Trailer Physics - Why a Single Wheel?

[paul ben]

I've been thinking of building a homebuilt trailer for a scooter but two images that I found on the web leave me perplexed because they both show a single-wheel trailer with the wheel situated well behind the payload.

My reaction is that this can't possibly be smart because if the payload is laterally unbalanced it will want to tip over and pull the bike with it. The second objection is that half the weight of the payload rests on the back of the bike and therefore it will tend to pull down on the back and raise the front wheel.

It seems to me that a proper trailer should have two wheels situated under the center of the payload. What am I missing? What are the advantages of a single-wheel?

[babblefish]

A single wheel trailer allows for a simplified hitch that doesn't have to pivot/rotate in the horizonal axis. A two-wheeled trailer, even with a rotatable hitch could cause instability issues if one of it's wheel hits either a large pot hole or object on the road. As examples, lets say the hitch has rotation stops that prevent a full 360 degree rotation of the trailer - if the trailer tries to rotate too far (because of that pot hole, or whatever), it would run up against that stop and try and twist the scooter with it, not good. Or, if the hitch allows for a full 360 degrees of rotation, then a large pot hole (or whatever) could cause the trailer to flip over, again, not good.
A single wheel trailer is also lighter, which allows for more cargo weight. If you think about it, the hitch weight on the rear of the scooter is no different than carrying a pillion (passenger). Actually, it would be less as the single wheel is taking half the load.
In actuality, both single and dual wheel trailers have their merits and problems. Determining which is better depends on intended use and being aware of their limitations.

The location of a single wheel on a trailer can only be in one of three places:
Front - this would make the hitch length too short, but most importantly, the weight of any cargo would tend to unload the rear scooter wheel which means possible lost of traction and stability.
Middle - not too convenient for carrying cargo as you'd be restricted to two small storage platforms, unless you tend to transport a lot of thin, long objects.
Rear - the best place for the scooter's stability and loading convenience.

[skipper20]

I've been thinking of building a homebuilt trailer for a scooter but two images that I found on the web leave me perplexed because they both show a single-wheel trailer with the wheel situated well behind the payload.

My reaction is that this can't possibly be smart because if the payload is laterally unbalanced it will want to tip over and pull the bike with it. The second objection is that half the weight of the payload rests on the back of the bike and therefore it will tend to pull down on the back and raise the front wheel.

It seems to me that a proper trailer should have two wheels situated under the center of the payload. What am I missing? What are the advantages of a single-wheel?

A single wheel trailer can have a significant disadvantage if the single tire hits a pothole or debris in the road. A few years back I was following a large RV on a major highway towing a single wheel at probably 55-60 MPH when the trailer's tire hit something on the pavement which caused the trailer to swing upwards. It was up just long enough for the wheel and tire to rotate sideways. When the trailer came down the sideways wheel and tire hit the pavement with a screech giving off a large cloud of rubber dust. I don't think the driver was even aware of it. I suspect that if the RV was going any faster the impact would have scrubbed the sideways wheel, tire and rotating axle right off the trailer. And, being right behind the RV I would have taken the full impact!

Bill in Seattle but currently wintering in Tucson
'15 170i Hooligan Titanium (AZ scoot)

[ScooterDave]

A few years back I was following a large RV on a major highway towing a single wheel at probably 55-60 MPH when the trailer's tire hit something on the pavement which caused the trailer to swing upwards. It was up just long enough for the wheel and tire to rotate sideways. When the trailer came down the sideways wheel and tire hit the pavement with a screech giving off a large cloud of rubber dust.

That is a special kind of stupid towing a single wheel behind a large RV.

[cummingsjc]

I've been thinking of building a homebuilt trailer for a scooter but two images that I found on the web leave me perplexed because they both show a single-wheel trailer with the wheel situated well behind the payload.

My reaction is that this can't possibly be smart because if the payload is laterally unbalanced it will want to tip over and pull the bike with it. The second objection is that half the weight of the payload rests on the back of the bike and therefore it will tend to pull down on the back and raise the front wheel.

It seems to me that a proper trailer should have two wheels situated under the center of the payload. What am I missing? What are the advantages of a single-wheel?

A single wheel trailer can have a significant disadvantage if the single tire hits a pothole or debris in the road. A few years back I was following a large RV on a major highway towing a single wheel at probably 55-60 MPH when the trailer's tire hit something on the pavement which caused the trailer to swing upwards. It was up just long enough for the wheel and tire to rotate sideways. When the trailer came down the sideways wheel and tire hit the pavement with a screech giving off a large cloud of rubber dust. I don't think the driver was even aware of it. I suspect that if the RV was going any faster the impact would have scrubbed the sideways wheel, tire and rotating axle right off the trailer. And, being right behind the RV I would have taken the full impact!

Bill in Seattle but currently wintering in Tucson
'15 170i Hooligan Titanium (AZ scoot)

All of the single wheel trailers designed to be pulled behind a scooter or motorcycle that I have seen have a fixed, not rotating, wheel so the issue you mentioned would not apply. Additionally, I think that a few of them have hitch mounts that only allow up and down motion on the hitch (with minimal side-to-side movement) that is in line with the vertical axis of the bike. The single wheel motorcycle trailers are designed to let the trailer lean with the bike through turns vice operating with different angular momentum than the pulling bike through a turn (disclaimer: I am not a scientist/engineer/mathematician so my terminology might be off.). Supposedly, these are easier to ride with since the bike behavior is is more natural than when pulling dual wheel wheel trailers.

[cummingsjc]

This is information on the supposed benefits of a single wheel motorcycle trailer (from the Texas Motorcycle Trailer website (http://www.uni-go-trailers.com/TrailerThirdWheel.html):

The most beneficial aspect of a single wheel trailer versus a 2 wheel is the fact that it leans with you at all times. This allows the center of gravity to always be in the center, as it is on the bike. You feel no side force or pull while cornering, not even while braking in the corner!

It is easy to avoid road hazards and obstacles. This means that any debris that you miss with the bike will also miss the trailer.

The trailer is no wider than the bike, giving you better aerodynamics than a wide trailer will. If the bike fits through an opening, so will the trailer!

[Syd]

A few years back I was following a large RV on a major highway towing a single wheel at probably 55-60 MPH when the trailer's tire hit something on the pavement which caused the trailer to swing upwards. It was up just long enough for the wheel and tire to rotate sideways. When the trailer came down the sideways wheel and tire hit the pavement with a screech giving off a large cloud of rubber dust.

That is a special kind of stupid towing a single wheel behind a large RV.

I agree with you, but at least one company is making them http://cruiserlift.dudaone.com/swivelwheel.

[skipper20]

A few years back I was following a large RV on a major highway towing a single wheel at probably 55-60 MPH when the trailer's tire hit something on the pavement which caused the trailer to swing upwards. It was up just long enough for the wheel and tire to rotate sideways. When the trailer came down the sideways wheel and tire hit the pavement with a screech giving off a large cloud of rubber dust.

That is a special kind of stupid towing a single wheel behind a large RV.

I agree with you, but at least one company is making them http://cruiserlift.dudaone.com/swivelwheel.

Thanks for the heads up. Based on my experience, if you see one on the highway, do not under any circumstances follow it.

Bill in Seattle but currently wintering in Tucson
'15 170i Hooligan Titanium (AZ scoot)