Tire Pressure for a layman

Its also indisputable that higher pressure degrades the ride, which could create more rattles and take its toll on the suspension bits.

 

Rain traction is my first reason for using pressure above the door placard. With rain nearly 180 days per year, water ponds in the tire ruts (make worse by studded tires and overweight trucks), and it isn't possible to skirt all of them. Wet traction becomes much more important than dry traction.

I don't over inflate tires, just push them close to full inflation.

 

If you care so much about rain traction, you go buy a set of new tires that has better tread design to begin with. Going 10psi higher than recommended is not overflating? What is?

 

The poll attached to the thread linked below indicates that most of us do indeed exceed the pressures recommended by Toyota. But I do not think of that as "over inflation" or anything close to it. To over inflate a tire you would need to exceed the pressure for maximum load printed on the sidewall (either 44 or 51 psi for our cars) and most of us are not doing that.

At any rate Toyota's pressures will provide a nice soft ride, although a bit squishy for my taste. I will sacrifice that for better steering response, and better wet weather performance.

http://priuschat.com/forums/gen-iii-2010-prius-fuel-economy/73682-prius-2010-only-tire-pressure.html

 

Not in any car I have ever owned, and the auto industry never advocated such nonsense.

 

An OVER INFLATED tire can more easily "Blow-Out" from an impact with a bump, pothole, or road debris putting ones life at risk, not to mention one's passengers and vehicles close by. An OVER INFLATED opinion of one's knowledge can lead to such dangers. Which is why seeking out car manufacturers, and tire manufacturers who would bear heavy Legal burdens for advocating Unsafe practices, like Over-Inflating tires, for their expert advice makes far more sense than taking the opinion of some unknown inflated opinion on an internet site.

 

Actually its a well known fact for automotive tires that generally speaking, higher pressures = less hydroplaning. You may not have ever measured it / tested it, but others (including NHTSA I believe) have.

 

I find it very difficult to portray anything under the MINIMUM pressure required for using the full tire load rating as OVERinflating. Is that load rating a piece of fiction?

 

According to Toyota (for most cars even), the pressure needed for maximum traction and the largest footprint is roughly 32-35psi. Unless you carry five Aretha Franklin with you or run at triple digits for a long time, 40psi is purely excessive. You lose grip, you decrease the feel of steering, you increase the chance of a blowout.

 

I hope we can agree to disagree - over max sidewall recommendation I would agree is excessive, but 40 is hardly that - for the 15" Yokohama's, they are 4 under the max sidewall inflation still.

The losing grip part (at 40 psi at least) I think is debatable, so I don't want to get into debating that. For cornering at least, you may actually see an increase in grip. Same goes for hard braking when all weight shifts forward. If you don't agree with me however, that is fine. I just have noticed similar effects esp. with my previous Honda that I ran higher psi in.

 

BINGO!!!! WE HAVE A WINNER!

This is why my tire practices are not based upon opinion postings on auto enthusiast web sites.

And thanks for the Goodyear descriptions about Over- and Underinflation. It confirms that my tires on previous cars used to be Underinflated, and have never been Overinflated. Now if you can post a link so that we can read it directly ...

 

It's not my car, you can run whatever you feel comfortable. At the end of the day, the recommendations are what they are and it seems to make a whole lot of sense to stick to it or at least be close to it. Your chances of blowouts increase and your grip decrease as your go higher on the pressure, those are all facts. Is it worth it for the extra 10 miles you might get? You make the call.

I forgot to mention tires can heat up 3-8psi in the summer, your 40psi can easily become 45-50psi on a hot day.
I have autox the Prius in stock form (actually won too), I can tell you the car doesnt handle better at 40psi than at 37psi. Sure you might get better steering response, but grip decreases as you go up in tire pressure.

http://www.popularmechanics.com/cars/news/4199963
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=1

 

Already factored in. All posted pressures, placard and sidewall, are cold pressures. They knew full well that tires will heat up in use.

If 40 psi cold is a hazard, Toyota needs yet another recall to check for widespread factory error or dealer negligence. My new Toyota was delivered with 40 psi, cold, and a thread last year indicated that many other new owners found similar pressures.

"To invoke the MythBusters"

No out-and-back at the same pressure. Trips two days apart with no controls over temperature, wind, warmup cycles, and with 17 extra miles attached to one direction under completely unknown conditions. And no accounting for the elevation difference in the two directions, which on my car would cause a quarter-gallon more consumption on the uphill than the downhill leg.

Due to uncontrolled and invalid experimental procedure, their mythbusting is itself Busted!

"If a vehicle's tires are underinflated by only 6 psi ... increasing the probability of irregular treadwear. "
"If a vehicle's tires are overinflated by 6 psi, ... experience irregular tread wear."

Thank you for yet again pointing to wear pattern arguments demonstrating that my old car's placard pressures are too low, and the new chosen pressures are more appropriate. (My Prius is too new to display tread wear.)

My favored independent shop (for the remaining old car) also strongly believes that tire pressure is a compromise among competing factors, and that placard labels overweight comfort and underweight efficiency and performance compared to their tastes. That is why the tires on that car were being set 10% over placard before I even asked.

