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Loading That Trailer
Height of your Center Of Gravity is just as important as load distribution

By: James R. Davis



Several recent discussions with bikers who pull trailers has prompted an article on trailer loading.

It is generally understood that however you load your trailer, the resulting load should result in a tongue weight of approximately 10% of the total weight of the loaded trailer. Some have argued that it is safe to pull a trailer if the tongue weight is anywhere between 10% and 20% of the total. I disagree.

Since very few trailers employ their own braking systems, it is generally thought that the tongue of a trailer cannot 'dive' like the front-end of your motorcycle when you brake - that the weight of the trailer simply pushes straight ahead into the rear of the bike, and because the hitch is so low that causes essentially no handling problems for the biker. Again, I disagree.

Finally, it was argued that no matter how high or low the trailer hitch is relative to the rest of the trailer, handling is not effected and the weight on the tongue of the trailer is not effected. Total rubbish!

Why is 10% a good target weight to be found on the tongue of your trailer? Well, let's assume that the fully loaded trailer weighs 300 pounds. That puts 30 pounds of weight on the ball of your hitch if it is loaded as recommended. That does far more than simply add weight to the rear wheel!

You have leveraged those 30 pounds because it is FAR aft of your rear-wheel hub. The consequences are that you have substantially moved the Center Of Gravity (CG) of your bike towards the rear. By placing weight aft of the rear-wheel hub you have actually reduced the weight on the front wheel as well as increased the rear-wheel weight. i.e., you have added more than 30 pounds of weight to the rear wheel and diminished weight (traction) on the front tire.

But 30 pounds is usually not so much weight that you will have control problems. On the other hand, consider that you might also have packed more in your luggage and may even be carrying a passenger for that trip. Both, like adding the tongue weight, result in moving your CG aft of where you are used to finding it. This argues that you should take it easy for a few miles as you re-familiarize yourself with the bike's now new handling characteristics.

The discussions I mentioned earlier involved full awareness of the need to place your heaviest cargo directly over the wheels of your trailer. This, to minimize tongue weight. But what was totally lacking in evidence in those discussions was a sense of the importance of managing the height of the CG of the trailer.

You will recall from other articles on this site that any weight you place above your CG results in raising that CG and any weight you place below it lowers the resulting CG. So, while simply placing all your heavy cargo above the wheels will certainly tend to minimize tongue weight, it also tends to raise the trailer's CG - particularly if you decide to place some cargo on top of the closed trailer.

Why is that important? Because of weight transfer when you accelerate or decelerate.

You will recall from earlier discussions that the amount of weight transfer, in the case of your motorcycle, is a function of the height of your CG as compared to the length of your wheelbase. In the case of your trailer, because it does not have its own brakes, the amount of weight transfer is a function of the height of the CG above the height of the ball on the hitch, as compared to the distance between the wheel and that hitch. Acceleration and deceleration cause weight transfer. Because slowing your bike effectively pushes against the trailer at the ball of the hitch, and weight transfer is through your CG, you can see that if the CG is higher than that hitch a torque will be created that will try to make the hitch dive, just like your front-end.

The higher the CG, the greater the weight transfer will be. Since we already know that your hitch is part of your motorcycle 'overhang', aft of the rear-wheel hub, that weight transfer is leveraged into greater weight on the rear tire and less weight on the front. That leverage can be PROFOUND!

And handling is similarly effected even without acceleration or braking. Consider what happens when the bike rides over a dip in the road. As the front-end of your bike rises, the ball is pulled DOWN.

So?

Let's assume that you have measured the horizontal distance between the ball on the hitch and the hub of your trailer wheels and that the ball is at the same height as is the hub of those wheels. Then if you either raise or lower that ball that horizontal distance must decrease. In other words, this is the same as saying that your 'wheelbase' has shortened. At the same time, unless the CG of your trailer and cargo is at exactly the same height as the ball/hub, it will shift along that 'wheelbase' a greater distance than you would expect.

Let me be specific for a change. In order for there to be 10% of the total trailer weight on the ball of your hitch the CG of the trailer MUST be 10% of the distance between the wheel hub and that ball (closer, of course, to the hub than the ball.) Now imagine that the CG is 1 mile above the trailer when the trailer is level and at rest. If you were to lower the ball only a few degrees the CG would move very far forward (indeed, in this extreme case it would move way forward of the hitch itself.) From this imagined scenario it is easy to see that the higher the CG is, the more significant will be the weight transfer to the rear of the motorcycle and off the trailer wheels.

Your trailer CG is not 1 mile above the trailer. But it IS higher than the ball of your hitch. So, instead of a 30 pound weight on that hitch, there could well be 100 pounds there if stopping fast or riding over a severe dip. That could easily make your front-end feel light and certainly will reduce its traction.

If while riding level or at rest there is 20% of the total weight of the trailer on that ball, then you could have upwards of 200 pounds added as a result of stopping or riding over a deep dip in the road. Surely the argument that 20% loading is OK is now seen to be ill-conceived?

  • Load your trailer with as close to 10% of the total weight of the trailer and cargo on the tongue as you can estimate (Obviously, it does not have to be exact.)


  • Place your heaviest cargo at the bottom of the trailer, as close to directly over your wheels as possible.


  • If you are going to place anything on top of your trailer make sure it is LIGHT!


  • Determine the weight distribution of your trailer while the tongue of the trailer is at the height it will be when connected to your bike.


  • The longer the tongue, the less will be the weight transfer, all else being equal.


  • If you are pulling a trailer, use it to store your cargo - empty your bike's luggage (saddlebags) as much as possible.


  • Always take it easy for the first several miles of pulling a trailer so that you can learn the new handling characteristics of the bike.


Copyright © 1992 - 2021 by The Master Strategy Group, all rights reserved.
http://www.msgroup.org

(James R. Davis is a recognized expert witness in the fields of Motorcycle Safety/Dynamics.)

     
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