Solution for Porpoising With 2012 GMT3500

Updated 02/25/2026 | Published 02/16/2026

Question:

Do you have any products that will reduce porpoising? Our MY2002 28 foot Outback has always been prone to porpoising when towed with Gvans which are primitive and jouncy anyway. Our Outback is rated for 7000 pounds which is probably at or beyond what the trailer axles, springs and tires are capable of, so not a robust design. Our MY2012 GMT3500 long wheel base Gvan has more than enough capacity. I have a scale to make sure tongue weight is acceptable. Ive tried no equilization and two sizes of equalizer bars. Ride is perfect on smooth roads, horrid on bad Michigan roads. Have tried two brands of van shocks, but not Bilstine yet. Very little aftermarket parts for Gvan since nobody cares. Ideas?

asked by: Paul R

Expert Reply:

Hey Paul! Have you tried adding air bags? The Air Lift LoadLifter 5000 Ultimate Air Helper Springs with Internal Jounce Bumpers # AL88245 fit the Savana and Express 3500 (not 100% sure which you have) and will help reduce the porpoising you're experiencing. Since they're adjustable you can play around with them until they're set just right for your setup. They're also compatible with your weight distribution hitch so you can use them together for the best results.

What trips or Michigan routes are you planning with the Outback this spring

expert reply by: Robin H

Air Lift LoadLifter 5000
(click to enlarge)

Paul R.

2/25/2026

So you recommend air bags for my long wheel base 3500 Express van even though the heavy rear leaf springs barely move with 600 to 800 unequalized pounds of tongue weight on the hitch (measured with my tongue weight scale)? I have experimented running unequalized, then with 750 pound Draw Tite bars and 1000 pound Draw Tite bars, measuring all four corners of the van for trim height and adjusting accordingly. The van porpoises like the front suspension is very underdamped, although no sway complaints. The Gvan isn't great for rough road ride quality without a travel trailer and can be really objectionable with the travel trailer. I replaced the low mileage OEM shocks with Monroes best shocks with no improvement. I towed an old Airstream years ago with a Gvan and that van seemed much better with the Airstream than with my MY2002 28 foot Outback. I tow an unequalized three axle flat bed trailer that is perfect, so tend to conclude my Outback travel trailer and my Gvans are for some reason incompatible with each other? What to do?

Robin H.

2/25/2026

@PaulR I wouldn’t have had any way of knowing your rear leaf springs barely move under load, that’s helpful information. Based on that, air bags likely aren’t the best solution since they would effectively increase rear spring rate even more and could make the motion worse rather than better. Given everything you’ve already tested with spring bars, trim height, and shocks, there isn’t another suspension product I can confidently recommend that would resolve the porpoising. At this point it’s most likely the dynamic interaction between that particular trailer and the GM van chassis on rough pavement. If you decide to try anything further, a firmer monotube shock like Bilstein would be the last variable I’d consider.