Trailer Frontal Area

[Raspy]

Originally Posted by Wayne Collins

But if you have to keep up with traffic, keep your tranny out of overdrive. Transmission (and engine) overheating is mostly caused by a slipping torque converter. The oil gets hot and transfers the heat to the radiator coolant via the Transmission oil cooler, which is in the bottom tank of the radiator, unless you have a "Towing Package" which includes a separate oil cooler. But that "radiator" is likely in front of the engine radiator so the air passing through it makes for less cooling for the engine.

All modern automatic transmissions have lockup clutches on their torque converters that eliminate the heat buildup in the fluid coupling. The newer the model the more aggressive the lockup programming is. New transmissions lock as part of the normal shift process in all but 1st gear. In my truck, for instance, no matter how hard it is working, the trans runs cooler than the engine and it is always locked up.

The trans should know what gear to be in, but it is a good idea to be in "Tow/Haul" if you have that option, when towing. Or just lock out the highest gear to keep the trans from hunting and to keep the engine near it's most efficient RPM.

[Carol H]

Originally Posted by Mr Lynn

I have long thought that there would be a market for underbody panels for cars and trucks, not only for aerodynamics, but to keep dirt and road salt away from the underside. But AFAIK, nobody has attempted to make and sell them.

/Mr Lynn

A number of trailers have underbody panels. Airstreams for example have them. While it does keep things smooth and clean looking you need to remove them in order to do any repairs to plumbing etc.

[steelypip]

Originally Posted by Raspy

All modern automatic transmissions have lockup clutches on their torque converters that eliminate the heat buildup in the fluid coupling. The newer the model the more aggressive the lockup programming is. New transmissions lock as part of the normal shift process in all but 1st gear.

All true. Further, there's a direct correlation between engine RPM and fuel consumption (Friction losses are linear with crankshaft speed and pumping losses are pretty linear to the torque peak). One of the most useful things I've done to my TV was to install a scangauge monitoring the calculated engine % load value. This number tells a Ford (which is what I have) whether it should unlock the torque converter or force a downshift.

The goal is always to stay in top gear and lockup as much as possible. Why? Because a) the engine is more efficient (which reduces cooling requirement, among other things, and b) the transmission dissipates the least amount of heat in lockup.

The key phrase is 'as much as possible.' The combination gets amazingly good fuel economy on the flat at 45 MPH, but I don't often tow at that speed. In reality, I find myself maintaining posted speeds on the level or downhill on interstates, can pull a 5% grade at posted speed without kickdown, and tend to let it droop to 55-60 MPH when climbing a gentle grade if it will let me keep the converter in lockup. If I need more speed, I force a converter unlock (load above 90% but below 95% for 3 sec) and that gets me 2-300 more engine RPM along with accompanying available power and fuel consumption. If I hit a steep grade or need to maintain speed, I'll allow the kickdown to direct from OD.

As a matter of reality, I have never observed a change in transmission temperature between direct (3rd) and OD (4th) or locked and unlocked. I suspect it's because the transmission has a factory-supplied enormous air-fluid intercooler with a thermostat.

The way this transmission (4R70W) is configured, a converter unlock is worth half of the difference between direct and OD, which effectively makes the four speed transmission a six speed. There's basically no transmission wear penalty for switching in and out of lockup, unlike a loaded gear change in or out of OD on this transmission.

Knowing the TV as well as I do, it's easy for me to compare different trailers it has towed: A UHaul 5x8' 1 axle cargo trailer (essentially a box on an axle) has almost exactly the same drag as my 16' Casita, for all that it's significantly shorter and has much lower frontal area. that's the value of the rounded corners on the Casita. My 6x12 utility/equipment trailer is essentially free to tow behind the TV if I leave the mesh ramp gate at home. With the ramp gate on it's a speed brake that puts the TV at full throttle to maintain 70 MPH.

I would guess that the mesh ramp has worse drag than a flat plate because it drags more air along than a plate would.

