The Operation Was a Success, But the Patient Died

[Don52]

Three years ago, I installed a cargo carrier on my minivan to haul around my wife’s scooter. The scooter weighs 305 lbs and the tongue weight rating of my 2018 Chrysler Pacifica is 300 lbs, so I figured that I was ok. On a recent trip the after hitting a pot hole while leaving a rest area (that was full of water and therefore invisible), I noticed that the cargo carrier started to sag. By the time I got home it was badly sagged.

1. Cargo carrier sagging


2. Sagging cargo carrier closeup


When I got home, I removed the fiber panel that covered the rear-bottom of the car, including the hitch, for visual access. It turns out that the rear bolts on the hitch pulled a piece of metal out of the frame as shown below. The following two pictures were taken on the driver’s side.

3. Split car frame 1


4. Split car frame 2


The following picture was taken on the passenger side. There is an Aluminum heat shield that wraps around the exhaust pipe that partially obstructs the view. The bottom of rectangular cross section of the frame was originally flat. In this picture you can see that the bottom of the frame is now cracked and domed.

5. Split car frame 3

[Don52]

I figured that this failure required further investigation. The first step is to find the equivalent force spread over the four back screws of the hitch due, to the rated tongue weight of 300 lbs. I chose the rear bolts for the calculation, because the frame failed at this point, so it is obviously the weak link in the system. The following calculation takes the summation of the moments about the front screws of the hitch. It turns out that the allowable force due to the tongue weight acting on the four rear screws is 790 lbs.

6. Force on rear of hitch


The next step is to calculate how much load I can safely put in the cargo carrier, which will match the rated load on the rear screws of the cargo carrier, which in turn will prevent exceeding the tongue weight rating for the car. The first step is to take the summation of moments about the front screws of the hitch. The following picture is to scale. You can see that the cargo carrier applies a huge offset load to the hitch. Solving the moment equation for Fc results in 193 lbs. This is the total load that can be applied to the location at the center of the cargo carrier, which won’t exceed the loading on the rear screws of the hitch. The cargo carrier empty weighs 76 lbs, so the weight that can be safely loaded into the cargo carrier is only 117 lbs. Unfortunately, our scooter weighs 305 lbs, so it turns out that I have inadvertently overloaded the car frame by 260 %.

7. Allowable load on cargo carrier


One solution is to haul the scooter on a utility trailer like the following. It has a weight of 434 lbs.

8. Utility trailer 1


9. Utility trailer 2


The recommended tongue weight for a utility trailer is 10-15 % of the total trailer weight.

Total trailer weight = Trailer weight + Scooter weight

434 lb + 305 lb = 739 lb

Now take 10 and 15 % of this

10% of 739 = 79 lbs

15% of 739 lbs = 111 lbs

Even the more conservative 15 % tongue weight results in a 2.7 factor of safety compared to the car’s allowable tongue weight of 300 lb.

Do you have any additional thoughts, comments or concerns, regarding my proposed approach to solving the problem of the overloaded cargo carrier?

-Don

[farmersammm]

Before we go any further...................Tongue Weight refers to one end of a beam, with 2 supports. It does not take into account a cantilever load.

I can't follow your calcs, so I don't know if you're figuring a vertical load, or a rotational load. Remember............the further the carrier extends beyond the van, the more torque it applies............making for a very exciting moment at the support.

In the old days(when stuff worked), it wasn't uncommon that engineers applied an "X" factor which relied on either experience, scientific study, or bare gut estimate. I believe 4X was common for something that had to endure impact load.

I'm surprised the Space Shuttle didn't fall out of the sky sooner, given the analytical prowess of the engineers.

[farmersammm]

Let me continue the rant.............................. I despise the bell curve, and how it's applied to failure rates. It might look good on paper, and good to the bean counters, but it costs lives.

Anyways..........................CANTILEVER....... ...........................Zen sound mmmmmmmmmmmmmmmmmmmmmmmmm.....Cantilever

[farmersammm]

Go to that plebeian app...........BeamBoyV2.2...........look at the moment a cantilevered, uniformly distributed load, generates. The moment is probably substantial. Look at the supports, and guestimate whether the supports will disintegrate under that amount of torque...........then you have your design parameters.

[farmersammm]

I play a fabricator on TV, but in reality.............I'm a hedge fund manager

[farmersammm]

Doooood................don't go Freebird on us.........ya can't justify the work. Notice ya been online for a while. Bad is bad (Huey Lewis).

Get over it, and do it right. You're not that bad at what ya do.

Unfair................You betcha, but I'm getting tired of substandard work here. Your work isn't substandard, considering your background, but it ain't right in this situation. You're not charging for your work. That's the red line I draw.

[Oxford1]

You should be ashamed of yourself, running your mouth over anybody’s work. You can’t keep the mice out of your welder, and you never seem to finish anything. How’s that hay making going, anyway?

