Q. We have a 1999 Dodge Ram 2500 Quad Cab shortbed with the
5.9-liter Cummins diesel. We use it to pull a 1999 30-foot Dutchmen
high-profile, single-slide fifth-wheel trailer. Our problem is that at
times it “porpoises,” especially on concrete roadways. I am aware that
this is somewhat normal on certain roads, however, is there anything we
can do to lessen the effect of the roadway? Perhaps things like adding
shocks to the trailer, or changing the hitch box to an air bag/air
spring?
— D.H., Savannah, Tennessee
A. A lot of the so-called “porpoising” is caused by an
interaction between the natural frequencies of the truck’s and trailer’s
suspensions, vehicle lengths and wheelbases. Often, frame flex, which
is impractical to cure, is a partial cause.
Both of the ideas you mentioned should help. In addition, premium
replacement shock absorbers on your truck may help. Also, measure the
weight on each axle and adjust tire pressure accordingly, using a
load/inflation chart, available at tire dealers. Try the least-expensive
items first and make one change at a time and test the result. — J.J.
Jeff Johnston, TL technical director, started RVing at age 6. During his more than 20 years
as a writer/photographer, he has worked for Truckin’ and Four Wheeler magazines before
joining TL’s technical staff in 1985. Johnston also has produced an award-winning travel
video and TV commercials.
