Fuel Milage Poll

Year 1985
Make Peterbilt
Model 359
Engine 3406B (stock oil pan and rocker covers)
Trans 18918B with Spicer 1750 4 speed aux
Gear Ratio 3.36
Tire Size 11r 24.5
Type of loads Cattle 102X53 Merritt Gold Line Cattle Drive
Normal Cruising Speed What ever the situation calls for
MPG (or GPM, LOL) 5.2MPG

I average above 6 MPG hooked to a timpte 78" side hopper at 80000 most loads. Avg 6.8 pulling a 53ft spread axle flat with hay. Ya I'm a poor boy I will hook to anything that pays money or fuel.
Normal Cruising Speed Above 70

 

13th double od flat ground, light loads and 12th single od with a heavy load. Super 13s have 2 od gears with 11th the direct drive gear.

Since February I have dedicated round trip 1200 mile run from NC to AL with no mtn grades fueling with southern non-winter blend dropping and hooking 53' dry vans lightly loaded weight with 2 of my last 3 runs over 10 mpg. The NASTC fuel discount fuel program has my cost around $3.55/gal that makes my fuel cost to gross revenue percentage between 20% to 23%.

 

DY...... What happens to your math if truck is loaded @80,000 lbs.

I'm trying to do the math now.....lol

 

I figured with the taller rear and only cruising at 57 you would rairly use double over, but i guess on the flats u can be int he bottom of 13th.

 

DOuble yellow,

Your math is missing some huge components. For 1, you mentioned yourself, the reason singles have less resistance then duels is less sidewall flex. By putting the lift down when its not needed, your adding twice as much sidewall flex. Secondly, how are you calculating how much less resistance the tag bearing and tire have then the drive?

If inflated properly, the RR on the drive tire doesn't change with weight. The bearing resistance could, but not more then the add resistance of 4 more side walls flexing.

The difference in RR between a drive and a tag tire is only about 14 RR. The tag is simply not that much less then the drive.

But, when bobtail or empty, the load is almost nothing. You dont need another axle to support the load.


Your math is simply flawed. First off, assuming bearing resistance is the same, that resistance does not half if the weight is halved. That math may be true when the truck is fully loaded, but there is an initial resistance of just turning that baring that would be the same if there was no weight on the tire. I dont even think that formula is true for a fully loaded truck. It seems to be missing some values. I agree that using a 6x2 vs a 6x4 makes sense to to lower rr tires on part of it.

What i dotn agree with is that the bearing and RR would halve with half the weight removed. What your basically saying is that a bobtail has only 1/4 of the rolling resistance of a fully loaded truck.

If your math were true, why would 4 axle heavy haul trucks lift 1 axle? Why would empty cement mixers lift one, or even the rear axles? Also, tire wear does not double with twice the weight. Running bobtail all the time would not give you 1.3 million miles on a set of drives insted of 300k. So, by having more rubber on the ground you replace your lift/tag tires more often.

If drive bearing, AND RR of the tire truly halved when you took alf the weight off, i would buy your premiss. BUT, that just not true. There is initial resistance regardless of weight. if you just draged a dead axle without putting any weight on it, there would be resistance. This resistance is still there on the drive if you halve the weight. The advantage of running part of you weight on a better rr tire is taken out by the fact that more sidewalls are on the road and the fact you have to spin up more bearings. Think about how hard it is to spin a tire when its jacked off the ground. No weight and its still pretty hard. According to your formula it should be frictionless and spin easily and forever.

 

Full size lift axle with same trailer tires? no difference in rolling resistance between tag & lift when the axle is down... The longer driveshaft of the tag will still have an additional ~ 0.001% or so drivetrain loss -- nothing you would notice on a 1-truck fleet.

Smaller axle with tires rated for 14k lb? tag with 445's would still have less rolling resistance but not as big of a difference as with the axle up. The smaller tires & axle might tare out better than the tag to partially make up for the difference in tire rolling resistance coefficients. Rotational inertia would be lessened. Longer driveshaft still present, & more complexity in getting the proper weight distribution between axles.

 

2007
Pete
387
06' C15 475 CAT
18spd
3.25 rear
295/75 22.5 M270 Bridgestones
Dry Van Weight Varies
Cruise 65/70
avg 7.1mpg.

 

No, you are halving the sidewall flex across twice the tires.

I'm not. It is not in the same order of magnitude as tire RR. It is a bit like asking a sniper if he's calculated the effect of a knat's fart downrange -- it can be done, but has no practical 'bearing' on the outcome.

