Tell me about fifth wheels

In My Humble Opinion
41

I think this is where I lose comprehension. My limited understanding reasons that power is power, no matter what type of engine (or electric motor?) is making it. That power is what turns the wheels. The transmission carries it to those wheels. Simple analogy: if I have to climb a hill on my ten-speed bike, from a complete stop, I might shift down to first gear. Couldn’t a similar solution apply to getting heavier loads rolling? This article from about a year ago says:

All but one of the Ford F-150’s engine selections come married with a 10-speed automatic transmission (with a six-speed auto on the base model).

source

Our class B weighs about 11,000 lbs at max and gets good acceleration from the V10. We appreciated it when trying to merge at interstate speeds and occasionally passing other cars. I remember reading something posted by a guy who drove one of those Toyota V6 campervans saying that getting on the highway was like taking your life into your own hands…they couldn’t get out of their own way. I do see your point, though. Towing would be a different game.

I found a few trailers that looked interesting but diving into the specs on them, they had a only 800-900 lb capacity for cargo, including the people using it when it’s parked. If you carry a full 30 gallon water tank, that alone would weigh 250 lbs. So it’s a high wire act all around.

Interesting, thanks for the links! It may be that it’s well worth it, depending on what we buy.

42

Same for ours. I can accelerate to 70 in pretty short order with the V8 and climb hills like a boss, but then I’m not towing 10-15K pounds of flimsy construction behind me. I had one of those 21 foot Toyotas with the V6 (a Winnebago Warrior), and you’re right. Going up a long hill was torture. I fully expected to see bicyclists passing me. I had a similar problem with my 27’ Bigfoot Class C with a V10. It was just underpowered for its weight. Luckily, most of the driving I did with both of those was in Alaska, where heavy traffic is not usually a problem, but the six months we spent on the roads outside of AK after we retired had some interesting days.

The F-150 is a light duty truck, for the most part. I’m not sure the frame or suspension would hold up to the strain. I’m not truck literate, but I do remember that we recommended heftier vehicles for the larger trailers. One anecdote: A couple of guys came onto our lot one day looking for a trailer. They of course didn’t really know what they wanted, so I spent a lot of time taking them in and out of various models. After over an hour of this, they finally said “Yeah! We want the Yukon!” (which weighs in around 16, 500) “Great!” says me. Then something occurred to me that I had forgotten to ask: “By the way, what will you be towing this with?” A big grin lit up his face: “Oh, I just bought a brand new Hummer!” Oops. I explained about towing capacities, and how an H1 is a gutless wonder, but he was still insisting on buying it. I had to tell him a) our shop would refuse to install a hitch on a vehicle that doesn’t have the proper capacity, and b) that knowing what he was going to try to do, my boss wouldn’t allow it in the first place. Bye-bye sale.

43

I read this description comparing horsepower and torque. Not sure if it’s accurate or not, but I hear torque is the big deal about diesels.

To illustrate, lets imagine you’re shopping for a new stereo system. You might consider how loud the stereo goes. The maximum listening volume possible for sustained playback is like the horsepower of an engine: a good indication of how much power that stereo has.

Now, consider the stereo’s bass. Bass is a part of the listening experience that plays into the maximum volume (horsepower), though the bass is more likely to be evaluated for its peak level of ‘punch’, created for a brief moment.

In this way, horsepower is like the volume of a stereo, and torque is like the bass: both attributes are worth knowing, and for certain types of music (or vehicles), one may be more important than the other.

Source (Toyota)

44

The problem is that when stopped there is zero power delivered. You have a chicken and egg problem How do you get moving so that the power can be delivered? The answer is that you need to consider the force applied, not the power. Torque is the rotational force applied. Measured as the linear force (newtons, or if you must pounds) pushing on an arm of a given length (feet or metres usually.)
If you are on your bicycle, but stopped. How do you get going? You push on the pedals. That force on the pedals is torque. (Force down on the pedal times the length of the crank.) Even when you are stopped, stand on a pedal, you are applying torque to the rear wheel via the crank and chain. The wheel pushes against the Earth, and you get your linear force pushing you along.
Same for an engine. There are pistons pushing on a crank. That force is what pushes you along by trying to rotate the wheels, and that is what accelerates your vehicle.

Once you are moving you have losses in the system. Frictional losses of all sorts. Road, air, internal losses in the transmission etc. To balance these losses you need to expend energy, and that is where power matters. At a constant speed your engine is delivering the power needed to balance the losses. Since the losses rise (dramatically) with speed you need more power to maintain speed. But it wasn’t power that accelerated you to that speed. It was force, and thus torque.

