Payload Weight and MPG's

[bobaloo]

I'm 3 months in on designing a camper build, similar in size to an Earthroamer. I plan on using a Ram 5500 crew cab chassis, and will extend the frame by approximately 2ft. Dimensions are not final, but the Earthroamer XV-LTS is the closest comparison so far.

In designing the build materials, as well as tank capacities, components, etc. I have come to a crossroads, that will effect the cost of my build. I'll be staying well within the payload capacities of the truck regardless which components I choose. What type of impact does weight have on the MPG, sway, and overall drivability (on and off road). For instance, everything adds up, so if I spend 2x as much on a lighted skin for the camper, I can cut 50% of the weight for the camper shell. This would be a difference of approx. 740lbs. Last night I read on Lipo batteries (lithium) vs. AGM. Again, another significant weight savings can be achieved. Water and fuel storage is another consideration. With better designed systems I can save right at 1,600lbs of weight. While this is a substantial weight savings, if I am still under the payload capacity of the truck, regardless of which build I choose, is the added cost worth it? I've set a ceiling price to my build, and surely can't use the best of the best in every aspect of the build. I hope to gain any insight so I can save weight where I can see the most 'bang for my buck'.

I've exhausted the search function on Expo, but can't seem to find any worthwhile data on the topic. Any links to existing threads are welcome.

 

[workerdrone]

Less weight is pretty much always better - better acceleration, handling, braking, mileage, not as easy to get stuck in the soft stuff,

less abuse on the chassis and drivetrain. About the only thing not much affected I think is top speed, which don't apply much to overland type vehicles.

 

[JHa6av8r]

For pretty much the reasons workerdrone stated, I chose to use a 1 ton with a 4,200 lb payload and a pop-up camper that used less than half the payload capacity when loaded. I think not being at or near GVWR is more important when off road due to less stress on the equipment.

 

[bobaloo]

Workerdone- I agree, and certainly will strive to design a light camper. I suppose I was looking for any comparisons someone may have done on different loads, tested over x miles with constant conditions. IE: If a f450 or ram 4500 with a full size lance camper could do a 10 mile highway section at 65mph unloaded and then load up an extra 2500lbs of weight and do another drive. Would be interesting to post specific results on mpg, sway, braking etc. Lighter is definitely better, but so is another $10,000 in my pocket by using heavier, but cheaper materials, if there isn't 'that' much of a difference.

 

[IdaSHO]

For pretty much the reasons workerdrone stated, I chose to use a 1 ton with a 4,200 lb payload and a pop-up camper that used less than half the payload capacity when loaded. I think not being at or near GVWR is more important when off road due to less stress on the equipment.

Double edge sword.

While I agree that having a truck built for the task, and running as light as you can get away with is good,
going big with the truck and light with the load can be a bit counter productive.

Dont get me wrong, I love my 1-ton.

But a 1-ton these days weigh a LOT. So take your 1-ton that weighs 8000 lbs, and add your 2k lb camper. 10k curb weight, right?

Why not take a modern 1/2 ton that can manage the same camper within its payload limits? 5000 lb truck and the same 2k lb camper.


Still well within the limits of the truck. Lighter, lower maintenance costs (arguably), certainly more friendly as a daily driver, and cheaper to get out the door.

 

[JHa6av8r]

Double edge sword.

While I agree that having a truck built for the task, and running as light as you can get away with is good,
going big with the truck and light with the load can be a bit counter productive.

Dont get me wrong, I love my 1-ton.

But a 1-ton these days weigh a LOT. So take your 1-ton that weighs 8000 lbs, and add your 2k lb camper. 10k curb weight, right?

Why not take a modern 1/2 ton that can manage the same camper within its payload limits? 5000 lb truck and the same 2k lb camper.


Still well within the limits of the truck. Lighter, lower maintenance costs (arguably), certainly more friendly as a daily driver, and cheaper to get out the door.

The OP isn't asking about building on a 1/2 platform. The build platform is a 5500. The thread is about using conventional materials or investing an extra $10,000 for lighter materials and saving 1,600 lbs off the build. Both option would be under GVWR. It would be nice to know how much under GVWR for both build options.

 

[IdaSHO]

Gee, struck a cord?

It was an analogy. And it still applies. Get over it.


Weather you are upgrading a truck to haul more, or spending $$ to go lighter, it is all a trade off.

 

[IdaSHO]

For instance, everything adds up, so if I spend 2x as much on a lighted skin for the camper, I can cut 50% of the weight for the camper shell. This would be a difference of approx. 740lbs.

If you want to do it, and do it right and within budget, you need more R&D

Just saying

I had more than a year into the design of my custom camper.
In that time I was able to produce a book of data with regard to every building material I used,
as well as a good amount of CAD drawings for the wife to sign off on it.

As you have found, it comes down to cost and weight.
Everything was accounted for with regard to weight, right down to the fasteners and the paint.

My rig happened to hit the projected weight and budget.

