Being new to towing a TT ( hadna 5th wheel), the porpoising of my GMC HD 2500 was driving me crazy. Trailer weighs 8000 pounds and with the WDH set up by the dealer, the whole rig rode like crap.It was strongly suggested that I add air bags, so I spent $400 on a set of airlift bags, and then the truck rides like a basketball....less porpoising but really bouncy ride an less stable. I tried several settings from 20 psi to 60 psi, and nothing worked. Either the truck porpoised badly or truck bounced....the wife got seasick on the last ride home.A guy at the last campground said I absolutely should not be having these issues with a 3/4 ton truck. He said NOT to rely on the dealer set up, and he helped me review the whole set up. The dealer had set the WD hitch chains on the last link, and he told me to go to the next link on the chains. So, I was able to adjust the chains by tightening up one more link. It sure made the ride a lot better....minimal porpoising now. When I go over a bigger bump it begins to porpoise but settles down quickly...I did measure the truck fender height at the top of the wheel well both unloaded and hitched up....based on what I have read, I think it is dialed in pretty wellFront unloaded 39 7/8" Hitched 39 3/4"Rear unloaded 41 3/4" Hitched 40 1/2"The front stays almost level, and the rear only drops an inch..from what I have read on this forum, and with the help of they guy at the campground, I this all is well now.We drove it home with no psi in the airbags, just to see how it would ride without them. Anyone want to buy a new set of slightly used airlift bags? HahaLesson learned...don't totally rely on the dealer's set up as every truck and trailer combination is certainly unique....

You need to keep at least 5 psi in those bags or risk ruining them... Also air bags are not connected to porpoising.... they are to adjust ride height with a load.. To properly adjust them simply measure the rear fender well without a load... apply load and adjust air pressure to return the truck to the same height... Be aware that if you use air bags you will need to re-adjust the W/D hitch to the new ride height...

It would SEEM like your spring bars on your WD hitch are likely the culprit. With spring bars not stout enough to handle your tongue weight, porpoising is the most common effect. Spring bars should be sized for a MINIMUM of the actual weight of your fully loaded trailer. If you don't have weights for the loaded tongue weight (not the empty weight on the brochure) then use 15% of the GVWR of the trailer. When the size is right and the adjustment is right you should experience ZERO porpoising without air bags or helper springs.Good luck / Skip

skipnchar wrote:It would SEEM like your spring bars on your WD hitch are likely the culprit. With spring bars not stout enough to handle your tongue weight, porpoising is the most common effect. Spring bars should be sized for a MINIMUM of the actual weight of your fully loaded trailer. If you don't have weights for the loaded tongue weight (not the empty weight on the brochure) then use 15% of the GVWR of the trailer. When the size is right and the adjustment is right you should experience ZERO porpoising without air bags or helper springs.Good luck / SkipHere is my 7700 lb. trailer with 1000 lb. spring bars.The only places where I have any porpoising are on certain streets in Kenosha and Brown counties in Wisconsin. Due to the way those roads were constructed I get porpoising without the trailer at any speed above 15 MPH.

Are your WD bars still parallel with the trailer frame??? Since you said you took up additoinal chain links I suspect not, as this is not the correct way to adjust a WD setup. YOu need to adjust the hitch head angle to increase tension on the bars. The bars need to be parallel with the trailer frame. Too many people pull them up tight and they do not work correctly in that position.A better set of shocks on the TV will help you with your porpoising more than anything. The stock GM shocks are garbage. I recommend Bilsteins...

Skip is right.You need to know the tongue weight of your rig to determine the correct distribution bars. My first TT (a toy hauler) had a tongue weight of about 1650. The dealer had installed 600 pound bars. Obviously, they didn't help much.Get the rig weighted and make a determination from there.Good luckBill

I think I solved Porpoising Problem | Good Sam Community

12 Replies

LarryJM
Explorer II
Jun 29, 2013
zogg wrote:

It was strongly suggested that I add air bags, so I spent $400 on a set of airlift bags, and then the truck rides like a basketball....less porpoising but really bouncy ride an less stable. I tried several settings from 20 psi to 60 psi, and nothing worked. Either the truck porpoised badly or truck bounced....the wife got seasick on the last ride home.

Well I sure don't think you got that "strong recommendation" on basically wasting that $400 on something that is unrelated to the porpoising issue you had. Too bad you didn't check here before spending that $$$ on something now you will have to mess with, but glad you finally got to the real cause and got it under control.'

Larry
Buzam
Explorer
Jun 29, 2013
Congrats! I had the exact same problem that was solved by increasing the tension on my WD hitch also. Feels so good to tow a properly set up rig, makes me wonder how many people are out there with sketchy dealer set ups...
BigToe
Explorer
Jun 28, 2013
I've experienced porpoising crossing Nevada's mining lands along 80. Trailer was light, almost empty, at about 4K to 5K. I ran the lightest bars I had for sway control 1,200. Tongue was less than 1K. I didn't bother to measure it, as I was towing with my 550. Horrible porpoising. Pulled over a couple of times, it was so bad. Right between 43-47 mph it would start. I was trying to drive 55, just couldn't push through the bouncing. Reminded me of those low riders that cruise the barrio.

