Quick question, what is this?

[Brewskier]

This is from Kodachromes thread, this is his rig.

That flap that runs from the top of the camper to the front of the windshield... is that to lower wind resistance? I was thinking about this the other day, how the wind will get caught in that gap between the camper and the truck. This seems like a good fix. Can these be bought somewhere and attached, or is it a custom job?

 

[brp]

He made it. There are some people who have fit non-custom fairings in a custom way. Yakima makes a fairing, but it is not very large. Also, some people do something like this

http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/23645900/print/true.cfm

Whatever you do, you want to accomodate any flex so that the parts don't hit each other when flexed on uneven terrain.

 

[MarkBC]

I'm interested in something like this, too.

I wonder how much difference it makes in fuel-mileage?

Must be lot's of opinion/info out there on camper-farings, as brp's link suggests.

 

[ski3pin]

Here's a thread that covered some of the wind deflector ideas.

Deflectors

 

[brp]

I am in the process of making one. I am only going to spend about $50. With gas prices what they are and going on multiple thousand mile trips, should pay for itself with even a small MPG increase. It also keeps the ride quieter. Some people have the problem of moisture getting under the front edge pop-top when driving hwy speed in the rain, I imagine it would cure that, although I have never had that problem.

I'll post up when it is made. I am using mostly schedule 80 PVC conduit for the frame, I don't have the actual fairing yet.

 

[chnlisle]

I'm interested in something like this, too.

I wonder how much difference it makes in fuel-mileage?

Must be lot's of opinion/info out there on camper-farings, as brp's link suggests.

I don't remember anyone giving a definitive answer on fuel savings with the fairing. I too would be interested in a real comparison.

 

[ski3pin]

I don't remember anyone giving a definitive answer on fuel savings with the fairing. I too would be interested in a real comparison.


A challenge for our scientists? Lets see some numbers. We want to know.

Last month we did a road trip up to Oregon. Drove Interstate 5. Noticed new skirts under the trailers on the big rigs. It looked like it was for wind deflection to increase aerodynamics and increase mileage. We hadn't seen these before.

 

[Overland Hadley]

Noticed new skirts under the trailers on the big rigs. It looked like it was for wind deflection to increase aerodynamics and increase mileage. We hadn't seen these before.


The rigs with the ''skirts'' have a lot less dirty air behind them. I have first hand knowledge of this because I bike on a road that has semis on it. At one point I read the fuel savings that theses rigs get, it was significant but I have forgot the exact number.

 

[MarkBC]

When I put my camper on my truck the mileage dropped from 14 to 12mpg. Without any real facts or knowledge my gut feel was that this drop was due to increased air drag rather than the increased weight.
This seems like such a basic/obvious question that it must have been answered before, if not on this site then on some truck or other camper site.

I bet one or more of the more truck-y guys on WTW already know...or have a definite opinion, at least.

 

[highz]

OK, Ski, here's a bit of underlying theory, which you may remember from physics class. I defer to our engineers and truck-y types for the practical application. Also, this is simplistic - Turbulence caused by the gap between camper and cab must play some role which the following doesn't address.

Aerodynamic drag is a force opposing the direction of motion, but we are really more interested in the power it takes to overcome this force. That power (force times velocity) is

(Thanks to Wikipedia for providing a cut-and paste formula )

rho is the fluid (air) density, A is roughly the cross section of truck and camper impacting the air, v is the speed of the truck relative to the air (not the ground, unless the air is still), and Cd is the drag coefficient. Turbulence probably means Cd becomes a function of speed for your particular rig, but considering that is beyond my pay grade. Turbulence is a hairy problem.

What's important here is not so much the values of the different constants, but the fact that the power needed to overcome this drag goes up as the speed cubed. The biggest effect you can make on fuel consumption is to slow down.

Even so, an air dam makes the ride quieter at high speed, and no doubt cuts down on turbulence, so even if it doesn't have a big effect on gas mileage, it may still be worth the cost of adding one. If the semi skirts improve fuel mileage by reducing turbulence, that's a good sign it could help.

 

[ski3pin]

Highz, thanks for the explanation! Science!

 

[highz]

When I put my camper on my truck the mileage dropped from 14 to 12mpg. Without any real facts or knowledge my gut feel was that this drop was due to increased air drag rather than the increased weight.
This seems like such a basic/obvious question that it must have been answered before, if not on this site then on some truck or other camper site.

