Optimal Tire Pressure
in [Toyota Prius]
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From: Marc Gerges on 25 Dec 2007 05:28 Bob & Holly Wilson <bwilson4use(a)hotmail.com> wrote: > > Thanks to Google books, I picked up the following graphs from: > > Gyenes, L. and Mitchell, C.G.B., "The Effect of Vehicle-Road Interaction > on Fuel Consumption," Vehicle-Road Interaction, ASTM STP 1225, B.T. > Kulakawski, Ed., American Society for Testing and Materials, > Philadelphia, 1994, pp. 225-239. > > http://hiwaay.net/~bzwilson/prius/pri_tire_010.jpg It actually took me a minute or two to figure out the units - and even then I'm not quite sure. The kN/m^2 are bar. 1 bar=10000 Pa = more or less atmospheric pressure Keep in mind though, that when referring to bars, one usually refers to relative pressure. If my tire has 2 bars, it means it has 2 bars overpressure vs the atmosphere. Therefore I'm somewhat unsure about the graph. And I guess that's why scientists love SI units like Pa ;-) > http://hiwaay.net/~bzwilson/prius/pri_tire_020.jpg No real relevance - I wouldn't expect Prii to come with anything but radial tires. I have a 40 year old classic that came with radials back then :-) > Tire rolling resistance is not a linear function with speed and > inflation pressure and these charts give a clue about tire drag. Keep in mind though that the higher the speed goes, the less important is tire drag relatively to air resistance. Latest at quick country road pace (100 km/h, about 60 mph) I wouldn't care so much about tire drag and more about keeping windows closed and not having any added air resistance. But then, a riced Prius with spoilers and the like is rather seldom. cu .\\arc
From: Bob & Holly Wilson on 25 Dec 2007 15:41 Marc Gerges <marc.gerges(a)gmail.com> wrote: > Bob & Holly Wilson <bwilson4use(a)hotmail.com> wrote: > > > > Thanks to Google books, I picked up the following graphs from: > > > > Gyenes, L. and Mitchell, C.G.B., "The Effect of Vehicle-Road Interaction > > on Fuel Consumption," Vehicle-Road Interaction, ASTM STP 1225, B.T. > > Kulakawski, Ed., American Society for Testing and Materials, > > Philadelphia, 1994, pp. 225-239. > > > > http://hiwaay.net/~bzwilson/prius/pri_tire_010.jpg > > It actually took me a minute or two to figure out the units - and even > then I'm not quite sure. > > The kN/m^2 are bar. 1 bar=10000 Pa = more or less atmospheric pressure > > Keep in mind though, that when referring to bars, one usually refers to > relative pressure. If my tire has 2 bars, it means it has 2 bars > overpressure vs the atmosphere. Therefore I'm somewhat unsure about the > graph. And I guess that's why scientists love SI units like Pa ;-) I found this converter page: http://www.centauro-owners.com/articles/psibar.html So at 1 bar, your running ~14.5 psi relative to the outside air? bar - psi 1 - 14.5 2 - 29 3 - 43.5 4 - 58 > > > http://hiwaay.net/~bzwilson/prius/pri_tire_020.jpg > > No real relevance - I wouldn't expect Prii to come with anything but > radial tires. I have a 40 year old classic that came with radials back > then :-) Use just the radial tire coefficient of drag. > > > Tire rolling resistance is not a linear function with speed and > > inflation pressure and these charts give a clue about tire drag. > > Keep in mind though that the higher the speed goes, the less important > is tire drag relatively to air resistance. Latest at quick country road > pace (100 km/h, about 60 mph) I wouldn't care so much about tire drag > and more about keeping windows closed and not having any added air > resistance. But then, a riced Prius with spoilers and the like is rather > seldom. There are non-linear effects that occur between 65-75 mph that can run my NH11 Prius mileage from a high of 53 MPG at 65 mph down to 39 MPG at 75 mph. Apparently tires also make a contribution along with a control law. This fall off in mileage does not follow the V**2, aerodynamic drag profile. The practical effect is to identify another performance limitation along with the control law that limits MG1 to 6,500 rpm. Larger diameter tires and wheels, say 10% larger, would still be subject to radial tire drag at higher speeds. However, they would shift the MG1 speed management laws up 10%. If this brings 50 mph through 75 mph, the results would be most excellent. Today, I'm using Sumitomo P175/65R14, T4, 919 rev/mi. It may be possible to fit P205/70R14, 831 rev/mi. and cut the vehicle rpm by 10% at any given speed. Thus an indicated 65 mph would be an actual 71 mph. This could defer the MG1 rpm control law but still be subject to the increased tire drag. Bob Wilson
