Hitch torture calculation - calling math geeks

I know I am over working my hitch on my 2019 Double Cab. But I want to know by how much with a reasonable degree of mathematical accuracy.


Here’s the facts.


I welded an aluminum double motorcycle carrier that weighs around 150lbs. It takes 2 bikes, one in front of the other. Each bike is 240 lbs. The weight of the first bike is placed 15” from the end of the receiver and the second bike is 35” away. See a picture of the configuration when my son and his buddy borrowed my carrier.


Additionally, some weight from the front axle transfers to the hitch and rear axle, which I can easily measure on a truck scale. I also have airbags which I usually leave at 4 psi unloaded.


My questions are:

1. My hitch is rated for 990 lbs. When hitches are “rated” what are the weight conditions / location? IOW, is the weight at the receiver? 5” from the end of the receiver? 10”?
2. What are the limits of the hitch system as it relates to the length of the hitch? This is a question of leverage and torque. What mathematical formula can I employ to calculate this?
3. The receiver hitch bracket that bolts to the frame is fairly long, distributing the weight over a longer distance. If the airbags centered over the axle are taking some of the load is this increasing or reducing leverage applied to the frame? I can add more weight to the frame above the axle by increasing air pressure to the bags which then shifts weight away from the spring perches fore and aft of the axle. Does this reduce strain or increase it?

My concern comes primarily from twisting my hitch and bending my frame where the hitch mounts.


Thanks.

 

I did the same thing except with a 4 wheeler and then had hitch on the back of carrier for a trailer. It was pretty white trash, lol. You should have zero issues with your load. I imagine without your airbags it would be petty bouncy. You will never fail your hitch, and even though Tundra have thin fragile frames again zero issues with your load.

I tow a 5800 lb boat with 600lbs tongue weight. The forces my hitch and attachment points experience starting, stopping, bouncing 70mph down freeway far outweigh your load...

 

I wouldn't worry about the loads you are talking about. Despite what people say, I have never seen an instance of a 07-21 frame bending from being overloaded whether its load in the bed or an oversize trailer. Plenty have bent in accidents, but that is to be expected. A google search will show others bending from loads, but never found a Tundra.

Disclaimer, I haven't looked into these kinds of engineering problems in a long time. I am most likely forgetting things and/or wrong about some of this.

To answer your questions:
1) EDIT See posts below. There is a chart I missed.

2) I don't think anyone can answer this without engineering data from Toyota (ie CAD models, etc). Calculating the moment is pretty easy, its force times distance. So if you have a 500lb tongue weight and your ball is 6in from the trailing edge of the hitch your moment is 250 lb/ft. Increase the ball length to 1ft and you get 500lb/ft.

3)This is probably your most complex question. You have to consider this in both static and dynamic conditions. Dynamic is really complicated and I don't remember the math tbh. Looking at it in static state, the airbags would create a third load point (call it "C") on each frame rail, shifting the stress away from the perches. With "A" being the front perch and "B" being the rear. As you add air, the stress will move from A and B to point C, essentially shifting the point load. Generally speaking the airbag is spreading the load over more points of contact and that will reduce stress. Leaf springs naturally distribute the load across the frame, the airbag can detract from this if you add too much air since it shifts more of the weight to one point (above the airbag). You would have to be pretty extreme before that was an issue though.

 

I think it might be @Sundog that has information that may be helpful on his signature. If using a phone you may need to rotate it.

Thanks Sundog!

 

At least one 2nd gen Tundra has broken its frame. It was a guy in a crewmax with a truck camper that hung over the end of his bed. Well behind the rear axle with no reinforcement. He hit the bottom of a washout and cracked the frame at the beginning of the C channel section. He has (had?) a YouTube overlanding channel. I forgot the name. He may have deleted all the episodes dealing with his frame saga. Ended up getting a new frame. I’m sure it wasn’t cheap.

 

If it really bothers you, use straps to go up over the tailgate and down to bed bolt d rings at the front floor with the other end on the rear of the carrier. Tighten the straps to take about 200lbs off the ass of the carrier. Which takes a bunch off the hitch itself.

 

It must have been taken down. I can't find anything on it. That said, those slide in campers seem to be really hard on trucks, really extreme loading in all the worst ways.

 

Wolfpac outdoors (youtube channel) had a similar thing happen. 2.5 gen crewmax with a flatbed and camper.

 

Not sure about your truck design, but in my case, I have a front receiver and could easily carry one dirt bike plus some gear up front.

Edit: OP says 2019, check Drawtite for a front receiver if you want one.

 

Please post pics of the carnage if/when the hitch snaps off

 

Much more secure in the BED of the truck.

 

That’s the one! Seems like he deleted some of the IG posts and YouTube videos as it happened and reposted about it later.

 

Guys, it won't snap. I haul a garden tractor on my DIY giant heavy (280 lb) cargo basket frequently. Zero worries.

 

Do you have to use Tow/Haul to do wheelies?

 

This is a great graph and it supports what I thought was happening. I’ve got around 650 lbs of static payload but it extends up to 35” past the receiver. The dynamic aspect of it is the surprise as you hit dips etc on the road. Thanks for the chats, gents.

 

LOL! With my aftermarket suspension, it doesn't even hardly begin to squat. Plus I have a Ranchhand heavy front bumper, front receiver, heavy E rated 35's, etc. Loaded or not, it's like driving on rails, it goes where directed without hesitation. Can't wait to seriously modify the front LCA's, UCA's, and spindles so I can really push it a bit more. More $$$, but more fun!

 

The 2018-2021 Tundra is rated for up to 10,200 pounds on the factory hitch setup. With the 10% rule that gets up to 1020lbs of tongue weight, so I'd follow the purple dashed line in the graph, then on the physics of the thing, the torque that the hitch experiences is the *average* distance of all the weight.
In other words, if you replace the two 240lb bikes with a 480lb block of lead, you would put it half way between where the bikes would have been. So, use the distance half way between the bikes as your lever arm length.
25" on the purple line is 450lbs, so yeah you're over by a bit. Is it enough to worry about? probably not... but I like the idea of throwing a couple straps over the bed to distribute some of that weight off the hitch and bring it further forward.
As for axle capacity and unloading the front axle, for that just drive onto the scales and compare against the axle capacity on your door card.

 

I never looked at it like that but that weight placement makes logical sense. Another helpful comment. Thanks.