ITT: You explain basic physics to a retard (me)

PaperclipGod

Banned
Apr 7, 2003
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Why don't road trailers have a couple caster-mounted wheels at the front end? Then you could have even small vehicles tow large trailers (albeit slowly) because the weight would all be supported by the trailer, not resting on the tow vehicles rear axle.

Is it some basic physical principle I'm missing, or is it more related to safety? i.e. if a caster freezes up on the highway it'd probably flip the entire load.

I was just thinking it'd be nice to be able to tow some stuff myself instead of having to hire someone, and the biggest issue seems to be the ability of the tow vehicle to support the trailers weight.
 

ElFenix

Elite Member
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Mar 20, 2000
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the trailer's weight on the hook up is what keeps the trailer and the tow vehicle connected.
 

FoBoT

No Lifer
Apr 30, 2001
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some types of trailers have electric brakes
big trailers have pneumatic brakes

it would be redundant
the back of any towing vehicle that is big enough to pull a big trailer load already has a back axle
adding a front axle to the trailer which would then be next to the tow vehicles back axle is redundant
 

Demon-Xanth

Lifer
Feb 15, 2000
20,551
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The trailer's weight on the tounge keeps the system stable and hooked up. Without the added weight there is no added traction. There are systems on big rigs using dollies that effectively do what you say, but they are used for connecting the second trailer to another trailer rather than the trailer to the truck. Weight balance on a vehicle is a good thing.

until_now.jpg
 

dullard

Elite Member
May 21, 2001
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It is the forces on the tow hitch that matter. A big trailer on a small vehicles tow hitch will create forces that the vehicle cannot handle (extra wheels or not). The vehicle isn't really supporting the much of weight of the trailer (there are some but not a whole lot). That is, on the hitch, the main forces to be concerned with are not the vertical forces (ie gravity). It is the horizontal forces that matter (ie tugging and pushing horizontal to the ground). No amount of extra wheels will change these horizontal forces significantly.
 

skyking

Lifer
Nov 21, 2001
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It is the forces on the tow hitch that matter. A big trailer on a small vehicles tow hitch will create forces that the vehicle cannot handle (extra wheels or not). The vehicle isn't really supporting the much of weight of the trailer (there are some but not a whole lot). That is, on the hitch, the main forces to be concerned with are not the vertical forces (ie gravity). It is the horizontal forces that matter (ie tugging and pushing horizontal to the ground). No amount of extra wheels will change these horizontal forces significantly.
What he said. You have not lived until you experienced the feeling of a "too big" trailer kicking your arse all over the road. In a perfectly smooth world it would not happen, but dips, bridge approaches, potholes can all start something you cannot stop without a sufficient amount of mass at your command. I tow a 40,000 pound trailer regularly with about 7500 pounds of tongue weight, but I have a 30,000 pound tow vehicle to work with.
 
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DesiPower

Lifer
Nov 22, 2008
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why don't chiks have diks so that can fvck themselves... :ROFL;

something are made with their counterpart in mind... also the the handling will me much more difficult, cost of extra wheels and the added drag/friction.
 

rockyct

Diamond Member
Jun 23, 2001
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why don't chiks have diks so that can fvck themselves... :ROFL;

something are made with their counterpart in mind... also the the handling will me much more difficult, cost of extra wheels and the added drag/friction.

Yeah, I would think that the front, hypothetical wheels would have to turn with the vehicle so these wheels would be even more expensive.

There's the main reason as alluded to above that a simple speed bump would unhitch the trailer.
 

PaperclipGod

Banned
Apr 7, 2003
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The trailer's weight on the tounge keeps the system stable and hooked up. Without the added weight there is no added traction. There are systems on big rigs using dollies that effectively do what you say, but they are used for connecting the second trailer to another trailer rather than the trailer to the truck. Weight balance on a vehicle is a good thing.

until_now.jpg


What if the hitch attachment used something like a vertically floating pintle instead of a ball? Then the connection wouldn't require the weight of the trailer to keep things connected.

