Trailers & Levers & Associated Arcana

Bill Combs (
Wed, 18 Aug 1999 22:02:41 -0500

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West Wight Potter Website at URL
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> However, I fail to see how making the tongue longer will lower
> tongue weight. I would surmise that tongue weight is the amount
> of weight in front of the axle that exceeds the weight in the
> back of the axle. Extending the tongue length would only
> increase tongue weight by the amount of the metal added forward
> of the axle.
> Am I missing something? Engineers please correct me! I have
> been considering extending the tongue on my trailer about 3 feet
> just to get the boat in the water further at the ramp.

OK, once again: the whole mess is a lever. The fulcrum is at the
centerline of the axle (levers rotate about the fulcrum). The load is
the weight of the boat, applied in a vertical line at the
center-of-gravity (CG) of the boat (that's the definition of CG). The
CG is forward of the fulcrum if the tongue of the trailer is on the
ground when not connected to a vehicle, making the model a class 2
lever. The applied force is your hand lifting the tongue exactly at
the hitch socket -- which is the same as the force downward on the
hitch ball on the vehicle when everything is hooked up. The weight of
the boat (W pounds) applied downward at the CG (X feet forward of the
fulcrum) creates a torque at the fulcrum of W*X foot pounds. In a
stable system, this torque must be offset by a torque of F*Y foot
pounds, where F is the upward force on the ball in pounds, applied at
the hitch socket, which is Y feet forward of the fulcrum. If the
offset were not exact, the system would not be stable; some rotation
would be occurring. Thus:

W*X = F*Y

a basic law of levers.

Solve that for F and get:

F = (W*X)/Y

So, if you want to lower the force on the ball (F), you can do so by
making X smaller (= moving the CG of the boat backward = moving the
boat itself backward) OR by making Y larger ( = lengthening the tongue
so that the ball is farther from the fulcrum. The third possibility is
to make W smaller (lower the weight of the boat).

This all agrees nicely with an intuitive feel for the situation, which
I'm sure you'd have if I could make a diagram ... but our level of
email won't support diagrams easily.

Clear as mud?


Bill Combs
WWP 19 #439 (Aug 1987)
"Ursa Minor"
Fort Walton Beach FL