Tire pressure myths, facts, observations, and improvements (canned post)

[ShadowWorks]

I am glad even if I feel everything on the road, I did 41 miles today on only 1/8 of a tank, that is almost double what I normally get.

I have see these tires made by a company called Vredestein they massive grooves 19mm wide, they claim it gives better water run out but I am thinking it helps with rolling resistance as well, I guy recommended them for FE.

[theholycow]

Hmm...that could possibly work. While the tread is still thick, they'll bulge slightly in the sipes without touching the ground; the footprint will be the size of a normal contact patch but the rubber will actually have less contact with the ground. The only downfall would be increased tread squirm.

[ShadowWorks]

This company make some excellent mountain bike tires , they are relativly new to making car tires.

I also remember seeing this no air tire made of plastic ribs and spokes with a rubber footprint, Tweel!

http://solo2.org/forums/thread/124490.aspx






Then I saw this polycarbonate ally wheel! 2 inches thick! it still has less mass then an alloy wheel.

[finfin]

I bumped mine up to around 40 psi and now the center tread on my tires is visibly worn down more than on the edges. The mileage increase was pretty good. I think I'll back off a little bit on the pressure. I would suggest using a tread gauge and check in for uneven wear... or doing the calculations and seeing if the tire cost is lower than the fuel savings. I hope they start publishing the RRC on tires before I buy my next set. Does anyone know of a good list or RRC's for different tires?

[theholycow]

finfin, I asked some questions about your tires in the other thread, but if you want you could put the answers here. They may be more appropriate in this thread.

I assume "RRC" means Rolling Resistance Coefficient? Someone posted a link to a PDF in another thread, but I think a lot of those tires were old / not on the market anymore.

I googled up this interesting post on a Prius forum:
http://priuschat.com/forums/prius-te...rc-4-data.html

One section of that user's analysis of the data supports some of my pet ideas about width and weight:

Per 2005 manufacturer-reported data list:
Integrity P185/65R14 has a Reported RRC/4 of .01278.
Integrity P225/70R16 has an RRC/4 of .00889. This tire wieghs 25 lb (vs 15 for the P185/65R14). I don't get why it has a lower RRC/4.
Comfortred P195/65R15 has .01139.
Best tires were Michelin Symmetry P225/60R16 at .00650 - a 27-lb tire, and
Michelin Tiger Paw AWP P225/60R16 at .00683 - a 25lb tire. On both these model lines, the smaller/lighter/narrower the tire gets, the higher its RRC/4.
Michelin Rugged Trail T/A P285/70R17, at .00709 is a 44lb monster that beats the RE92 by 33%?

[finfin]

Sure thing! P205/75 R15 Goodyear Regatta tires. I see the center wear on the back tires mainly. Not so much on the front from just looking. I'll get a tread gauge and report back later.

[theholycow]

More rolling resistance data:
http://onlinepubs.trb.org/onlinepubs/sr/sr286.pdf

finfin, as I mentioned in the other thread, your tires are huge for your vehicle -- not aesthetically but functionally. Your size tire is rated for far, far more load than they are carrying; I suspect that in the rear they're carrying less than 1/3 of their safe capacity for being at 40psi. I doubt that you'd lose any FE by reducing the pressure in the rear tires a little bit.

[theholycow]

Okay, that brings me to this: Perhaps optimum FE/wear pressure can be determined simply by percentage of load capacity. So, here's an analyzation of a few of my past experiences:

Case 1. 1997 Pontiac Grand Am SE V6. Curb weight somewhere between 3000 and 3500 pounds (according to Google). Aggressive driving, occasional tire rotation. Tire size: 205/55-16. Load capacity for that tire size: ~1,300lbs at 44psi. Pontiac's recommended pressure: 30 or 35psi (memory fails me).

- OEM Goodyear Eagle RS (or were they S, or RSA, or what, I don't remember)
- Pressure: Pontiac's recommendation.
- Result: Edge wear but not terribly so.
- Life: 45,000 mile before sidewall deterioration caused a blowout.

- Bridgestone Turanza T
- Pressure: Pontiac's recommendation.
- Result: Same as OEM tire.
- Life: 70,000 miles before sidewall deterioration, I don't remember if it caused a blowout or if I caught it before it was too late.

- Bridgestone Turanza T
- Pressure: 44psi, sidewall-marked maximum
- Result: Perfect wear, better handling
- Life: 80,000 miles and still fine when I sold the car.

