Tires-Why Are Rolling Circumference And Loaded Radius Not The Same?

npalen · Nov 2, 2016

In other words, why isn't dividing rolling circumference by 6.28 (two times pi) equal to loaded radius? I guess what I'm asking is the exact definition of loaded radius and rolling circumference.

Edit: An example http://www.titanstore.com/info/412373

Rolling circumference shows 118" so dividing that by 6.28 equals 18.8 but the specs show 17.7 for the rolling radius.

Edit: Should read "loaded radius" previous sentence.

dickfoster · Nov 2, 2016

npalen said:
In other words, why isn't dividing rolling circumference by 6.28 (two times pi) equal to loaded radius? I guess what I'm asking is the exact definition of loaded radius and rolling circumference.

Edit: An example http://www.titanstore.com/info/412373

Rolling circumference shows 118" so dividing that by 6.28 equals 18.8 but the specs show 17.7 for the rolling radius.

The tire is flat on the bottom when loaded, thus the loaded rolling radius while the circumference is just the circumference of the tire unloaded.

bcp · Nov 2, 2016

I've read that the RC measurement method is different for ag tires than for highway tires. Maybe it allows for tire lug penetration into the ground when working.

Bruce

dickfoster · Nov 2, 2016

Loaded radius depends on variables like load and tire pressure, Just go measure it. Use a tape measure, yard stick or even a string with knots in it to measure from the ground to the center of hub.

npalen · Nov 2, 2016

I always assumed that the loaded radius is the distance from the center of the wheel/tire to a hard surface when maximum rated weight is applied at the recommended tire PSI. I'm guessing that most would agree with that.
So why wouldn't rolling circumference be calculated using that same loaded radius?

Here's some info that would lead me to believe that the rolling circumference SHOULD be determined by the loaded radius x 2 x pi. So why is that not the case in the Titan spec in post #1?

zzvyb6 · Nov 2, 2016

Tires 'slip' when they roll. Slip is affected by pressure, load, camber and steer angle. Sometimes its excessive and will show up on a concrete surface as rubber tread marks.

All tires slp, thats how they react to a round tire running on a flat road. Even steel train wheels have this characteristic. Not much, but that's also one reason why they can't stop quickly (besides the coefficient of friction for steel on steel). And your pi is not accurate enough for the rocket scientists.

npalen · Nov 2, 2016

That's interesting! I ran across the info below on tirerack.com which seems to coincide with your comments: It sounds like "rolling circumference" would factor into tire diameter matchup for FWD tractors.

Pi - Wikipedia (To satisfy the rocket scientists:laughing

Revolutions Per Mile
Revolutions per mile indicates the number of times a tire revolves while it covers the distance of one mile. Depending on the tire manufacturer, revolutions per mile may be either measured in a laboratory or derived from calculations based on their previous test experience.

Tire revolutions per mile cannot be calculated by simple math because the tire tread and sidewall bend and stretch (deflect) when the load of the vehicle presses the tire against the road.

Since the resulting loaded or rolling radius is less than half the tire's published overall diameter (which would only reflect the tire's unloaded radius), calculating the tire's absolute rolling circumference isn't possible.

Additionally, a tire transitions from an unloaded to loaded state as it rolls, continuously flattening where the tread footprint comes into contact with the road. These continuous transitions result in some tread slippage, again increasing the tire revolutions per mile beyond what simple math would indicate.

Texasmark · Nov 3, 2016

zzvyb6 said:
Tires 'slip' when they roll. Slip is affected by pressure, load, camber and steer angle. Sometimes its excessive and will show up on a concrete surface as rubber tread marks.

All tires slp, thats how they react to a round tire running on a flat road. Even steel train wheels have this characteristic. Not much, but that's also one reason why they can't stop quickly (besides the coefficient of friction for steel on steel). And your pi is not accurate enough for the rocket scientists.

That's the first explanation I have read that makes sense on using something other than the actual circumference of the tire when considering things like matching front to rear tires on 4wd machines. Will give the AG tire sinking factor the #2 spot......course both depend upon soil/contact conditions so there is some tolerancing in the numbers.

Worst case I saw was a coworker with a long bed Ford P/U years ago. Put hot dog tires on his hunting truck and in 4wd on concrete/asphalt, the front end would actually hop in a hard turn......bad mismatch.

zzvyb6 · Nov 3, 2016

Most of the slip (in going forward) occurs at the trailing edge of the contact patch surface. That's where the highest tread heat develops, rolling resistance is highest, and belt fatigue appears. Keeping pressures up helps eliminate problems with fuel economy, tread separation and tire longevity, but reduces stopping ability (smaller patch area) and hurts ride (because the spring rate increases).

I live with the ride penalty but like the better wear and fuel economy of higher pressures in all my vehicles because most of my driving is on highways (expressways). I usually run then 5 psi over door placard. Don't worry, grampa, tires can take a LOT higher pressure than that. Well above the sidewall rating. In fact, its the wheel and suspension parts that are most likely to fail if you ran above 60 psi in some vehicle's tires. That excludes PU trucks (usually 3/4 or 1 ton trucks on singles instead of duals). They have to run 90 psi or so because of their load rating.

And remember folks, the bottom of the tire doesn't hold the wheel up, it hangs from the top part of the tire. Take an old bicycle wheel and cut the spokes starting at the bottom. It's OK (when stationary) until you start removing the uppermost spokes. Air pressure keeps the suspending tire ring structure intact. When it collapses (buckles), you are gonna get hurt.