Physics/Forces on two bike trailers: one centered, on off-centered
I recently wrote a review for two bike trailers (you can see it here:http://www.twowheelingtots.com/burley-dlite-2/
). One of the big differences between these two high-end trailers is that one has a tow arm that has a C-shaped bend in it, so the trailer is centered directly behind the adult bike. The other trailer's tow arm is L-shaped and off-centered behind the adult bike. We preferred the centered trailer for various reasons. We contacted the manufacturer, Burley, regarding the design of their tow arm, and this was their response:
"Burley has aligned the hitch along the centerline of the trailer and not along the centerline of the bike. This allows the trailer to track straight and makes towing a Burley trailer easier. If the trailer was designed to pull directly behind the bike with the centerline of the trailer aligned with the centerline of the bike, the hitch would have to angle to the left side of the trailer to hook to the bike. This would make the trailer want to constantly drift out and the trailer would not track in a straight line."
We had a little trouble understanding exactly what they meant by this, since the centered trailer performed just as well (if not better, for other reasons), than the Burley trailer.
So my question is -- is there any difference in forces on a trailer whose hitch is on the trailer's centerline, versus one whose hitch is off-centered a bit but subsequently centered behind the bike?
I'm attaching two aerial photos of the different trailers so you can see what I mean.
Thanks in advance!
There is a detail that I can't determine for certain from the photos. I'll describe my interpretation and proceed from there. I was uncertain whether the tow arms are rigidly fixed where they are attached to the trailers or if they can pivot left/right at the trailer connection. It is clear that they are free to pivot at the forward end, at the bike's rear axle. Studying the photo of the 2 tow arms side-by-side has me 90% convinced that neither of the arms can pivot left/right at the trailer connection.
Allowing the negotiation of a turn is the reason for at least some of the bends in the arms. Unfortunately those extra bends complicate study of your question. If, only for the duration of this study, we ignore that we occasionally need to turn, we could straighten the arms somewhat. Imagine that for both Thule and Burley, we keep constant the relative positions of the trailers with respect to the bikes, as shown in your attached photos. Then assume we build new tow arms that go straight from the hitch to the connection point at the left front corner of the trailer, without any bends between those points. (The bend in each arm at the trailer end, that is associated with coupling with the trailer, would remain.) This change in the bends in the arm would not change the force applied to the trailer. The arm could be a corkscrew as long as it is rigid and has the correct angles at the connection points.
With the arms simplified in that manner, the arms would go straight from the hitch to the trailer connection point -- straight but not parallel to the bike's centerline. The new Thule arm would be closer to parallel than the new Burley arm because of the Burley centerline offset preference. The point of this simplification was to be able to more easily visualize the interaction. It seems clear to me that on the road there would be a tendency for both trailers to want to shift to the right so that the straight section of the arm points forward. Since the straight section of the new Burley arm has a greater angle to the bike's centerline, I expect that tendency to be stronger for the Burley. Because of the bend in the arm required by the connection at the trailer, this would force the trailer off of perfect tracking. If both the wheels were on swivels, the trailers surely would shift to the right, but the direction of the wheels is fixed, pointing in the same direction as the sides of the trailer; and therefore the wheels would fight that tendency.
I would expect the result to be a compromise between 2 tendencies: the tendency for the trailer to shift so the tow arm is parallel with the bike's line of travel, and the wheel's tendency to move the trailer so the wheels track in the direction of travel. I believe the result of the compromises that the 2 bike/trailer setups achieve is that there would be a small angle between the centerline of the trailer and the bike's line of travel, causing some scrub at the trailer tire/road contact. I believe the angle discrepancy of the Burley would be greater. How significant would that be? Probably small for both, but smaller for the Thule. I believe the actual arms need to make the same compromise. You indicated slightly faster times for the Thule and credited aerodynamics. This phenomenon I have described may have been part of the Thule's better times.
I agree that a trailer (any trailer) should center on the bike for safety reasons, and for efficiency reasons my preference for the location of the arm's connection point at the trailer would slightly to the right of the left front corner. (I am talking real world now, so the arms would have to regain the shape that allows the bike and trailer to manage turns.) The result would be that the arm mounts to the trailer directly behind the hitch and the trailer centers on the bike's line. But perhaps mounting the arm at the corner was dictated by the structure of the trailer chassis. I feel that I need to say that my career was mostly in testing things. I have thought of tests that could be made to verify my interpretation. Without those tests, the above contains some possibility of misunderstanding.
I hope this helps,