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From: Jeff Strickland on 5 Nov 2009 22:48 "Tegger" <invalid(a)invalid.inv> wrote in message news:Xns9CBAE1FF91E2Etegger(a)208.90.168.18... > "badgolferman" <REMOVETHISbadgolferman(a)gmail.com> wrote in > news:xn0ghau681ojn70000(a)news.albasani.net: > >> Tegger, 11/5/2009,4:03:51 PM, wrote: >> >>> "badgolferman" <REMOVETHISbadgolferman(a)gmail.com> wrote in >>> news:xn0ghaisbeui4k004(a)reader.albasani.net: >>> >>> > Jeff Strickland wrote: >>> > >>> > > 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.) >>> > >>> > I don't want anything changed on my car. I was just wondering if >>> > there is a difference between tires with thin sidewalls and those >>> > with very wide sidewalls when it comes to odometer/speedometer >>> > readings. And I'm not talking about one inch differences, more >>> > like 45s vs. 85s. >>> > >>> > After you guys work out all this technical minutia let me know what >>> > the consensus is. >>> > >>> > >>> > >>> >>> >>> You'll never get "consensus". >>> >>> The fact is: >>> Given a specific sidewall height and overall tire diameter, if you >>> change ONLY the sidewall height (bigger wheel), you WILL change the >>> rolling circumference. >>> >>> If you go from a 60 to a 50, you probably won't notice the >>> difference. If you go from an 80 to a 45, there would be a big >>> difference. >> >> So will the wheel with the 80 tire rotate more or less than the wheel >> with the 45? Which one will show more than the correct speed and >> which one will show less? >> > > > The OEM wheel, tire size, and speed rating will be closest to the > manufacturer's intended speedometer calibration. > That is not entirely true. For example, the Camry has several different tire sizes that can be fitted by virtue of the different trim levels and various option packages. The speedometer is not calibrated for each different size of tire package, it is calibrated for the largest tire package. Smaller tires will have greater speedometer error than the larger tires. If the base model Camry gets a 195 / 60 x 15, the fully loaded can have a 215 / 45 x 17, and the diameter of the tire is different by less than one-half inch. The speedo calibration can be identical across the model line, meaning that one could buy a base model car then get upgrade tires and wheels and still use the same speedo. > When it comes to aftermarket, I wish I could pin that question down for > you, but I can't. The answer depends on the actual rolling circumference > that each tire describes when loaded the way your car will load it. > > One maker's 225/45-16 won't necessarily measure the same actual unloaded > diameter as another maker's 225/45-16. And those tire-shop tire-size > equivalency charts are fraught with uncertainty on account of that. > > But: Given two differently-seriesed tires with identical unloaded > circumferences, the one with the shorter sidewall will have the larger > rolling circumference and the lower speedometer reading. > > The upshot is that the difference will likely not be enough for anybody > to notice unless he's very specifically looking for that difference. > Even the cops recognize that, which is partly why they give you 5mph > grace (or more depending on the jurisdiction) before whacking you. > That's certainly true. The differences you are telling us about are there, maybe, but utterly insignificant UNLESS one tire is different than the rest. The small difference could be enough to trigger the Low Pressure Monitor if said monitor derives its information from the speed sensors.
