35s? is the hit to mpg from the extra weight or the extra width?

[AcesandEights]

From the article you linked:

"If other vehicular factors remain constant..."

you changed one, right

"...increasing a tire's overall circumference is the only way to enlarge the contact patch."

Are you now saying that the article was wrong and changing wheel size impacts contact patch area too?

No, I'm not saying anything about the article, except what I wrote previously was, that all other things being equal, and you changed the variables. If I say 2+2=4 and you say ok, but I used a calculator and input 2+3 and it didn't equal 4, that's ok with me. We can both be right, or wrong, I don't care.

Sponsored

 

[roaniecowpony]

All of this contact patch stuff only matters on a flat concrete slab. As soon as you put the vehicle on a soft surface like loose dirt, sand, mud, etc., wider and taller is better.

Which is better: wider or taller? I suppose that depends on how wide you start with and how wide you go to and how tall you start with and how tall you go. But, I recall tests, conducted by Hondo's research company a few decades or more ago, on motorcycle tire sizes. In the sand tests, the best improvement in front steering was with taller tires. It had to do with the approach angle of the tire as it meets the surface. A small diameter will plow more rather than roll and steer.

 

[STW]

since we're talking about traction, my understanding is there are a couple kinds of traction relevant to offroading (and probably to road driving too)...

1. friction coefficient. This one is about how much weight is concentrated on the spot you want to have friction, and this is the reason sometimes a narrower tire has more grip--sometimes in snow and ice--because more weight is concrentrated on a smaller area increasing the coeffient of friction.

This is one reason snow tires designed for snow/ice only are often narrower than you'd want for a performance summer road tire.

In absolute terms this is why a hockey skate blade can push off on such a slippery surface. Coefficient of friction is maximized so all the skaters weight is on one line of contact--the blade.

People who choose tires for mud or snow are often deciding whether to choose a narrow tire to sink best to the harder surface below and get better traction on it, or choose a wider tires hoping for flotation.

2. individual grippy facets of a tire. Can't remember if this is called mechanical traction? But it's the grip exerted by all of the edges of all of the lugs on the tire, gripping the roughness of the rock surface (or road surface). This represents most all of the tire grip a rock crawler will use, and it's why often the best tires for rock crawling are the widest. A wider tire can have more edges of lugs in contact with more rough spots on the rock and generate more friction--grip at each of those many contact points.

It's part of the reason airing down to low pressure makes such a difference. It allows more of those grippy lug edges to lay on the rock face.

It's why even a narrow snow tire has sipes and many edges so that each can be gripping whatever roughness in the ice is available on a slippery icy surface, while the narrowness of the tire makes as much friction coefficient overall as you can.

Obviously, most of the times we want our tires to balance between these two forms of grip. Some surfaces demand more of one kind than the others. Some trails have multiple conditions where you want a tire that is good and more than one thing. A trail that is all slick rock might need only the second kind.

Sponsored