Fuel Milage Poll

By "Tag" you mean the axle is always down with no power to it right? 6x2 with the axle always down? I want to do that but lift the dead axle up if weight allows.

how do you get less rolling resistance from having an extra axle down? the lift axle isnt powered in a 6x2 config so drive train resistance should be the same if its tag or lift. less rubber on the ground when empty means less rolling resistance. I almost never bobtail. the idea is to reduce RR when empty or carrying insulation (6000 pounds). I would never spec a truck to do better bobtail when i almost never bobtail. If traction be came a real issue, i could always lower it, but having it up will put more weight on my drive axle and thus improve traction on the drive axle.

 

Year: 2004
Make: Kenworth
Model: T600
Engine: CAT C-15 MBN factory set was 475 HP, now prev owner tuned up to 500 HP
Trans: Eaton 10 SP
Gear Ratio: 3.70
Tire Size: 285/75R/24.5
Type of loads: Dry Bulk frac sand
Normal Cruising Speed: HWY: 69 mph, Mountains: 45-60 mph
MPG: 5.73 lifetime mpg

Recently valve lash adjustment on inlet, exhaust, and jakes. Was 5.16 mpg prior to adjustments.

 

Tag = trailer axle (non-driven) in the rear-most position. You can install a valve to dump air to it to increase traction on the drives (e.g. when starting from standstill on slippery surfaces/sliding stubborn tandems), but the axle is always down.

Physics. Rolling resistance is determined by:

coefficients of rolling resistance (ground surface, axle, wheel, tire, air-pressure -- often all combined into a single coefficient)
The normal force (on flat ground this is equal to weight on axle)
Speed

Of those factors, tire RR, weight, & speed are the only ones of any real significance. You could make a case for bearing resistance, but microblu remains as unproven as turbo3000, & synthetic oils/greases only show an improvement when the axle is cold.

A tractor lift axle in the up position increases weight on the drive axle -- which has the tires with the worst rolling resistance (& probably the bearings with the worst resistance since they are designed to handle drivetrain torque too). A tag axle halves the weight on the drive tires, placing the other half on a trailer tire (best rolling resistance).

Now a lift axle in the up position does has some advantages over a tag:

1) reduced tolls
2) reduced unsprung weight / better weight to unsprung weight ratio (better ride)
3) reduced moment of inertia (basically each wheel/tire is a flywheel that takes energy to spin up to speed. If you do a lot of stop-n-go driving, this could be a factor. If you tend to maintain a steady speed it is not)
4) reduced weight? unusure, but I suspect this would be the case -- especially if you used 7k-rated tires instead of full wide 10k-rated singles (but if you did this, you would need to add complexity in the leveling system so that the drive axle got 20,000lbs and the lift axle got 14,000 rather than each getting 17,000)

Driveshaft efficiency is inversely proportional to length. A pusher setup has a longer driveshaft than a tag axle. Of the efficiencies I've already mentioned, this one is least material (wouldn't be a factor for a single truck -- but it would make a difference on a fleet of 3,000)

False (Even though counter-intuitive). And you're probably thinking how a tridem pulls worse than a tandem, but a tridem is heavier. You're going to have that axle anyway, up or down, so the weight is still there. You won't be spinning 2 wide singles up to speed -- and this is probably what you would notice -- but that only has an effect during acceleration (& deceleration) and the force at play is rotational inertia, not rolling resistance.


And trust me when I say you will notice the decreased traction when 6x2. Rarely when at speed on the road, but in parking lots and docks and dirt/mud/snow. You will have to get a run at little obstacles you never thought twice about. Personally I think it is still worth, but I can see why fleets like Con-Way are moving back to twin screw.

 

But that's the latest and greatest thing. Keep in mind where he's located. He lives up where them high $ toll roads are. Might be better for him to go that route, but its still going to take a while to break even on it. But since he wants to swap a set of gears it'll recoup faster if he does them both at the same time, least hell only have 1 gear set to buy as opposed to 2. So I suppose there's a cost savings there.

I agree with the tag more than the lift myself, less cost overall when all is said and done to get the same effect. But like Isaid tthat's the new hip thing so everybody's gotta have it.

 

The only way you save on tolls is with cash. Can't use ezpass.

 

Year: 2014
Make: Western Star 4900ex daycab
Engine: 3406 550hp
Trans: 13spd
Ratio: 3.42

Trailer: dry bulk tanker (cement)
Avg length of haul: 75 miles ( uphill both ways lol)
Avg speed: 35-75 mph
Mpg: lifetime average 5.3 mpg

I only run approximately 400 miles a night on average, usually haul three loads of cement or lime, and typically have three hours a night running a PTO driven blower

 

You must run different toll roads than I did, I got charged by the axles that were on the ground. If it's $12.50 for 6 axles and $10 for 5 axles how am I not saving money by flipping the switch and lifting a tag axle.

 

there are scales on every tool that determine how many axles.