I think the old towing rule about keeping engine RPM up while towing has a lot more to do with road speeds and cooling systems of 1940-1970 than anything else. Cooling fans were crankshaft driven, so spinning the engine faster pulled more air over the radiator, and (to the limit of cavitation) pumped coolant faster through water jacket and radiator. Staying in a high gear meant more torque converter slip and trans fluid heating with the sloppy converters used with one, (Dynaflo) two, (powerglide) and three (everything after 1965 or so) transmissions of that era.

I've observed no relationship between engine RPM and temperature at highway speeds with this TV, which is why I drive for best fuel practical fuel economy.

[samclem]

Originally Posted by steelypip

I think the old towing rule about keeping engine RPM up while towing has a lot more to do with road speeds and cooling systems of 1940-1970 than anything else.

I think it also has a lot to do with modern automatic knock sensors and other electronics that can automatically retard the ignition timing, richen the mixture, etc. Before these became available, "lugging" an engine under heavy load was a great way to cause detonation (knock, ping) and blast a a hole through piston, throw a rod, or otherwise destroy an engine in short order. Today's electronics can prevent this and the only hint most folks will have is a drop in fuel economy. Unless I have some visibility into what is actually going on with the electronics and the compromises that are being made to prevent damage, I would prefer to run in a lower gear (i.e. slightly higher RPM) than cause the system to retard the timing or select a richer mixture in order to prevent knock. At the very least, I'd be sure to use the highest octane fuel recommended for that engine if I decided to run in higher gears with a load.

[Raspy]

[QUOTE=steelypip;615844)

I think the old towing rule about keeping engine RPM up while towing has a lot more to do with road speeds and cooling systems of 1940-1970 than anything else. Cooling fans were crankshaft driven, so spinning the engine faster pulled more air over the radiator, and (to the limit of cavitation) pumped coolant faster through water jacket and radiator. Staying in a high gear meant more torque converter slip and trans fluid heating with the sloppy converters used with one, (Dynaflo) two, (powerglide) and three (everything after 1965 or so) transmissions of that era.

I've observed no relationship between engine RPM and temperature at highway speeds with this TV, which is why I drive for best fuel practical fuel economy.

[/QUOTE]

There's more to it. Older engines did not have variable valve timing, so they had a specific torque curve that meant they ran more efficiently at a bit higher RPM. Higher RPM means more internal friction, but it can also mean more efficient breathing that more than offsets the extra friction. A common mistake is thinking RPM equals mileage, when it's actually total work done and how efficiently that work is being done. So if you run it as slow as possible you can easily be below an efficient RPM. Then you have very high piston temperatures, detonation (in gas engines) and poor cooling. Detonation is very damaging and inefficient and changing the timing or mixture to counteract it makes the engine very inefficient.
When running at a higher RPM the cooling system can circulate more coolant and more oil is pumping through the engine. This helps to cool the pistons as well as cool the bearings and head. The torque converter is seeing less torque for a given horsepower. Throttle response is better with more power on tap. Engine braking is stronger on gas engines and engine brakes work better on diesels.

So yes, it is a good idea to run the engine faster when under heavy load. More power on tap, better cooling, likely more efficient and less internal stress. Plus the torque converter, locked or unlocked, is seeing less torque for a given load. The same amount of work is being done, but everything is happier even though there is a small additional amount of internal friction.

There is no way to accurately see the relationship between temperature and RPM. The gauge only reads water temp as it exits the engine, but not at various points in the engine, like piston temp or the difference in temps between different cylinders. As long as the overall cooling capacity is not exceeded, the temp will read normal. RPM is not equal to the total energy delivered to the cooling system and therefore not a gauge of efficiency. But the cooling system will work better at higher RPM.

With older gas engines, it is easy to look up the maximum torque RPM and then run it somewhere near that figure if working it hard and below that if running it easier. With modern diesels the maximum torque is a programmed number that is achieved at a very low RPM, but doesn't represent the engine efficiency or best speed to pull at. For instance, Cummins engines achieve max (programmed) torque at about 1400 RPM, but should run at about 1800-2200 RPM when working hard. This allows for more boost and cooler pistons. In other words, they will pull the load at 1400 RPM, but they don't like it. So you can be blissfully pulling a long grade at the lowest RPM and not realize you're causing unfair heat loads in the cooling system and in the pistons. Meanwhile the torque converter is barely holding the torque, and the throttle response is not there if you want more power. All in the name of better mileage, which probably isn't happening anyway.

