On tire pressures, the Yaris, and the Prius

[1stToyota]

Originally Posted by BailOut

I never put to much stock into what other people's liability lawyers tell me.


@Steve: That's great info, and it's interesting to see how the recommended pressure is linear between the two vehicles. One thing that's worth adding is that for hypermilers lowering the rolling resistance is just as much about increasing coasting distance as it is for making powered driving more efficient.

On a different note, the first folks I ever knew of that were jacking their tire pressure up to 60 PSI was a segment of the 1st generation Prius crowd.

I put zero stock in someone thinking that they've outsmarted Goodyear's and Toyota's engineers.

[sbergman27]

Originally Posted by 1stToyota

I put zero stock in someone thinking that they've outsmarted Goodyear's and Toyota's engineers.

The 0.3% fuel economy per 1 psi rule of thumb applies to underinflation. One would expect to see diminishing returns as one increases the pressure above the placard values. Plus, I would expect to see a significantly shallower slope (less return per psi change) for the Prius' LRR tires than for regular tires.

Increasing and decreasing pressure is a trade-off, even between the placard pressure and the sidewall pressure. The tire probably won't fail under normal conditions. But impact resistance of the tire *is* reduced. Shock stresses on the wheel *are* increased. (Think "bending" in the case of steel, and "breaking" in the case of alloy.) Shock stresses on suspension components, such as the strut mounts, *is* increased.

It's not the normal driving that is likely to get you. It's that sheer 3 inch transition you hit in the construction zone, where the not yet repaved road meets the newly laid concrete, that you absolutely were not expecting.

I used to run my Sprint at 44 psi... and did sustain wheel and strut mount damage.

And, of course, stress on nerves and dental fillings is increased.

-Steve

[1stToyota]

Originally Posted by sbergman27

The 0.3% fuel economy per 1 psi rule of thumb applies to underinflation. One would expect to see diminishing returns as one increases the pressure above the placard values. Plus, I would expect to see a significantly shallower slope (less return per psi change) for the Prius' LRR tires than for regular tires.

Increasing and decreasing pressure is a trade-off, even between the placard pressure and the sidewall pressure. The tire probably won't fail under normal conditions. But impact resistance of the tire *is* reduced. Shock stresses on the wheel *are* increased. (Think "bending" in the case of steel, and "breaking" in the case of alloy.) Shock stresses on suspension components, such as the strut mounts, *is* increased.

It's not the normal driving that is likely to get you. It's that shear 4 inch transition you hit in the construction zone, where the not yet repaved road meets the newly laid concrete, that you absolutely were not expecting.

I used to run my Sprint at 44 psi... and did sustain wheel and strut mount damage.

And, of course, stress on nerves and dental fillings is increased.

-Steve

True, and what would probably worry me most is being forced to do a panic stop and getting to skid and crash due to the fact that stopping distance would be increased when tires are overinflated to 60psi, and mine didn't come with abs brakes.

[sbergman27]

Originally Posted by 1stToyota

and mine didn't come with abs brakes.

That brings up another interesting trade-off when it comes time to buy new tires. Rolling resistance, tread life, and traction form a sort of triangle. If you want more of one, you likely have to give up some of one or both of the others. I suppose that adding money might help. But still.

I don't have ABS either. And I just recently had to travel for many miles on a layer of sheer, naked ice on I40 between Moriarty and Albuquerque. (The snow plow removed all the snow, right down to the solid ice layer.) This is in the Rockies, of course, so 4% and 6% grades were included. I didn't realize it at first, and happened to hit the brakes, just lightly, to slow down a bit... and all 4 wheels immediately locked. (Ye Gads!!!) Road conditions stayed that way for miles and miles.

I'd been assuming that my next set of tires would have the lowest RR value available. But I've reconsidered that. And now I'm wondering if it is at all possible to *retrofit* ABS.

-Steve