# Car trailer which pushes the car.



## Pisoila (Feb 28, 2021)

I want a car trailer as small as possible but which to be equipped with an electric motor and to push the car.


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## john61ct (Feb 25, 2017)

Seen them for bikes but not cars.

I think in most jurisdictions getting past the regulatory authorities would be even trickier than the engineering.


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## brian_ (Feb 7, 2017)

There are many good technical reasons why this is a bad idea, and that's why it is uncommon. This has been tried with a gas engine as an assist device for an electric car, and while that's a bad idea I can see why people would want it... but why would you want an electric pusher?


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## Pisoila (Feb 28, 2021)

brian_ said:


> There are many good technical reasons why this is a bad idea, and that's why it is uncommon. This has been tried with a gas engine as an assist device for an electric car, and while that's a bad idea I can see why people would want it... but why would you want an electric pusher?


Can you please tell me what were the reasons for giving up this idea?


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## brian_ (Feb 7, 2017)

Pisoila said:


> Can you please tell me what were the reasons for giving up this idea?


It's unstable.
The trailer doesn't have enough traction.
The vehicle structure is not designed to be pushed by the towing hitch.

There are probably more, but any one of these is enough to make this a bad idea.


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## Pisoila (Feb 28, 2021)

Some vehicles have front-wheel drive, others have rear-wheel drive.
We can say that they are pushed from behind.

Even very long buses, not cars.

Normally, cornering acceleration behavior is very different.


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## Duncan (Dec 8, 2008)

The problem is the "what are you trying to do"??
If you have an electric car and you are using it as a range extender then just fill the trailer with batteries

If the car is a petrol car then throw away the petrol engine and put the electric motor in the car

The petrol engine in a pusher trailer as a range extender makes sense and is nowhere near as dynamically difficult as some people think

But an electric motor in a push trailer ????????


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## brian_ (Feb 7, 2017)

Pisoila said:


> Some vehicles have front-wheel drive, others have rear-wheel drive.
> We can say that they are pushed from behind.


Rear wheel drive vehicles are not hinged in the middle like the combination of a car and trailer, so they are driven by their own wheels, not pushed from behind - that's the important difference. Also, the rear wheels hold up about half the weight of the car, so they have traction; a trailer's tires carry only the trailer weight. The situation is very different.


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## Duncan (Dec 8, 2008)

brian_ said:


> Rear wheel drive vehicles are not hinged in the middle like the combination of a car and trailer, so they are driven by their own wheels, not pushed from behind - that's the important difference. Also, the rear wheels hold up about half the weight of the car, so they have traction; a trailer's tires carry only the trailer weight. The situation is very different.


True - but the result in the real world is not very much worse than towing a normal trailer 

Its more unstable than just a car - but I have towed all sorts of things for a lot of miles - just take care and you will be OK - most of the time


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## TeZla (Feb 18, 2021)

The only 'common' example of a road vehicle driving from the 'trailer' that i can think of is an articulated bus. They have a large 4 wheel bus for the front section, with a 'trailer' on a single axle that contains the engine and drive wheels. The connection between them is a solid engineered piece, Its nothing like a car trailer hitch.
They need to be big and heavy to take all the power of the engine, that puts the whole connection under compressive forces as the trailer tries to push itself into the front.

Trying to do this on a car is utterly stupid. It'll be unstable and difficult to drive, let alone pants shittingly terrifying if you were dumb enough to try drive it at highway speeds.


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## MattsAwesomeStuff (Aug 10, 2017)

"Push Trailers" were all the rage 10-15 years ago for DIY EVers. Few got built.

I heard nothing but positive experience with them, but it might have suffered from evangelical blindness that many early EVers blathered on about.

Back then batteries were shit, and the equation of how much lead acid to carry didn't have a solution. The weight penalty devoured its own utility. So vehicles were packed full of lead acid, which was just enogh to commute, and then some built gas-powered pusher trailers for road trips.

I wouldn't use one in any type of performance environment, but most highway driving is long, straight, dry, and uneventful. Yeah I suppose you've basically built a jackknifing machine, but, I doubt it's serious unless you're pushing it to its limits. The EV part can still do its thing if it needs to, too. "Not designed to..." doesn't worry me much. It's no different than braking with trailer behind you. The structure is fine, the driveline is probably fine.

