Hi,
I don't know of any set number like the 75-80% of tow ratings.  There
is a formula for motor homes that is talked about and that is wheel
base divided by the overall length.  Anything over 50 is preferable,
the higher the number the better.  This is because a value of 50 or
over means the motor home has a shorter overhang behind the rear axle
which will usually indicate that it drives better and handles wind
better.

For a tow vehicle the worst case would be a short wheel base vehicle
with a long overhang from the axle to the bumper.  That is because the
long overhang would give the trailer even more leverage over the tow
vehicle.  If you think of the tow vehicle and trailer as a lever with
the hitch as the pivot point you can see that the shorter the wheel
base of the tow vehicle and the further the hitch is behind the axle
the more leverage the trailer is going to have.   It doesn't matter
when the trailer is tracking straight behind the towed vehicle.  When
it matters is when the trailer starts moving to one side or the other.
Then the forces are not pushing the tow vehicle straight but trying to
push sideways.  The more it moves to the side the harder it pushes the
tow vehicles rear to the opposite side.  The more leverage it has the
less it has to move to effect the tow vehicle.  Then the weight of the
TT matters too as the heavier it is the easier it will be for it to
push.

 If your tow vehicle has a longer wheel base and short overhang it
will take more force to move the tow vehicle to the side because the
trailer will have less leverage.  There is no tow vehicle towing a
travel trailer that is immune to the trailer swaying or pushing the
tow vehicle to the side no matter what the wheel base.  It's just the
nature of the beast.  But a longer wheel base tow vehicle makes the
combination more stable, tow vehicle weight would help some too.  The
best thing you can do if towing a travel trailer is get a Hensley or
Pull Rite hitch.  These effectively move the hitch point from the rear
of the tow vehicle to the axle of the tow vehicle making the travel
trailer behave more like a 5th wheel.  Which have much less tendency
to push the tow vehicle sideways then conventional travel trailers.

 I used a Hensley to tow my 30 foot TT and the difference between
towing with it and a  Reese weight distributing hitch was amazing.
The high sides of a TT make a difference too as they supply more
surface area for wind and semi's air streams to push against to get
things started.  Anti sway devices like the friction control device is
a band aid, IMO.  If a trailer is swaying it is doing so because
something isn't right. Those devices work both ways, in other words
they act like a brake to keep the trailer from turning to the side.
But if it does turn it acts like a brake to keep it from returning
straight behind the tow vehicle, especially bad in wet and slippery
conditions.

It used to be common to see large cars and SUV and pickups towing pop
ups with the pop ups swinging back and forth with nothing apparently
happening.  Except maybe wearing the heck out of the pop ups tires and
hitch and ball.  But pop ups used to be light, normally a lot lighter
then the tow vehicles.  But today some pop up and hybrids are almost
as heavy as some TT.  And cars and SUV's are lighter/smaller with
short wheel bases.  You get a 2500 or 3000+ lb. popup/hybrid swinging
back and forth and you could be in real trouble if you have a short
wheel base vehicle.

Of course there are a lot of other things that effect how a trailer
tows such as proper hitch set up proper weight distribution etc.  But
a longer wheel base will always be preferable to a short one.  IMO :)

Take care and Happy Campin...



 
RichA
"We Get Too Soon Olde and Too Late Smart"

Here is the chart you are looking for. While it is not etched in stone, it is a
helpful guide:

http://www.rvtowingtips.com/how-long.htm

Good Luck,

Mark Filice
2004 Homestead Settler 255RS
1999 Chevrolet Suburban 2500

There is an article providing two guidelines on this subject at:
http://rvtowingtips.com/how-long.htm

The first guideline, which I have seen elsewhere is:
For the first 110" of wheelbase, this allows you 20' of trailer.
For each additional 4" of wheelbase, this gets you 1' more of trailer.

The second guideline:
The distance from the coupler to the rear trailer axle should be no more
than twice the wheelbase of the tow vehicle.

The article has nice tables.  There are no scientific references given.

Chris you said:  "There a common rule of thumb cited here that you
shouldn't exceed 75% of the towing capacity of your tow vehicle..."

I realize that my reply to this is off-topic, but I want to share
another method.  Every TV has a GCWR and a GVWR.  If you subtract the
GVWR from the GCWR, your answer will be the largest trailer GVWR that
you should pull.

(GCWR-GVWR=GVWR)

This formula was given to me by another RVer, and many will argue that
it doesn't hold true to all vehicles.  But arguing aside, it is just
another "rule of thumb" to keep in mind for those buying a trailer to
match to the TV they already have.

Once I owned a Ford E-150 conversion van, which had a tow rating of
5000#.  The shame of it was, a 5000# TT was too heavy for the van.
After the fact, I learned the formula GCWR-GVWR=GVWR.  That applied to
the E-150 looks like this:  10,000# - 7,000# = 3000#  Anyone can see
that 3000# is much different than 5000#.

I replaced the E-150 with a E-350 with a tow rating of 10,000#.  When I
applied GCWR-GVWR=GVWR, I got much better numbers:  18,500# - 8700# =
9800# trailer.  The E-350 pulled the 5000# trailer like it wasn't even
there.

George