Wheels - 13 or 14 for better efficiency?

[hypermiler01]

Originally Posted by Frank Lee

Where's the evidence that skinnier is always lower r.r.?

If everything else is equal then it will be. Less mass, less volume of material, and less frontal area.

But a light silica based 14 tire may be a better LRR tire than a standard 13. For example, the Michelin Harmony is available in 13, but the HydroEdge isn't.

(According to Michelin customer support, those are the two best LRR tires that Michelin makes.)

But it comes as a trade-off with less traction.

[Christ]

a 205/40R16 often will have less rolling resistance than a 175/70R13.

The contact patch is 30mm wider, with more frictional area as a result, but you're going from 122.5mm of squishy sidewall on the road side of the tire to 82mm and much stiffer sidewall (Z rated tires).

Traction is not compromised, Rolling resistance is still less. Ride quality on the other hand... well, that's a problem with nearly any 40 series tire.

From what I can tell about rolling resistance from comparison of tires, most of the resistance comes from flex in the rubber compound of the tire... therefore, harder compound tires should flex less. Lower profile tires also flex less, so it stands to reason that even though you have a larger contact patch, you're still saving on RR.

Someone please correct me if I've misconceived something here.

[Frank Lee]

Originally Posted by hypermiler01

If everything else is equal then it will be. Less mass, less volume of material, and less frontal area.

There are too many variables to have a blanket statement that says skinnier is better for r.r.

[Christ]

Originally Posted by hypermiler01

If everything else is equal then it will be. Less mass, less volume of material, and less frontal area.

Some of the variables of a tire actually have to be unequal to create equal circumstances. This kind of throws off the intuitive side of things, because of things like Aspect Ratio (the second number) which determines the sidewall height based on the Width of the tire's tread bearing area.

So would "all things equal" mean a 175/60 tire versus a 155/60 tire?

Tire a has 105mm sidewall, while tire B only has 93mm, but the aspect ratio is equal.

So if you make the sidewall height equal, the aspect ratio compared to the width of tread bearing surface is then unequal. Add into this that each aspect ratio and width comparison has different flex characteristics, and it quickly becomes obvious that "all things equal" does not exist for tires.

I would have to agree with Frank Lee here.

[hypermiler01]

Originally Posted by Frank Lee

There are too many variables to have a blanket statement that says skinnier is better for r.r.

Yes, there are a lot of other variables, and IF ALL THOSE OTHER VARIABLES (same rim diameter, the same sidewall height, the same rubber compound, the same construction, etc.) ARE THE SAME, then yes, skinnier is better 100% of the time.

Why do you think racing bicycles have tires less than an inch wide with tread less than an eighth inch thick?
Can you accept that less frontal area is better for aero?

Less friction ALWAYS loses less energy than more friction.
Less air resistance ALWAYS loses less energy than more air resistance.
Less mass ALWAYS has less rotational inertia at the same radius.
And narrower tires ALWAYS have those three effects.

The amount of flexing is not the most important thing, so you are wrong when you imply that less flexing is always better.

What is important is how much energy is absorbed into the tire as heat caused by the flexing. Generally, tires with thinner cross-sections and more supple compounds will have less energy loss when they deform. That is why most LRR tires are using silica compounds now, and why tires with thinner tread cross-sections roll easier, even though they are more flexible.

The old balloon tires with thin tread and cord reinforcement had much less rolling resistance than a modern steel-belted radial, even though the steel radial has much less deformation.

And by thin, I am not referring to the width, but the measurement from the inside of the rubber to the outside.

[Frank Lee]

"and IF ALL THOSE OTHER VARIABLES (same rim diameter, the same sidewall height, the same rubber compound, the same construction, etc.) ARE THE SAME, then yes, skinnier is better 100% of the time."

I found on a Michelin site once upon a time, where they said the optimum contact patch geometry for low r.r. is a "square" patch. Go too skinny and the patch is no longer square. Go too skinny on bicycle tires and r.r. goes UP. Ever notice the special tires made for super high mileage competition vehicles? They are actually WIDER than commonly available bicycle tires. Hmmmm.

"Can you accept that less frontal area is better for aero?"

Maybe, maybe not. Since most of the time the vehicle is operating in a yawed condition...

[hypermiler01]

Bicycle tire width is also influenced by traction requirements. Road racing tires need a larger contact patch than track racing bicycles to account for sand and other obstacles on the road, as well as more grip for fast cornering speeds.

Originally Posted by Frank Lee

Since most of the time the vehicle is operating in a yawed condition...

Not on the highway it isn't.

[Frank Lee]

Originally Posted by hypermiler01

Not on the highway it isn't.

Oh yes it is.