My point is, I don't know that heavier rated spring bars is the universal solution to porpoising. i didn't need spring bars at all, I just had them for the dual cam sway.

I had Rancho 9000 shocks, and only about 20K miles on them. I changed the shocks anyway when I made it back, to the newer XL models. Ruined two tires... well, feathered them severely. One blew out later, so whatever.

Not entirely sure why the porpoising in that one stretch of roadway, and not the other 1,500 miles of the trip. I think it can be the road sometimes.

It can be an elusive problem to solve.
BenK
Explorer
Jun 28, 2013
Porpoising is the compression of the TV's rear suspension (mainly) to have it
release the compressive energy at a frequency that matches the natural harmonics
of 'that' setup

Lots of things play in this. Shocks (the biggie), springs, rating of the TV RGAWR,
tongue/Pin weight, tires, WD Hitch system, with lots of etc's

Since a 9.1K GVWR, on the higher class TV's that normally has a higher natural
harmonic than a softer sprung for ride quality half ton (spans from around
6.2K up to 8.1K for the fake half tons)

Shocks 'dampen' the rate of change for the suspension. Installing higher rate
will help to solve, but ride quality will go down. If really heavy tongue/pin
then that will wear out the TV's rear shocks sooner

Changing the spring rates will also move the harmonic. Higher harmonic with higher
rate springs (leaf spring pack, coil springs, air bags, higher rate WD bars, etc)
Why you got lots of advice to change the spring rates, but most don't think it
that way...just increasing the overall spring rates is the norm

Increasing the WD bar tension and/or changing to higher rate bars, to even
reducing the WD tension moves the natural harmonic around

Moving the natural harmonic 'around' so that whatever is causing (expansion
joints, ruts, etc) 'that' harmonic will not be tuned into 'your' TV's natural
harmonic

Why changing tire pressure works on some, as that changes the harmonics.

Speeding up, or slowing down a ditto in changing the harmonics vs your setups
natural harmonic.

Ride quality isn't on my have to have list and close to the bottom of my nice
to have list...so on my setup have HD shocks (KYB Mono's). Harsh for most, but
okay for me and am noodling going double shocks soon (welding up my own kit)
RCMAN46
Explorer
Jun 28, 2013
Increase your tongue weight and I bet your problems will go away. Number one cause of Porpoising with TT and chucking on 5th wheels is inadequate tongue/pin weight. How did you determine your tongue weight? You indicate about 10%. I would increase that to 15%.
zogg
Explorer
Jun 28, 2013
I bars are rated at 1400 pounds and the tongue weight is around 850 pounds. The bars a parallel with the trailer frame, and since the front stays the same loaded or unloaded, and the rear only drops about an inch when hitched up, i am not sure there is anything else that can be done.

I towed it once with no air in the bags to see how it towed.....really much happier. I removed the air bags and have them for sale....i hated the ride with them at any pressure.
MO_Trout_Bum
Explorer
Jun 27, 2013
Personally, I'd be headed back to the dealer to make sure that it was fixed. I.e. correct size trunnion bars based on Gross TT weight, correct head pitch angle, level bars, level TT front to back. Any dealer worth its salt should be able to do this.
Fishbreath
Explorer II
Jun 26, 2013
Skip is right.

You need to know the tongue weight of your rig to determine the correct distribution bars. My first TT (a toy hauler) had a tongue weight of about 1650. The dealer had installed 600 pound bars. Obviously, they didn't help much.
Get the rig weighted and make a determination from there.

Good luck

Bill
N-Trouble
Explorer
Jun 25, 2013
Are your WD bars still parallel with the trailer frame??? Since you said you took up additoinal chain links I suspect not, as this is not the correct way to adjust a WD setup. YOu need to adjust the hitch head angle to increase tension on the bars. The bars need to be parallel with the trailer frame. Too many people pull them up tight and they do not work correctly in that position.

A better set of shocks on the TV will help you with your porpoising more than anything. The stock GM shocks are garbage. I recommend Bilsteins...
camperpaul
Explorer
Jun 25, 2013
skipnchar wrote:
It would SEEM like your spring bars on your WD hitch are likely the culprit. With spring bars not stout enough to handle your tongue weight, porpoising is the most common effect. Spring bars should be sized for a MINIMUM of the actual weight of your fully loaded trailer. If you don't have weights for the loaded tongue weight (not the empty weight on the brochure) then use 15% of the GVWR of the trailer. When the size is right and the adjustment is right you should experience ZERO porpoising without air bags or helper springs.
Good luck / Skip

Here is my 7700 lb. trailer with 1000 lb. spring bars.

The only places where I have any porpoising are on certain streets in Kenosha and Brown counties in Wisconsin. Due to the way those roads were constructed I get porpoising without the trailer at any speed above 15 MPH.