Double Yellow's Company Driver to Independent Thread

Nope, not doing anything at the front of the engine. I wouldn't be as opposed to dipping into debt for elective preventative maintenance if there weren't so many big repairs still lurking on the horizon:

1.6 million on the original meritor 10d transmission (which I plan to replace with eaton 13 sOD when it goes)

1.6 million on rear end (which I plan on converting to a trailer axle + 23k single axle when it or front drive axle goes)

1.6 milion on (unserviceable) meritor steer axle (which I plan on upgrading to a 13k+ axle when it goes).

 

Its great for short hauls where you go out loaded and return empty, but it isn't a good fit for OTR for the following reasons:

1) Lifting a tractor axle outfitted with trailer tires will increase rolling resistance (assuming the drive axle has drive tires)
2) A lift axle adds weight
3) A tag axle is slightly more efficient than a pusher axle (shorter driveshaft)

But for short hauls, you have the following advantages:

1) Less tire wear
2) Less rotating inertia -- a factor that becomes important the more rapidly & more frequently you accelerate
3) Less tolls

The 10C is the meritor 10speed overdrive? The 10 speed direct isn't bad -- I'd like it if 10th and 9th were just a little closer, but the gap between 8 and 9 really drops my speed uphill...

 

The physics of MPG (part 1)

I've been meaning to post this for a while, but it is one of those seemingly simple topics that has lots of special cases. But with people learning more and more about it from here, or KR, or the facebook 9mpg club, I think its time to go into a little more depth.

This is the formula for power demand (ignore the constants, they are for cars):


Don't let the length of the equation throw you, it simplifies considerably if you stipulate steady speed (no acceleration) and flat ground:

Power (flat ground, no acceleration) = m*v*g*Cr + 0.5*p*Cd*Af*v^3

That's a little better, right? It can break down even more:

Power (flat ground, no acceleration) = rolling power + aerodynamic power

where rolling power = m*v*g*Cr

and aerodynamic power = 0.5*p*Cd*Af*v^3

And here are some generic constants for a semi truck and trailer:

mass: 36287 kg (80,000 lb) -- this should be kg, not metric tonnes.
velocity: 29 m/s (65mph)
g: 9.8 m/s^2
Cr: 0.0086
p: 1.2 kg/m^3
Cd: 0.7
Af: 10.66 m^2 (13'6" x 8'6")


How accurate is that formula?

Most of you have probably seen this graph from the Cummins MPG whitepaper:



This is the EPA formula using the constants I provided:



The only real difference between the two graphs is that Cummins separated tires from other rolling resistances (tires make up ~75% of rolling resistance).


Factors that affect rolling resistance:

m * v * g * Cr

This is a linear equation, which means if you halve any of the terms, the result is cut in half. For instance, if you were to go from 80,000 lb to 40,000 lb, the rolling resistance power demand is halved:



with aerodynamic drag:



Likewise, if you cut your speed in half, you cut the rolling resistance in half... But what about g?

G is a gravitational "constant," but it isn't 100% constant. In the USA it varies from 9.796 to 9.804 -- a ~0.1% difference. There isn't anything you can do about it, but you might as well know that rolling resistance is 0.1% less in Denver than New York City...

Coefficient of rolling resistance? Now this is where modifications come into play.

Tire rolling resistance of a typical truck:
Steers: 110
Drives: 140
Trailer: 100

That averages 118.5 -- (12,000 * 110 + 34,000 * 140 + 34,000 * 100) / 80000

Using the lowest rolling resistance tires currently on the market brings that down to:
Steers: 78
Drives: 89
Trailer: 75

average: 81

Which brings the overall coefficient of rolling resistance from 0.0086 to 0.00656:





So what about a tag axle vs a pusher axle with a lift?


Tag:
Steers: 78 * 11,500 lb
Drive: 89 * 10,000 lb
Tag: 75* 10,000 lb
Trailer: 75* 20,000lb
----
average rolling resistance: 78

Pusher with lifted axle:
Steers: 78 * 11,500
Lift: 75 * 0
Drive: 89 * 20,000
Trailer: 75 * 20,000
----
average rolling resistance: 81

But but but the man on the radio said lifting an axle lowers rolling resistance... He's wrong.

But but but I saw a study of B-trains where they got better mpg lifting an axle... Those studies lifted a trailer axle, in which case it transferred weight among tires with equal rolling resistance -- it made zero difference in rolling resistance. What lifting an axle does do is removes 2 giant flywheels which lowers the rotational inertia making it easier to accelerate:



...but when velocity is constant, acceleration is zero which means that whole term gets wiped out (multiply something by zero and you get zero).

If you are doing shorter hauls or are on roads with a lot of stoplights, this inertial factor can become significant. But if you accelerate once to 60mph and stay there for 5 hours at a time, it is essentially meaningless.


But what about bearings?

Remember that 75% of rolling resistance is tires, so of that initial 0.0086 coefficient of rolling resistance, 0.00215 of it is from bearings. Changing to synthetic fluids, for example, increases bearing efficiency by ~5% at 100F -- changing its contribution to the coefficient of rolling resistance from .00215 to .00204.

Unfortunately, bearings are extremely efficient so that 5% improvement is imperceptible when looking at the big picture:



Keep this in mind when listening to a salesman promise large mpg improvements if you switch to their magic bearings or lube.


... aerodynamics to come later