mythbusters tests tyre / tire pressure - Page 5 - Fuel Economy, Hypermiling, EcoModding News and Forum

euromodder · 05-12-2011, 05:25 AM

Quote:

Originally Posted by hu_man

The high tire pressure will increase the stiffness of the ride increase the chance of the car to push a little more and it will most of all increase the wear on the center of the tire...

Tyre pressure indicated by the car manufacturer are often too low.

If higher inflation spreads out the wear better, higher inflation can reduce tyre wear.
Obviously you need to check and monitor it over time.
At 3.5 bar / 51psi I'm finally seeing even tyre wear across the thread, in addition to reduced fuel consumption.
YMMV.

Aside from reduced rolling resistance, higher inflating leads to reduced tyre deformation, resulting in less heat in the tyres, and less heat means less wear.

Quote:

Watch your tires closely the cost saved at the pump could be less then a new set of tires or a fender.....

That's something that every driver should do, regardless of the pressure in their tyres.

Hypermilers are looking at their tyres far more often than the regular John & Jane Doe.

We'll be fine, thank you

euromodder · 05-12-2011, 05:33 AM

Quote:

Originally Posted by Enginator

Running tire pressure at 45 psi or higher increases danger of loss of control, bouncing and the suspension is NOT tuned for it.

I've only experienced bouncing when going beyond the max. rated tyre pressure - 51 psi (as rated by the tyre manufacturer)

Quote:

It can affect stopping distance.

I don't have the slightest indication that it actually has increased my stopping distances.

Low rolling resistance tyres have reduced my stopping distances over the sportly rubber I used to mount before.
The later pressure increase doesn't appear to have had a further deteriorating effect.

Quote:

Plus the tires will not last as long.

Everything is pointing to my tyres lasting longer @ 51 psi than @ any lower pressure before.

NeilBlanchard · 05-12-2011, 06:43 AM

The thing is, with higher cold pressure, the amount of heat added when you drive is less, and therefore the amount of additional pressure is much lower, than if you start at a lower pressure.

The bottom line is, running at the highest safe pressure (sidewall maximum is safe), means you minimize the rolling resistance, which means you can use less energy. Though you need to learn to ecodrive, so as to take advantage of this.

moorecomp · 05-12-2011, 09:52 AM

Quote:

Originally Posted by jtbo

And with energy you used to be an ass, you could of provide simple answer which seem to be very clear to you, wear to your keyboard would been less, also usage of electricity etc.

So you did not achieve nothing else than being an ignorant person.

jtbo · 05-12-2011, 12:29 PM

What is the sound of one hand clapping?

SlideWRX · 05-12-2011, 02:35 PM

Quote:

Originally Posted by hu_man

I was just trying to get my message out that it is not safe to over inflate the tires.

I have been racing cars for over 35 years now ...

I haven't been racing for 35 years, but I can tell you that my best times at auto-x had the street tires set around 38-40 psi. Considering that is ~10 psi over the settings in the owners manual, I wonder if one can go up another 10 psi and get back to roughly original grip levels. I'd like to see some data on that.

mcguire · 05-12-2011, 04:08 PM

Quote:

Originally Posted by capnbass91

isn't that idea a fact? when over-inflated only the middle of the tire will have contact with the road and that part will wear out faster than the outside; it will wear unevenly.

This is exactly what my F-150's tires do if inflated to the sidewall pressure (45psi, IIRC). It took a couple thousand of miles to become obvious. On the other hand, they are fine at 42psi front/40psi rear and rotated regularly.

A vehicle with a better weight balance or with less of a difference between normal load and max load probably would not show as much of a difference.

fidalgoman · 05-12-2011, 04:18 PM

Forgive the slight rabbit trail but race cars might have between thirty and fifty PSI more or less depending on tires, down force and a world other parameters. It is a rule of thumb that for each pound of tire pressure it is equivalent of fifty pounds of spring rate. That's in tuning the suspension balance and changes the dynamics of the car. It's also why pressure balance is so important.

The difficulty is not so much keeping the pressure even side to side but what about front to back and it's effect on oversteer/understeer. Many factors such as static vehicle balance, spring rates, changing payload, etc determine handling characteristics. Tires slightly low and soft promotes good ride and tends to mush the vehicles handling making it docile to drive. Anybody who's played with tire pressures a bit knows how quickly things can change. Wet vs. dry handling etc. Snow is an area where hard tires make good skis. Wet as well can be an issue if inflation pressures are too high or too low. It's not one way or another and you couldn't possibly expect your local tire store to know how to balance your vehicle under all driving conditions and thus the rule of thumb tire pressures.

All I know is running in the forties makes the ride a little bit harsh, improves fuel economy, and stiffens up the cars responsiveness. On soft surfaces like sand and snow, softer makes the tires conform more and have a lot better traction. More pressure can be good until it is too much then it's very bad if traction is diminished too much. Too hard to facilitate low rolling resistance can be as bad as too skinny and loss of directional control. Baby steps. Each car is different.

bobstad · 05-12-2011, 04:20 PM

I'm still enthralled with the 63 plus mpg recorded one day in my '91 Festiva L five-speed; which if not from being spaced out and entering a bogus figure somehow I've gone over and over again and again without any suggestion of that; seems from the temperature being over a hundred degrees from long before sun-up, and that the station in Lake Stevens, WA where I filled up has tanks below the lake level, thus very cool gasoline.* And/or the freeway station near Toledo, WA I filled up at that afternoon was selling their stuff on the cheap? That is redneck heaven with lots of truckers mostly who buy there; so one has to factor in for paranoiac fantasy, in reasoning about what otherwise is very hard to explain; about twenty mpg over the car's otherwise absolute a few times, best milage.

Oh hey; yeah, I try to keep the tires at 42 pounds front, 38 rear.

