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Re: Hypermiling and SUV max tire pressure
Originally Posted by shiba3420
(Post 176978)
Sorry, I have a good old book on race cars. I wish I could find some similar quotes on the internet. The short version was talking about lateral G's associated with turns. Because we are pulled to earth at one G, it was assumed that car's couldn't turn at more that 1 lateral G or they would break traction. An absolute rule that couldn't be broken...of coarse it can. Today's modern slicks are almost like glue and easily allow more than one lateral G...and that's withou applying any downforce tricks. The last thing I read was early written in early/mid 90s are refered to a car from late 80's that achived a record 1.4 to 1.6 (don't remember exact number) of sustained lateral G's. And they were careful to demonstrate that there were no wings or vacuum effects creating "additional artificial gravity". It really is fun to read about some misconceptions associated with basic physics, although this is one of those that if I had been asked in high school if was likely to be true....I might have said yes.
Thanks MMooney, I'll definatly go check out the texts on this. MM |
Re: Hypermiling and SUV max tire pressure
"Example: ice skaters, they have excellent traction on little tiny bits of metal."
Apples and oranges here, but.... Ice skaters, like snow skiers, have (desire, want..) virtually ZERO, ZIP, ZILCH, longitudnal traction, but stupendous lateral traction. In both cases the trick is to turn that lateral traction into longitudnal traction when/as needed. In a car, as long as you don't need to change direction, or hold "direction" due to a skid, you could be a bit like the skater, but in reverse, stupendous longitudnal traction.... Ah, compromises... |
Re: Hypermiling and SUV max tire pressure
Originally Posted by MMooney
(Post 176858)
Willard:
Think of the water in the puddle as ink. After rolling through the puddle, the tread pattern that is transferred to the ground is the measure. No, what you should be doing is wetting the tire, driving a short distance and then jacking the tire up to measure the level of contact beginning at the leading edge of the contact "patch". Look at it this way. You could probably over-inflate the tire just barely to, not beyond, the point where all the way across the tread directly underneath then axle you will see full contact. But that WILL NOT tell which part of the tread, center vs outside, is carrying the car's weight, or an inordinant share thereof. On belted radials, the length of the contact patch actually changes very little from inflation pressure. Further it has no bearing on center wear, which will be indicated if the tire is inflated to the point that it is picking up the edges. No need to jack up the car to do this. Just drive through the puddle, continue driving until you leave no tracks, then walk back along the path viewing the contact area. |
Re: Hypermiling and SUV max tire pressure
Shiba3420...
May I first say.....WOW....??!! Obviously you do have an ax to grind. But no, I do not, at least not on this subject. The horrible dangers of so many folks driving FWD or F/AWD, yes, YES. Towing... If what you are towing puts so much extra weight on your vehicle that you need additional tire inflation then you need to revisit the tow hitch weight distribution issue. Extra heavy loads. Think of your tires as being underinflated when you really load up your vehicle, the center of the tire will "buckle up, upward, just as it would due to actual under-inflation with normal loads, loading. High speed driving. This is where the Explorer story comes into play, if you are going to do an extended period of high speed driving you will want to give up a small portion of traction in return for less HEAT from tire flex. Just think of the explorer as being constantly driven TOO FAST for the level of inflation the manufacturer recommended for the tires. I feel people who drive their cars beyond what is legal, are entirely on their own..legally. Absolutely, unequivicably NOT...!! NEVER...!! While not a legal issue in the US (YET..!!) think of all the unthinking owners/drivers that are opposed to DRLs, some disconnect them, many more want to know how. They are needlessly putting other drivers and passengers at risk. IMMHO the same goes for over-inflating tires simply to improve FE, legal until it is discovered that the injury's and deaths resulting from an accident were caused, even partially by over-inflation. |
Re: Hypermiling and SUV max tire pressure
Originally Posted by shiba3420
(Post 176936)
I didn't realize we were speaking specificly about edges at that point. Thank you for explaining.
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Re: Hypermiling and SUV max tire pressure
"While I find the previous curious, I find the following ansurd, why would you suggest both...
