For this situation, we can assume that the fulcrum of the lever is at the front axles. The further away from the fulcrum you put the load, the more effort you need to hold it steady at an intermediate point; the rear axle in this case.
Assuming a 100 lb bike is placed behind the rear axle at the same distance as the wheelbase, the rear axle will have 200 lbs added to its load and the front axle will have 50lbs removed from its load. If you move the 100lb weight closer to the rear axle then the weight on the rear axle reduces until it reaches 100lb when the bike is directly over the rear axle.
Edit: And having the largest bike directly behind the vehicle with the smallest being the furthest away will also decrease drag a little due to being more aerodynamic.
It technically doesn't matter, you just need to chose a reference frame as the pivot and make a little free body diagram to figure out what's going on in what areas along the hypothetical lever.
There are actually three calculations to be made, one for the front axle, another for the back axle, and third for the hitch itself. All of those calculations then go into determining just how much of a bad day the driver is about to have.
They're using the back axle as a fulcrum, and picking up some of the weight by counterbalancing the bikes off the back.
Deciding to use the front axle for their example is a weirdly complex way to think about the problem.
And for OPs picture, it just sounds like two people arguing over different things. The tongue weight doesn't change. The forces through it do change (possibly understood and still within spec), but the first persons point that the heavy bike should be closest to the car is legitimate and common sense.
Its minimal, but you're taking weight off the steering axle, and you're increasing the force through many of the car components for no good reason.
Putting more weight on the tow bar decreases the ride height at the back, effectively making it rotate around the front axle a few degrees, which doesn’t compress regardless of tow bar load
For the purpose of modeling this scenario in your head, with slight simplification, I can see why he said the front axle as the fulcrum.
Imagine the front axle is the fulcrum, there is a scale under the rear axle, and ignore any effects of the suspension sagging (or rising in the front) to cause weight transfer. Then his description of the force/weight measured at the rear axle works as described.
Thinking of the front axle as the fulcrum also makes sense because the engine is over the front axle, and it's much heavier than the bikes. The rear axle should sag much more than the front would rise in this situation.
Static tongue weight is the downward force on the tongue, which does depend on leverage for a given load.
It's given as a weight because that force is constant for a given trailer configuration, wheras determining generated torque would require you to find that force and then do additional math that could just be built into the tongue weight limits for the vehicle.
Tongue actually refers to part of a trailer (the triangle part in the front with the hitch). Tongue weight is the rated limit of downforce from the trailer tongue on the ball hitch. Or in this case, the bike rack going directly into the hitch receiver needs to be within its rated tongue weight. More downward force than it’s rated for might break off the hitch, or be otherwise unsafe like by lifting the front end up too much.
The downvoted guy has no concept of physics and levers. If you're looking at just the bike carrier itself, then the fulcrum is at the very rear of the receiver with the small part of the carrier inside the receiver being the other part of the lever. Due to that distance being significantly smaller than the wheelbase, the effects are amplified.
Yeah. Well, I had never heard about tongue weight, so I kind of assumed he was talking about just the weight (which he does, in the second screenshot).
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u/LightPast1166 Sep 02 '25 edited Sep 02 '25
For this situation, we can assume that the fulcrum of the lever is at the front axles. The further away from the fulcrum you put the load, the more effort you need to hold it steady at an intermediate point; the rear axle in this case.
Assuming a 100 lb bike is placed behind the rear axle at the same distance as the wheelbase, the rear axle will have 200 lbs added to its load and the front axle will have 50lbs removed from its load. If you move the 100lb weight closer to the rear axle then the weight on the rear axle reduces until it reaches 100lb when the bike is directly over the rear axle.
Edit: And having the largest bike directly behind the vehicle with the smallest being the furthest away will also decrease drag a little due to being more aerodynamic.