Enthusiast

Ok, first I think I should define what a "center of mass" is:
(From wikipedia) "is the point at which the system's whole mass can be considered to be concentrated for the purpose of calculations. The center of mass is a function only of the positions and masses of the particles that compose the system. " It's kind of like an average. For example, a person's center of mass is located in their abdomen near their belly button.

The center of resistance is a similar concept, except instead of masses and their positions, it deals with the forces of resistance(i.e. rolling resistance of tires and aerodynamic drag) and their positions.

A simple example: An arrow flying through the air. Most of the weight is concentrated in the sharp metal tip so the center of mass is located somewhere along the center of the shaft, closer to the tip than the tail. The fletching at the aft end creates more air resistance than the arrowhead and shaft, which shifts the center of resistance aft. Having the center of resistance behind the center of mass makes the arrow stable. This is because the drag force is a ******ing one, pulling back on the arrow from the center of resistance. In addition, the greater the distance between the centers, the more stable the arrow, as the drag force has a longer lever arm to keep the arrow flying true.
I found another explanation (with graphics!) online.

Now to apply these concepts to a bike tourist astride their bike, towing a trailer. The rider, bike, and trailer each have a center of mass and then all combined have a center of mass. Lets say this theoretical center of mass is somewhere aft and down from your bellybutton due to the position and weight of your bike and gear. The exact position will depend on just how much your bike and gear weigh relative to you. For the purposes of this post, I'm also going to ignore the vertical and transverse (side-to-side) position of the centers. It's complex enough and this is an internet forum.

The center of resistance has the following main components: air drag of you, your bike, your gear/trailer, and the rolling resistance of all tires touching the ground. The position of your tires rolling resistance is obvious (where they touch the ground) but the position of the center of resistance from air drag for each component is complicated. So I'm assuming that the center of resistance for the rider, bike, and gear is the same as their centers of mass. (I should also mention that the force of braking for each wheel can be thought of as part of that wheel's rolling resistance) Combined, we end up with a single center of resistance.

I will also mention that air drag increases exponentially with speed and rolling resistance increases linearly with speed. So air drag increases faster than rolling resistance as you gain speed.

Let's send that bike tourist down a steep hill. As they gain speed the air drag of the rider and bike increase quickly (the trailer is partially shielded from the wind), this shifts the center of resistance forward. If the center of resistance passes in front of the center of mass, the system becomes unstable! On a touring bike with panniers the bike will feel twitchy and may shimmy as it becomes less stable. With a trailer you have the additional risk of having the trailer fish tail or jack-knife. Think of it as if you and that heavy trailer are an arrow flying in reverse! The further back the center of mass, the sooner you will reach this point of instability. A light rider pulling a heavy load in a trailer is a prime example!

A quick word about damping: damping is resistance to motion (especially oscillation) in a system. It is quite common for bike tourists to ride bikes that are "unstable" according to the example above without any problems because we as riders "damp" out unwanted motions mostly unconsciously. The damping we as riders provide depends on a whole bunch of stuff: familiarity with the bicycle, fatigue, tension, etc. Our body weight relative to our bike and gear can also play a role in how much damping we provide.

I hope this all makes sense, but I caution that it is not the whole picture. Based on my explanation it may seem prudent to shift the center of mass forward and shift the center of resistance aft but this is not always so. I would hesitate to shift more than 60% of the weight to the front wheel for instance.

valygrl

Wow, thanks so much.

Enthusiast

My other post was huge so I thought I'd include more trailer advice here. It is based off what I learned when I built my own single wheel trailer and took it on a 1500 mile tour.

*Letting out the air in your trailer tire to the min necessary for the weight/terrain will increase stability. I've only done this ahead of really scary steep downhills, otherwise it's too annoying.
*I've also let my tarp flap in the breeze off the trailer to further shift the center of resistance back.
*Getting the trailer weight as low as clearance issues allow helps with stability.
*Bungees to keep everything still

To keep everything under control on a downhill:
*Get into a tuck position, even when braking. This helps a lot.
*To dampen out speed wobbles, don't try to keep the bike straight, force it to make sweeping turns

As for MTB touring, speeds are lower and rolling resistance has a relatively larger influence on the center of resistance, so many of the stability issues trailers see on the road are less likely on the dirt.

barturtle

Just to clarify, I believe you are referring to total weight, not just the luggage load, right?

For example in my earlier statement I stated that it is often recommended that 60% of your touring load be placed in the front. with 40% in the rear. To use a simple example:

on a bike with a 42 inch wheelbase and 17 inch chainstays and a 175lb rider, whose position puts his center of mass above the bottom bracket: which would put more of the riders weight on the rear tire than on the front. 45%F/55%R or 78#F/96#R. If a 50 pound touring load was then split 60% front 40% rear, you would have 30#F/20#R of touring load, moving the total distributed balance to 108#F/116#R. If that same load were to be all placed at the rear, you end up with a 78#F/146#R or 35%F/65% rear.

Of course this assumes that the center of both the front and rear loads were kept within the wheelbase, something that can be harder to do when only loading one end of the bike (normally that end would be the rear), and any weight placed outside the wheelbase would act in a way as to lighten the other end, for example extra large rear panniers, rack trunks or duffelbags that extend well to the rear, placing their center of mass behind the rear axle.

balto charlie

Excellent posts. I am not small(175lb/6ft) so can't reply to the OPs original post. I can say that having a 2 wheel trailer prevents a lot of the problems mentioned above. I have hit speeds above 40 mph on downhills and never felt any shimmy pulling a burley flatbed. Obviously this is not an option for single track but is an excellent possibility for paved touring.

Enthusiast

barturtle, yes I was referring to total weight. My 60% figure is not exact; I was trying to demonstrate that folks shouldn't allow the brief engineering lesson I gave to blind them to some common rules of thumb which have served tourists for decades.

balto charlie, thank you. 2 wheel trailers do reduce a trailer's tendency to shimmy or jack-knife. (in part because they create more air drag and rolling resistance than a single wheel trailer) They can be more prone to roll over issues when high speed turning though. Packing gear low becomes even more important for a two-wheel trailer than with a single-wheel!

redbike72

I'm sure you will get a different answer from every single person on the list. Like most of cycle touring, it all depends on you and your attitude. If you are in the right frame of mind you can do an Alpine tour on a bike made of old gas pipe banded together with duct tape.
To find out for yourself, you may want to borrow a trailer (or two or three) to see if they work for you on a weekend ride. Don't be afraid to try for yourself even if it goes against the popular vote.... at least you will know for yourself.

rnorris

Should have checked back in here earlier, but was out riding too much .This has turned into a really interesting discussion! Thanks, enthusiast, for your explanation of the dynamics of a rider-trailer system. I've dealt with similar dynamics in sailboats, except there it's center of effort in the sails vs. center of mass of the boat. (and bad dynamics lead to much wetter crashes).

In my experiences with carrying loads, I find I have to be careful when riding with a rearward weight bias on my main commuter (Marin Point Reyes) as it tends to start a resonant wobble in the front at higher speeds- particularly if the weight is centered fairly high, such as in a rack bag without panniers below.

I'd like to try a trailer, even knowing ahead of time it may not work- but then I tend to try lots of things just because I'm incorrigibly curious about everything bicycle related.