Hyperinflating tires

[IndyIan]

Originally Posted by PaleMelanesian

About treadwear: Wayne at Cleanmpg has the original tires on his Accord, 100,000 miles later. Completely even treadwear and still 4-5/32 tread depth. He's had them at about 60 psi since new. They are NOT worn down in the middle.

I know it's just one example, and not representative, but there it is.

Even if the tire does wear unevenly, I think most tires are due for replacement long before they wear out. The compound hardens over time and wet weather traction goes way way down. I'd think that tires even 4yrs old have far less traction than new tires, at least this is what I've found.
Hopefully our tire expert can confirm or deny my theory, but I've tossed quite a few tires that are nowhere near the wear bars because they are just unsafe in the rain and I don't mean aquaplaning either.

This is kind of a balancing act as we are accepting slightly lower traction to increase mpg and save the environment and $$ but buying new tires more often costs both...

Its a multidimensional math problem for sure. Traction vs. mpg vs. monatary cost vs Enviro cost...
Ian

[NeilBlanchard]

Greetings,

All you folks that "over" inflate your tires: are you driving primarily at highway speeds, or are you staying below, say 40mph most of the time? Just curious.

The reason I'm asking is that if you are driving primarily at highway speeds, then aero drag swamps any gains due to reduced rolling resistance. And as measured by MetroMPG, there is about a 10% gain to be had to go from normal(ish) pressure up to the sidewall max, and about another 3% by going above this -- but nothing is gained by going more than 6psi above the sidewall max.

As we all know aero drag goes up by the square of the speed, so:

30mph = 900
40mph = 1600 (almost 2X 30mph)
50mph = 2500 (almost 3X 30mph)
55mph = 3025 (3.3X 30mph)
60mph = 3600 (4X 30mph)
65mph = 4225
75mph = 5625 (6.25X 30mph & more than 2X 50mph)

I've got to think that a 10-13% gain is swamped by the drag, even at just 40mph?

[mattW]

But RR is still proportional to velocity, so higher speeds means more RR losses. At 40mph RR is probably half your FE so a 10-13% gain is likely to be a 5-6% mpg increase, still very much worth it.

This thread is proving very inconclusive, we haven't seen why:

• Higher pressure means it is more likely to have tyre failure? we've yet to see that higher pressure increases tyre stress (since it means less deformation), obviously extremely high pressures beyond sidewall. Though we have seen that the stems may be a problem.
• Higher pressure reduces traction? since friction is proportional to the force and the coefficient of friction, not area. I have yet to see why contact patch makes a difference.
• Higher pressures i.e. 10% over sidewall are dangerous? in light of the above two points.

So does anyone have anything concrete about why higher pressures are dangerous or is it all just a gut feeling? I'm welcome to being wrong, I just need to be convinced and I haven't been yet...

[Gregte]

Originally Posted by NeilBlanchard

Greetings,

All you folks that "over" inflate your tires: are you driving primarily at highway speeds, or are you staying below, say 40mph most of the time? Just curious.

The reason I'm asking is that if you are driving primarily at highway speeds, then aero drag swamps any gains due to reduced rolling resistance. And as measured by

For many of us at least half of our miles are city miles, i.e. low speed. Low RR may mean a lot for city FE and certainly does not hurt highway FE

[RH77]

Originally Posted by NeilBlanchard

I've got to think that a 10-13% gain is swamped by the drag, even at just 40mph?

But -- if these pressure weren't in use, that high-speed drag would be even more dramatic. I drive quite a bit in 70 mph zones (that generally translates to an average of 60 mph up and down hills).

Plus, A/C use lately can be "free" with TOI (I had to develop a TLA for Tire Over-Inflation).

RH77

[Duffman]

Originally Posted by mattW

This thread is proving very inconclusive, we haven't seen why:

• Higher pressure means it is more likely to have tyre failure? we've yet to see that higher pressure increases tyre stress (since it means less deformation), obviously extremely high pressures beyond sidewall. Though we have seen that the stems may be a problem.
• Higher pressure reduces traction? since friction is proportional to the force and the coefficient of friction, not area. I have yet to see why contact patch makes a difference.
• Higher pressures i.e. 10% over sidewall are dangerous? in light of the above two points.

Short ugly answer is they do
A) Ever seen a balloon that was overfilled until it burst? Why did it fail?
B) Dont believe this one, do dragsters, F1 or Nascar run wide or narrow tires? I have seen a load, slip angle and traction plot for a racing tire and thier is a point where a tire is overloaded wrt the traction it can produce. Also tire traction is very temperature dependent and it may be hard to believe but a tire can exceed a Cf of 1.0. The basic 100 level physics you took university do not hold well for tires.
C) See A) & B)

[mattW]

A) Obviously there is some point where tyres burst because of pressure but I'm sure its not anywhere near the sidewall rating or even 10% over...