If your mileage computer is accurate, try running a long grade at lugging RPM in top gear and then go back and do it again in the next lower gear at a more reasonable RPM and see the calculated difference. It's probably better mileage in the lower gear, but maybe not if you are lightly loaded and running premium fuel with a higher octane number for less pinging. While you're at it think about the better driveability in the lower gear and whether the higher octane fuel is worth the cost.

[Mr Lynn]

Interesting posts. My 2013 Ford Expedition EL 4x4 seems odd: It has a nominal 6-speed transmission, but only four shift positions (1, 2, 3, D). Unlike the automatic transmissions on my previous vehicles (except for a 1964 Rambler many years ago), the lower (non-D) positions do not simply limit the top gear, but start in whichever gear you select! So if you select 3, you are stuck in 3rd gear willy-nilly (I don't know if that's really 3rd gear, or some other place in the 6-speed lineup). Starting in 2nd or 3rd can be useful in slippery conditions, but I might like the tranny to downshift when driving. So I don't really know if there's a way to lock out the higher gears to increase RPMs when towing, unless I just hold it in 3rd position (whatever gear that really is) and downshift manually to 2nd or 1st when I need to.

I would like to find the most efficient mode. Leaving the tranny in D certainly didn't help gas mileage, towing our Casita 17—went down from 14 or so to only 10. But, aside from the fixed 1-2-3 positions, and the tach, I don't really know what's going on.

Re the original thread topic: The Ford towing guide says 60 square feet trailer frontal area is the maximum for my Expedition. The Casita may be more than that, but it's rounded, so not a problem, I reckon.

/Mr Lynn

UPDATE: I didn't realize it, but the Expy has a 'Tow/Haul' button that (according to the manual) delays upshifts and automatically downshifts for grades. Next time when we take the trailer out, I'll have to try it. The Expy also has a computerized anti-sway program that supposedly will detect sway and try to minimize it by applying brakes on individual wheels. Would that help?

[Raz]

The lower gears are useful for braking, especially on long down hill grades.

[frank_a]

What kind of Ford?

Originally Posted by steelypip

This number tells a Ford (which is what I have) whether it should unlock the torque converter or force a downshift.

[DavidG]

[QUOTE=samclem;615752]No, I haven't done any calculations. With stuff like this it normally boils down to actual wind tunnel tests, very complex computer modeling, full-scale tests, or "rules of thumb." But quite a bit of money has been spent modeling tractor-trailer rigs, with the results being what we see on the roads today: Virtual elimination of slab-front cabover tractors for long-haul use, the incorporation of "skirts" to reduce drag underneath the trailer, and now we see some good foldable fairings on the back of trailers to significantly reduce drag (improving total fuel economy by about 10% at highway speed, which is tremendous when we think about how much fuel these trucks burn).
Some snippets: From this Canadian government paper:
-- Under-trailer side-skirts are good for an approx fuel burn reduction of about 3-7% (so, maybe a drag reduction of about 10%?)
-- Fairings to reduce the gap between tractor and trailer have produced results that vary. The upper practical limit appears to be a drag reduction of about 7%.
-- Rear-body fairings: These have been studied a lot, and for trucks the problem is designing one that does a good job but also can easily be gotten out of the way so that the doors can be opened. A "perfect" rear fairing would smoothly come to a point well in back of the truck, but this would obviously be impractical due to the maximum trailer length issues, etc. But, as far as "bang for the buck," just some flat panels that form a "boattail" at the top and sides of the trailer can do a pretty good job. More in this wind-tunnel study of many tractor-trailer devices done by Lawrence Livermore. Interestingly, int at study, the under-trailer skirts saved about 150% more than the aft-body boat tail fairings.
-- BTW, some folks believe that small vortex-generators on the sides/tops of trailers right at the eft portion can do some good, but from what I read they don't reduce drag very much. They do apparently help reduce turbulence when passing other trucks, which can make big trailers unsteady.
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I've tried to put up a reply with photos of truck streamlining, but photos don't show. Will try again : the rear panels that fold and allow use of the doors but still direct air, controlling the dead air space . I think that all trucks that use the rear panels also use side skirts, which would look funny on a fb trailer but might be effective, along with a 'deflector' on top of the PU cap. No comments on the effectiveness or even purpose of the smooth covers over the battery and propane tank area of a fb trailer ?