I attribute that to a good deal of luck. Being fairly OCD helps too


BTW, if you want to see the entire build (still not QUITE done...) google IDACAMPER2.0

Youll find it :chef:

 

[Kiomon]

I have a pretty good example. We have a Unimog based camper, and when we were building it we had to load all of the shop materials and tools inside it and drive it across country. We were significantly overloaded. I did weigh us and we were 2000 lbs heavier than our final build weight. We compared the mpg from that trip and another interstate trip and it was only a 1 MPG difference. And I partially attribute that to error.

So for us it didn't make a huge difference in mpg. But it did make a noticeable difference in the feel of the truck. We had to load the box from floor to ceiling on the back, so there was weight up high from the chassis and it was not fun. You could feel the stability penalty.

From our experience I would always go lighter and GVWR's are not all created equal. If you are heaving off road and going to do some hard bogging, it's very hard on bearings, axles, and other components. Most of the time the gvwr is rated on smooth road use. But 10k isn't anything to sneeze at.

Personally for me, it would come down to where the weight is going to come from. If it's coming from up high, then I would go for it. If it's coming from down load, e.g. from the agm switch to lithium batteries whcih would normally be sitting on the frame rail, then I am not sure it would be worth it. 10,000 is 3 months of good overland fun on the road for two people. We are all intoxicated to obsess over our rigs, and want them perfect for every occasion, but in reality, most won't get stressed to their max and a lot of time is spent on the road. The key liberator is cash in the bank, that's going to be sure you go further and see more.

Just my two cents.

 

[Healeyjet]

Are you keeping it on DRW or switching to SRW? Air suspension and lifted?
Ward

 

[highdesertranger]

for any given vehicle wind resistance has more to do with mpg then weight. slow down any vehicle and your mpg will go up. this works down to about 40-45 mph. below that and aerodynamics don't make much of a difference. highdesertranger

 

[Dan Grec]

As highdesertranger said the physics tells us that payload only impacts mileage when accelerating.

Once you're at cruising speed, it's all about aerodynamics and rolling resistance.

Of course, you'll want to keep the weight down for all the other reasons you mentioned, but it's not going to impact mileage a great deal.

-Dan

 

[southpier]

maybe worth consideration:

what is the total cost for the finished build (research/ labor/ materials) in dollars?

what percent of that is 10K$ ?

while light is good, are there easily accessed components that could be switched over time to sting less & maybe resold to recoup some of the $$$. obviously this wouldn't be raw materials or things like built-in tanks, but batteries apparently have a somewhat predictable lifespan. maybe in three years you could switch to the high priced spread? maybe you'd be having so much fun the focus would be different?

 

[foxhunter]

My U500 weighs 23k loaded, which is 10k under max GVWR. At 55 mph I get near 9mpg. At 65mph I get 7.5 mpg and that can go to 5mpg in a headwind.
My dodge 3500 CC with flatbed and 4:10 rear axle gets 15mpg at 70mph on the road, 10 mpg pulling anything(wind resistance at work). Payload has minimal effect. I would guess with wind resistance in a 5500 you should get about 10mpg on the road at highway speeds.
My opinion is that vehicles loaded to near their GVWR don't handle nearly as well as when you keep them lighter. Also, any farmer will tell you to never by a bigger tractor than you need for the job, as it is a waste of money and horsepower. The same applies here.

 

[Mwilliamshs]

Tool for such calculations

http://ecomodder.com/forum/tool-aero-rolling-resistance.php

unless you know the Coefficients of drag, rolling resistance, etc for your particular rig you can just leave them alone and change the weight to whatever you think it will be with "normal" materials and then to whatever you think it will be with "lightweight" materials and compare the mph, hp demands, etc based on that sole variable. Pretty handy.
.
For example, changing only the weight to 7,500 lbs and then 9,100 lbs (OP states 1,600 lbs spread between normal and lightweight) gives results of 28.77 mpg @ 60 mph @ 7,500 and 26.05 mpg @ 60 mph @ 9,100 lbs. These are obviously not repesentative of the actual mpg of your rig but assuming the shapes and dimensions remain the same for both construction styles, the percentage of change (>10%) should be comparable and frankly, I'd consider ~10% a fairly big deal but it'd still take a long time to pay back $1,600 to get it.
.
My proposed Central America trip of 25,000 miles will burn 2,500 gallons @ 10 mpg and if fuel were constant $2.00/gal (yeah right), that's $5k in fuel.
.
Same trip, 25,000 miles, would require only 2,264.49 gallons @ 11.04 mpg so $2/gal = $4,528.99
.
Spending $1,600 to save $471.01 doesn't make sense to me. Even if the quick and easy $2/gal changed to the realism of my local fuel cost today of $3.199/gal the savings go up as well, but the savings is still only $753.39. Now of course these numbers are theoretical at best but there is a lot of very real science in that calculator I linked to. The folks at ecomodder take such things very very seriously and through my own experiments on several vehicles it has proven to be quite accurate. That being said, the higher your fuel costs the faster the $ spent to save on fuel will be paid back and since nobody expects fuel to ever be appreciably cheaper in the future than it is today, there are worse investments than those in fuel savings.