15 more PSI = 7% better MPG

[qx4dude]

Originally Posted by Frank Lee

I will laugh just as hard at any litigation as I do at freaking out over 4 more psi. It just ain't gonna happen.

Frank, you make good points, and I agree with you. You are a voice of reason. I'm not "freaking out" over 4 PSI. My door sticker says 36, the max sidewall (on my tires) is listed at 51. I'm at 42. For me, that allows for variance in temperatures, load, and road conditions. What others choose to do is entirely their own business. I also think it's important that people do their own research (and use common sense) before running their tires at pressures beyond manufacturer's maximum specifications.

[CapriRacer]

2 thoughts:

1) While there is some data on the affect higher inflation pressures has on fuel economy, there is little data on the other affects - traction, wear, ride, failures, etc. The only thing we have is anecdotal data.

A caveat - No data is no data. Lack of data is neither a positive nor a negative. It is, however, a reason to be cautious.

2) Does anyone know where the maximum pressures listed on the sidewall of tires comes from?

No, testing is NOT the correct answer. Please note that there is a pattern to the pressures and part of the question I am asking has to do with why there is a pattern.

- OR -

Why the static burst pressure is so high compared to the recommended inflation pressures?

And don't say "over-design" or "safety factor". The question is "why design something with apparently so much excess strength?"

-

[qx4dude]

Originally Posted by CapriRacer

2 thoughts:

1) While there is some data on the affect higher inflation pressures has on fuel economy, there is little data on the other affects - traction, wear, ride, failures, etc. The only thing we have is anecdotal data.

A caveat - No data is no data. Lack of data is neither a positive nor a negative. It is, however, a reason to be cautious.

2) Does anyone know where the maximum pressures listed on the sidewall of tires comes from?

No, testing is NOT the correct answer. Please note that there is a pattern to the pressures and part of the question I am asking has to do with why there is a pattern.

- OR -

Why the static burst pressure is so high compared to the recommended inflation pressures?

And don't say "over-design" or "safety factor". The question is "why design something with apparently so much excess strength?"

-

Also,

3) Assuming higher PSI = better FE, why do most auto makers choose a door sticker PSI number approximately 30% less of max sidewall pressure. If tires are designed to handle 200 PSI, wouldn't raising the pressure be the first place they go to help their cars achieve CAFE standards?

And don't say "it's a conspiracy".

[mcrews]

Originally Posted by qx4dude

Hey, it's a free country. If you want to inflate your tires to 200, or modify your car to run with bicycle rims & tires - fill your boots. I'm sure someone on the Internet says it's perfectly safe to do so.

REALLY!!!!!
bicycle tires.....
that's your next argument!!???
again, the 'hypothetical'.....
no....bs

[cbaber]

Originally Posted by qx4dude

Also,

3) Assuming higher PSI = better FE, why do most auto makers choose a door sticker PSI number approximately 30% less of max sidewall pressure. If tires are designed to handle 200 PSI, wouldn't raising the pressure be the first place they go to help their cars achieve CAFE standards?

And don't say "it's a conspiracy".

It's all about comfort. Even at 45 PSI, the ride becomes more rigid. You feel more bumps. Low pressures absorb some of the bumps so less is transferred through the suspension. Consumers would complain about the rough feeling ride, they would not sell cars.

[mcrews]

cbaber beat me to the obvious!

Originally Posted by qx4dude

Also,

3) Assuming higher PSI = better FE, why do most auto makers choose a door sticker PSI number approximately 30% less of max sidewall pressure. If tires are designed to handle 200 PSI, wouldn't raising the pressure be the first place they go to help their cars achieve CAFE standards?

And don't say "it's a conspiracy".

that's an abstract and 'simplton' question....you assume only one static situation.

getting better FE is not the function of the tire. Infact it is probably down the list.
the tire is designed primarily to make contact with the road, work as part of the suspension system (softening the ride) and have a reasonable wear rate.

adding more psi to increase FE will decrease the softness of the ride.