And would traction really be that big of a deal for the tow vehicle? I mean, a guy with a rope can haul on a trailer and as long as nothing is keeping the wheels from turning he can get it moving. Not very fast, but the point is that his feet don't slip all over the pavement trying to get some grip.

Theoretically I don't think that stopping would be an issue, as the trailers brakes would take care of the trailers mass. Although, both brakes better have the exact same braking power, or it'd probably jack-knife the whole rig pretty fast....

Anyway, the whole reason I started thinking about this wasn't to try towing a 40ft trailer with a Kia, but to give a moderate increase to the towing capacity of an SUV or something like that. e.g., Perhaps a 3k lb truck with a towing capacity of 2 tons could actually haul a 6 ton trailer if it didn't have to carry all the extra weight on its rear axle.

Or how about this -- the front "caster" wheel only engages the road when the tow vehicles axle is loaded to a certain point. Sort of like how dump trucks often have an extra axle that only hits the ground when its fully loaded.
 

Sluggo

Lifer
Jun 12, 2000
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Well, the worst part is that a trailer like that would be nearly impossible to back-up. Ask anyone who has flat towed a car or towed a car on a car dolly, trying to back those things up is an exercise in futility.
 

peasant

Banned
Nov 22, 2009
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I'm pretty much sure that tractor units and trailers in the UK are the same in operation as tractor units and trailers in the US, although the design may be different. In effect, the rear wheels of the tractor unit do what you ask for in your original question, the tractor unit slides under the trailer, which then mounts on a platform mounted behind the front wheels but in front of the back wheels of the tractor, I guess if it was a car, you'd call it mid engined.

I have seen the arrangement you pictured, and I'm sure that an experienced driver knows how to handle the reversing scenario with ease, having said that though, I have seen some lorry drivers, unhitch, reverse, reconnect, with the front pinion and drive the trailer into position.
 

Colt45

Lifer
Apr 18, 2001
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Apart from not sitting on the hitch right (main flaw), a tiny caster is going to spin at a bazillion RPM and cook it's bearings on short order.
 

dullard

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May 21, 2001
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Anyway, the whole reason I started thinking about this wasn't to try towing a 40ft trailer with a Kia, but to give a moderate increase to the towing capacity of an SUV or something like that. e.g., Perhaps a 3k lb truck with a towing capacity of 2 tons could actually haul a 6 ton trailer if it didn't have to carry all the extra weight on its rear axle.
Like I said above, it isn't the weight on the rear axle that is the problem. It is the fact that your transmission will be destroyed that is the problem. I have a good friend who tried something very similar. His dad has a decades long hobby of making trailers, so they were confident they could get an SUV to tow one of their trailers beyond the SUV's limits. 500 miles later, while stranded from a broken transmission, it only required $3000 in transmission repairs and nearly $1000 more for the rental of a proper vehicle for the trailer size to finish their cross-country haul.

Go ahead and reduce your vertical weight. Your transmission on a good SUV will die. Or who knows what would happen to the frame on a Kia car.
 

Glayde

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Sep 30, 2004
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I'll explain this properly, but we have to start at the beginning.

"It is a warm summer evening in ancient Greece..."
 

peasant

Banned
Nov 22, 2009
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The weight distribution thing here, is of no import. The weight is distributed along the whole length of the trailer. For a point, you do not have for eg, 40 ton loaded onto the trailer pivot point, nor do you have 40 ton weight distributed upon the rear axles exactly.

OK, for a trailer 40 ft in length, bearing 40 ton, eight wheel trailer, 3 axles plus tractor. Even given pure weight distribution per axle ( which doesn't exist, the weight being spread over 40 ft) gives 5 ton per axle. Dynamically this is corrext, kineticically it is a fallacy. In exactly the same way, you wouldn't load the trailer 30 ton on the left and 10 ton on the right.

This now, has gone beyond explaining physics, to explaining what should be basic common sense. There must be a simple element of comprehension to begin with. I cannot believe that is lacking, or if it is, it may explain the parlous state of the American education system.
 

