Receiver Hitch Snapped off at bolts!

[john98026]

I see a lot of responses regarding leverage and increased stresses due to drop draw bars but I wonder how many are actually engineers? I was a designer for a large aircraft manufacturing company for almost 38 years. Please note that I did not say I was an engineer. I just worked with a lot of them and learned while doing. Increasing the distance between the toad and tow vehicle has minimal effect on the load on the hitch. If anything, it would increase the chances of buckling of the a-frame during a hard stop but, that would be a really hard stop. Does a drop draw bar increase the loads on a hitch? Sure but, not much. The majority of forces on a hitch are in a straight line. Adding a drop changes the forces on the hitch pin from shear to a moment due to the now torsional nature of the load path. That said, the moment would be transmitted to the hitch and then to the vehicle frame because of the relatively close fit of the draw bar into the receiver. Side loads would be relatively low unless there was an adverse condition causing side loading (steering wheel locked, hitting a curb, etc). Without having access to the actual parts, we are all guessing. My guess is that the bolts weren't torqued properly, allowing movement between the hitch and frame. Over time, this fatigued the brackets. Once a crack starts, they can go quickly. Thankfully, nobody was hurt.

[09 harley]

Looks like a cheap hitch. If you make your decisions on parts, as manufacturers do, based on cost then you end up with this type of problem.
Look at the hitch on the new Super Duty and you will see what’s quality hitch look like. Poor engineering and parts selection looks like the problem.

[Mded40]

Inadequate lateral support for the hitch mount probably compounded by the drop down distance of the brackets leading to fatigue. Beef it up big and add supports in all directions at least twice the thickness of the square tube thickness. Hitch should have at least 4 brackets , 2 north/south and 2 east/west

[lllkrob]

Originally Posted by john98026

I see a lot of responses regarding leverage and increased stresses due to drop draw bars but I wonder how many are actually engineers? I was a designer for a large aircraft manufacturing company for almost 38 years. Please note that I did not say I was an engineer. I just worked with a lot of them and learned while doing. Increasing the distance between the toad and tow vehicle has minimal effect on the load on the hitch. If anything, it would increase the chances of buckling of the a-frame during a hard stop but, that would be a really hard stop. Does a drop draw bar increase the loads on a hitch? Sure but, not much. The majority of forces on a hitch are in a straight line. Adding a drop changes the forces on the hitch pin from shear to a moment due to the now torsional nature of the load path. That said, the moment would be transmitted to the hitch and then to the vehicle frame because of the relatively close fit of the draw bar into the receiver. Side loads would be relatively low unless there was an adverse condition causing side loading (steering wheel locked, hitting a curb, etc). Without having access to the actual parts, we are all guessing. My guess is that the bolts weren't torqued properly, allowing movement between the hitch and frame. Over time, this fatigued the brackets. Once a crack starts, they can go quickly. Thankfully, nobody was hurt.

As an engineer, for 27 years, with a major automotive manufacturer I agree with your assessment of the probable failure cause along with a possible metallurgical failure do to constant hitch vibrations.

[ARB357]

Probably cheap Chinese bolts. Only use USA stainless steel for important connections.

[StageCoachDr]

Contact Buyers, the manufacturer. They appear to be a quality company specializing in heavy duty, commercial hitches.

[bigb56]

Originally Posted by ARB357

Probably cheap Chinese bolts. Only use USA stainless steel for important connections.

So your'e saying cheap bolts caused the steel to crack and break away?

[renegade]

Unless you know the rating of your stainless steel bolts, they tend to be NOT as strong as alloy steel grade 8 or stronger. High strength stainless is available but you are not going to find it at local hardware store. The fasteners in this incident held, they were not an issue. From the pictures the thickness of the side plates looks marginal but without seeing it first hand or having measurements that is just a guess.

[Finance]

Originally Posted by Kountry_star

Reasons not to use drop / rise??? Our hitch manufacture says NEVER exceed 3" +/-. If it is exceeded you MUST use a drop or rise to bring it within spec.. We pull a Jeep Grand Cherokee Overland. It requires a 6" rise to just get within spec.. Roadmaster says good to go (just reconfirmed) they make up to 10". He did mention to reduce "play" as much as possible with accessories such as there "quiet Hitch". Manufacture says hitch is designed to take loads of drop / rise. I debate whether forum engineers should over ride tow system engineers. Thinking maybe: run your ENTIRE system by your manufactures (Neumar / Roadmaster in our case) to get about as good answer as is possible about what is safe. As a side note, I must say the hitch on our coach appears to be at least double the thickness of the failed one in the original post.

No one is saying towbars shouldn't be leveled; only to be aware of the effect on other components. And you've got no worries, because 4+6 is not the same as 4-6. If your receiver design is typical then likely the vertical lever and resulting forces on the receiver to frame fasteners is reduced, not increased.

[Renipladlo]

https://www.blueoxtowbars.com/produc...ceiver,BX88128

According to BO, the part to lower the towed vehicle to proper towed height, is rated at 5K, it extends the 5-10K rating, does not de-rate it.