1. that Ford, not the tire manufacturers, was at fault with its inappropriately low numbers. 2. and that car manufacturers know more that (than) tire manufacturers. Isn't the first a perfect example that Ford knew less??" No, absolutely not. The court documents indicate that Ford made the too low tire inflation recommedation primarily to reduce the statistical probability of the Explorer for rollover. There is NO indication that they didn't know EVERYTHING about the tire that Firestone knew. Ford simply had their own goals in mind above all else. Did Firestone know that absent the inappropriately low inflation pressure that Ford was proposing more Explorers would be subjets of rollover accidents. Clearly not something Ford would willingly disclose. "Isn't the first a perfect example that Ford knew less..??" It appears that the Ford engineers simply didn't realize (didn't want to know??) that there was a "flip" side of the low tire inflation recommendation. I suspect had they told the Firestone engineers of their plan and why then the Firestone engineers might have gone looking into engineering nooks and crannies that the Ford engineers didn't want to go looking for, or simply didn't bother. I have seen no indication that Ford wasn't given full and complete knowledge surrounding the use, appropriate use of the tire by Firestone. |
Re: Hypermiling and SUV max tire pressure
MMOONEY:
So. I say HELLO, you say GOODBYE... Or: I say TRACTION, you say FRICTION... Look it up. |
Re: Hypermiling and SUV max tire pressure
Originally Posted by wwest
(Post 177017)
MMOONEY:
So. I say HELLO, you say GOODBYE... Or: I say TRACTION, you say FRICTION... Look it up. Dictionary.com Unabridged (v 1.1) - Cite This Source - Share This trac·tion http://cache.lexico.com/dictionary/g...una/thinsp.png // Audio Help /ˈtrækhttp://cache.lexico.com/dictionary/g...una/thinsp.pngʃən/ Pronunciation Key - Show Spelled Pronunciation[trak-shuhhttp://cache.lexico.com/dictionary/g...una/thinsp.pngn] Pronunciation Key - Show IPA Pronunciation –noun 1.the adhesive friction of a body on some surface, as a wheel on a rail or a tire on a road. |
Re: Hypermiling and SUV max tire pressure
wow west your still going over this? if you follow toyo's recommendation of 35 / 33 psi in your 03 prius hahahaha i hope your in for a nasty surprise...
economy tires on a car that weights as much as a mid size sedan, boy the engineers were on crack i'd say! |
Re: Hypermiling and SUV max tire pressure
Here are excerpts from a study by Hunter, the company that makes suspension and tire test equipment. In this study they measured the effect of tire pressure on tire spring rate, and road adhesion over bumps.
. Tire spring rate is measured as the number of pounds required to compress the tire one inch. Under 32 psi the OEM MXM4 has a spring rate of about 1100 lb/in. Each 1 psi of pressure above 32 increases the spring rate about 35 lb/in. Inflating to 50 psi gives the RDX tire a spring rate of 1730 lb/in...a 57% increase. Note below how the 2 performance tires at the top of the graph go from spring rates around 1100 lb/in at 30 psi to 1600+ lb/in at 45 psi. Why is the tire spring rate important? Because the tire spring and the suspension spring act in series. Increasing the tire spring rate transfers it's bump absorbsion to the suspension spring. The suspension assembly compresses faster and the tire contact patch loses more adhesion with the road. You can see this in the next graph. The K2 in parenthesis is the tire spring constant in lb/in. The middle line at K2(1040) lb/in (28 psi) strikes the bump and immediately drops to about 80% adhesion. Then, as the suspension oscillates it recovers and drops further to 65% adhesion. Adhesion between 61 and 100% is considered "excellent" dynamic tire contact. Now look at the spring constant line for K2(1560) lb/in. This represents a tire over-inflated by 15 psi. Adhesion drops to <40%. Adhesion between 21 and 40% is "fair" dynamic tire contact. You can extrapolate that a tire at 50 psi, or K2(1730) lb/in would drop adhesion exponentially to <20%. This is the range of "poor" dynamic tire contact. Additionally, higher pressures create a longer and narrower tire FAR (footprint aspect ratio). The front of the tire footprint is the contact zone and the rear is the slip zone. It is now agreed that a long narrow footprint has a larger slip zone than a wider short footprint. This is why wider tires have better grip and why over-inflation reduces grip and degrades safety. Impressive post...Evem more IMPRESSIVE information. |
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