Re: [a/s] Fear This

[Jim Dunmyer <jdunmyer@xxxxxxxxxxx>]

Mike,
 Actually, the truckers themselves call those multiple trailers
"Wigglers". <<grin>>

Now, start up that movie projector in your head so you can see what's
going on:

The main reason that semis are much more stable (even with multiple
trailers) is because of the hitch geometry. The trailer hitch (5th
wheel) is nearly directly over the axle of the tractor or dolly. When
the tractor initiates a turn of any sort, nothing at all happens to the
trailer until the tractor actually moves off its line of travel. Then,
the trailer just follows. Also, the trailer wheels are very near the
rear, not in the middle. This is far better for both stability and
load-carrying than a typical travel trailer configuration.

Now, look at your travel trailer setup. When your tow vehicle initiates
a turn, the hitch (way back from the axle) moves the OPPOSITE direction.
Sometime later, it begins to move in the direction of the turn. You can
probably envision that this isn't really desirable from a stability
standpoint. You can also see that the trailer, attached to this long
lever arm, is going to push the tow vehicle around more than if it was
hitched to a point very near the center of the rear axle. Because the
TTs wheels are near the center of the trailer, it makes it easier for
the trailer to pivot, adding to the instability. Picture a 2X4, balanced
on something; you can spin it around like a top. Put the 'balance' or
support under one end and it's totally different.

One thing that a typical travel trailer hitch does that's good is make
it easier to make sharp turns at slow speeds. That hitch ball swings
out, making the trailer follow and not cut the corner quite as much. If
you were pulling a 5th-wheel trailer, you'd have to swing a bit wider so
the trailer wouldn't run over the curb (or whatever) on the inside of
the turn.

Everything's a tradeoff: a travel trailer must be made the way it is
because we aren't using a semi-tractor to pull it with, our vehicles
simply can't carry the weight. And, it's nice to have the
manueverability when we need it. Have you ever looked at the
relationship of the wheels to the body of a typical motorhome,
schoolbus, and Greyhound type of bus? The latter is much better for ride
and stability, but it takes a lot of room to turn them around. Both the
M.H. and the schoolbus are built to handle a bit more like a car. (even
at that, it's known that getting the ratio of wheelbase to overall
lenght below 54% makes for evil handling in a M.H.)

One thing that I do when I'm trying to envision and understand how
something works is to wildly exaggerate one or another component.
Picture your vehicle with a 6-foot ball mount sticking out the back and
go from there and you'll see what I mean.

                                               <<Jim>>