B) The reason I think race cars need wide tyres is so that the heat is spread over a large area and doesn't affect the properties of the rubber, narrow tires at high psi with normal driving shouldn't get even close to temperatures where the heat negatively affects traction. In fact the higher the pressure the lower the temperature would be because of less deformation.

If you can tell me what is actually behind there being less friction I will believe you, I just haven't seen any decent reason besides 'race cars have wide tires'... Why do race cars have wide tires.

My question is why are you recommended to increase the psi of the front tires to reduce understeer? If higher pressures reduce grip then increasing pressure would make it worse...

Here is a quote from a tire info place about overinflation:
"An overinflated tire is stiff and unyielding and the size of its footprint in contact with the road is reduced. If a vehicle's tires are overinflated by 6 psi, they could be damaged more easily when encountering potholes or debris in the road, as well as experience irregular tread wear. Higher inflated tires cannot isolate road irregularities as well causing the vehicle to ride harsher and transmit more noise into its interior. However, higher inflation pressures reduce rolling resistance slightly and typically provide a slight improvement in steering response and cornering stability. This is why participants who use street tires in autocrosses, track events and road races run higher than normal inflation pressures."

Other than the threat of pothole damage (offset by longer following distance and more attention on the road for hypermilers) its seems overinflation could actually be safer than normal levels, albeit more uncomfortable...

[Duffman]

Originally Posted by mattW

A) Obviously there is some point where tyres burst because of pressure but I'm sure its not anywhere near the sidewall rating or even 10% over...

B) The reason I think race cars need wide tyres is so that the heat is spread over a large area and doesn't affect the properties of the rubber, narrow tires at high psi with normal driving shouldn't get even close to temperatures where the heat negatively affects traction. In fact the higher the pressure the lower the temperature would be because of less deformation.

If you can tell me what is actually behind there being less friction I will believe you, I just haven't seen any decent reason besides 'race cars have wide tires'... Why do race cars have wide tires.

A) A case of a balloon is a 1 cycle to failure. Tires would not be failing from one loading but from fatigue from multiple loading cycles. Fatigue strength is reduced from both the amplitude of the cycle but also from higher mean values as well. There is your 10% overinflation.

B) No sorry I cant explain why, tires are more complex then people give them credit for. Back to the wide vs narrow, a dragster runs for 5+ seconds, that is not enough time to modify the properties of the rubber so your counter argument does not hold. Drag racers have been lowering tire pressures for traction (how low they go might blow your mind). Sorry I can not do better than "it just is" but "it just is" has held across North American dragstrips for as long as we have had dragstrips.

edit: Best guess as to why lower pressures yield more traction is the tire can better mold itself to the irregular surface of the pavement.

[mattW]

A) still doesn't make sense to me because at the same rpm you have the same number of cycles with less deformation. If it is the movement of the rubber back and forth that causes it to crack and leak then higher pressures would be better, if it is the opposing compression forces of the road on one side and the air on the other then lower would be better. I have no idea which one is worse for tyre failure so its inconclusive until someone enlightens me...?

b) Dragsters also do a quick burn-out to warm up their tyres, so obviously temperature is important and the tyres can heat up pretty quickly. I think there is warm which is good and hot which is bad. I wouldn't be surprised with significant temperature differences in the tyres across 5 seconds.

I have heard that the reason for the big low psi tyres on dragsters is because the rubber acts as a torsional (rotating) spring, that absorbs some of the initial force when the clutch comes out and releases it when the rpm/torque drops a little bit when the clutch actually engages. I don't think the peak friction of the tyre is increased, its just smoothing out the engine input to keep it close to the peak for longer...

[Duffman]

A) IMO a tire is not going to fail from a defect on the tread patter but from a failure of the structure that contains the pressure from the tire. I dont know if you missed what I said but a higher aplitude of the mean stress reduces the fatigue strength. A 10% higher inflation pressure should correspond to a 10% higher mean stress on the tires casing.

B) Yes they warm the tires and this is accomplished through tire sliping (spinning) but the dragster does not spin the tires for the entire 1320 feet of its timed run so they stop generating a significant level of heat after they stop spinning. If traction was dependent only on temperature then tire width or pressure would not matter as temperature can be totally controlled by the time of the burnout before the run.

Tire pressure carries no torsional load only vertical load. The shock that the tires adsorb is related to the height of the sidewall, again width plays little to no role.