I not buying this. My research shows that the RR of a tire doesn't varry that much with weigh. I don't buy that turning an extra bareing AND tires is going to make a big difference on the drive's resistance, but it will significantly add. Bob tail my truck only has 8 k on the drives. 4k or 8 k on one axle makes no difference since it way less then the tire and axle is rates for anyway. It less then half its max. Your also not factoring added tire wear on tag/lift. Also, your tires are super over inflated when empty since most of us leave them inflated to loaded weight. putting less weight per axle would make them even more over inflated. If your theory were true, why wouldn't cars have another axle? Less rubber on the ground = less resistance. Its simple physics.

 

It's your money, spend it however you want.

I'm tempted to post a snarky "have you actually done any research?" but I'll give you the benefit of a doubt. The coefficient does not change with weight. Rolling resistance does.

Source:

http://www.engineeringtoolbox.com/rolling-friction-resistance-d_1303.html

Factors? Weight, Velocity, Air Pressure, & Coefficient of RR

You are thinking of it as if you are dragging around an empty axle that is doing no work. But that axle is supporting the weight of the load -- it is removing work from the other axles. If you don't need the axle -- don't add it at all -- its just extra weight and two big flywheels to spin up to speed. But if you need the axle AND if that axle assembly has less rolling resistance than the alternative use it!

Q) Do they need the extra axle to be legal?
A) No, therefore adding an extra axle will add unnecessary weight, cost, and complexity. In your case, you sometimes do need the extra axle therefore you have to carry around the weight of two additional wheel assemblies, an axle, suspension, etc all the time anyway.

Q) Would the extra axle offer lower rolling resistance than the current axles?
A) No, cars generally use the same tires at all position and the differences in bearing resistance is insignificant. Your truck also has insignificant difference in bearing resistance, but drive tires almost always have more rolling resistance than trailer tires. So the tractor has the benefit of being able to place some weight on a lower rolling resistance option -- which is a benefit not present in cars.

False false false false. Widely believed, very very wrong.

http://www.schwalbetires.com/tech_info/rolling_resistance



Still not convinced? Let's work an example:


Steer Tires: Michelin X Multi Energy Z -- 102
Drive Tires: Michelin XOne Line Energy D -- 89
Tag & Trailer: Michelin XOne Line Energy T -- 74


Now lets take your bobtail example. Say 10K on the steers, 8K on the drives.

Lift axle:

10,000 lb * 102 (Crr tire) * Crr axle * Crr wheel * Crr road surface * Velocity
8,000 lb * 89 (Crr tire) * Crr axle * Crr wheel * Crr road surface * Velocity

For simplicity, we will combine "Crr axle * Crr wheel * Crr road surface * Velocity" into a single coefficient "C." This assumes the bearing friction is the same steer vs drive (it will be very close), the wheel coefficient will be the same (it basically will), the road surface is the same (it is), and the velocity is the same (it is).



So you have a total resistance of:

Steer: 10,000 * 103 * C
Drive: 8,000 * 89 * C
-------------------
Total: 1,742,000 * C


Now with a tag:

Steer: 10,000 * 103 * C
Drive: 4,000 * 89 * C
Tag: 4,000 * 74 * C
--------------------
Total: 1,682,000 * C


The difference? (1,682,000-1,742,000)/1,742,000 = -0.0344


So the tag axle setup has 3.44% less rolling resistance when bobtail.


But what about with a partially loaded trailer?

Steer: 11,500 * 103 * C
Drive: 18,000 * 89 * C
Trailer1: 9,000 * 74 * C
Trailer2: 9,000 * 74 * C
--------------------------
Total: 4,118,500 * C


vs

Steer: 11,500 * 103 * C
Drive: 9,000 * 89 * C
Tag: 9,000 * 74 * C
Trailer1: 9,000 * 74 * C
Trailer2: 9,000 * 74 * C
--------------------------
Total: 3,983,500 * C


Difference?

(3,983,500 - 4,118,500) / 4,118,500 = -0.0328

So a 3.28% reduction in rolling resistance with a tag axle vs a lift axle.



Now if you notice, having a lift axle on the trailer will make NO difference in rolling resistance (because the tire rr is the same). So you would get all of the benefits (less rotational inertia, better sprung to unsprung weight ratio, lower tolls) without the hit to rolling resistance. You still have the extra cost & complexity though.




So there you have the physics and the numbers in black & white. You're free to throw your money away chasing claims that KR admitted today he *thinks are true but hasn't tested*



PS: The astute observer will also realize that lower rolling resistance can be obtained (for free) by running the trailer tandems as far forward as permissible (shifting weight from higher RR tractor tires to lower RR trailer tires).

 

Year 07
Make Freightliner
Model Century
Engine S60
Trans 13 speed single od
Gear Ratio 3.64 (? Not sure)
Tire Size 22.5 yokos 577
Type of loads Dry van (tend to avoid over 20k)
Normal Cruising Speed 61mph (rolling road block lol)
MPG (or GPM, LOL) around 8 (depending on the wind)

I bought into the FleetAir filter, Donaldson high flow muffler and trailer skirts. Which was a waste of money imo. I seen no improvements.