By simply using Michelin's rolling resistance #'s and plugging them into the physics formula I already presented. I don't need to solve for everything else to prove the Tag is superior to a raised lift by ~3%.

Sir Isaac lie still! The formula is there for you to see, or on wikipedia, or eco-modder, or 100 different bicycling sites. The coefficient doesn't change appreciably -- but the rolling resistance does. F=W*C -- you can't just remove the W. It is the whole reason a loaded truck takes more power to maintain the same speed on flat ground. The wind resistance didn't change....

And as a percentage? 15 (not 14) divided by 89 is 16.9%

No you don't. But you're carrying the weight of the assembly anyway and the tires have less RR than the drives therefore your physical resistance is lessened with them down.

Says who? Kevin Rutherford? He's far from an expert on physics. The formula is there, it is a simple linear equation with no slope-intercept -- instead of y=mx+b it is just y=mx (where b would be a fixed resistance just for adding the other axle)

a) There is weight on the tire -- half the drive tire's load in fact.
b) Your claim is the crux of KR's argument for lift axle. I've presented the physics that clearly contradict his claim. If he were buying someone else's product that made a claim contrary to physics, he would ask for proof. But does he offer any? No. He has some poor couple spend $200,000 on a t660 when much of that was due to this lift axle. No proof of a benefit -- let alone a benefit large enough to offset that cost. But go ahead and be a guineau pig. At least you have been warned. And if you buy it anyway do 10+ coast-down tests before and after see for yourself -- it won't cost you anything extra...

1/4 is a crude estimate, but even still consider that math for 1 second. At 55mph ~1/2 of your power goes to RR and ~1/2 to aerodynamic resistance (source: http://www.bridgestonetrucktires.com/us_eng/real/magazines/ra_v13_i3/ra_popular.asp -- although this does vary depending on how aerodynamic the truck) . Now a bobtail is a bit more aerodynamic than a tractor-trailer (dry van), but it basically has the same frontal area and the shape itself is awful. Lets say it has 80% of the wind resistance. Crude guess...

So (again crudely estimated) at 55mph a bobtail has 1/4 the RR and 4/5 the AR. 1/4 of 50 is 12.5 and 4/5 of 50 is 40. So a bobtail should use 52.5% -- (40+12.5)/100 of the power at 55mph that a loaded truck does.

In real life at 55 I get 12.5 mpg bobtail & 8.5 mpg loaded -- which equates to 47% less power used when bobtail. Not bad for crude guesses -- we are at least in the right ballpark & the error can easily be attributed to our rough assumptions.

Less rotational inertia to spin up to speed means they burn less fuel every time they accelerate. These tend to be local & regional operations where they aren't running on an interstate at the same speed for 8hours at a time. And those that do probably associate better fuel mileage from the rotational inertia with the axle being up so they leave it up. And they save on tire wear.

True, so this now becomes an economics question. At 55mph, a 3% reduction in rolling resistance means 1.5% improvement in fuel economy. So at $70,000/year in fuel, you save $1050. You can buy 2 new michelin Wide singles for that -- and I can tell you tag axle tires last a very long time (they're basically the same as trailer tires). So any increase in tire wear is more than offset by the decrease in fuel.

And this is the crux of our difference. I've explained each of your points as best as I can. I do admit this is counter-intuitive. But now at least you know what to research on your own before you break out the benjamins. If you don't want to "do the hard work" you could probably just send an email to a couple different physics professors -- their contact info is usually publicly posted on university web sites.

Do think about it. It takes quite a bit of work to get it up to speed (fighting rotational inertia). Once up to speed, it takes very very little work to keep it going (the actual bearing resistance) -- especially on an application with drum brakes (another KR pet project that decreases fuel mileage is disk brakes -- but that's another rant).

"Ah ha!" you say. I am admitting that there is *some* bearing resistance to keep it spinning at a constant speed. But of course. There is also *some* weight on the axle from the tire/wheel assembly. You are carrying this weight around with a lift axle -- you are just transfering that tiny bit of steady-state resistance from the lift axle bearing to the drive axle bearing -- and in doing so, you give up all the benefit of the trailer tire's improved rolling resistance.

 

With a fully loaded truck dumping the air from a tag axle, if only for a moment, will severely overload the drive axle and will eventually cause damage. The drive axle is rated at 20,000# and it is already loaded to 17,000#. With a pusher axle, a percentage of the weight from the drives would be transferred to the steering axle.

 

True, but you pretty much never need to dump air to the tag when fully loaded. The traction problems come into play at light loads (& at slow speeds).