A battery trailer is a poor solution in my eye. You're paying for all this extra capacity that you use a few days a year. No point. You'll never break even. Just rent a truck.

Towing a generator is another idea. It avoids the safety issues of a pusher trailer.

I saw a couple of pusher trailers that were literally just the front of a FWD car, cut off before the windshield. It has the advantage over a generator trailer in that you're not doubling up on the generator mass. It's just combustion to wheels.

It's janky though, and of questionably legality most first world places.


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## Pisoila (Feb 28, 2021)

Compare transporting a generator in a trailer with a car engine replacement with an electric motor, which most give advice here. How is this from the point of view of legality? How many of them have approved their cars transformed into electric?

I need approval to replace the engine with the same type of engine, from the same manufacturer with the same characteristics. (I'm not doing this, just as an idea)

What I proposed is not something practical!

It's a hobby, a brainstorming and from many ideas that do not work it is possible to appear a good one!
.
Second hand Hybrid cars have become quite cheap(They have run out of batteries, but you can replace them)I thought, maybe wrong, that we are here to say new ideas.
Some ideas are not good, that's right!


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## brian_ (Feb 7, 2017)

Duncan said:


> True - but the result in the real world is not very much worse than towing a normal trailer


if you have extremely low drive force, and drive only on good surfaces (no ice or even rain), sure.



Duncan said:


> Its more unstable than just a car - but I have towed all sorts of things for a lot of miles - just take care and you will be OK - most of the time


Towing things which are not driven is a very different situation... and even then it is not as stable as a single vehicle, but of course it's not a problem when done properly.


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## brian_ (Feb 7, 2017)

TeZla said:


> The only 'common' example of a road vehicle driving from the 'trailer' that i can think of is an articulated bus. They have a large 4 wheel bus for the front section, with a 'trailer' on a single axle that contains the engine and drive wheels. The connection between them is a solid engineered piece, Its nothing like a car trailer hitch.


True. And an excellent illustration of an articulation mechanism - thanks  That one looks like it has active hydraulic control of the articulation; I wasn't expecting that.



TeZla said:


> Trying to do this on a car is utterly stupid. It'll be unstable and difficult to drive, let alone pants shittingly terrifying if you were dumb enough to try drive it at highway speeds.


With just a ball hitch, yes. Even the buses have problems in low-traction situations - articulated buses occasionally jackknife themselves as they try drive up hills in ice and snow, when the rear wheels of the front section slide sideways due to the force from the rear section. Currently in Canada transit operators are buying double-decker buses instead of articulated, despite the many problems of headroom and requiring people to take stairs, because the articulateds are such a pain.


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## brian_ (Feb 7, 2017)

MattsAwesomeStuff said:


> "Push Trailers" were all the rage 10-15 years ago for DIY EVers. Few got built.
> 
> I heard nothing but positive experience with them, but it might have suffered from evangelical blindness that many early EVers blathered on about.


In the world of people who build this kind of crap, I think "it worked great" should be interpreted as "no one died".



MattsAwesomeStuff said:


> It's no different than braking with trailer behind you.


But a trailer of any significant weight legally and rationally requires its own brakes, so the pushing force on the hitch is small. And even then, people lose control with trailers.



MattsAwesomeStuff said:


> A battery trailer is a poor solution in my eye. You're paying for all this extra capacity that you use a few days a year. No point. You'll never break even. Just rent a truck.
> 
> Towing a generator is another idea. It avoids the safety issues of a pusher trailer.
> 
> I saw a couple of pusher trailers that were literally just the front of a FWD car, cut off before the windshield. It has the advantage over a generator trailer in that you're not doubling up on the generator mass. It's just combustion to wheels.


All of this is about assisting an EV with some sort of trailer... but Pisoila wants to build a trailer _with an electric motor_. We still don't know why.


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## brian_ (Feb 7, 2017)

Pisoila said:


> I want a car trailer as small as possible but which to be equipped with an electric motor and to push the car.


Agan... *why*?

If we understood what you are trying to achieve, we could discuss the solution more constructively.


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## MattsAwesomeStuff (Aug 10, 2017)

brian_ said:


> In the world of people who build this kind of crap, I think "it worked great" should be interpreted as "no one died".


Whoa whoa whoa. Let's not jump to conclusions. We have no information about what happened to all those trailers and the vitality of their owners. Only that while they were alive and still using them, they were highly regarded.