Case 2. 2002 GMC Sierra 4x4. Curb weight: 4,400 pounds (weighed). Less aggressive driving, sometimes very heavy load/rough work -- hauling, towing, off-roading. Tire size: 245/75-16. GMC's recommended pressure: 35psi.

- OEM Goodyear Wrangler ST. Maximum load ~2271 pounds.
- Pressure: 44psi, sidewall-marked maximum
- Result: Slight edge wear.
- Life: 50,000 miles until tread was too worn and winter was coming

- Off-brand, don't remember. Load range E. Maximum load >3000 pounds.
- Pressure: 80psi, sidewall-marked maximum
- Result: Great wear
- Life: 50,000 miles plus whatever miles were on them when I mounted them. They were rotting on a parked van for a few years before they went on my truck.

- Nexen Rodian HT. Load range E. Maximum load >3200 pounds.
- Pressure: 80psi, sidewall-marked maximum
- Result: Wear is currently almost perfect, possibly a little center wear on the rear and a little edge wear on the front, rotation will even it out
- Life: I think they're over 50,000 miles already and still have some life left.

Edit: I forgot to make some conclusions based on the data. It looks like the optimum pressure for treadwear is the pressure where the tire will be at 40% of its safe maximum load (remember, safe load depends on tire size and pressure; see a load-inflation chart to find the right pressure).

[theholycow]

WOW! Here's a couple zingers from that PDF I linked a couple posts back:

Individual tires that start out with different rolling resistance—whether OE or replacement tires—will not retain the same differential over their service lives. Rolling resistance generally diminishes with tire use, and differences among tires will change. The many physical changes that tires undergo as they are used and age will modify rolling resistance over their life span. In particular, the loss of hysteretic tread material due to wear causes rolling resistance to decline. The rolling resistance of a properly inflated tire will typically decline by more than 20 percent overits service life.

So, worn tires are more efficient. That sure makes buying used tires sound more cost-effective, though I'd still rather not have to change them any more often than necessary.

Also, they make it sound like tread squirm is responsible for more of rolling resistance than I thought.

How well tires are maintained has a critical effect on their rolling resistance. Proper tire inflation is especially important in controlling rolling resistance because tires deform more when they are low on air.

...and then immediately after that they support the theory about sidewall deformation.

More interesting stuff:

For typical passenger tires inflated to pressures of 24 to 36 pounds per square inch (psi), each 1-psi drop in inflation pressure will increase rolling resistance by about 1.4 percent. Hence, a drop in pressure from 32 to 24 psi—a sig- nificant degree of underinflation that would not be apparent by casually viewing the shape of the tire—increases a tire’s rolling resistance by more than 10 percent. At pressures below 24 psi, rolling resistance increases even more rapidly with declining inflation pressure.

Tire rolling resistance is dependent on inflation pressure and load.

Edit: More data. Very interesting.

http://www.energy.ca.gov/transportat...ency_Study.pdf
- Besides RR questions, pages 4 and 5 have an interesting analysis of where the energy in gasoline actually goes.
- It has some formulas for RRC that I don't quite understand.

http://ecomodder.com/forum/28668-post35.html

- Larger diameter tires have a lower RRC than smaller ones.
- The lowest RRC tire width is the closest one to the tire diameter times 14 (ignoring units of measure). (At max sidewall pressure and max load)
- The lowest RRC aspect ratio is lower as tire diameter gets larger. It appears as the the lowest RRC Diameter/Aspect ratio are: 13/80, 14/75, 15/70. (At max sidewall pressure and max load).

Blowout risks related to sudden pressure rises in overinflated tires are non existent as radial tires will not unbed under 150 psi.

http://www.motiva.fi/attachment/f16d...ce_on_road.pdf
http://www.energy.ca.gov/transportat...%20Testing.pdf

Edit: The data keeps coming. There's more information out there than I thought.
RR values for a few models: http://www.greenhybrid.com/discuss/f...w-tires-17500/
Interesting info on how rubber compound affects RR and its relationship to treadwear and traction: http://www.geocities.com/barrystiretech/rrandfe.html
- I agree with his bottom line, to choose based on treadwear if your interest is long-term cost savings; but I disagree with his statement about just how much extra inflation to use to reduce RR.

[ShadowWorks]

Guys I have been reading about Nitrogen air filled tires and it seems to me that it is a better gas to use all round than normal air.

http://www.youtube.com/watch?v=iLUI6...-nitrogen.html

Its only down fall is the cost