[Oxford1]

I think that’s a way better solution, but how are you going to fix the frame? The same mounting supports the ball in the receiver hitch, no?

[Oldiron2]

Doooood................don't go Freebird on us.........ya can't justify the work. Notice ya been online for a while. Bad is bad (Huey Lewis).

Get over it, and do it right. You're not that bad at what ya do.

Unfair................You betcha, but I'm getting tired of substandard work here. Your work isn't substandard, considering your background, but it ain't right in this situation. You're not charging for your work. That's the red line I draw.

Samm, your multi-post ranting has noting to do with anything Don has written here. Maybe he made unwarranted assumptions about the strength of the car in the first place, but this thread isn't about that. Maybe you need more of the "white pills" to keep your mind in check.

Don, my main question too is how does one/can you repair the car to make it stronger than as built? Or at least as good? How much is plastic vs metal and are those areas readily accessible?

[farmersammm]

360# distributed over 45". Fella once said "gimme a long enough lever, and I can move the world"

Actually, it's not all that uncommon. I see quite a few folks overloading carryall style platforms attached to a trailer hitch. It's not a problem on a pickup truck, but mini vans don't do well.

My prehistoric software doesn't take into account magnitude when impact is involved(it's a static load calculator). I'll generally, for safety reasons, go 2-4X amplitude.

A trailer will work, but it's a PITA to deal with. Life is hard enough. I used to have to haul a wheelchair around for my Grandmother, so I'm not blowin' smoke....... She actually was the one who hated it. She was stubbornly independent, and felt it was just making work for others.

Reinforce the hitch................maybe. Spread the load over a wider area, or beef up the members on the van.

[farmersammm]

Sort of fancy, but any lowboy trailer ought to work. Note that the above trailer is unsprung in order to lower the profile.

The set back axle, which makes backing easier, might be a problem as far as weight distribution goes. It would limit you to a trlr/load combined weight of around 600ish pounds

[farmersammm]

https://www.scootatrailer.com/

[duramax-rob]

what most people seem to forget is that a unibody auto, van, few trucks do NOT have a "frame". every piece of the thin metal inclu0ding the high strength thin steel, comprise the equivalent OF a frame. thats why trucks with a seperate frame don't have issues like OP van.

[Louie1961]

You should be ashamed of yourself, running your mouth over anybody’s work. You can’t keep the mice out of your welder, and you never seem to finish anything. How’s that hay making going, anyway?

I think the serial/machine gun style posts are a bit over the top, but Samm is spot on in his first post. Tongue weight is not at the end of torque arm. Moving that 305 lbs out a couple of feet probably more than doubled the stress on that hitch. AND the OP was already overloaded with just the weight of the scooter. It seems he didn't consider the weight of the platform he built which adds to the overall forces being applied.

[whtbaron]

what most people seem to forget is that a unibody auto, van, few trucks do NOT have a "frame". every piece of the thin metal inclu0ding the high strength thin steel, comprise the equivalent OF a frame. thats why trucks with a seperate frame don't have issues like OP van.

Yep... those unibody supports aren't much of a frame rail at all. I'd be looking inside , where I assume you'd be looking at the recessed tub for the spare tire. If you need to spread the load, I'd think a flat plate inside the trunk and bolt right through the "frame rail" and floor using the plate to spread the forces. Unfortunately everything you add, also adds to the weight and still doesn't address Samm's concern that you are overloading the vehicle. The trailer although inconvenient, may be the best choice short of a van with a wheelchair hoist. I'm currently dealing with the mobility issues of my wife and mother in law, so I'm familiar with the problems you are encountering.

[mrmikey]

I'd actually be scared of even 300 lbs tongue weight on body metal.
Not to rag on anyone or any trade but saying that....Engineers can tell you and prove to you it's strong enough, experience will tell you it isn't.
Where I used to work, we were making a s**t ton of 6" x 6" x 3/8 plates with 4 pieces of #3 weldable rebar, bent to an 'L' welded to one side to be imbedded into concrete precast panels for lifting and fastening together. A couple of us dumb a$$ welders/fitters that didn't know s**t commented to the engineer that the rebar was too small but what do we know and we were ignored.
Yup, month or so later when the panels were poured and ready to be lifted, a couple of them let lose at he welds, luckily with no injuries. "Bad welding, wrong procedure' we were told, The engineer proved on paper why it should have worked, we proved it by experience that it wouldn't.

[_Weldman_]

One thing could not and will never be accounted for is how much downward force was put on the load based on spring rate of the mini van, how deep each pot hole could be or was, speed bumps and the height of them. So cantilever weight or not and as impressive all the formulas put up in the presentation of what might have went wrong there is no definitive answer of max load based on unseen variables.
Question is where is the fudge factor, that's where this should of been built on, worst case scenario not best case scenario. Three hundred five pounds in a spot meant for 300 lbs and you don't know if the manufacturer put in a fudge factor or not. Pull with a winch capacity based on the weight only of the load and you will fail.