As an aside, our present coach was built in 06, model year 07. In 2012 going though Phoenix, a driver lost her tire (blowout-tread failure), she slammed into the side of our motorhome in the radiator stack, and everything (Drivers side-Rear) was toast, no frame damage. She bounced off there and hit the side of my Jeep Wrangler, and then bounced off that and hit the guardrail. I felt the 1st hit, and looked into DS Side Mirror, and hit the camera button, Jeep was on two wheels for about 4 seconds, and then righted itself. If that did not stress the hitch with the 2" drop, nothing would. I have since had MH repaired and used it till March 2019, where we have delivered it to dealer for trade for the new one. Point of the above, I don't believe side to side or up/down movements contributed to this. One poster back in the first few comments said something about the grain of the metal, and how it's bent, and I would bet, that is what happened in this case, Plus, the thickness of the metal does not appear to be thick enough to really rate it to the 5K the manufacturer says. Metal fatigue is the cause here, but my guess, its because of the "way it was made" not the forces it was used under. My (2009 was in accident-currently tow 2013) Wrangler has been towed for at least 8 trips across the USA, with no issues period. But this is a HUGE WAKE UP CALL for all of us to inspect our towing stuff, at least annually, if not every 6 months. Mechanical Stuff sitting around in the elements is not good for it.

[arcaguy]

First thing, this break happened fairly quickly as there is no rust or the OP lives in a very dry climate. From the looks of the break to me it was originally caused but lateral motion. There also seems to be a design defect in that the side plates were flat steel when they should have been angles to keep them from flexing during lateral loads. There may also have been some issues with the hardness of the steel, but we'll never know unless the OP sends us a piece to test or the he has it Rockwell tested and posts the results. I'm betting neither will happen. (It wouldn't happen if it was my hitch either). The bolts appear to be fine as I did not see any evidence of them being loose. The take away for me is make sure that your hitch is well made and well designed. Flat steel isn't too strong, bent steel is.

[NeilV]

Unfortunately most hitches on Motor Homes are not bolted to the OEM frame rails from the chassis manufacturer but the mild steel extensions from the coach builder. Most times that cuts the capacity stamped on the hitch in half right off. Since most of those frames are mild steel they can flex more than the harder steel the hitch is made up of if you exceed the coach builders reduced as installed hitch capacity so you really can't go by the capacity stamped on the hitch by the hitch manufacturer as it could say 10,000 lbs but as installed only be sufficient for less than half that amount due to being mounted to a frame extension or even the lower basement frames. Add a drop hitch and capacity can be further reduced depending on the severity of the drop. A 1 inch drop probably would have minimal impact compared to say a 6 inch or greater drop.

A GMC Terrain of any year is probably near the max capacity of many motor home rear frame extensions especially if its the AWD version since empty it can be over 4,000 lbs and if you use it to carry any incidentals that you do not want to have in your basement or under the bed can be a tad over 5,000 lbs.

Sadly there are a number of folks that don't even realize that most hitches installed on motor homes can't take the stress of an equalizing hitch either.

Even more unsettling is many hitch installers don't understand the reduced strength of a motor homes frame extensions and upgrade your hitch to a much higher capacity telling you its all good when its really not. I have inspected used coaches on dealers lots for friends and found a great many with rear cross members cracked and rear frames sagging having a hump in the floor, where the factory frame ends, from overloading the back of the coach or towing heavy trailers with a high tongue weight. Some salesmen offer to correct this by adding air bags which only masks the damage however the real fix is to jack up the house frame to take the load off the extensions/cross member and repair or replace the damaged frame extensions and cross member which can crack like a failed wing strut on an airplane.

Too many get lulled into a false sense of strength when they look at the bulk/size of a Class A coach however compared to trucks of the same size and weight designed for carrying freight they are relatively delicate vehicles.

[hillbilly3]

Originally Posted by john98026

I see a lot of responses regarding leverage and increased stresses due to drop draw bars but I wonder how many are actually engineers? I was a designer for a large aircraft manufacturing company for almost 38 years. Please note that I did not say I was an engineer. I just worked with a lot of them and learned while doing. Increasing the distance between the toad and tow vehicle has minimal effect on the load on the hitch. If anything, it would increase the chances of buckling of the a-frame during a hard stop but, that would be a really hard stop. Does a drop draw bar increase the loads on a hitch? Sure but, not much. The majority of forces on a hitch are in a straight line. Adding a drop changes the forces on the hitch pin from shear to a moment due to the now torsional nature of the load path. That said, the moment would be transmitted to the hitch and then to the vehicle frame because of the relatively close fit of the draw bar into the receiver. Side loads would be relatively low unless there was an adverse condition causing side loading (steering wheel locked, hitting a curb, etc). Without having access to the actual parts, we are all guessing. My guess is that the bolts weren't torqued properly, allowing movement between the hitch and frame. Over time, this fatigued the brackets. Once a crack starts, they can go quickly. Thankfully, nobody was hurt.

You don't have to be an engineer to realize changing the point of leverage can negatively affect the loading of the hitch, an example, take a 2' crowbar, the amount of force is x, change to a 4' crowbar, the same amount of force is multiplied. This would be side to side, as well as front to back. In a turn, if the toad has minimal or no braking, the forces would be sideways as well as forward.

[Golfalot]

When metal breaks like that it is called a "Brittle Fracture". A severe and sudden overload causes this. If that is a drop hitch in the top picture and it looks like it's been scraped, then the hitch probably hit the pavement (usually when entering or exiting a private drive).