D
Dennis Becraft
Guest
Pusher push trailer design considerations
Congratulations on your electric start! That's rare amongst motorized bikes.
You wrote
"The hitch is probably the strongest part of the build. While it do have less than 1/16" play in the vertical and horizontal slide, it has Zero rotational play. That caused my previous hitch to wobble like no other."
You also mentioned that stiffening your suspension reduced "a jerking feeling" that was due to the lifting action of the upward fraction of the push from your trailer to the seatpost.
I built my pusher as light as I could which revealed issues that needed to be addressed. Mine, like most or all single-wheel pushers I've seen here, tried to flop laterally because it was topheavy, also due to flex in the U-joint hitch and loose tolerances because the hitch was made from two gate hinges at right angles to each other.
Instead of making a beefier hitch to eliminate all "rotational play", I solved the problem by making the trailer stand upright on its own by pendulum effect. This is much easier to do with a seatpost hitch due to its height. You need more weight below a line from your hitch to the ground contact patch of your tire to make it stand upright. Then your hitch doesn't need to FORCE it upright. My lightweight flexy hitch worked perfectly when I solved the topheaviness. Look at the attachment to see what I mean. Notice it's longer than other pushers. Also notice the wheel is entirely above the line, so you need enough engine weight below the line to more than compensate for that.
You might think a beefier hitch is preferable to a longer trailer but there are other benefits. Leverage from a further back wheel results in less lift of the engine weight when hitting bumps. Look at the illustration and imagine the tire going up over a bump. The engine doesn't have to lift as far up as the tire does when you have a longer trailer.
Also a further back tire has less weight on it, allowing you to reduce tire pressure, which helps eliminate bouncing on rough roads. The longer trailer allows more frame flex. In my case the flex is in the conduit tongue. This flex adds more suspension effect. The first little bit of suspension is the most important (and the trailer doesn't need much).
In your case an important factor is that your powerful engine is lifting your suspension because it is pushing upward to some extent due to the height of the seatpost. Making a longer trailer also helps solve this problem because the push from a tire further back acts at a reduced angle, reducing the lift. This solves a minor problem with a pusher on a rear suspended bike.
Just like the suspension helps you lift the rear of a car a few inches, the bike's suspension increases the lifting effect of a short, powered trailer. That's why stiffening it reduced the problem. In effect, you eliminated some of your suspension. Just like you'd have to be extremely strong to lift a car bumper even an inch without help from its suspension, your pusher would have to be extremely powerful to lift your bike if not for its suspension.
So pendulum design by increasing trailer length, (especially between engine & wheel) can solve several problems, resulting in a simpler, lighter build that doesn't bounce, flop or "wobble" and doesn't significantly lift a suspension bike. It requires an easily built hitch that doesn't have to have zero tolerance axially. The trailer can be built without worries about axial torsion anywhere else in its construction either.
In short: DON'T MAKE IT TOPHEAVY ! Unnecessary topheaviness can even roll a 2 wheel pusher when cornering sharply.
I've posted most of this before and no one has really commented, agreed or disagreed. Please respond if you have opinions.
http://www.youtube.com/watch?v=XjEgsKqfi74 Here's my son on it at about age 10-12, about 18 yrs. ago.
Congratulations on your electric start! That's rare amongst motorized bikes.
You wrote
"The hitch is probably the strongest part of the build. While it do have less than 1/16" play in the vertical and horizontal slide, it has Zero rotational play. That caused my previous hitch to wobble like no other."
You also mentioned that stiffening your suspension reduced "a jerking feeling" that was due to the lifting action of the upward fraction of the push from your trailer to the seatpost.
I built my pusher as light as I could which revealed issues that needed to be addressed. Mine, like most or all single-wheel pushers I've seen here, tried to flop laterally because it was topheavy, also due to flex in the U-joint hitch and loose tolerances because the hitch was made from two gate hinges at right angles to each other.
Instead of making a beefier hitch to eliminate all "rotational play", I solved the problem by making the trailer stand upright on its own by pendulum effect. This is much easier to do with a seatpost hitch due to its height. You need more weight below a line from your hitch to the ground contact patch of your tire to make it stand upright. Then your hitch doesn't need to FORCE it upright. My lightweight flexy hitch worked perfectly when I solved the topheaviness. Look at the attachment to see what I mean. Notice it's longer than other pushers. Also notice the wheel is entirely above the line, so you need enough engine weight below the line to more than compensate for that.
You might think a beefier hitch is preferable to a longer trailer but there are other benefits. Leverage from a further back wheel results in less lift of the engine weight when hitting bumps. Look at the illustration and imagine the tire going up over a bump. The engine doesn't have to lift as far up as the tire does when you have a longer trailer.
Also a further back tire has less weight on it, allowing you to reduce tire pressure, which helps eliminate bouncing on rough roads. The longer trailer allows more frame flex. In my case the flex is in the conduit tongue. This flex adds more suspension effect. The first little bit of suspension is the most important (and the trailer doesn't need much).
In your case an important factor is that your powerful engine is lifting your suspension because it is pushing upward to some extent due to the height of the seatpost. Making a longer trailer also helps solve this problem because the push from a tire further back acts at a reduced angle, reducing the lift. This solves a minor problem with a pusher on a rear suspended bike.
Just like the suspension helps you lift the rear of a car a few inches, the bike's suspension increases the lifting effect of a short, powered trailer. That's why stiffening it reduced the problem. In effect, you eliminated some of your suspension. Just like you'd have to be extremely strong to lift a car bumper even an inch without help from its suspension, your pusher would have to be extremely powerful to lift your bike if not for its suspension.
So pendulum design by increasing trailer length, (especially between engine & wheel) can solve several problems, resulting in a simpler, lighter build that doesn't bounce, flop or "wobble" and doesn't significantly lift a suspension bike. It requires an easily built hitch that doesn't have to have zero tolerance axially. The trailer can be built without worries about axial torsion anywhere else in its construction either.
In short: DON'T MAKE IT TOPHEAVY ! Unnecessary topheaviness can even roll a 2 wheel pusher when cornering sharply.
I've posted most of this before and no one has really commented, agreed or disagreed. Please respond if you have opinions.
http://www.youtube.com/watch?v=XjEgsKqfi74 Here's my son on it at about age 10-12, about 18 yrs. ago.
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