Power delivery is force times distance. The faster things are moving with a given force the more power is delivered. And vice versa. So at low speeds, engines deliver less and less power. So you have a gear box. But you still have the fundamental problem, you need the engine to be turning fast to deliver power. In order to accelerate the system you don’t need power. You need force. (So when you are turning a wheel, torque.) You choose the right gear and right throttle opening, and you end up with a system where the force applied via the wheels (torque) balances the opposing forces from all the losses in the system. That force times your speed equals the power being delivered. The vehicle designer chooses an engine and gearbox so that this balance is available under the expected driving conditions.

Infernal combustion engines deliver maximum power at (roughly) maximum revs. But they deliver maximum torque at lower than maximum revs, when the combustion in the cylinders is working to give maximum effect. And for diesels, this can be much lower revs. The much higher compression and general thermodynamic nature of diesels means that overall they can create much more torque, and at lower revs than a gas engine. Because they are usually rev limited, they don’'t manage the same peak powers, but they can deliver torque forever.

Nowadays, with the continued uptake in electric vehicles, the nature of electric motors compounds this difference. These motors can deliver maximum torque when stalled. It is just like you standing on a bicycle pedal when at rest. There is no power being delivered, because you are not moving. But the acceleration is maximum. ICE of all kinds can’t do this because they need to breathe, and if they are not breathing, they cant work.

45

To chime in here with this bit,

I know that for the diesel engines in semi-tractors, the lubrication technology is pretty good. From greasing things like the fifth-wheel to ball joints (and anything else lubricated with grease) to motor oil, the science behind the lubricants allows for tens of thousands of miles between maintenance stops. There are motor oils designed specifically for long haul diesel motors.
What I can’t tell you is how this applies to an RV puller that isn’t being driven multiple tens or even hundreds of thousands of miles per year. Might be worth looking into though.

Oh, and as long as you’re not doing something like modifying things for coal rolling, diesel engines have some fairly strict emissions regulations these days, so they are far less dirty than the old days.

46

Applause.

47

The steel bars are spring steel. That’s not necessary for the simple theory of load-leveling, but it’s practically important. Also, in practice, the bars are long out towards the trailer, and tensioned with chains. That’s also not important for the theory – it would work even better with long forward bars – but cars and trailers aren’t built for that.

If they aren’t tensioned to start with, they aren’t doing anything. And if there wasn’t some springiness in the bars, they’d just bend the first time you went over a bump.

48

Just to be clear about what I wrote. Energy is force times distance. I mean energy delivery above. Which is playing fast and loose with the terms. Power is energy per unit time. And distance per unit time is speed.

So, if fuel in a cylinder burns, it pushes down on the piston with a given force, for a given distance. The force on the piston pushes the crank around (torque), which goes via the gearbox, and eventually pushes the driving wheels around, which pushes your vehicle along. More force from the burning fuel, more acceleration. Faster moving piston means the force on the pistons covers a longer distance in a given time (more revs) so more energy is produced over that period of time. So the engine is developing more power.

49

Thanks for all the replies!

That physics aspect is where I fail to grasp. If you told me that power was applied to move the object, I’d believe you. Terms like “power” have a special definition of course in the jargon of science.

Per the info that I posted (Toyota), isn’t torque a subset of horsepower, so to speak? If a gas engine needs higher RPMs to produce the torque you want, could you pick a lower gear…?

If that’s hard on the transmission, would a beefier transmission help? Or is it too hard on the (gas) engine?

I don’t understand the physics but I’ve seen that before—at low revs diesels will produce the torque needed.

@Chefguy mentioned the difficulty getting people who work on diesels. But we had problems getting Ford dealers to work on our E350 based campervan. Once, the driver window had come out of its track and I couldn’t get it back in. I found this service below and used it. A mechanic came to where we store it, removed the panel (tricky because of the power mirror controls, power locks, power windows, etc., fixed the problem and put it all back together. When you figure what a tow to a dealer would cost…pretty sweet! And if you’re a thousand miles from home in need of help at the side of the road, it could be a lifesaver. Caveat: I don’t know if they’d work on diesel engines.

Right, that’s what I assumed. The “wind up” part threw me off…I imagined some sort of cranking motion.

It occurs to me that I’m probably assuming some sameness that isn’t warranted. If you dismantled a gas engine and a diesel engine of the same displacement, would there be other differences? For instance, would the pistons weigh the same, basically? Displacement and RPMs constant, pushing a heavier piston in one would require more energy. Are there inherent differences to take into account?