From: Bob & Holly Wilson on 25 Dec 2007 16:07 Elmo P. Shagnasty <elmop(a)nastydesigns.com> wrote: > In article <1i9niau.1d5kfmu3377upN%bwilson4use(a)hotmail.com>, > bwilson4use(a)hotmail.com (Bob & Holly Wilson) wrote: > > > > You know, Mythbusters showed that . . . > > > > Exploding truck tires, can be a hazard; breathing air from a tire > > underwater doesn't work; and driving in reverse on ice is ineffective > > tire traction improver. However, Gyenes and Mitchell have shown: > > > > > http://hiwaay.net/~bzwilson/prius/pri_tire_010.jpg > > > Tire inflation lookup table: > > > kN/m**2 - psi > > > 200 - 29 > > > 225 - 33 > > > 250 - 36 > > > 275 - 40 > > > 300 - 44 > > > 325 - 47 > > > 350 - 51 > > > 375 - 53 > > > > > > http://hiwaay.net/~bzwilson/prius/pri_tire_020.jpg > > > Speed lookup table: > > > km/h - miles per hour > > > 80 - 50 > > > 90 - 56 > > > 100 - 62 > > > 110 - 68 > > > 120 - 75 > > You didn't quote me all the way. I stayed on topic, "Optimal Tire Pressure" and ignored the rest. Perhaps you might start a thread on tailgating. >. . . > So in addition to pumping those tires up to 1000psi to get crisper > steering and handling, in addition to the fuel economy benefits, >. . . Posted on the sidewalls is the maximum pressure. For my Sumitomo T4s that pressure is 51 psi. BTW, there is an essay by the Tire Rack: http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=1 "Disadvantages of Overinflation An overinflated tire is stiff and unyielding and the size of its footprint in contact with the road is reduced. If a vehicle's tires are overinflated by 6 psi, they could be damaged more easily when encountering potholes or debris in the road, as well as experience irregular tread wear. Higher inflated tires cannot isolate road irregularities as well causing the vehicle to ride harsher and transmit more noise into its interior. . . . " This is something folks who live with bad roads should consider. Fortunately, North Alabama is blessed with good roads that seldom have potholes. With good roads and a morning commute that seldom exceeds 55 mph, I run my tires at their maximum pressure rating, 51 psi front and 49 psi rear. ". . . However, higher inflation pressures reduce rolling resistance slightly and typically provide a slight improvement in steering response and cornering stability. This is why participants who use street tires in autocrosses, track events and road races run higher than normal inflation pressures." The reduced rolling resistance pays at the pump but the steering and cornering stability is especially nice. I regularly take turns and curves at +5 to +10 mph over what other vehicles can handle. For example, one of my favorite curves is at: 34 42' 31.00" -86 40' 11.05" (use Google maps) This curve is rated at 45 mph for most cars. But I enter it at 55-60 mph on cruise control and don't blink an eye. Tailgaters often wind up backing off quite quickly because they have to. Bob Wilson
From: Marc Gerges on 26 Dec 2007 12:05
Bob & Holly Wilson <bwilson4use(a)hotmail.com> wrote: >> >> Keep in mind though, that when referring to bars, one usually refers to >> relative pressure. If my tire has 2 bars, it means it has 2 bars >> overpressure vs the atmosphere. Therefore I'm somewhat unsure about the >> graph. And I guess that's why scientists love SI units like Pa ;-) > > I found this converter page: > http://www.centauro-owners.com/articles/psibar.html > > So at 1 bar, your running ~14.5 psi relative to the outside air? > bar - psi > 1 - 14.5 > 2 - 29 > 3 - 43.5 > 4 - 58 Yep, seems like it. > >> >> > http://hiwaay.net/~bzwilson/prius/pri_tire_020.jpg >> >> No real relevance - I wouldn't expect Prii to come with anything but >> radial tires. I have a 40 year old classic that came with radials back >> then :-) > > Use just the radial tire coefficient of drag. Obviously there's a bunch of development going in them, and I'd expect tires built for low resistance to score differently to tires built with other characteristics in focus. The tires my car came with are V rated, that's 240 km/h, around 150 mph. I presume that's not because they'd expect me to drive that fast, but because they wanted the stiffest possible tire construction for minimizing rolling resistance. cu .\\arc |