: alt.autos.toyota Sent: Wednesday, November 04, 2009 7:26 AM Subject: Re: wheel circumference > "badgolferman" <REMOVETHISbadgolferman(a)gmail.com> wrote in > news:xn0gh8siuolk2q001(a)news.albasani.net: > >> Is there any difference in speedometer/odometer performance between >> wheels that have the same overall circumference but one has thin >> sidewalls and the other has wide sidewalls? I know there is a handling >> performance and comfort difference. I've always preferred the looks of >> the extra wide sidewalls with small wheel hubs to the more popular huge >> wheels with thin tires. > > > > Yes, there would be a difference in speedometer and odometer calibration. > > The operative wheel/tire radius is not the nominal overall radius as > measured from hub center to tire tread, but a smaller "working" radius > line > that is traced from the hub center to the /road/. > > > Notice how the tire bulges at the bottom? That's where the working radius > line ends. This working line is much shorter than the nominal radius line, > resulting in a much smaller circumference than nominal. That's how ABS- > based low-tire-pressure systems know which tire has low pressure. > > The shorter the tire sidewall (as for those larger wheels), the stiffer > the > sidewall, the less the bulge at the bottom of the tire and thus the less > the reduction between nominal radius and the working radius. While agree that there may be a small difference (a very small difference) in rolling diameter for tires of the same overall diameter but with different inside (wheel) diameters, I don't entirely buy you explaination of why. For sure you are right about how the ABS based low tire pressure sensors work, but they take miles of driving to detect a very significant difference in tire pressure. BUT..... Modern radial tires are not like hard wheels, they are like tank treads. The rolling diameter is mostly based on the diamter of the steel belt in the tire as long as the tires are properly inflated. This belt has to make one revolution per revoltion of the tire. I know this is obvious, but think about the implication. There is very little stretch in the steel belt. Now think about how this relates to the tank tread analogy....Even though the tire flattens out at the road surface, every portion of the belt still passes over the road in a almost exactly a 1 to 1 ratio. This would only be false if the belts stretched a lot - which they don't or if they took a really strange wavy or concave shape at the road interface, which also doesn't happen - at least if the tire is anywhere close to properly inflated. Any how back to the original question: Here is a chart for Michelin Pilot Sport Tires that relates tire size, overall diameter, and revolutions per mile: Revolutions Overall Per Tire Size Diameter Mile 245/40ZR17 24.7" 841 225/45ZR17 24.8" 837 255/35ZR18 25.2" 824 225/40ZR18 25.3" 821 265/35ZR18 25.3" 819 255/40ZR18 25.9" 770 285/35ZR18 25.9" 804 255/45ZR17 26" 798 255/45ZR17 26" 798 255/40ZR18 26" 800 285/35ZR18 26" 800 225/40ZR19 26.1" 797 265/40ZR18 26.6" 782 245/45ZR18 26.8" 780 245/45ZR18 26.8" 780 245/45ZR18 26.8" 775 255/45ZR18 26.9" 771 285/40ZR18 26.9" 771 235/50ZR18 27.2" 762 In general I don't think you can decide that tires with the same overall diamter but different wheel sizes will have greater differences in rolling diameters than tires of the same size but different brands or models (construction techniques). In any case the difference is very small (1% or less). Ed
From: Tegger on 5 Nov 2009 09:49 "C. E. White" <cewhite3remove(a)mindspring.com> wrote in news:IdednamjeIFc2G_XnZ2dnUVZ_gKdnZ2d(a)earthlink.com: > > > While agree that there may be a small difference (a very small > difference) in rolling diameter for tires of the same overall diameter > but with different inside (wheel) diameters, I don't entirely buy you > explaination of why. > > For sure you are right about how the ABS based low tire pressure > sensors work, but they take miles of driving to detect a very > significant difference in tire pressure. BUT..... > > Modern radial tires are not like hard wheels, they are like tank > treads. The rolling diameter is mostly based on the diamter of the > steel belt in the tire as long as the tires are properly inflated. I think you need to go do some actual observation and measurement. Go outside and measure those distances on your own tires. I'll use my own car's front tires as an example for illustration: My tire size is 195/60-14. That gives me a nominal diameter of 23.21". An actual (as best as I can eyeball) diametrical measurement reveals 23", when measured across the unloaded portion of the tire from front-to-back. That means the unloaded radius is 11.5". If I measure from dead-center of the hub to the road (the loaded, or "working" radius) however, I get 10.625". That's 7/8" difference, or about 8% less than the unloaded radius. Now... Consider my REAR tires. The car has 61-39 front/rear weight distribution. Same tire, same pressure, much lighter loading. The working radius here I measure at 11.0625". The rear tires thus have a 4% larger working radius than the fronts. It is impossible to have a contact patch on the road unless the tire develops a "flat spot" where the tire contacts the road. This has nothing to do with the steel belt or anything else. Even a hard-rubber towmotor tire has a flat-spot and a contact patch. The center of the contact patch is the end point for that "working" radius line. If you have bigger wheels and shorter, stiffer tire sidewalls within the same unloaded diameter, the loaded distortion will be less, which means the wheel/tire assembly will have a larger working circumference. How much more? Possibly up to four or five percent, possibly as low as one or two percent. Depends. But there WILL be a difference. -- Tegger