[samclem]

Originally Posted by DavidG

Will try again : the rear panels that fold and allow use of the doors but still direct air, controlling the dead air space . I think that all trucks that use the rear panels also use side skirts, which would look funny on a fb trailer but might be effective, along with a 'deflector' on top of the PU cap. No comments on the effectiveness or even purpose of the smooth covers over the battery and propane tank area of a fb trailer ?

Yes, the picture shows the fold-up "boat-tail" fairings that are becoming more popular. They do an okay job of reducing the turbulence at the back of the trailer caused by abrupt end of the trailer body. From a theoretical perspective they would be better if the hinge was smoother (rather than being a sharp angle) and if they didn't have the cavity in the back but instead came to a point, but that's simply not practical due to legal restrictions on trailer length and practical considerations of getting them out of the way so the doors can be opened. For a typical FGRV with large-radiused corners/edges (Scamp, Casita, etc), a foldable boattail like this would likely have considerably less value than it would for a trailer with sharper edges at the back (e.g. Lil Snoozy). While I'm sure there's still a lot of flow separation behind a Scamp, with resultant low pressure across the back ("suction", which "pulls" the trailer back), the smooth edges do help smooth the flow past the corner, which can help reduce vortices--another source of drag.

Smooth covers over the propane tanks, batteries, etc: I don't have any data on that, but my guess is that they don't do much of anything for the aerodynamics. Where they are located (low down, well in the slipstream "shadow" of the tow vehicle), the air reaching them is probably of low velocity (compared to the vehicle's speed) and so "messy" that it seems unlikely fairings would do much good. They are probably more useful in a practical sense in keeping snow and road spray off the propane regulators, battery terminals, etc. And they do "look aerodynamic," so that may help in marketing.

[sokhapkin]

Originally Posted by Mr Lynn

UPDATE: I didn't realize it, but the Expy has a 'Tow/Haul' button that (according to the manual) delays upshifts and automatically downshifts for grades.

Very frequent user error Nobody reads the F..g manual. I use the feature a lot on steep downgrades when towing with my 2013 F150.

[Carl V]

We bought our Trillium a 1.5 year ago, and it's been parked in the backyard since, and I'm still working on it.
The only time I towed it was the day we bought it and towed it home.
On that day, it was pouring rain.
One interesting thing I noticed when towing the trailer, was how the water sprayed under the front of the trailer. The water spray made the airflow more visible. In my mirrors, I could see that there was a significant downward and outward flow of air near the lower front corners of the trailer, blasting down hard on the pavement just in front of the trailer. Looks to me when the air hits the front of the trailer, some of it curves up above it and to the sides of the trailer, but also a significant portion drops between the tow vehicle and the trailer. Then it hits the tongue area, where it is forced to split apart and then evacuates on each side where I could see it. So I would be inclined to think that a more streamlined tongue area would be beneficial (by a small percentage, but still) as a lot of air seems to hit the tongue from above.

[samclem]

Originally Posted by Carl V

One interesting thing I noticed when towing the trailer, was how the water sprayed under the front of the trailer. The water spray made the airflow more visible. In my mirrors, I could see that there was a significant downward and outward flow of air near the lower front corners of the trailer, blasting down hard on the pavement just in front of the trailer. Looks to me when the air hits the front of the trailer, some of it curves up above it and to the sides of the trailer, but also a significant portion drops between the tow vehicle and the trailer. Then it hits the tongue area, where it is forced to split apart and then evacuates on each side where I could see it. So I would be inclined to think that a more streamlined tongue area would be beneficial (by a small percentage, but still) as a lot of air seems to hit the tongue from above.