I bet one or more of the more truck-y guys on WTW already know...or have a definite opinion, at least.


Hey, Mark. The Eagle on the Ranger dropped my mileage from 20 to 18 mpg (low speed driving). I found this link about how extra weight affects mileage:

http://www.fuelecono...l#remove-weight

One to two percent per 100 lbs. That's agrees with the mpg drop we see.

 

[chnlisle]

Hey, Mark. The Eagle on the Ranger dropped my mileage from 20 to 18 mpg (low speed driving). I found this link about how extra weight affects mileage:

http://www.fuelecono...l#remove-weight

One to two percent per 100 lbs. That's agrees with the mpg drop we see.

That has always been my guess as well. Fuel economy is more tied to what your hauling and how hard the truck has to work to get there. When I had the camper on my T-100 the milage went from 18mpg to 15mpg. On the Ram CTD I really don't notice any change in fuel economy. Still if a fairing would deliver some sort of substantial savings I'd be interested.

 

[highz]

That has always been my guess as well. Fuel economy is more tied to what your hauling and how hard the truck has to work to get there. When I had the camper on my T-100 the milage went from 18mpg to 15mpg. On the Ram CTD I really don't notice any change in fuel economy. Still if a fairing would deliver some sort of substantial savings I'd be interested.


Right. I agree with what you say. I see the ten percent drop in mpg at low to moderate speeds, but see worse mileage when I am forced to take the interstate. That is where aerodynamic drag really starts to kick in (or in a strong headwind). Since I prefer low speed roads away from cities, I can usually minimize the aerodynamic drag effect. I haven't felt the need to modify my rig with a fairing yet. If I had to drive a lot of high speed roads, I might change my mind - for a quieter ride, if nothing else.

 

[ski3pin]

Right. I agree with what you say. I see the ten percent drop in mpg at low to moderate speeds, but see worse mileage when I am forced to take the interstate. That is where aerodynamic drag really starts to kick in (or in a strong headwind). Since I prefer low speed roads away from cities, I can usually minimize the aerodynamic drag effect. I haven't felt the need to modify my rig with a fairing yet. If I had to drive a lot of high speed roads, I might change my mind - for a quieter ride, if nothing else.


This has been my experience also. I really notice a difference driving above 60 mph or dealing with a strong wind.

 

[SunMan]

This has been my experience also. I really notice a difference driving above 60 mph or dealing with a strong wind.


My Scanguage seems to support this. The faster I go (above 65ish) the lower my realtime MPG is and I can feel the resistance more. When i'm really pushing it is awful to watch, really makes you factor the value of time vs. $ in your driving.

 

[MarkBC]

I see that the weight effect -- 1-2%/100lbs -- would matter if you were accelerating or climbing (which, as Einstein taught us, is ~ the same thing; Who knew that you'd need General Relativity to explain our truck's gas mileage!

). But for those long stretches on cruise control -- driving I-5 between Redding and Bakersfield (my condolences to anyone who does that

) or on I-70 between Denver and Kansas City (never done it but it can't be much fun, either) -- seems like weight can't matter much, or at all.

Like some of you, I agree about the wind-drag cost of driving fast -- I don't want to pay that cost either, which is why I'm usually in the right lane when driving on rural interstates in NV or UT. In Oregon we're not allowed to drive faster than 55 except on Interstate Freeways, and then the max is only 65. (That may be for the same reason that we're not allowed to pump our own gas: Assisted suicide is legal in Oregon, and we're not going to squander that right by dying prematurely in a horrible accident. Well, that's one theory...

).
When driving I-80 across northern Nevada where the speed limit is 75mph I keep it close to 65 -- never above 70, though many are exceeding 80mph. When I'm only getting 12mpg at best, I am just too thrifty to want to spend even more on gas just to cut my driving time by 10 or 15% -- not worth it to me. And then there's the Planet to consider... (though if that was really a priority I wouldn't go on out-of-town camping trips at all, would I?)

Or that's the way I see it.

 

[EdoHart]

I see that the weight effect -- 1-2%/100lbs -- would matter if you were accelerating or climbing (which, as Einstein taught us, is ~ the same thing; Who knew that you'd need General Relativity to explain our truck's gas mileage!