This is always something which makes me feel that the car's performance is enhanced in several ways; so that my cornering is appreciably improved for instance. The mileage always perks up a little; as I record every gas station stop with amount delivered, miles from the most recent previous gasoline taken on, date, place and gas mileage: When often having the car sit for months without much driving, so that I'll space out the tire pressure which always sags a little below the 35 psi, which is the minimum I feel competently conveys the car down the road.

There are definitely feelings of worse than slightly degraded gas mileage at say 30 psi, though I've never allowed less pressure, or even below 35 psi for that matter except rarest occasion.

Thirty-five mpg is pretty normal when having to keep up with freeway traffic of 70 plus mph running down to Seattle and back; though one Bellingham to Olympia tour got me over forty-three or so round trip, when there'd been icy enough conditions both the rolling resistance was minimal and my speed around fifty-five. Tearing up the road in comparison to the laggards about me; except right at the last I chickened out passing a big semi until another dainty enough rig zipped by us both to demonstrate I'd nothing to fear from diving into the big leviathan's wake too.

"Mr. Smarty-Pants" ain't so bright after all, eh? I got over that pretty quick; thinking of how long since motoring under such conditions: As global warming and/or periodic weather patterns made for spring like conditions moving from Eureka, CA to B'ham in eight round trips from February to June of '08; when I found the Festiva Motor Sports "racing/lowering" springs and 13" SR-15513 tires instead of the stock 12" ones with KYB damping, made pick-up like loads using a rolled metal futon frame for a roof-rack an entirely reasonable set-up.

That yielded 30 mpg pressing always to maintain 60 mph, with no problems despite aerodynamics of decidedly lumpen style. To the point catching a cat-nap during the month of March at the I-5 rest area south of Olympia, WA I awakened to morning gawkers speculating on my endangerment of their sanctity; as part of the on-going denizens of the rolling zoos of humanity, up to not much good I can manage to imagine?

One time doing a run back north I'd spaced tying down well enough out a 16" wide little flap of 3/8" plywood on the roof which worked a little loose then caught lots of air; that dropped the mileage down to the worst of those journeys, of around 25 mpg. When I noticed that in a rest area in southern Oregon I just about #&@% which was in tune with the whole foray that particular mission, where I'd not had enough space to hardly curl up over the two front seats; as someone used to having the whole back-end behind the front seats, with my Therma-rest under me to be quite comfortable upon while traveling to snooze with.

I'd say as many have before me; that manufacturer's tire psi reflects mostly their desire to make people as happy as possible about their car's ride qualities, fudging a little on what would be the most rational tire pressure to do so.

Yet, if that were really much of an issue; long ago the whole industry would've adopted the type of an oil-based suspension which was used unique in all of automobilia on the first car I ever drove and learned to drive in, the family '64 MG 1100.

These cars had large pods in place of shock absorbers, each filled with oil and connected by a line to the one either in front or behind; so that when a bump in the road was hit by the front end this would send pressurized oil to the pod on the rear for an instant, to better absorb the jolt.

This worked so well the car could be driven at 70 mph over the worst wash-board or pots holes, as if almost on the freeway; but, the engineering if exceptional in concept and design also suffered massively from poor execution, as the thing always leaked oil after the new wore off. So that typically there was a visible sag on one side and nearly no effective suspension on that side either.

I keep wondering when someone will start building and marketing decent after-market conversions applicable to any variety of vehicles; but, I suspect law enforcement of suppressing these cars and that technology.

Which, one wonders about as far as gas mileage economy; if this could also be enhanced by advanced suspension design?

Not too many years ago Formula One race cars were getting exceptionally fast due to computer controlled suspensions; though last I knew of, these were banned to keep the track speeds from getting so excessive the driver's safety was becoming ludicrous were someone to crash; which a person would imagine would also be easily applied to consumer vehicles as well.

That MG's system was so interestingly simple though; shared by the Austin America, whose only difference was a less classy grille and lack of wood dash, done in walnut on the Morris Garages examples. The French Citroen has a similar suspension; with air as the animate medium, rather than more viscous oil.

*I heard of a couple characters obviously disgusted with their livelihood; who'd got caught pumping heated fuel into airplanes at the Spokane airport during cold weather, to take advantage of the differential. Which was supposedly enough to cause significant diminishment of range compared to a normally fueled plane.

Check out Mrs. Hudson, one of the country's four hundred richest people; who got caught setting the pumps to read a little too much in her favor in her chain of gas stations, and had to do community service for a year as a slap on the wrist.

Though her amazing perfidy in another far less prosaic offense is daunting to try to imagine as simply bucolic if benign; where huge storage tanks of a chemical highly corrosive and four times as lethal as cyanide, had been nearly about to burst due to lousy maintenance and inattention over years, when discovered accidentally; that would've rendered a four state area basically inhabitable for decades to come if not longer, centered around Ohio.

Well, maybe four counties; but, certainly sleazy-on worse than Chernobyl levels. We'd guess ended up teaching disadvantaged pre-school children how to cut-out doilies over that?

bobstad · 05-12-2011, 04:41 PM

Quote:

Originally Posted by fidalgoman

..."rule of thumb"...

I used to use this one sometimes; until learning that "rule of thumb" was the size allowed for rods to beat one's wife with. No larger than the circumference of the husband's thumb.

Another one interesting is "buck" for the lowest denominator of paper currency.

In a land made the richest, most powerful nation in the world, based upon genocide of the native peoples combined with one of the most vicious forms of chattel slavery ever allowed in the species' social evolution.

Saw a photo one time of a condor kept in captivity for thirty-seven years; sitting on the same perch indoors someplace at a residence. Imagine the clean-up often enough: Speaking of "chickens" coming home to roost.