DrPizza

Administrator Elite Member Goat Whisperer
Mar 5, 2001
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Slowly, as the OP stated, it's not a problem. If you're going just a couple miles per hour - I can't see any downside. In fact, at two of the marinas I go to, they do exactly what the OP has suggested with small 4 wheelers pulling large boats around the marina; far more maneuverable that way to move big boats into tight spots.

However, on public roads - there's no way to accomodate vehicles moving around at 3 mph. And, at higher speeds, it's pretty obvious why it's a bad idea, particularly in an area where the roads aren't perfectly straight and level.
 

PaperclipGod

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Apr 7, 2003
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The weight distribution thing here, is of no import. The weight is distributed along the whole length of the trailer. For a point, you do not have for eg, 40 ton loaded onto the trailer pivot point, nor do you have 40 ton weight distributed upon the rear axles exactly.

OK, for a trailer 40 ft in length, bearing 40 ton, eight wheel trailer, 3 axles plus tractor. Even given pure weight distribution per axle ( which doesn't exist, the weight being spread over 40 ft) gives 5 ton per axle. Dynamically this is corrext, kineticically it is a fallacy. In exactly the same way, you wouldn't load the trailer 30 ton on the left and 10 ton on the right.

This now, has gone beyond explaining physics, to explaining what should be basic common sense. There must be a simple element of comprehension to begin with. I cannot believe that is lacking, or if it is, it may explain the parlous state of the American education system.

WTF, why'd you go and get yourself banned? :/

Well, maybe someone else can continue for ya:

I assumed the weight distribution would be one of the most important factors, because if you really load up the rear axle of your tow vehicle, then it's going to reduce your traction while trying to steer the rig with the (now lightened) front-end.

If, instead, 90% of the weight is riding on the trailers axles, it shouldn't mess up the handling of the tow vehicle.

Colt45, as for the caster, I mostly just meant using a caster-style attachment, not necessarily a caster wheel that's as small as the ones underneath an office chair. Or, instead of that, imagine an attachment like the picture posted above, except your tow vehicle is only towing one trailer while using that dolly.

And dullard, I have no idea what it is about a transmission that makes it suitable for towing or not. I mean, assuming you dont have to worry about the physical stress on the parts of the drivetrain, I'd imagine that even a 20hp vehicle could get a larger trailer moving at a good clip, it'd just take it a much longer time to accelerate to a given speed than a 100hp vehicle.

Actually, if it is just all about stress on the transmission, then wouldn't that be alleviated by just accelerating at a much slower rate than if you were using a "proper" towing transmission? If a car transmission is rated for 100 lbf of towing capacity, then it could theoretically still tow a 10,000 lb load, as long as it accelerates slowly enough that the force on the hitch is never greater than 100 lbf, right? Likewise, that same tow vehicle could put the pedal to the floor if it was only pulling a 10lb load.

I know all this is getting pretty far removed from the realm of practicality, but I'm just curious.
 
Dec 19, 2009
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@paperclipgod, I assume you actually read the post carefully, the answer to your point is within it. You would no more overload the trailer from side to side, than you would put the entire weight on one axle, weight distribution should be even throughtout the length of the trailer.

In reality, the brunt of the weight/force is actually taken by the pin, not by the axles/wheels. The weakest point of any chain is the link.
 

PaperclipGod

Banned
Apr 7, 2003
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@paperclipgod, I assume you actually read the post carefully, the answer to your point is within it. You would no more overload the trailer from side to side, than you would put the entire weight on one axle, weight distribution should be even throughtout the length of the trailer.

In reality, the brunt of the weight/force is actually taken by the pin, not by the axles/wheels. The weakest point of any chain is the link.

So, the reason that some vehicles have smaller tow limits than others is because of the strength of the connection between trailer and truck?
 
Dec 19, 2009
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Not solely, other design factors come into it, maybe an issue for the manufacturers to deal with. Not only vehicle design, but also trailer design.

I guess one one to look at it though, is that you wouldn't use a cardbord egg carton to support the weight of your house. Design limitations and stress factors all play a part in the overall result.