Rear tires on truck wearing much faster than front tires

[Wayne]

Today I stopped at an auto parts store and bought a tire depth gauge. I have observed the rear tires wearing considerably faster than the front tires and I wanted to know how much each tire has worn from new. The truck and tires have 30,000 miles with about 20,000 of that towing. The spare has zero miles and measures a tread depth of 14/32". The fronts measure 13/32" and the rears measure 9/32". If the condemnation tread depth is 3/32", then the rears have worn about 50% of the useful tread, while the fronts still have more than 90% useful tread left. The tires have never been rotated.

I would assume this to be normal wear when pulling an 8,000 lb trailer and carrying the stuff in the truck we do.

My question is, should I rotate or not? If I don't, then I will replace the rears when they get to 3/32" and the fronts should still have 12/32". I've heard people voice their opinion both ways, rotate and don't rotate. What do you think?

 

[dnewton3]

Rotate. You maybe should have done so a bit sooner, but it's still not too late.
I rotate mine every 10k miles.

And while tongue weight has something to contribute, it's really more a function of the adhesion traction element pulling 100% more load in forward thrust. You have a very capable drivetrain which can really generate significant thrust forward. Now you've added a lot of resistance to that forward movement. That makes the tires wear much faster with every passing mile. The front tires are just coasting along for the ride, as are the trailer tires. The rear tires are now moving 8000 lbs of truck plus another 8000 lbs of trailer (nominal weights), so you've doubled the amount of resistance to forward movement. Hence the engine and trans will do what you ask, at the expense of tread-life.

On a flat level road, you can estimate the resistance force as perhaps .15 coefficient of rolling resistance (CoRR). That means the 8000 lbs of truck would generate 1200 lbs of resistant force. But trailer doubles that; it also contributes 1200 lbs of rolling resistance. Now those rear tires are having to overcome 2400 lbs of force; this has to be generated by those same two rear tires to move forward. Now add in wind resistance as you increase speed, and you still have to apply that force through those same two tires.

The greatest risk in not rotating tires (not only front to back, but side to side) is that you can end up with some fairly serious cupping and feathering of tires, because if you leave them in the drive-thrust positions for long, it will affect not only tread depth, but tread profile.

Some folks will rotate trailer tires, and I will do that occasionally, but they don't "drive" any forward thrust forces and they are so close together that the share approximately the same loads. The rear tires will "scrub" a bit more on twin axle rigs, and that affects wear a bit, but nothing like the drive wheels on the tow vehicle!

I have written a fairly complex spreadsheet to help me estimate thrust forces when pulling loads. It's not rocket science; just HS physics in real world applications.

 

[Wayne]

I have written a fairly complex spreadsheet to help me estimate thrust forces when pulling loads. It's not rocket science; just HS physics in real world applications.

I would like to see that spreadsheet.

On a flat level road, you can estimate the resistance force as perhaps .15 coefficient of rolling resistance (CoRR). That means the 8000 lbs of truck would generate 1200 lbs of resistant force. But trailer doubles that; it also contributes 1200 lbs of rolling resistance.

This is a complete surprise to me that the rolling resistance is that high! Does the rolling resistance of the trailer mean that the tractive effort of just the trailer's rolling resistance is 1,200 lbs cruising down the highway?

 

[dnewton3]

Disclaimer ... I made a mistake ... I induced a magnitude error in the calcs in my previous post. CoRR is .015, not .15. Too late in the evening to be doing math by memory in my head apparently. My apologies; I should have caught that immediately.

Tow topics to discuss here; the CoRR and CoF (coefficient of rolling resistance vs. coefficient of friction). The CoF is the adhesion effect whereas the CoRR is the rolling effect; they are different.

The CoRR is the effect of the resistance to movement due to the effect of road surface, tire contact patch, tire tread design, tire air pressure, wheel speed, even temperatures can effect CoRR of the tire compounds, etc. The generally accepted CoRR on concrete can be anywhere from .010 to .015 depending upon those other factors. The CoRR is applied to the "normal" force of the vehicle on the road; the force of load applied perpendicular to the road surface. As shown above, 8000 lbs of truck weight on a flat road will result in approximately 120 lbs of "pull" needed to make it move in a constant fashion. Even cruising at 60 mph, it still takes 120 lbs of forward thrust force to continue that forward movement, or the truck's rolling resistance would cause the truck to start slowing. Truck plus trailer is now 240 lbs given the total GCW of 16k lbs. Note that the speed induced change in CoRR is non-linear.

The CoF is the percentage effect of the available grip of the surface (be it concrete, asphalt, sand, dirt, wood or whatever you have present) when one object touches another object. It's consistent for the given conditions and does not change with speed (at least not appreciably in normal road speeds; we're ignoring racing cars for this conversation).

There are times when the CoRR can either overcome or succumb to the CoF. Consider what happens in summer versus winter. Ice does not provide nearly as much traction as asphalt or concrete. The CoRR changes just a little due to cold tires, but the CoF changes greatly due to ice and snow.


The mistake I made last night was getting the CoRR magnitudes confused with the CoF magnitudes.
CoRR is measured in hundredths whereas the CoF is measured in tenths; a matter of decimal placement.
See these links:
https://www.engineeringtoolbox.com/rolling-friction-resistance-d_1303.html
https://marules.com/wp-content/uploads/Adj-FrictionCoefficients-VariousRoadwaySurfaces.pdf


The reality is that when you pull a trailer such as your condition (doubling the weight of the truck by the trailer weight; 8k to 16k lbs) will essentially double the wear rate on the rear tires because those two tires now have to apply twice the force to move the truck/trailer combo. There are eight tires in contact with the road, but only two of them are providing forward thrust.

 

[Wayne]

Great information - thank you.

I would also be curious how much air resistance there is pulling my trailer at 65 MPH down the highway. My guess is that it's a lot more than the rolling resistance. The combined rolling resistance and air resistance probably add up to a tractive effort of 1,000 pounds or more, but that's a wild guess.

 

[dnewton3]

wind resistance is reasonably negligible below 40mph. However, the force to overcome is the square of the speed; double the speed and you quadruple the wind load! At 65mph and higher, it can become pretty significant.

the Airstream is more "aerodynamic" than the typical brick-shaped RV (like mine), but it still presents a load.

when you get back, Wayne, we can PM and I'll go over the spreadsheet with you