That said, no one dying is a decent benchmark. No accidents cause would be one I'm happy with. A sacrifice in performance and handling would be acceptable to me.



> But a trailer of any significant weight legally and rationally requires its own brakes


I dunno, a tent trailer doesn't have its own brakes, and plenty of times is loaded to 1-2 tons. Seems well within ability.

Again, performance-wise, you don't want any hard accelerating. And you definitely don't want it to be your main source of propulsion. And you don't want it to be something you use in situations with low traction.

But those are easily accommodated by only using it to maintain highway speed for longer range on the highway.



> so the pushing force on the hitch is small. And even then, people lose control with trailers.


Hmm, yes, but, that's perhaps like saying "A t-shirt is not designed to be worn backwards!" true yes, but that only introduces the topic, it's not sufficient to say it's catastrophic to do it. Maybe, like a t-shirt, it's perfectly fine to push on the rear frame or on the pin or or on the ball, just as it's perfectly fine to pull on them. I myself don't see much of a difference or things that would be true one way or the other... but I don't have a good understanding of why it even might be an issue.



> All of this is about assisting an EV with some sort of trailer... but Pisoila wants to build a trailer _with an electric motor_. We still don't know why.


Yes this is a weird use case.

Well, in one sense, it's building an EV with nearly zero modifications to the original vehicle, and zero conversion downtime. Seems to be the worst tradeoffs, towing a trailer around in town all day long is my version of hell.


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## brian_ (Feb 7, 2017)

MattsAwesomeStuff said:


> I dunno, a tent trailer doesn't have its own brakes, and plenty of times is loaded to 1-2 tons. Seems well within ability.


Not legally behind a car in Alberta. And if you try to stop a two-ton trailer with a two-ton tow vehicle and only the tow vehicle's tire braking, you will have half the acceptable emergency braking ability and poor stability while braking. Lots of people do it, but lots of people do lots of stupid things.



MattsAwesomeStuff said:


> Again, performance-wise, you don't want any hard accelerating. And you definitely don't want it to be your main source of propulsion. And you don't want it to be something you use in situations with low traction.
> 
> But those are easily accommodated by only using it to maintain highway speed for longer range on the highway.


In other words, it's nearly useless and a terrible way to achieve more range. It might even not extend range at all.



MattsAwesomeStuff said:


> Hmm, yes, but, that's perhaps like saying "A t-shirt is not designed to be worn backwards!" true yes, but that only introduces the topic, it's not sufficient to say it's catastrophic to do it. Maybe, like a t-shirt, it's perfectly fine to push on the rear frame or on the pin or or on the ball, just as it's perfectly fine to pull on them. I myself don't see much of a difference or things that would be true one way or the other... but I don't have a good understanding of why it even might be an issue.


Not really. The dynamics of pushing forward on the ball and pulling back on the ball are very different. I can get into why... but so far I don't see a point. It's like getting into the details of a university physics course in a conversation started by the question "do you have to apply force to something to get it to move". Umm... yeah, and when the conversation gets way further ahead we can get into the fine details.

For now, just think about the possible direction of the force on the ball, compared to the direction of motion of the tow vehicle and where it is pointed. Think about the vertical component, too.



MattsAwesomeStuff said:


> Yes this is a weird use case.
> 
> Well, in one sense, it's building an EV with nearly zero modifications to the original vehicle, and zero conversion downtime.


Is it? I still don't know what the use case is. Is the vehicle driven by an engine, and if it is then is the engine on while the electric trailer is driving? Is the vehicle electric, and the electric trailer is another whole EV slaved to it? Where does the electrical power for the trailer's motor come from - a battery on the trailer, a generator on the trailer, the tow vehicle? None of these scenarios make sense, but why the idea is bad depends on which nonsensical design is being considered.



MattsAwesomeStuff said:


> Seems to be the worst tradeoffs, towing a trailer around in town all day long is my version of hell.


That I agree with!


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## electro wrks (Mar 5, 2012)

Some ICE pusher trailers have been tried. One, by this wanker, JB-something or other: EV Pusher Trailer
I'll have to look up if he or his vehicles ever amounted to anything.
And then there's Mr, Sharky's(hardly a wanker, and always a good read) pusher adventures: EV Pusher | MrSharkey.Com
Or, in another case, pushing a 14,000lbs truck:EV Pusher Enhancements | MrSharkey.Com
The practical takeaway I get from these two examples is that this system works if there are 1000lbs or so of batteries (or a heavy truck!) holding down and stabilizing the rear axle. Does anyone know of examples of how this system worked where this was not the case?


