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  • Jackknifing not actually bad?

    I talked to a GD representative today and while we were talking suspension, I figured I would ask just how bad Jackknifing is. He said that is a common myth, but that it is not bad at all for the suspension. He said that compared to the dynamic forces from bumps and regular driving aberrations, the forces (lateral and otherwise) during jackknifing are tiny. That all the creaking, popping and ugly looking angles are all normal operation.

    At first I couldn't believe what he was saying. So I re-asked 2 or 3 different ways , making sure he knew what I was talking about. But he insisted he understood and that it's harmless unless there is already issues with the system (he mentioned a few parts that could break if rusted as an example).

    What do you guy say to that?
    2021 Grand Design Momentum 381m
    2020 Ram 3500 8ft

  • #2
    I will politely disagree with the GD Rep on this. Sideways force on the spring hangers , especially in a jack knife situation, is bad in my opinion. Repeating this action may cause the spring hangers to break. I know there may be circumstances that this may be required but I would avoid this at all costs.

    Brian
    Brian & Michelle
    2018 Reflection 29RS
    2022 Chevy 3500HD

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    • #3
      Well, I know it looks bad. I don't know for sure if it is bad. But I tend to agree with Brian.
      I would be willing to agree with the GD rep if you are on dirt/loose gravel/sand instead of pavement or concrete?
      2018 Dodge 3500 6.7 Cummins w Aisin and 9 cup holders
      Not to brag or anything about my finances, but my bank calls me about every day to tell me my balance is OUTSTANDING!

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      • #4
        Originally posted by timelinex View Post
        I talked to a GD representative today and while we were talking suspension, I figured I would ask just how bad Jackknifing is. He said that is a common myth, but that it is not bad at all for the suspension. He said that compared to the dynamic forces from bumps and regular driving aberrations, the forces (lateral and otherwise) during jackknifing are tiny. That all the creaking, popping and ugly looking angles are all normal operation.

        At first I couldn't believe what he was saying. So I re-asked 2 or 3 different ways , making sure he knew what I was talking about. But he insisted he understood and that it's harmless unless there is already issues with the system (he mentioned a few parts that could break if rusted as an example).

        What do you guy say to that?
        I have video from under my Imagine that shows the lateral forces from sharp turns on concrete cancels out. The axles are laterally moving in opposite directions. The shock loads with spring wrap occur mostly on the rear hangers from hard braking along with front axle rise IMO.

        Jim
        Last edited by Guest; 05-07-2022, 07:00 AM.

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        • #5
          I would like to see the engineering test data that shows "compared to the dynamic forces from bumps and regular driving aberrations, the forces (lateral and otherwise) during jackknifing are tiny." Comparing the lateral loads on the spring hangers while forcing a sharp turn, to the vertical loads while towing/braking in the forward direction is an "apples to oranges" comparison. These are completely different loading scenarios (in my opinion).

          Rob
          Cate & Rob
          (with Border Collies Molly & Angel + Kitties Hazel & Elsie)
          2015 Reflection 303RLS
          2022 F350 Diesel CC SB SRW Lariat
          Bayham, Ontario, Canada

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          • #6
            I agree with Rob. Someone would have to show me the data.

            Likewise, normal driving loads are all applied to the strong direction of the hangers. Turning loads however are applied to the weak axis of the hanger (even with the angel braces between) since the bolt and spring do not effectively make the vertical plates act as one.
            Joseph
            Tow
            Vehicle: 2024 GMC K3500 Denali Ultimate Diesel
            Coach: 303RLS Delivered March 5, 2021
            South of Houston Texas

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            • #7
              Originally posted by Jlawles2 View Post
              I agree with Rob. Someone would have to show me the data.
              Engineers....always wanting facts to work with....
              2018 Dodge 3500 6.7 Cummins w Aisin and 9 cup holders
              Not to brag or anything about my finances, but my bank calls me about every day to tell me my balance is OUTSTANDING!

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              • #8
                I agree the data would be helpful as well since we are data driven. But that will never happen. I would especially like to see the data on the rear hangers versus the center hangers when its known the center hangers exhibit the highest lateral motion while the rear hangers exhibit the highest failure rates.

                Same with springs and tires. Most failures are on the rear axle.

                Many times engineers use engineering logic in analyzing an issue. Can anyone guide us in how most all failures occur on the rear axle and rear spring hangars?

