Porpoising: What it is, and how to stop it

author image

March 23, 2009

A guy called me up the other day and told me his Dolphin motorhome was porpoising. I told him it sounded like it was having an identity crisis! (insert rim shot here).

Seriously though, porpoising is something we get quite a few calls on, and it is one of those terms that you hear about mostly with regard to RV’s. Porpoising is front to rear bounce; the front hits a bump, then the rear hits the bump, and they both bounce independently of each other, creating an oscillating motion. If the problem is severe enough, it can feel like you’re going to pull the wheels off the ground.

The problem is more typical in shorter coaches (low 20-foot to low 30 foot) than long ones. You don’t have as much trouble when you head up to 37 foot and beyond, and we definitely have more problems controlling it on a short wheelbase coach. The reason is that there is more time for the bump to settle out between the front and rear wheels on a longer wheelbase coach.

Anything from the springs on down is called “unsprung weight.” The heavier your axles and tires are, the more difficult it is to make them change direction. Because of the high amounts of unsprung weight on a motorhome, it’s been a real challenge for us. Between the wheel, tire and brake, you’ve got 300 lbs. there, and when it’s spinning, it’s like a gyroscope; it doesn’t want to change direction.

Changing out your shocks and sometimes your springs will usually diminish the problem significantly. On some of the IFS coaches, our Motion Control Unit (MCU) can help cut down on porpoising as well. Sometimes controlling airflow into and out of the bags can help in some applications, but not always. For example, MCU’s on some Freightliners; we’ve managed to get the side to side movement out of it, but then it started porpoising. I had a guy with a Spartan IFS chassis that he cut down some porpoising by putting on the MCU’s on the front.

On non-airbag coaches, good shocks will typically do the trick provided you’ve still got some decent springs. In situations such as these, we use the Koni FSD, and for the ultimate, the RoadKing 2-5/16 bore shock. FSD’s are good for rebound control, and the frequency selective damping takes some of the harshness out of the ride. The FSD valve is a secondary valve that they put in the piston head that dissipates the initial shock load. Most shocks are a linear rate; the harder you work them, the stiffer they get. That’s an age-old dilemma; you can make anything handle well, but it can end up riding like a lumber wagon.

We were fortunate to be involved on the protoype testing on the RoadKing. When we first experienced these shocks on a P30 chassis, it felt like we pulled 20 psi out of the tires and improved handling at the same time compared to a competitive shock. We’ve been working with King shocks for over ¬†three years. What we discovered with them is, the bigger the piston the more range of control, and the smoother the transition could be from small amounts of control to great control.

The RoadKing is a mono tube, high-pressure design which is the standard in high performance vehicles. It has the largest bore size on the market, so it has up to three times the volume of standard sized shocks. Standard pistons have 1.5 square inches of surface area; the RoadKing has 4.2 square inches-that’s 280% more surface area. The increased volume allows engineers to lower the required gas pressure in the cylinder, which gives you a smoother ride; the bigger piston also has more surface area to absorb the bumps. You also don’t have to run such tightly stacked disks in the piston head to control movement (check out roadking.com for more info).

RoadKing shocks aren’t cheap, but they last longer than anything else and they’re also rebuildable. The Koni FSD’s are a 100 thousand mile shock; The RoadKings are run 350,000 or more in the trucking industry before requiring a rebuild.

Leave a Reply

12 comments

  1. Pingback: How easy is it to install a ProPride Hitch? – Traveling Workerbees

  2. Julie Rea

    OK. Sounds like I should take a “road trip” down there sometime. I will call for appt. in few months. Thank you so much.

    -1
  3. Hello Julie,

    I apologize about not getting back to you sooner. Life goes on and I do miss Dad. He instilled some very important values in my life and gave me an opportunity to make the world a better place. I’m looking forward to seeing him again when I get home. Thank you for the condolences.
    I was hoping you would get the new rotors. We are still waiting for a Workhorse to show up with cracked rotors.