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## MattsAwesomeStuff (Aug 10, 2017)

brian_ said:


> And if you try to stop a two-ton trailer with a two-ton tow vehicle and only the tow vehicle's tire braking, you will have half the acceptable emergency braking ability and poor stability while braking.


Yes, but that's an extreme situation. A pusher trailer is not going to be 2 tons. Braking with a trailer is a separate issue that can be resolved or addressed on its own.

My point was that the vehicle frame is going to handle those forces just fine, as it does in any other trailer situation when braking.



> In other words, it's nearly useless and a terrible way to achieve more range. It might even not extend range at all.


Not sure if you quoted the wrong part, but, an ICE pusher trailer is certainly going to be useful and massively extend range. EV pusher trailer, yeah, I'm having doubts.

If you have an EV where you want an ICE pusher trailer for longer trips, where you aren't looking to push the car to its limits, where you don't even need to use the trailer until you're up to speed, I think it would have great use.

One of the examples Electroworks mentions says he built his for $400. The savings on battery alone, in addition to the ability to refuel on trips, makes it a pretty solid idea in my opinion.



> Not really. The dynamics of pushing forward on the ball and pulling back on the ball are very different.


I'll accept that there's unknown unknowns for me here, but I don't see how it's different than towing a trailer normally and braking.



> For now, just think about the possible direction of the force on the ball, compared to the direction of motion of the tow vehicle and where it is pointed. Think about the vertical component, too.


I can't see anything different than towing a (much heavier) trailer, towing or braking downhill, with the weight of the trailer shoving the vehicle forward.



> Is it? I still don't know what the use case is. Is the vehicle driven by an engine, and if it is then is the engine on while the electric trailer is driving? Is the vehicle electric, and the electric trailer is another whole EV slaved to it? Where does the electrical power for the trailer's motor come from - a battery on the trailer, a generator on the trailer, the tow vehicle? None of these scenarios make sense, but why the idea is bad depends on which nonsensical design is being considered.


Indeed.


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## motas (Apr 21, 2017)

I think a better way to think about this would be a "drag reduction trailer" rather than a "pusher trailer", the big difference to me is whether there is a pushing or pulling force on the tow ball.

I can definitely see the benefit of a drag reduction trailer that works the opposite of override brakes. Basically use a force controller at the coupling which provides enough power to the trailer wheels to reduce the force on the coupling, but not to negate it. Regenerative braking would do the exact opposite by reducing the pushing force. This should extend the range quite substantially and not introduce any stability issues. 

There was a final year project at RMIT that focused on something similar.


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## Major Jeff (Jun 5, 2011)

electro wrks said:


> Some ICE pusher trailers have been tried. One, by this wanker, JB-something or other: EV Pusher Trailer
> I'll have to look up if he or his vehicles ever amounted to anything.
> And then there's Mr, Sharky's(hardly a wanker, and always a good read) pusher adventures: EV Pusher | MrSharkey.Com
> Or, in another case, pushing a 14,000lbs truck:EV Pusher Enhancements | MrSharkey.Com
> The practical takeaway I get from these two examples is that this system works if there are 1000lbs or so of batteries (or a heavy truck!) holding down and stabilizing the rear axle. Does anyone know of examples of how this system worked where this was not the case?


I do hope you are joking about JB-something or other.. ;-)


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## MattsAwesomeStuff (Aug 10, 2017)

motas said:


> I can definitely see the benefit of a drag reduction trailer that works the opposite of override brakes. Basically use a force controller at the coupling which provides enough power to the trailer wheels to reduce the force on the coupling, but not to negate it. Regenerative braking would do the exact opposite by reducing the pushing force. This should extend the range quite substantially and not introduce any stability issues.


Wouldn't that do no better than to break even? I.E. Free trailer towing, but no extra.

If the trailer isn't pushing on the coupling, it's not extending any range. It's at best acting like an empty hitch again, no?


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## motas (Apr 21, 2017)

MattsAwesomeStuff said:


> Wouldn't that do no better than to break even? I.E. Free trailer towing, but no extra.
> 
> If the trailer isn't pushing on the coupling, it's not extending any range. It's at best acting like an empty hitch again, no?