So from a human behavior point of view, if the car 'feels/ride' harsh ALL the time....it's hard to remember it's getting me 2 more mpgs. How many drivers really 'know' that 40psi is going to get better mpg? I just think the car rides like shxx.
But if I am highly concerned about mpg, I can live with the self induced pain of a harsher ride.

Pretty obvious really.

[qx4dude]

Originally Posted by cbaber

It's all about comfort. Even at 45 PSI, the ride becomes more rigid. You feel more bumps. Low pressures absorb some of the bumps so less is transferred through the suspension. Consumers would complain about the rough feeling ride, they would not sell cars.

I get the low PSI = soft ride / high PSI = harsh ride concept. But again, CAFE is what started the race to high MPG that all automakers are doing - so wouldn't it make sense for automakers to raise door sticker pressure another 5-10 PSI while tuning / compensating the suspension for a softer ride.

MPG is typically the #1 factor that new car buyers consider, so again, if running tires at max pressure is "safe", why do most manufactures have door stickers in the 32-36PSI range?

For some comic relief, here's a fun video on the effects of handling when the tire's contact patch is altered. They ran a Mercedes AMG with P125 / 60PSI tires around a track. Again, it's an extreme example, but it does illustrate the effect that changing the contact patch has on handling.

http://www.youtube.com/watch?v=HPh90yNX-mY

Sorry, couldn't find a video with bicycle tires...

[qx4dude]

Originally Posted by mcrews

that's an abstract and 'simplton' question....you assume only one static situation.

Pretty obvious really.

Please help me understand what makes the question "abstract" and "static". Automakers could easily soften the suspension to compensate for higher PSI - while giving their customers what they want - higher MPG.

PS> "simplton". I think you actually misspelled the word "simpleton". Now THAT is funny!

[Frank Lee]

Originally Posted by qx4dude

MPG is typically the #1 factor that new car buyers consider...

Unless we're in the middle of a gas price spike, MPG barely makes the Top Ten List.

[cbaber]

Originally Posted by qx4dude

I get the low PSI = soft ride / high PSI = harsh ride concept. But again, CAFE is what started the race to high MPG that all automakers are doing - so wouldn't it make sense for automakers to raise door sticker pressure another 5-10 PSI while tuning / compensating the suspension for a softer ride.

MPG is typically the #1 factor that new car buyers consider, so again, if running tires at max pressure is "safe", why do most manufactures have door stickers in the 32-36PSI range?

For some comic relief, here's a fun video on the effects of handling when the tire's contact patch is altered. They ran a Mercedes AMG with P125 / 60PSI tires around a track. Again, it's an extreme example, but it does illustrate the effect that changing the contact patch has on handling.

Sorry, couldn't find a video with bicycle tires...

Which is easier, designing a suspension to work better, possibly requiring more expensive parts, or simply keeping the tire pressure low for similar effects? While a lot of consumers are concerned about fuel economy, a bigger factor is price. It's cheaper for companies to keep pressures in the mid range to promote a better ride than it is to develop better suspensions and possibly have to increase the price of the vehicle.

I know your example was not meant to be taken seriously, but increasing the pressure inside the tire is not the same as configuring a vehicle with tires not up to the challenge of guiding the vehicle. Even at 60 PSI, there is still the small bulge where the tire meets the pavement on my tires. That would indicate to me the tire is still making good contact, and the tire is not acting like a bicycle tire that is higher in the center than the edges.

BTW, now that I think about it, guess how many PSI are in my bicycle tires? I keep them at a nice 125 PSI, as recommended. If my tiny bike tires, that are much thinner than car tires, and do not have the type of steel belt construction can handle 125 PSI, with no suspension, then my tires at 60 PSI have no risk of a blowout simply due to the pressure. Are the loads completely different? Yes, but again the load on my tires hasn't changed. If they could handle my 2400 pound car before at 30 PSI, 60 PSI isn't going to blowout the tire.