From: C. E. White on 5 Nov 2009 10:26 "Tegger" <invalid(a)invalid.inv> wrote in message news:Xns9CBA63CD94FA9tegger(a)208.90.168.18... > "C. E. White" <cewhite3remove(a)mindspring.com> wrote in > news:IdednamjeIFc2G_XnZ2dnUVZ_gKdnZ2d(a)earthlink.com: > > >> >> >> While agree that there may be a small difference (a very small >> difference) in rolling diameter for tires of the same overall >> diameter >> but with different inside (wheel) diameters, I don't entirely buy >> you >> explaination of why. >> >> For sure you are right about how the ABS based low tire pressure >> sensors work, but they take miles of driving to detect a very >> significant difference in tire pressure. BUT..... >> >> Modern radial tires are not like hard wheels, they are like tank >> treads. The rolling diameter is mostly based on the diamter of the >> steel belt in the tire as long as the tires are properly inflated. > > > > I think you need to go do some actual observation and measurement. > Go > outside and measure those distances on your own tires. > > I'll use my own car's front tires as an example for illustration: > My tire size is 195/60-14. That gives me a nominal diameter of > 23.21". > > An actual (as best as I can eyeball) diametrical measurement reveals > 23", > when measured across the unloaded portion of the tire from > front-to-back. > > That means the unloaded radius is 11.5". > > If I measure from dead-center of the hub to the road (the loaded, or > "working" radius) however, I get 10.625". > > That's 7/8" difference, or about 8% less than the unloaded radius. > > Now... > > Consider my REAR tires. The car has 61-39 front/rear weight > distribution. > Same tire, same pressure, much lighter loading. The working radius > here I > measure at 11.0625". The rear tires thus have a 4% larger working > radius > than the fronts. > > It is impossible to have a contact patch on the road unless the tire > develops a "flat spot" where the tire contacts the road. This has > nothing > to do with the steel belt or anything else. Even a hard-rubber > towmotor > tire has a flat-spot and a contact patch. The center of the contact > patch > is the end point for that "working" radius line. > > If you have bigger wheels and shorter, stiffer tire sidewalls within > the > same unloaded diameter, the loaded distortion will be less, which > means the > wheel/tire assembly will have a larger working circumference. How > much > more? Possibly up to four or five percent, possibly as low as one or > two > percent. Depends. But there WILL be a difference. I tried really hard to get you to see this differently. Go back and rear my prior post. Tires are not hard wheels. The revolutions per mile is not directly related to the loaded radius of the tire in the manner you are suggesting. It is likely a factor, but a very minor one.... Think! Car tires are not hard wheels like forklift tires. How can what you are saying be true? Look at it a different way...every time the tire goes through a revolution, there is apporximately a one to one relationship between the tire and the road surface (every part of the tire contact the road). One revolution of the tire will move the car forward by the circumfrence of the tire. If it doesn't you have to have slip (which will be true if you spin the tires, but is a negligible factor if you are cruising straight ahead on a level road at a moderate sped). The fact that the tire flattens out at the road surface doesn't change this. Of course rubber is flexible and can stretch/shrink, which is why the really important circumfrence is the circumfrence of the steel belts inside the tire. The treads area can stretch and flex (think about how tank treads move) but the steel belt stretchs only by tiny amounts. Unless the tire is so underinflated that the tire assumes a concave shape at the road surface, the effective rolling radius is not significantly dependednt on the distance from the road to the wheels center axis. Like most real world things, there are other factors that come into play, so I don't doubt that there is some effect on revolutions per mile related to tires of the same outside diameter with different inner diameters (i.e., wheel size), but the I don't think the loaded radius is the prime reason. One more thing to think about. If you are right, where does the extra tire go? You are suggesting that one revolution of the tire will result inthe car moving forward by a distance equal to the loaded radius of the tire times 2 times pi. But the actual circumfrence of the tire is the unloaded diameter of the tire times pi. Since 2 times the loaded radius of the tire is less than the unloded diameter of the tire times pi, this implies something is happening witht he "extra" circumfrecne in your senario. Where is it going? You might convince me that the rubber portion is stretching and shrinking to accomodate your theory, but what is happenign with the steel belt in the tire? One last analogy - think conveyor belt..... Did you look at the chart I included with the prior note? Ed