                Jim
                Last edited by Guest; 05-07-2022, 01:42 PM.

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                • #9
                  Hey guys, don't shoot the messenger! I was very skeptical too, but he insisted it's overblown. No idea, but would be great to hear from other tech's.

                  I know there are alot of anecdotal accounts of "hey look at what happened to my frame/bearings/hangers"... But how many of them could be verified to for sure be done by the sharp turning? Maybe it was done on the road from bumps/rocks/etc and it was noticed after taking a close look after sharp turns? I don't think he ever denied that it puts stress on parts... Just that the parts get orders of magnitude more stress at speed or with dynamic actions.

                  If you guys ever call GD for something else, maybe throw in this question at the end. This rep said he has been working with trailers, including during fabrication part, for over 15 years. But lets see if a different rep has a different take!
                  2021 Grand Design Momentum 381m
                  2020 Ram 3500 8ft

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                  • #10
                    If the wheels came as far out of alignment when travelling down the highway as they do when backing the trailer . . . the tires would not last a thousand miles. The side loads are completely different. Obviously the hangers (usually) survive the sideways bending during backing and return to alignment for travel . . . but, this does not mean that highway loads are many times greater than backing loads.

                    Rob
                    Cate & Rob
                    (with Border Collies Molly & Angel + Kitties Hazel & Elsie)
                    2015 Reflection 303RLS
                    2022 F350 Diesel CC SB SRW Lariat
                    Bayham, Ontario, Canada

                    Comment


                    • #11
                      Originally posted by Cate&Rob View Post
                      If the wheels came as far out of alignment when travelling down the highway as they do when backing the trailer . . . the tires would not last a thousand miles. The side loads are completely different. Obviously the hangers (usually) survive the sideways bending during backing and return to alignment for travel . . . but, this does not mean that highway loads are many times greater than backing loads.

                      Rob
                      Agree Rob. I believe most failures at the rear axle (tires, springs etc.) and hanger are due to heavy braking that applies significant loading onto the rear axle and rear most hanger due to front axle rise. The rear hanger is the only solid anchor point to the frame under this condition. Slipper springs would cure this issue. Since most all input to the suspension is while traveling I could see this fellows point but it would be difficult to quantify due to all the variation.

                      Jim

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                      • #12
                        Guest
                        Hi Jim,

                        I agree that the most heavily loaded and most likely to break hangers are the rear ones. It happened to my 303! But . . . the break line in the “heat affected” bend adjacent to the weld is a classic “fatigue” break migrating slowly across this area. This would indicate repetitive side-to-side flex . . . not a failure due to longitudinal overload. I admit that I don’t understand why this is . . . but I have seen pictures of this flex fatigue break from a dozen or more trailers. The “V clip” reinforcement stops the side-to-side flex . . . and stops the hanger breakage.

                        Rob
                        Cate & Rob
                        (with Border Collies Molly & Angel + Kitties Hazel & Elsie)
                        2015 Reflection 303RLS
                        2022 F350 Diesel CC SB SRW Lariat
                        Bayham, Ontario, Canada

                        Comment


                        • #13
                          We have referred to this LCI video in the past . . . but it is worth a "revisit" for those who may not have seen it. https://www.youtube.com/watch?v=Bp5iRVPwYZg In particular, note the constant side-to-side flex of the spring hanger at the start of the video and then later on you can see the forward tire stop rotating as it lifts from the ground with moderate braking. This is the interaction through the equalizer that Guest Jim has referred to. From some analysis done by Gyrogearloose Pat, a while back, this phenomenon seems to have become more pronounced when the RV industry went to springs over the axles rather than under the axles (to gain ride height) without fully understanding the dynamic affect.

                          Rob
                          Cate & Rob
                          (with Border Collies Molly & Angel + Kitties Hazel & Elsie)
                          2015 Reflection 303RLS
                          2022 F350 Diesel CC SB SRW Lariat
                          Bayham, Ontario, Canada

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                          • #14
                            That video is unsettling, the side flex on the hangers is crazy.
                            2021 Reflection 337RLS, 2021 Silverado 3500HD 6.6 gas. Nellie the wonder boxer

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                            • #15
                              Looks like the 'equalizer' doesn't really equalize the suspension loads.
                              2018 Dodge 3500 6.7 Cummins w Aisin and 9 cup holders
                              Not to brag or anything about my finances, but my bank calls me about every day to tell me my balance is OUTSTANDING!

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