That's an interesting observation, and a lot can be learned from looking at spray, or even small beads of water on the surface of a trailer (airplane testing sometimes uses small tufts of string or yarn attached to the skin of the plan to get an idea of which way the air is moving and how smoothly). If the air is coming straight down onto the hitch area, it's not directly slowing down the trailer as a result of what it hits (since only the positive pressure on forward surfaces or "negative" pressure on rearward ones have any >direct< impact on increasing the load). It's even possible that impeding the flow downward in this area could >reduce< drag (to the degree the air "piles up" and increases in pressure in the TV/trailer gap, it might help keep air flowing past the "gap" between the TV and the trailer, rather than having it enter gap, get turbulent, create vortices, etc).
Other than the obvious stuff (antennas, awning housings, rooftop AC units and vents, getting the bicycles off the roof, etc) it's very hard to know for sure what will work without some testing.

[DavidG]

Someone mentioned not too practical to extend the rear of a square trailer except with the folding panels. Quite true, but I occasionally see these in Calif, the rear end evidently swings open. 2 - 3 companies use these; the front is pretty streamlined EXCEPT they have a big overhead overhang in front like a BIG overhead bed on a camper. Must be light wt cargo because they have small tire wheels and underframe storage, or skirts. But this shows that someone thinks these are economically smart, possibly after testing. The rear looks a lot like a fiberglas trailer, doncha think ?

[steelypip]

Originally Posted by frank_a

What kind of Ford?

In this particular case, an EEC 5 controlled 4.6L SOHC Ford modular V8 with a 4R70W transmission.

So no variable valve timing, but the transmission is fully controlled by the PCM, and it makes transmission shifting decisions based on that %LOD number. The PCM strategy used is quite similar to the well-documented CRAI8 strategy.

Yes it's old, but it's a lot newer than the 1992 Casita it's towing...

[frank_a]

When you say it's old, just what year is it?

I'm looking at used trucks and just curious.

Thanks!

Frank

Originally Posted by steelypip

Yes it's old, but it's a lot newer than the 1992 Casita it's towing...

[steelypip]

Originally Posted by frank_a

When you say it's old, just what year is it?

I'm looking at used trucks and just curious.

Thanks!

Frank

It's a 1999 with about 160K miles on it.

[frank_a]

A number of years ago I bought a 2001 F150 same motor as what you have, with 94,000 miles on it. It performed well for a long time till I had a garage change the plugs at about 115,000 miles. Turned out #3 cylinder was helicoiled, which I did not know when buying the truck. Within a week after getting the plugs changed the plug in the heli coil blew out and a part of the heli coil fell into the cylinder. I sold it for junk. I knew these motors had a problem like that, but usually it affects the 5.4. I was just unlucky.

Good luck with yours!

Frank

[samclem]

Originally Posted by frank_a

Turned out #3 cylinder was helicoiled, which I did not know when buying the truck. Within a week after getting the plugs changed the plug in the heli coil blew out and a part of the heli coil fell into the cylinder.

BTW, there's a new(er) method of thread repair that many folks believe is far superior to helicoils. "Time-serts" do the same job, but consist of a complete socket of steel, rather than just the threads. They are so good that some folks who race cars with aluminum heads put them in when the engines are new, before they are even run. More info here.

Disclaimer: I have no commercial interest in this company or product, but it is what I'd demand be used if a mechanic found a stripped spark plug hole.

[frank_a]

The truck is long gone. I got less than 1/4 of what I had it in it when I sold it. It hurt, trust me.

I'll keep the time serts in mind. Thanks!

Frank

Originally Posted by samclem

BTW, there's a new(er) method of thread repair that many folks believe is far superior to helicoils. "Time-serts" do the same job, but consist of a complete socket of steel, rather than just the threads. They are so good that some folks who race cars with aluminum heads put them in when the engines are new, before they are even run. More info here.

Disclaimer: I have no commercial interest in this company or product, but it is what I'd demand be used if a mechanic found a stripped spark plug hole.