Drivers with other makes/models (none of my old cars are/were Toyotas) and in different climates may find different results.

 

New vehicles have higher tire pressure when delivered to the dealerships. It is because the automakers want to avoid flat spotting of tires during transportation (especially from Japan vis ships). Not that automakders endorse such high psi.
Upon delivery of vehicles, the dealers should have lowered the tire pressure and top off all fluids. That is the standard procedure. However, occassionally, some guys got lazy ... zzzzzz

 

In a few other similar threads I have suggested that people read http://www.hunter.com/PUB/UNDERCAR/SAEATS13/SAEATS_2.htm,
which gives a technical discussion of suspension parameters, including the impact of tire pressure on the vehicle. To me, what it boils down to is whether the possibility of saving a few 10's of dollars per year in gasoline is worth the risk in damaging the car or injuring someone.

As the paper states: "The maximum pressure for maximum load that is printed on the tire sidewall ... should not be used for ... regular driving. The optimum tire pressure consists of a compromise between adhesion (traction), ride harshness isolation, rolling resistance, and tire life. The tire pressure can also differ for the front and the rear wheels, depending on the vehicle design and load, and should be recommended by the vehicle and tire manufacturers for different loads."

In case there is any doubt as to which factor is more important for safety: "The minimum adhesion relates to the safety of a vehicle at a predefined road input." Then as long as you believe that traction is important, you might want to note that: "the relationship of tire pressure to adhesion is an inverse relationship in the proper range of inflation and load on the tire."

While there is a detailed discussion in the paper, the effects of overinflation on traction boils down to: "A change of one p.s.i. of tire pressure can change the minimum adhesion by 0.5% to 2.2%. From testing over 100 different vehicles, an increase of one p.s.i of tire pressure decreased adhesion by an average of 1.2%." So, if I understand correctly, a 10 psi overinflation would reduce traction by about 11% on average, but maybe as high as 20%. Either seems significantly unsafe to me.

The paper also discusses other consequences of increasing pressure, such as increased wheel hop, as well as other interactions between components.

As for the possible benefits for reducing occasional hydroplaning, reducing speed to match the road conditions is probably far more effective than increased tire pressure on the stock tires. If standing water on the road is a daily/seasonal problem, then why not mount special tires -- not unlike many of us who mount snow tires for the winter?

 

This raises an interesting point. For the huge portion of folks in regional driving cultures expecting speeds 5-15 mph over speed limit (17% to 56% increased stopping distance) and car spacings much shorter than safety guidelines demand (50-70% less stopping room available), an 11% change in (adhesion) braking traction could push them over the edge of safety margin.

At these higher speeds, they likely need a bit lower pressure to maintain adhesion. Unfortunately, the lower pressure also increases tire heating, increasing the risk of tread separation or blowout.

The most effective solution, wet or dry, would be to slow down. But that is so un-American.
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NB - see my new posting below. When I originally wrote 'adhesion', this word was mistakenly being interpreted as equivalent to braking traction. Further review of the terminology of the article in question reveals that author used the word for a much different meaning, disclosed on another page.

 

The recommended tire pressure is far from the "low" tire pressure you are talking about.

 

I think the loss of adhesion would be most serious during a panic stop. That is more likely to occur driving around town or in a neighborhood (where a pedestrian or a child might walk into the road), than on the open highway. So to me the trade off is between possibly not stopping in time to avoid hitting someone and saving maybe $50 per year on gas. I just don't see how that possible savings out weighs the potential risk.

 

Re-reading this SAE article, including the definitions of terms on the Terminology page, I now see that the small piece excerpted to this thread was not being understood correctly.

For this article, 'adhesion' is a minimum vertical contact force during induced vertical oscillation cycles on the author's suspension tester. It varies enormously among tested vehicles with different tires, suspensions, suspension tuning, damper condition and settings, spring constants, loads, unsprung mass, and mechanical resonances. Adhesion becomes zero when suspension hop causes the tire to lose contact with the ground.

In the terminology of this specific paper, 'adhesion' is not an average braking traction. And absent vertical oscillations of the vehicle suspension, it has no relation to braking traction whatsoever.

For this testing, increased tire pressure does increase 'tire spring constant'. So do wider tires with narrow aspect ratios, unless pressure is sufficiently reduced. And so do stiff tires.

In this testing, a new 1992 VW Jetta with known good dampers but stiff tires displayed extremely low adhesion, while 1982 Ford Ranger pickup with its shocks removed performed 'fair', i.e. significantly better.

The only take-away I can interpret from this article is that if one's tire pressure changes cause increased suspension hop, pick a different pressure. Or get rid of tires that are too wide, too low profile, or too stiff. Or change shocks. Or change springs. Or change cars.

 

I keep my tires at 40 PSI front and 38 PSI back...

The Prius steering feels heavy and sluggish with the tires at lower pressures.

I was driving a light, zippy Toyota Celica before I got my Prius. So that might be a factor for my steer-sluggish sensitivity.

But the Prius feels great to drive with higher pressures!