confused about tire pressure

All that's been reported so far is that the sensors trigger yet another warning light when the pressure in one tire drops by 25% or more.

 

Has anyone taken the time to run some tire temperature tests using 36/34, 38/36, 40/38, and 42/40? h34r: I am really tired of hearing all this "sage" advice about where to run tire pressue but never really seeing any OBJECTIVE data about the optimum pressure vs. footprint.

Yes, I know that it will vary slightly for each car/driver/load combination but instead of just pumping the tires to some off the wall number I'd really like to see what it means as far as the tire footprint on the road.

(I run mine at 40/38 but the ride is a bit rougher. On the other hand I feel the car isn't quite as squirrely as at 35/33pressures.)

End-Of-Rant

 

Feel free to post the scientific data when you have it!

There's little doubt that softer tires will get heat up more with use because of the friction of the flexing. But I have no idea how MUCH more they heat up. I do have a pyrometer here... just no Prius. Maybe I can give you some actual data when my car shows up. Still haven't figured out how to measure the footprint, or just what that translates into regarding anything we care about.

From my own experience tires last longer, perform better and offer lower rolling resistance when they're harder.

 

I have the Prius but no pyrometer.

I read that the way to measure the footprint is to set the pressure (accurately) and then in a large parking lot drive in a constant circle at a constant speed in the same direction for a given number of "laps". Then stop, hop out and measure the tread temperature at 3 places across the tire. Two points about 1" in from the edges of the tread and then in the center of the tread. Measurements have to be taken quickly because the tires cool down real quickly and a pin pyrometer is best because it gets the internal temp. Once you have a set of readings in one direction you repeat the process in the other direction. This should provide a relative temperature across the tire and from this you can determine if the contact patch is uniform. If the outside is hotter (compared to the inside) then the tire is rolling over. If the center is hotter then the patch is not optimum.

I'll try to find the specific information on how to do the measurements and post the link.

 

I have essentially the same question, except regarding the Nokian WR tires I just bought for winter driving. I spoke with the dealer who sold me them, who said they can handle up to 41lbs inflation pressure, and that would give a harder ride, probably better mileage and no loss of traction.
Also, can anyone recommend a specific brand of wheel cover (hub cap) for the steel rims I bought? I bought one set of 4 that doesn't fit really tight, and have lost 2 in 3 days of relatively gentle driving.
Thanks. Mark

 

I'm an idiot! When I first got my 2006 Prius I checked everything but my tires. I like an idiot that I am figured the dealer is going to have the correct tire pressure. I know, I know, I'm an idiot.
After all the car rides smooth, mpg running around 47-49 in the wet cold weather and I'm an idiot. My son even gives me a electric air compresser for Christmas. Do I use it? No, because I'm an idiot.
Today I check my tires. Why, I don't know, I'm an idiot. 32#, 32 pound psi, thirty-two pounds!
They are now 42/40. I feel like such an idiot.
Kathleen

 

no your not an idiot. This happens all the time, and I even catch myself forgetting to check them. Last time they were down to 37-39 psi. Pumped them to 42-40 and enjoyed the nice stiff ride again. When I picked mine up that was the first thing I noticed, said to self, go to gas station and pump the tires up, first one, yup 28 psi same for all the rest. Wonder what would have happened to someone else's gas mileage, who did as I did and drive 1833Kms over the next 2 1/2 days.

 

Back To 35/33

I ran my 05 Prius TRAC rental at 40/38 for 4 months.

My 06 came with 35/33 and that's where it's staying. Lower tire pressure combined with the 06 suspension improvements are delivering a comfortable and quiet(er) ride while still getting 47-48 MPG, which is the MPG I was getting with the 05 at 40/38 AFTER it was broken in.

 

Everyone seems focused on increasing tire pressure to increase mileage. A secondary thread relates to concerns about wear. After tracking my NSX for several years at texas World Speedway, I found that sometimes modest changes in tire pressure has a surprising effect on cornering AND braking. Increasing the tire pressure to a point that reduces the footprint would certainly have some adverse effects and not just on ride harshness. Is increasing gas mileage worth decreases in safety? Sounds nuts to me. Finally, has anyone done a thorough (systematic) testing of performance at different tire pressures?

 

<div class='quotetop'>QUOTE(mehrenst @ Jan 20 2006, 01:52 AM) [snapback]193867[/snapback]</div>

I've got some results to share. The following graph shows my results:




I have Sumitoro HRH T4s and I'm using an IR themometer bought from Harbor Frieght:

> Date: 9/9
> Cold pressure: 53 psi
> Test tire: passenger side front
>
> Test speed: 65 MPH
> Runup: 6 miles
> Outside -> Inside (min. 116F)
> 00 00 07 03 00
> 02 03 06 03 00
> 00 00 03 02 00
>
> This was 'practice' but it is curious there was no inner bead
> temperature rise.

> Date: 9/10
> Air temp: 88-90F (Prius temperature gage)
> Cold pressure: 47 psi
> Test tire: passenger side front
>
> Outside -> Inside (in driveway)
> 103 98 97 95 94 (was on sunny side of vehicle
>
> Test speed: 65 MPH (52.3 MPG)
> Runup: 12 miles
> Outside -> Inside (min. 109F)
> 05 07 12 09 14
> 05 05 10 03 07
> 01 01 05 00 03
>
> Test speed: 70 MPH (44.5 MPG)
> Runup: 8 miles
> Outside -> Inside (min. 122F)
> 01 02 07 03 09
> 01 01 05 02 09
> 00 01 02 00 04

NOTE: in addition to air drag, the tires appeared to heat up too.

>
> It still looks over inflated so:
> Date: 9/11
> Air temp: 88F
> Cold pressure: 44 psi
> Test speed: 62 MPH (49.9 MPG)
> Runup: 25 miles
> Outside -> Inside (min. 115F)
> 04 04 07 03 10
> 00 00 03 01 06
> 00 00 03 00 03

Although I'm only testing one tire, the inner bead temperature is making me wonder about the toe-in and camber. My understanding is camber can lead to one tire heating and 'toe-out' will cause both tires to heat on the inner bead. Time to switch measurement tire.

The real challenge will be convencing the shop that my data indicates there is a problem.

Bob Wilson