    To answer your question about your “porpoising” Dolphin. It certainly is not normal in my book. If you hit a dip in the road and it wants to launch like you are describing the shock upgrades we are talking about willl certainly help. For the rock and roll the sway bars and or track bars will help greatly.
    Next time down let’s go out for a Road Performance Assessment and see how your coach is performing.

    Once again thank you for the privilege to serve you, and thank you for the kind words.
    Robert

  4. Julie Rea

    My condolences on the loss of your father. I understand the difficult time this is for you and your entire family.
    I want to thank your staff for their courtesy and helpfullness when I brought in my Dolphin to have the rotors checked. I feel much better that I now know the brakes are all in great shape, just disappointed that I cannot be part of the study group for the new rotors from Workhorse.
    Since I am new to the maintenance and mechanical side of motorhoming, I am not sure what porpoising is. My Dolphin really bounces when going over rough roads, or RR tracks. There have been a few times that I am sure the passenger would have been launched if not wearing a seat belt. Is this porpoising or is this normal? It is a 2003, 32 ft. gas Dolphin. Seems like it has been like this since it was new. Does it need different shocks, or springs, or is this the way it should be?

  5. Eric

    I will look to see what I can find in your archives on 5th wheels and this problem. Just in case I can not find anything it would be great if you would talk about this problem with 5th wheel trailers sometime. Thank you.

  6. Ride quality is a significant issue with the deteriorating road conditions. Tire pressures , ride height and shock absorbers are all very important. RV’s have a high amount of unsprung weight ( all the weight on the suspension components beneath the springs like axles, hubs & drums-rotors-brakes, wheels and tires) and when all that weight hits a bump it takes a lot of energy to change directions. Alloy wheels help cut down on this weight. The Road King with it’s huge bore and the Koni with the FSD technology are great choices depending on your pocket book.
    On the 5th wheel application we work with various aftermarket products to improve ride quality. If you would like me to do a future blog on this let me know. I believe you will find some info in the archives on some of the things we do on trailers.
    I am very excited to let you know that we are introducing a line of Koni FSD shocks for pick-ups.
    We are at your service,
    Robert

  7. Catchesthewind

    I drove I-40 west to east through Oklahoma and I have never experienced a more miserable ride in my 38fy Winnebago Voyage. It felt like both axles hit the bumps at the same time. I was literally forced to drive below 55mph by the condition of the road all the time looking at oil well pumps pumping. My eye balls are still jiggling up and down.

  8. Jim

    How to stop porposing on a motorhome chassis? Koni FSD’s plain and simple…….deisel chassis ride on a gas chassis!!

  9. Larry Gallagher

    I had a class “a” 31 ft motorhome and not only did it porpoise but in any significant wind it would wear you to a frazzle fighting the wheel. I traded it in on a 29 ft class “c” and have never experienced those type of problems driving since! I know the class “a” was roomier but I guess there’s a give and take in everything. I enjoy getting to our destination and not feeling like a limp dish rag. Larry Gallagher, Maryville TN.

  10. Eric

    We have this same kind of problem with our 35 foot fifth wheel. Any ideas on how we can fix this problem?

  11. bob

    I installed the KONI FSD on my workhorse W20 and the ride was 1,000 percent beter and it handels very well now. You have road controll…. best $700 I spent. bob

  12. Oops. I meant to say we were involved in the prototype testing of the Koni FSD shocks on the P32 Chassis. We used to offer a quad shock kit for the P32 chassis. Once we we tried the Koni FSD shocks on the P32 we felt like we did not need to keep building them anymore because they worked so well that coach.
    The Koni FSD is an excellent shock for the money.
    One more thing I want to mention is that Road King is introducing a new type of valving called “Constant Force Valving”for the Independent Front Suspension Diesel Pusher Coaches like the Country Coach motorhome has.

    Thank you,
    Robert

    PS The founder and father of Henderson’s Line-Up, Floyd Henderson passed away March 15. I am so grateful for the parents God has blessed me with. I owe so much to them for the values they instilled in me. Thank you and best wishes , Robert