50

Thanks, I’ve bookmarked that. I had a similar service come to where I was encamped in Wisconsin one time years ago. My water heater quit and I was in a bit of a panic. The guy showed up and (literally) blew the cobwebs out of the ignition point. Seems a spider had set up shop in there. He came back a few days later after a guy limped into the park with a 5th wheel that had a broken axle. Yeesh, what a mess.

I’ve never had a problem with a Ford dealer working on my E-350 chassis, unless it needs to go up on a lift. The close-in dealers can’t lift it :roll_eyes: or just don’t want to mess with an RV. I did browbeat them into working on a Ford warranty issue, however. When I wanted to get the coach windows resealed, I finally found someone who would do it located about 30 miles from here. It was worth the drive to stop moisture from getting in. I could possibly have done it myself, as it’s not all that difficult to remove the coach windows (they’re just bolted in) but just didn’t feel like going through the angst.

51

I like too that they can give you a quote beforehand…even if you aren’t really experiencing an emergency, you can use the site to see what a repair might typically cost, compare it to the quote your shop is giving you. You pay for the service call but you’d pay to get in line at the repair shop. When the mechanic comes to you, you’re the only client.

Gas or diesel? Hey, how about this hybrid? 0-60 in 5.2 seconds? And 570 ft-lb of torque?

Ride quality was also quite good for a truck with a 7,350-pound gross vehicle weight rating and 12,400-pound towing capacity.

52

It is unfortunate that we get a common usage of “power” that leads us astray here. In many ways power is actually the least important.

Power is a measure of work done over time. So it is a useful metric to understand what is happening, but it isn’t the thing that does the work.

If you want to move something, you apply a force. Pushing on something is applying a force. If you want your car to move, you push it. When it moves, you are transferring energy into the car, and the rate at which the energy is transferred is the power. (If the darned thing won’t budge, rather counter intuitively, no work was done. So no power.)

So when you say “power is applied” just swap out power for force. You can’t apply power to anything. You apply force. Power is how you measure the rate of energy transfer.

Think of the entire car’s system in the same way you do a bicycle. The pedals are the pistons, the crank is - well - the crank. You have a chain transmission with gears, same as the gears in a car, and eventually you try to rotate the driving wheel. Expanding hot gas pushes down pistons, your leg pushes down on a pedal. Nothing else is much different. The mass of the pistons is like the mass of the pedals. They are pretty inconsequential in this view of things. They need to be strong enough, but they are coupled to the rest of the vehicle, so the expanding gas doesn’t see the mass of the piston, it sees the mass of the vehicle. Same as your foot doesn’t see the mass of the pedal as you push down. It sees the mass of you and the bike. (Mass of the piston does matter, but where it really matters in on top of the upstroke, when it tries very hard to pull itself off the end of the coupling rod. Your bike doesn’t have this problem. You can’t pedal fast enough for this to matter.)

As above. Torque and power are related, but one isn’t a subset of the other. Torque is the force that is pushing things around. Power is a measure of the rate of work done. You can be in a situation where you are applying a lot of force (starting the whole rig moving) and thus need a lot of torque, but the actual power delivered is very low, since you are moving quite slowly. It swaps at highway speeds. You only need enough force to balance the air drag, friction, and transmission, but you are moving quite fast. Thus the power delivered may be quite high. Eventually you can’t go any faster. (As a rough approximation, car designers used to work out the balance of these losses and choose the transmission ratios, so that in top gear, your engine is delivering maximum power when the car is travelling at its top speed. This ends up with poor fuel economy, so they add overdrive ratios, that get you good fuel economy at highway speeds, but are too tall to allow the car to reach its maximum speed. So your maximum speed is achieved in say 5th in a 7 speed box.) The balance of peak torque and peak power in an ICE is tweaked to give good driveability. Diesels provide a better starting point for getting the balance right for moving lots of mass. Gas engines are more flexible, and provide a good balance for lighter, zippier, things.

You can pick a lower gear, and this is exactly what you do. But your gas engine may be revving higher than you like, and generally not as happy as a diesel. (It will be wearing much faster for a start.) It depends upon how it is designed and tuned. Neither engine type is ideal for all circumstances, but generally diesels are good at low rev torque, and are happier for this task.