From: Tegger on 6 Nov 2009 06:55 "Jeff Strickland" <crwlrjeff(a)yahoo.com> wrote in news:hd069m$b9l$1(a)news.eternal-september.org: > > "Tegger" <invalid(a)invalid.inv> wrote in message > news:Xns9CBAE1FF91E2Etegger(a)208.90.168.18... >>> >> >> >> The OEM wheel, tire size, and speed rating will be closest to the >> manufacturer's intended speedometer calibration. >> > > > That is not entirely true. For example, the Camry has several > different tire sizes that can be fitted by virtue of the different > trim levels and various option packages. The speedometer is not > calibrated for each different size of tire package, it is calibrated > for the largest tire package. Smaller tires will have greater > speedometer error than the larger tires. If the base model Camry gets > a 195 / 60 x 15, the fully loaded can have a 215 / 45 x 17, and the > diameter of the tire is different by less than one-half inch. The > speedo calibration can be identical across the model line, meaning > that one could buy a base model car then get upgrade tires and wheels > and still use the same speedo. But they're all OEM, which is my point. -- Tegger
From: Tegger on 6 Nov 2009 07:08 "Jeff Strickland" <crwlrjeff(a)yahoo.com> wrote in news:hd05hr$6ok$1(a)news.eternal-september.org: > > "Tegger" <invalid(a)invalid.inv> wrote in message > news:Xns9CBAE381511EEtegger(a)208.90.168.18... >> "Jeff Strickland" <crwlrjeff(a)yahoo.com> wrote in >> news:hd044q$sph$1(a)news.eternal-september.org: >> >>> >>> "badgolferman" <REMOVETHISbadgolferman(a)gmail.com> wrote in message >>> news:xn0ghaisbeui4k004(a)reader.albasani.net... >>>> Jeff Strickland wrote: >>>> >>>>>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.) >>>> >>>> I don't want anything changed on _my_ car. I was just wondering if >>>> there is a difference between tires with thin sidewalls and those >>>> with very wide sidewalls when it comes to odometer/speedometer >>>> readings. And I'm not talking about one inch differences, more like >>>> 45s vs. 85s. >>>> >>>> After you guys work out all this technical minutia let me know what >>>> the consensus is. >>>> >>>> >>> >>> >>> The short answer is, no, there is no difference. >>> >>> The long asnwer is that the overall diameter has to be the same for >>> the short answer to be true. For example, my truck has a 265/75x16. >>> I could go to a 265/35x20 and the tire should be the same overall >>> diameter, therefore no affect on the speedo. >>> >>> >> >> >> Supposing the two sizes have identical unloaded diameters, the 35 >> will have >> the larger rolling circumference and the slower speedometer reading. >> > > > NOT true. Well, not entirely true. > > For every inch increase in the rim, the aspect ratio is decreased 5%. > The result is a overall diameter, or radius, or circumference > (depending on the specy you prefer) will be virtually identical. The > difference -- if done on all four corners -- is insignificant. There > will be a small change to the speedometer -- a change measured in > single digit percentages. As I said all along. The point is that it is unlikely that there will be NO change. Another example for illustration: Assume a tire with 23.21" diameter (my 195/60-14). The sidewall of that tire measures 4.6". The front tires drop about .875" at the contact patch. That's about 19%. Now assume a tire of the same diameter, but a hypothetical size of 195/35-18. This gives the sidewall a height of 2.7". Now imagine that this same tire has a drop of .875" at the contact patch. That's /one-third/ of the sidewall height of only 2.7"! NO 35-series tire drops 7/8" at the contact patch. It cannot. Any owner of that tire would be convinced his tire was flat! And this means the effective rolling circumference of such a tire would be /larger/ than the original 195/60-14, giving a slower speedometer reading.. > > The speedometer would be more sensitive to the circumference, but > since circumference is calculated by diameter X pi, or radius X 2 X > pi, then all are the same. > > I assume proper inflation, and ignore any argument that assumes > improper inflation. Improper inflation causes a variance, and the > variance is measured by the speed sensors, not the speedometer. > > If the wheel is turning at a speed other than what the speedometer is expecting, the speedometer will reflect that. Period. It may not be enough to notice in everyday driving, but it will be reflected. ABS-based speed sensors only track DIFFERENCES between wheels, not absolute speeds. And if a lower pressure makes a difference to the ABS, it will also make a difference to the speedometer, provided the lower pressure tire is on the wheel that's on the "solid" side of the diff. -- Tegger