Correct, I was thinking basically so you don't kill your range if you have to take a trailer somewhere, plus the car wouldn't need to be as powerful to tow a heavy trailer around. 

If the idea is to hook on the trailer to make the car go further/faster than without a trailer on at all that seems rather inconvenient.


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## Infomaniac (Feb 1, 2021)

This is actually my first post here. I don't see why this is such a bad idea. I tend to be a glass half full thinker. I try to direct my thinking toward making something stronger rather that finding a way to dismiss it as should or can't be done.

My first thought is not being comfortable with the fwd force being on the locking tab of a ball hitch. I would reverse that.

If someone were to "need" a trailer to haul whatever load it could be powered to nullify it's own weight. And transition into some amount of push when desired.

I would add anti-sway bar load leveling system to one if I were to attempt a build. The fwd force could be shared with the trailer ball.


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## brian_ (Feb 7, 2017)

motas said:


> I think a better way to think about this would be a "drag reduction trailer" rather than a "pusher trailer", the big difference to me is whether there is a pushing or pulling force on the tow ball.
> 
> I can definitely see the benefit of a drag reduction trailer that works the opposite of override brakes. Basically use a force controller at the coupling which provides enough power to the trailer wheels to reduce the force on the coupling, but not to negate it. Regenerative braking would do the exact opposite by reducing the pushing force. This should extend the range quite substantially and not introduce any stability issues.


I understand the concept, but it assumes that a trailer is being towed for some reason (to carry cargo, or as an RV); the original poster of this thread has provided no indication of the purpose of the trailer.

If a trailer doesn't do anything useful, it only makes the vehicle's range worse (even if not much worse) and is worse than useless.


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## B307 (May 29, 2021)

I’m sorry your thread got trolled so hard. I don’t know what made these people decided to comment on this post when they don’t know anything about them. What you’re talking about is called a trailer pup and everyone I’ve known to have one used electric ones that functioned like the electro motors on a hybrid and pushed the car around at the control of a computer tuned to increase the gas mileage in city traffic. They didn’t just attach at the ball though because it had to be at an angle for it to have enough traction. I’ve only heard good things about them and they they’re super effective on motorhomes. As long as you aren’t one of those people who think of their vehicle as some kind of investment it would be great fun to make and make your commuting way more less expensive in a motor home. You can alway take it off and install it on another motorhome too if you get another


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## brian_ (Feb 7, 2017)

I didn't notice this post at the time...



Infomaniac said:


> My first thought is not being comfortable with the fwd force being on the locking tab of a ball hitch. I would reverse that.


The Andersen "No-Sway" WD hitch system pulls back on the hitch head with chains and pushes forward on the ball with the coupler. Yes, pushing forward continually with the coupler causes problems for the latching jaw of some couplers.



Infomaniac said:


> I would add anti-sway bar load leveling system to one if I were to attempt a build. The fwd force could be shared with the trailer ball.
> 
> 
> View attachment 122006


An anti-sway bar and a weight-distributing hitch are two different things. The illustration is of a weight-distributing hitch (WDH), which is incorrectly called an "equalizer" by people who don't understand how they work. A WDH does not transmit force to the tow vehicle horizontally - using one with a pushing trailer would not share pushing force with the ball.


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## Borrink2021 (Oct 8, 2021)

Did you all catch this news report from Airstream? Airstream Is Developing Electric Powered Trailers To Help You Save Fuel According to the article, Airstream is ... "now looking at developing an electric drivetrain into a camper, allowing extra driving force; basically assisting the engine of the car it's attached to." Maybe not so crazy after all!


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## brian_ (Feb 7, 2017)

Borrink2021 said:


> Did you all catch this news report from Airstream? Airstream Is Developing Electric Powered Trailers To Help You Save Fuel According to the article, Airstream is ... "now looking at developing an electric drivetrain into a camper, allowing extra driving force; basically assisting the engine of the car it's attached to." Maybe not so crazy after all!


The idea of a powered trailer is that the tow vehicle doesn't have to pull as hard, not that the trailer would ever push the tow vehicle. "Assisting the engine" is very different from "taking over for the engine".


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## john61ct (Feb 25, 2017)

And the engineering challenges even then would be more than a DIYer should take on.


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