42/40 Air pressue unsafe???

<div class='quotetop'>QUOTE(tomdeimos\";p=\"96362)</div>

Tom:

Me too.

My Dunlop Graspic DS-2 winter tires have *way* more sidewall flex than my Michelin Harmony "all season" tires. The Graspic tires also have a deep, soft, and squishy tread.

What I found surprising is that running at Trans Canada posted speeds, the Graspic only appeared to lose 3 MPG over the Harmony, at factory recommended inflation. Also surprising was no real change in handling, except delayed cornering response.

Due to the condition of the roads here, I'll keep my tires at the factory recommended inflation. Running the higher pressure, at the most I only gained around 2-3 MPG and that was not worth the bone-jarring ride quality.

Jay

 

I must have spent way too many years driving 3-cylinder econoboxes, because "bone jarring" doesn't describe how the ride felt on the OEM tires at 42/40.

The TripleTreds at supposed to be at 42/40 (adjustment requested during oil change this past weekend) and the ride still feels fine.

If air was free at gas stations, I'd play with the pressure, but at 50 cents to rush and do 4 tires, I'll skip that. Next time, the place I got my oil change might have their nitrogen equipment installed. If they "top off" tires for free, I might experiment...

 

I'm currently using the hydoedge tires and have for about 8K miles. I've probably developed every rattle that any Prius owner has experienced and some of that is due to my 42/40 tire pressures. I currently have the dash to display rattle that the dealer has tried to fix with that black putty they used to call "pokey" putty in the glass industry. Looks like I'm going to have to try to fix that one.

I have noticed that my experientc is that the car wanders more at the higher pressure and the ride is of course more firm. The one thing that really surprised me is that it's much easier to get the tires to squeal when taking corners faster than normal which implys that it's easier to loose traction with the higher pressures. A controlled repeatable test might confirm this. I have to admit I continually go from 42/40 to 35/33 when I remind myself that the 1-2 mpg advantage (if that) at the higher pressure is just not worth the trade offs (rattles, etc.). So tonight I'll go fill-up and try a more sensible pressure.....maybe I'll go to 37/35 this time!

 

QUOTE "A couple of last notes. A five psi reduction from the manufacturers recommended can change your hydroplaning speed by ten MPH. " END QUOTE

Please explain this as I think that it is misleading if not totally incorrect.

It has always been my understanding that Basic or Dynamic Hydroplaning is experssed as: NASA critical speed = 9 * (SQRT P)

or

9 times the square root of the tire PSI will equal Basic or Dynamic Hydroplaning velocity when water depth equals tire groove depth.

Therefore, a 5 PSI reduction would only lower your basic hydroplaning velovity by 4 kts, not 10 kts as stated at the start of the post.

A 10 kts differential would be 2.5 times off the true hydroplaning velocity of any given tire PSI.

Another formula that is somewhat more accurate, though more difficult to calculate, is: 7.95 * Sqrt(tire pressure * contact patch width / contact patch length). This formula shows that the wider the contact patch is relative to its length, the higher the speed required to support hydroplaning, however, the math relationship remains the same.

 

<div class='quotetop'>QUOTE(Tideland Prius\";p=\"100593)</div>

Yea. Not one gas station I've been to in the South has free air. 50 cents for a few minutes is pretty-much the standard. I do have a 12V air/vac/jumper in the trunk, but it's only really suitable for emergency use after trying it a few times.

I also used to complain about how we pay sales tax on food, and pay property tax on cars, but now that I own a house and realize what I'd be paying in property taxes if those other taxes weren't there, it's not as huge a deal...

 

around western WA, i believe the last free air station went missing about 1994 or so.

 

On the East Coast at least Wa-Wa's has free air!

 

wa-wa's??

i wonder if we cried about it enough, if they would start doing it around here again?

hehe

 

LOL good one Dave!

 

I think in NY (for example), it's considered a mandatory 'service' like restrooms in restaurants, and is required to be free. Pay toilets died a long time back, too.

When I was a kid, we discovered that some state we'd travelled to (Massachusetts, maybe) didn't require restrooms and we had to wait for another place to go.

I guess the 'service' thing is a local concept, but since people want it everywhere, it is provided as long as you pay...

 

Some stations will give you free air if you buy gas from them. The attendant can turn it on for you.