From: C. E. White on 6 Nov 2009 08:36 "Tegger" <invalid(a)invalid.inv> wrote in message news:Xns9CBA8755D37D3tegger(a)208.90.168.18... > "C. E. White" <cewhite3(a)mindspring.com> wrote in news:hcur0a$o5e$1 > @news.eternal-september.org: > > > >> >> 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. > > > > The /working/ circumference, which changes with all sorts of > factors, > pressure, weight, sidewall height. > > As I said before, ABS-based low-pressure warning systems > _could not work_ unless this was true. Very ture, but they work by comparing the response of similar tires and take a relatively long time to figure out that one is runnnig at a much lower pressure than the others (miles of driving). Lots of thing affect the number of revolutions per mile for a tire, but most are minor. Even the speed of the car has an effect. My only point is, you can't treat a tire like a hard rubber wheel. The loaded radius (distance from the axle centerline to the ground) is not the most significant factor in determining the rolling diameter 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). > > > > Not so. It's called "scrub" and is the reason tires wear even in a > straight-ahead, steady-state position. > > > >> The fact that the tire flattens >> out at the road surface doesn't change this. > > > > But that makes /all/ the difference. > > > >> 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. > > > > Then how can an ABS-based warning system work if it cannot depend on > changes in working radius? And monitoring changes in rolling > circumference due to changed working radius _IS_ how such systems > operate. I did not say that there was no effect, just that it is a major effect. The ABS tire monitoring systems compare the response of the tires. A tire with a much lower pressure than the others will on average have a different rolling radius, but it is a very small difference. Thise systems takes miles to figure out a tire is underinflated. >> 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? > > > > Where did that 1/4" go? I don't know. I suspect there's a minuscule > stretch or displacement around the unloaded portion of the tire. But > in > order to confirm that, I'd need to jack the wheel off the ground and > take some really careful measurements both loaded and unloaded. > > My contact patch is about 4.25" front-to-back (about 45deg of > rotation). > On my tire, a line traced from front-to-back on that flat spot is > about > a quarter-inch shorter than the curve would be if the tire was not > flattened by the load. > > If that 1/4" were distributed evenly around the unloaded portion of > my > tire, each of the 315 degrees of unloaded tire would have to > displace or > stretch about .0008", which is plausible. Plus some of the > distortion > would be compressed into the flat spot, so the .0008 might be an > overestimate. > > > >> 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? > > > Well, I have one guess: The steel belt is a woven assembly with the > wires on a bias. It's possible the weave distorts under compression > and > extension, much like rope, cloth or window screen can. > > > >> >> One last analogy - think conveyor belt..... >> >> Did you look at the chart I included with the prior note? >> > > > I did, but the chart misses the point. The point was that the rolling circuimference of tires with similar outside diamters but different rim diamters are usually very close.. > My trump card is ABS-based low-pressure warning systems. I know for > a > fact that they work by sensing a low-pressure tire spinning faster > than > one with higher pressure, which can ONLY happen if the working > radius > (and circumference) can shrink and grow on an otherwise inextensible > tire. Again, I agree that these ABS systems work by detecting changes in the rolling radius of the tire, but the effects are very small and the system take miles to figure out that the tires are different. The rolling radius is affected by many things, temeprature, speed, inflation pressure, even the particular road surface, but the ABS based systems work by comparing tires to other tires on the car. They can detect samll changes that affect one tire comapred to the others. Therefore the small effect on the rolling radius due to a large change in air pressure (>20% decrease) is detectable by these sysems I am sure you are greatly over emphasizing the contribution of the loaded radius on the rolling diamter of tires, but don't know of any other effective arguements. Maybe an experiment would convince you. If you have the time, measure the loaded radius, mark the tire, move the car for 100 revolutions of the tire, and then measure the distance moved...You will find that it moved a significantly greater distance than 2 x pi x loaded radius x 100. Ed
From: Jeff Strickland on 6 Nov 2009 12:51
