check it out - new equipment

My experience with Lift axles is; it's whilst there's a fuel & tyre saving.
It's a several year payback time.
if you factor in the oportunity cost on the Money, It's really a "lineball' exercise....

 

When I looked into the lift axle thing, I could see a benefit of about $1300 a year at current fuel prices. Assumptions included about 1/3 of total mileage possible with the axle up, and a 1.0 mpg savings for those trips. I'm still considering rigging up something manual, as the hundred or so dollars in hardware and effort to do it would pay back quick. The cost of installing or buying new with a fancy automated chassis to do that automatically, not so much.

 

1mpg doesn't seem plausible to me. Going from 7 to 8mpg is a 14% improvement which is just massive. The physics of lifting an axle just aren't that compelling:


P = m*v* [ ##(1+i) + g*grade + g*CR ] + 0.5*ρ*CD*FA*v3

where:

P: is the power demand in watts
m: is vehicle mass in metric tonnes
v: is vehicle speed (assuming no headwind) in m/s (or mps)
a: is vehicle acceleration in m/s^2
i: is mass factor accounting for the rotational masses
g: is acceleration due to gravity (m/s^2)
grade: is road grade
CR: is rolling resistance coefficient
ρ: is air density
CD: is aerodynamic drag coefficient
FA: is the frontal area in meters^2



When you are on flat ground at a steady speed, that simplifies to:

P = m*v*g*CR + 0.5*ρ*CD*FA*v3



Contrary to Rutherford's teaching, lifting an axle itself does nothing to improve rolling resistance -- it just shifts weight (and rolling resistance) to the other axle.

You go from 5 axles: P = v*g [ m*CR + m*CR + m*CR + m*CR + m*CR ]

to 4: P = v*g [ m*CR + m*CR + m*CR + (2m)*CR ]

but either way it is the same result: P = 5*v*g*m*CR


What a lift axle does is reduce that inertial mass factor in the original equation -- ##(1+i) -- but by no more than 20%. And that inertial mass factor is only in play whenever you have acceleration (the "a").

That can be significant you are running local and hitting lots of stop signs/lights, but if you're pretty much getting up to speed once and then cruising all day on the freeway, you won't see a significant improvement in mpg.


A 2010 SAE study showed a 4.7% mpg gain when lifting 3 out of 5 axles on local trucks when returning empty ( http://papers.sae.org/2010-01-1917/ ). That would suggest a maximum 1.6% gain with 1 lift axle -- again in a local setting where acceleration becomes a factor.



Cliffs: Don't buy a lift axle for mpg. Buy it for tolls, or to save on tires, or to have the coolest ####. But for mpg's you'd be better off throwing money away on turbo3000's and airtabs (wouldn't be as broke).

 

Thank you so much for the CLIFFS! lol

 

I don't know how KR came into the discussion, but so be it. Yeah 1.0 mpg might be on the optimistic side, but until I lift one and see what happens, it's just a guess. My ad-hoc reading on the topic, mostly unscientific results from a few people that have rigged up a manual lift, suggest anywhere from 3/4 to a full mpg benefit by lifting one axle on a standard 5-axle semi. Those could even be misleading, given the fact that you're hauling light in the first place in order to not go over 20k on the single. Or any other number of variables.

The formulae presented are interesting, but also include some notable assumptions. Like rolling on a perfect surface at standard temperature and pressure, for example (I presume this would be the premise of some or all of the constants like RR). The SAE study was interesting. But also specific to a B-train configuration, which has other dynamics in play than just the axle count.

You're also missing the additional friction and tire wear from dragging a 2-axle tandem through a turn. Or for that matter, the additional friction and drag from pulling 4 extra tires across the pavement going in a straight line. Of course that would assume perfect alignment and exactly equal tire pressures, hardly ever found outside a lab.

To me, the real killer (for me) in the value proposition is twofold. 1. My fuel is costing an average of $2.25/gal, including taxes. Most claims monetize the savings at $4/gal. Even when cash prices were up there, that's not what I was paying. If the fuel you're not burning costs less, your cash savings are reduced. 2. Percentage of miles I could utilize a single axle. I haul reefers that weigh about 10k on the trailer tandem empty. So already less available for cargo than someone like RC with a dry van, or someone else with a flatbed. Ideally, my cargo weight would have to be less than 20k and well balanced to leverage the single axle back there. Off the top of my head, that is about 1 out of 4 or 5 loads, plus deadhead. Of course having put that out there, my last two loads in a row were less than 20k and could have been hauled on a single trailer axle LOL.

 

Lineball? Oh , maybe that's an Australian thing. Hahaha.

 

Double Yellow gets the award for most impressive post. Formulas and explanations, too.
I see Watts is part of the formula. If i understand correctly, a thumpin' stereo could improve my mpg?

 

But seriously, if i buy some new equipment, I think I'll take a serious look at disk brakes.

 

Sorry, Lone ranger! But yes It's an Australian term.
Ever watched the Tennis?
If the balls on the line, the Umpires decision can go either way..

--------------------------------------------------------------------------------------------------

Double yellow, There's a couple of Fields missing from your Formula....

HF= Human Factor

ML= Murphy's Law (what can go wrong will go wrong)

What I'm trying to say is that;

Theory & Reality, are ; Two completely different animals..

My real world experience with a Lift axle being raised 50% of the Time. the Payback was 3 Yrs.

If the axle Lift time drops to 25% of the time, conversely to payback time blows out 6Yrs..

Unless you plan on running empty 45-50% of the Time,& keep the Trailer 8-10 yrs.

The reality will overrule the theory....

PS: as Red correctly points out todays Low fuel prices (which won't be around for ever) further upset the calculations..