Getting moving this is hard on the transmission, especially the clutch. An ICE can’t do anything if it is stopped, so you need to bridge the gap. That means a sliding clutch or a fluid coupling (aka torque converter in an auto transmission.) Either way there is a mechanism of allowing the engine to run whilst apply torque to a stationary vehicle. Getting past the use of this as fast as possible reduces the wear. Slipping the clutch trying to get a large rig moving will work, and kill the clutch real fast if you keep it up. Or kill it first time you try if you get it wrong. Automatics are dissipating heat as well, and they is why you often need a transmission cooler for towing. All the energy from the engine not going into moving the rig is going into heating the fluid. It can’t keep heating up forever. So the answer is to get the transmission solidly engaged asap. That means having an engine that is able to deliver the needed torque to accelerate you at as low a rev rate as possible. That favours a diesel. Or electric. Cybertruck maybe? :slight_smile:

53

Like trains, big mining trucks were diesel-electric many years ago, and probably still are. The electric motors used develop maximum torque at minimum speed, and are directly coupled to the wheels. The diesel just runs a generator, which doesn’t run at the same speed as the wheels at all.

54

Thanks for taking the time to explain it. I had no problems with chemistry when I was in school and I was a math whiz. Physics always somehow eluded me.

In (ten pin) bowling, an 8 lb ball traveling at 15 mph doesn’t have the same impact as a 16 lb ball traveling at 15 mph. I know diesels are supposed to be pretty heavy and I assume it’s a durability issue in part but I thought maybe it was similar—if a diesel moves heavier pistons.

That Ford hybrid is a very interesting idea. I’m not sure about buying one during the first model year, though. Some say the Tesla truck might produce 1000 lb/ft of torque. Of course there was that armor glass debacle…

55

Regarding the Tesla armored glass demo, which everyone seemed to think was a massive failure, I think it was widely misinterpreted. The projectile never made it past the glass. I don’t think anyone should expect the glass to remain untouched.

56

The difference is that the pistons are not free rolling. They are coupled to everything else, and can’t move freely. Their job isn’t to contain all the energy inside themselves as they move, it is just to provide a surface for the expanding gas to push against. Just like bike pedals, their only job is to provide a surface for your foot to push onto. In many ways pistons are just like gear teeth. They buzz about transmitting force. They need to be strong enough not to break, but that is about it.

57

There is a difference. For cars and trucks, the weight of the pistons is much less than the weight of the car or truck, but in analyzing the engine, or for specialist applications, the weight of the pistons is part of what determines how fast you can spin up an unloaded engine.

58

I’ll have to take some time to try to digest all this, but again, thanks for taking the time to explain it.

I was doing the “build and price” on the Ford website earlier and swapping a gas V8 for the turbodiesel added $10,000+ to the proposed bill.

59

Curiously there is a time when the bowling ball analogue works. The individual gas molecules inside the cylinder are behaving like bowling balls. The hotter the gas, the faster they are moving, and it is fast gas molecules bouncing off the top of the piston that imparts the force to move everything. Faster they move, the more force. So the hotter they need to be. (Pretty quickly this gets us to the Carnot Cycle and efficiency of engines.)

60

That’s pretty much what we found when shopping in 2017. I don’t think most of us “recreational” tow-drivers really need diesel – unless we’re really racking up the miles. I was shopping/building only gas trucks during our hunt, but we found an aggressively marked down diesel at a nearby lot the week before Christmas, and ended up buying it. The truck was an end-of-model-year with middling upholstery, no carpet, and a long bed. In short, a difficult set of options to move in Da Big City (DFW). The dealer admitted it was a special order who’s buyer had backed out, for somewhere in the hinterlands, that ended up on their lot due to a series of trades. The dealer discounted it to a price below the gasser we were considering, so we’re now diesel owners.

Let there be no doubt, the diesel IS better for towing, but it’s not better during the other XX weeks per year that you’re using it unloaded. Assuming you hire most maintenance: Oil changes are $100+, changing the dual fuel filters runs $300 each time at the dealer, and new batteries (installed) run about $700. In addition, you need to ensure the DEF tank is kept full (or carry a few gallons) because the truck will go into limp mode (5 mph max) if you let that get too low. I bought new tires a few months ago, and they were well past the $1000 mark. If you use this as a daily driver you’ll discover it’s cost per mile is pretty high (for most of us anyway). I don’t know what the break-even point is, but I’d bet you’d need something like 50,000 miles/year for these beasts to make economic sense. If you’re only using it for towing and hauling, and not commuting, it’s more sensible. FTR: I have an old Corolla that’s used for daily errands. The Nimitz* sits unused for days at a time.

*Wife’s name for it.