"Tegger" <invalid(a)invalid.inv> wrote in message news:Xns9CBB488E24A98tegger(a)208.90.168.18... > "Jeff Strickland" <crwlrjeff(a)yahoo.com> wrote in > news:hd05hr$6ok$1(a)news.eternal-september.org: > >> >> "Tegger" <invalid(a)invalid.inv> wrote in message >> news:Xns9CBAE381511EEtegger(a)208.90.168.18... >>> "Jeff Strickland" <crwlrjeff(a)yahoo.com> wrote in >>> news:hd044q$sph$1(a)news.eternal-september.org: >>> >>>> >>>> "badgolferman" <REMOVETHISbadgolferman(a)gmail.com> wrote in message >>>> news:xn0ghaisbeui4k004(a)reader.albasani.net... >>>>> Jeff Strickland wrote: >>>>> >>>>>>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.) >>>>> >>>>> I don't want anything changed on _my_ car. I was just wondering if >>>>> there is a difference between tires with thin sidewalls and those >>>>> with very wide sidewalls when it comes to odometer/speedometer >>>>> readings. And I'm not talking about one inch differences, more like >>>>> 45s vs. 85s. >>>>> >>>>> After you guys work out all this technical minutia let me know what >>>>> the consensus is. >>>>> >>>>> >>>> >>>> >>>> The short answer is, no, there is no difference. >>>> >>>> The long asnwer is that the overall diameter has to be the same for >>>> the short answer to be true. For example, my truck has a 265/75x16. >>>> I could go to a 265/35x20 and the tire should be the same overall >>>> diameter, therefore no affect on the speedo. >>>> >>>> >>> >>> >>> Supposing the two sizes have identical unloaded diameters, the 35 >>> will have >>> the larger rolling circumference and the slower speedometer reading. >>> >> >> >> NOT true. Well, not entirely true. >> >> For every inch increase in the rim, the aspect ratio is decreased 5%. >> The result is a overall diameter, or radius, or circumference >> (depending on the specy you prefer) will be virtually identical. The >> difference -- if done on all four corners -- is insignificant. There >> will be a small change to the speedometer -- a change measured in >> single digit percentages. > > > > As I said all along. The point is that it is unlikely that there will be > NO change. > > Another example for illustration: > > Assume a tire with 23.21" diameter (my 195/60-14). The sidewall of that > tire measures 4.6". The front tires drop about .875" at the contact > patch. That's about 19%. > > Now assume a tire of the same diameter, but a hypothetical size of > 195/35-18. This gives the sidewall a height of 2.7". Now imagine that > this same tire has a drop of .875" at the contact patch. That's > /one-third/ of the sidewall height of only 2.7"! > > NO 35-series tire drops 7/8" at the contact patch. It cannot. Any owner > of that tire would be convinced his tire was flat! And this means the > effective rolling circumference of such a tire would be /larger/ than > the original 195/60-14, giving a slower speedometer reading.. > > > > >> >> The speedometer would be more sensitive to the circumference, but >> since circumference is calculated by diameter X pi, or radius X 2 X >> pi, then all are the same. >> >> I assume proper inflation, and ignore any argument that assumes >> improper inflation. Improper inflation causes a variance, and the >> variance is measured by the speed sensors, not the speedometer. >> >> > > > If the wheel is turning at a speed other than what the speedometer is > expecting, the speedometer will reflect that. Period. It may not be > enough to notice in everyday driving, but it will be reflected. > > ABS-based speed sensors only track DIFFERENCES between wheels, not > absolute speeds. And if a lower pressure makes a difference to the ABS, > it will also make a difference to the speedometer, provided the lower > pressure tire is on the wheel that's on the "solid" side of the diff. > > > I don't see that it matters if all tires are the same equivelent of the original specification, or that they are all the same non-equivelent -- physical fitment issues aside. ABS does not look at different speeds of the tires, it looks at some tires turning and some tires not turning, it makes the not-turning tires turn. The speedo would not care about one tire turning at a different speed, ever. But none of this is in the realm of answering the OP's question. His question was not framed very well, but that's another discussion all by itself. In the example of the 195/60x14 and the 195/35x18, the sidewall goes from 4.6 to 2.7 inches, but the tire diameter goes from 23.2 to 23.4, and the tire circumference goes from 72.9 to 73.4, and the revolutions per mile go from 869 to 863. It's possible that in such a small sidewall that the pressure monitor might not work very well, or at all, if it's based on feed from the speed sensors. This might be an undesirable affect. But the speedo would be unafftected for all practical purposes. Since the new tire is miniscually larger, then the speedo would slow a tiny amount -- a current reading of 70 might be an actual speed of 66 and the new reading of 70 might be an actual speed of 68, which is a desirable change as opposed to an undesirable one. |