From: Jeff Strickland on 5 Nov 2009 12:34
"Tegger" <invalid(a)invalid.inv> wrote in message news:Xns9CBA63CD94FA9tegger(a)208.90.168.18... > "C. E. White" <cewhite3remove(a)mindspring.com> wrote in > news:IdednamjeIFc2G_XnZ2dnUVZ_gKdnZ2d(a)earthlink.com: > > >> >> >> While agree that there may be a small difference (a very small >> difference) in rolling diameter for tires of the same overall diameter >> but with different inside (wheel) diameters, I don't entirely buy you >> explaination of why. >> >> For sure you are right about how the ABS based low tire pressure >> sensors work, but they take miles of driving to detect a very >> significant difference in tire pressure. BUT..... >> >> Modern radial tires are not like hard wheels, they are like tank >> treads. The rolling diameter is mostly based on the diamter of the >> steel belt in the tire as long as the tires are properly inflated. > > > > I think you need to go do some actual observation and measurement. Go > outside and measure those distances on your own tires. > > I'll use my own car's front tires as an example for illustration: > My tire size is 195/60-14. That gives me a nominal diameter of 23.21". > > An actual (as best as I can eyeball) diametrical measurement reveals 23", > when measured across the unloaded portion of the tire from front-to-back. > > That means the unloaded radius is 11.5". > > If I measure from dead-center of the hub to the road (the loaded, or > "working" radius) however, I get 10.625". > > That's 7/8" difference, or about 8% less than the unloaded radius. > > Now... > > Consider my REAR tires. The car has 61-39 front/rear weight distribution. > Same tire, same pressure, much lighter loading. The working radius here I > measure at 11.0625". The rear tires thus have a 4% larger working radius > than the fronts. > > It is impossible to have a contact patch on the road unless the tire > develops a "flat spot" where the tire contacts the road. This has nothing > to do with the steel belt or anything else. Even a hard-rubber towmotor > tire has a flat-spot and a contact patch. The center of the contact patch > is the end point for that "working" radius line. > > If you have bigger wheels and shorter, stiffer tire sidewalls within the > same unloaded diameter, the loaded distortion will be less, which means > the > wheel/tire assembly will have a larger working circumference. How much > more? Possibly up to four or five percent, possibly as low as one or two > percent. Depends. But there WILL be a difference. > > None of that matters to the ABS if all tires on the ground are the same size, even if the tires are not the size that is specified for the car. The ABS/Low Tire Pressure systems look at differences in the rotational speed of one tire vs. the others. If all four tires were the same, then the speed of the tires would also be the same and the ABS would function properly. The tire size _can_ influence the speed display on the speedometer, but as has been shown in several charts, if the stock size is changed for an aftermarket size that is properly equivelent, the influence over the speedometer is insiginficant, and if the new tire is a few percent larger, the actual effect on the speedometer is desirable -- it makes the speedo display the proper speed instead of display a lower speed than one is actually travelling. My car came with a 205/50x16, the car before it came with a 225/55x15, I put a set of 225/45x17s on both of these cars, and all three tires are functional equivelents for one another. (I had a car that had the 15s on it, I put on the 17s, and a lady in a double-know pants suit and blue hair slammed into me. I bought the car with the 16 on it, and move the 17s to it). The affect on the speedo was that the 17's changed the error at 80mph from almost 5mph to just under 2mph. Foremrly, the speedo would read 80 when doing 75-ish, now it reads 80 when doing 78-ish. But since all four tires are the same size, and functional equivelents, the ABS system still works right. The OP wants a larger sidewall for some reason, not a smaller one. (I don't get the logic he's using, but it's not my decision.) If he wants to go down an inch on the rims he's using, then he would go up 5% on the aspect ratio (the center figure in 190 / 55 x 15) to keep the same tread width. If he wanted to keep the same rims but get a larger sidewall, then he's have to get a thinner tire. On second thought, I'd have to run the numbers, I'm not sure he can get more sidewall on the same rim because if the width got less but the aspect ratio went up, the result would be the same sidewall -- 190 x .55 and 180 x .6 are different by 3.5mm, hardly a useful difference for any practical reason. He would get 10mm less rubber on the ground and virtually no change in the sidewall. |