[farmersammm]

I could have handled this better. Being a jerk doesn't solve problems.

I believe Don can fix the hitch.

Remove the hitch, place a jack under the damaged sheet metal, and lift it to close the crack.

Don't get all technical about open root, full penetration welds. Close it up, and place a good MIG weld along the entire crack, with the weld standing proud of the metal. In other words........let it build up a bit, and make it wide enough to get some tooth in the metal adjoining the crack.

I don't like how the hitch is bolted on. It places all of the stress at single points,, instead of spreading it over the entire length of the "frame".

Go ahead and rebolt it after you fix the crack. This is strictly for locational purposes. Then run short 1" welds, spaced about 2ish" apart along the hitch surface you can reach. Back to front, get as many stitch welds as you can on that metal. This spreads the load over the entire surface.

Then...............make that damn trailer if you have the time to do it. I know y'all have to be able to move around, so that might be a real factor............she can't wait a few weeks for the trailer to be finished.

Trailers aren't magical, nor are they rocket science. Your welding is serviceable in the flat position. Small trailer like this, you can flip it to weld it. If you're comfortable with out of position welding, then go that route. It saves time, and stress on your body.

While I thought the axle was unsprung.................I noticed that they said, in the video, that the trailer had independent suspension. They may be using torsional style axles, which will also make for a very low profile.

BTW........................ Personally, I like steel for the trailer frame, and sides(you can substantially lower the sides to save weight). Aluminum might be lighter, but I've never seen an aluminum trailer that didn't start showing cracks after some service. Aluminum has a lower fatigue life than steel IIRC. If it comes down to determining the weight difference between the two materials......remember that the axle, and tires, do not figure into the load factor. Sure.........they add to inertia, but only marginally. All you're concerned with, is what's riding on top of that axle when determining tongue weight.

For piece of mind, I use a tongue scale. I don't rely on calculations. I like to see exactly where I can place the axle for ideal weight distribution, and transfer.

[whtbaron]

Looking at that van and the space taken by the scooter has me wondering. Do you use the back seat often or is it mostly just you and the Mrs? There "might" be enough room there to use light weight ramps to run the scooter into the back of the van with the back seats in their most forward position. Still a little on the heavy side, but easier on the chassis than what you've been doing...

[_Weldman_]

Looking at that van and the space taken by the scooter has me wondering. Do you use the back seat often or is it mostly just you and the Mrs? There "might" be enough room there to use light weight ramps to run the scooter into the back of the van with the back seats in their most forward position. Still a little on the heavy side, but easier on the chassis than what you've been doing...

Little of modification, they do use minivans down here to transport mobility scooters as such, some even go through the side door too.

[whtbaron]

I was thinking about the side door too... but that would eliminate any rear seating.

I managed to bend the rear suspension in a RAV 4 bringing 3 bingo grannies and their luggage home from the airport after a trip to Mexico. Seems they are only rated for 600 lbs of cargo. SUV's and mini-vans are not built like trucks.

[CAVEMANN]

The Ambulance company had a 97 K2500 Chevy SUBURBAN, it was decked out with the lights, siren, and rescue equipment and a 12k electric winch mounted on a winch rated BULL BAR. it was hell for stout when it was built, or so it seemed. The bull bar mounted to the frame horns on both sides, but instead of mounting it on the web it was mounted to the bottom of the flanges which were no more than an eighth inch thick at the end there were no reinforcement from the bar to the sides of the frame. After a few years of that thing bouncing around on the end of the rails it started splitting the frame rails. That all occurred after I started working there, except building the truck. I didn't have a welder yet so I pulled the winch and got it welded up, I replaced the BULL BAR with a proper winch bumper and solved that problem. The truck got nearly totaled after the service director hit a horse running balls to the wall wide open. I got hte frame straightened and put a used doghouse on it and drove the hell out of it for the rest of my time there. That rig had the crappiest brakes of any vehicle I have ever driven, K2500 with Hydroboost & you never knew if it was gonna stop or not!

[farmersammm]

This has been bugging me "don't go all technical about a full pen weld". That was just plain stupid. A MIG welder is very capable of doing a full pen weld on that thin stuff. Just widen the crack a skosh with a razor wheel, and have at it.

Big Business Sucks!!

I think I found the type of hitch installed on the van.

https://www.etrailer.com/hitch-2018_...w-3500_lbs_GTW

Yeah sure..............the damn HITCH is rated for 350........BUT WHAT ABOUT THE FRIGGIN' VAN???? These bastds will sell you anything.

They will even sell you a Class III hitch for the same vehicle. https://www.etrailer.com/Trailer-Hit...leID=201845395

I call this CRIMINAL!

[whtbaron]

I still have a brand new hitch for the 94 F150 sitting in a box. Because of the location of the rear spring hangers they made it in a big horseshoe shape with a lot of leverage on the bottom part of the frame rail. Made my own with more mounting points spread over the frame and never looked back.