). But for those long stretches on cruise control -- driving I-5 between Redding and Bakersfield (my condolences to anyone who does that

) or on I-70 between Denver and Kansas City (never done it but it can't be much fun, either) -- seems like weight can't matter much, or at all.

Like some of you, I agree about the wind-drag cost of driving fast -- and I don't want to pay it either, which is why I'm usually in the right lane when driving on rural interstates in NV or UT. In Oregon we're not allowed to drive faster than 55 except on Interstate Freeways, and then the max is only 65. (That may be for the same reason that we're not allowed to pump our own gas: Assisted suicide is legal in Oregon, and we're not going to squander that right by dying prematurely in a horrible accident. Well, that's one theory...

).
When driving I-80 across northern Nevada where the speed limit is 75mph I keep it close to 65 -- never above 70, though many are exceeding 80mph. When I'm only getting 12mpg at best, I am just too thrifty to want to spend even more on gas just to cut my driving time by 10 or 15% -- not worth it to me. And then there's the Planet to consider... (though if that was really a priority I wouldn't go on out-of-town camping trips at all, would I?)

Or that's the way I see it.

I think you mean Newton. Anyway...
I don't remember Newton's laws of physics by their numbers, but IIRC
Law 1 is: An object at rest tends to stay at rest, and an object in motion tends to stay in motion provided that no external force is appplied.

Let us remember that weight (pounds) is a measure of force. Also, force can be applied in any direction. Force which pushes us forward is often called thrust, and force which retards us is often called drag.

The more a vehicle weighs, the more force it applies to it's wheel bearings and tires, which means more drag, thus one needs to apply more thrust to maintain a constant speed.

 

[MarkBC]

I think you mean Newton.
No, I meant Einstein. I was referring to the equivalence of acceleration and gravity (where I said "accelerating or climbing"). See here, for example.
Newton quantified the effect of gravity (pretty close), but he didn't understand it.

The more a vehicle weighs, the more force it applies to it's wheel bearings and tires, which means more drag, thus one needs to apply more thrust to maintain a constant speed.

This must be an effect too...but I bet the acceleration in normal driving (that is, not cross-flat-country on cruise control) would be a major effect, since accelerating more weight (mass) requires more energy which means more fuel. And people accelerate every time they start from a stop or regain their target speed after slowing down.
I don't have any quantitative facts to back this up -- I'm just making it up, gut feel...and it's an engineer/scientist gut, not an auto/truck-guy gut, so my gut may not apply.

I feel more sure in saying that the relative effect of vehicle weight on mechanical drag would vary depending on the vehicle...

...Just as the effect of wind-drag varies with different vehicles: I know wind is a bigger drag -- requiring a greater %-increase in fuel to overcome -- on my F250-with-camper than on my Honda Civic sedan. That big square back of the camper-pickup is just asking for a storm of drag-producing turbulence!

 

[EdoHart]

No, I meant Einstein. I was referring to the equivalence of acceleration and gravity (where I said "accelerating or climbing"). See here, for example.
Newton quantified the effect of gravity (pretty close), but he didn't understand it.


This must be an effect too...but I bet the acceleration in normal driving (that is, not cross-flat-country on cruise control) would be a major effect, since accelerating more weight (mass) requires more energy which means more fuel. And people accelerate every time they start from a stop or regain their target speed after slowing down.
I don't have any quantitative facts to back this up -- I'm just making it up, gut feel...and it's an engineer/scientist gut, not an auto/truck-guy gut, so my gut may not apply.

I feel more sure in saying that the relative effect of vehicle weight on mechanical drag would vary depending on the vehicle...

...Just as the effect of wind-drag varies with different vehicles: I know wind is a bigger drag -- requiring a greater %-increase in fuel to overcome -- on my F250-with-camper than on my Honda Civic sedan. That big square back of the camper-pickup is just asking for a storm of drag-producing turbulence!

I love this stuff. Keep it coming.

Maybe as member administrator you can spin this off to another thread?

I remember reading that the only way to describe the orbit of Mercury was to include Einstein's General Theory of Relatviity. I can explain Einstein's general theory and special theory of relativity in layman's terms, but I don't comprehend either in the slightest.

Newton observed and predicted the effects of gravity in a Newtoniun Universe.
Einstein observed and predicted the effectis of gravity in an Einsteinian Universe.

Both were correct, yet neither desciibed gravity.