Prior to starting new plans, we do research to understand the landscape, then look for opportunity to improve and make a better product. That is certainly the case with our upcoming Car Hauler & Equipment Transport Trailer design. New plans are coming soon. – (Photo above from M&G Trailer.)
There are a lot of Car Hauler trailers with various design ideas and configurations. So, over the past while we have studied them, thinking about the good ideas, and about areas to improve. Of course, every trailer has a target purpose, so things don’t always compare. Yet, for this general category, there are some key similarities.
For this article I want to share JUST ONE finding that has me both confused and, a little shocked. With the perspective of trailer frame failures, I always look at the materials and think about the Engineering. Here are some things to think about.
In the Car Hauler trailer / Equipment Transport world, it is very common to identify and perhaps brag about the material size for the main beams. Names like H6X 6″ CHANNEL BUGGY HAULER, or the 6″ Channel Equipment Tilt (T6) are two examples. I like that concept, but for people that don’t really know, the name is a bit deceiving because it implies strength.
Along that same line, I like the trend in the trailer industry to be open about materials in general. Many websites and videos talk about material and spacing, for things like the frame, cross members, the tail, and deck, etc.. I think it’s great.
But, there’s a catch. Since you don’t really know what’s Strong Enough and what is not, it poses an interesting deception. What if the names and claims are smoke and mirrors? That won’t help our new Car Hauler Design.
Are we concerned if they are actually bragging about how weak the trailer is?
Car Hauler Loading & Design
Here’s what I mean. Let’s look at the capacity claim, then a load they intend. So, after creating a simple CAD model to represent the situation, I ran some numbers. My past experience says the large trailer manufacturers do a lot of bragging without the ability to back it up, so it’s worth it to check.
I have a particular interest in the tilt top car hauler design. I think they offer a nice simplicity, yet they have a design challenge.
The challenge is holding the entire load on a center pivot. But it’s more than that. The load is a car (or jeep, or truck, or tractor). The load to the trailer is therefore at only 4 points, and those 4 points are a long distance apart.
For the sake of argument, let’s use the dimensions of a pickup truck or large SUV. The wheelbase of a Ford F-150 SuperCrew® 4×4 is 157.2″. Weight is right around 5000 lbs. To compare, a Chevy Suburban has a wheelbase of 134.1″ and a weight of ~5700 lbs. Other large vehicles are similar.
If you had one of these vehicles, would you put it on a 7,000 lbs capacity trailer? A 5000 lbs vehicle would probably be near the limit (because the actual vehicle will weight more with fuel in the tank, a driver and perhaps some other stuff in it). For the Suburban or a larger vehicle, you’d really need a 10,000 lbs trailer.
While most 7,000 lbs Car Hauler trailers brag 5″ Channel in the design, some go as small as 4″ Channel. The 10,000 lbs trailers are a mix, but most 14,000 lbs trailers usually brag 6″ Channel. Here’s what I found.
Second Guessing Their Designs
The analysis model is a simple tilt deck. For simplicity, we are examining only the Tilt Deck, not the whole trailer.
For these analysis images, the crossmembers and other components are NOT shown. A pivot bar IS shown. While crossmembers are important on the actual trailer, once the vehicle is on it, only the cross members immediately under the wheels hold the load.
The condition we are evaluating is somewhat transitional, because it is the point of load balance. Since this is a broad look, we’ll acknowledge other minor factors, but we won’t analyze them. Admittedly, for driving, the load must be forward of the pivot to get proper tongue load. That can reduce stress by 15% or so, static, but it is also subject to driving dynamics like bumps and potholes. That is a different analysis.
We will use 3 vehicles to compare. While these numbers are not exact, they do represent typical loads and uses of a car hauler design trailer.
- An F-150 with a 157.2″ wheelbase at 5000 lbs.;
- A Suburban with a wheelbase of 134.1″ and a weight of 5700 lbs.;
- Just for grins, a Porsche 911 with a wheelbase at 97.6″ at 3800 lbs.
First, this is the result with the Porsche 911 on a trailer with 4″ C-Channel for the main beams.
That seems reasonable. Replacing the Porsche with the F-150 — 5000 lbs at 157.2″ — we get this result for the trailer with 4″ C-Channel main beams.
If the trailer is 7000 lbs capacity and it weighs less than 2000 lbs, then both of the above are within the load specs. However, the trailer frame would certainly fail with the F-150. From this analysis, even as simplistic as it is, I would NEVER have a car hauler with a 4″ C-Channel frame design. — Even if it was a double beam like you can see in the image at the very top of the page.
(The top photo is an aluminum car hauler design, so it’s a little different. I’m not seeing the double beam design in most.)
Reading the Results
Let’s take a minute and explain the graphics. First, the analysis setup is the image above with the tilt frame all in gray. The purple arrows pointing down are the location of the 4 wheels contacting the trailer deck. They represent the vehicle on the deck. Green arrows show the pivot axis. It’s a 16′ deck, but that doesn’t really matter because we are not using the ends.
Wheel placement is not perfectly accurate since it assumes a 50%-50% split for load front and back. However, the beam loading (moment) is the same near the center pivot at the balance point even if the actual vehicle is 60%-40% or some other number. Please remember, this is simplified to see the concepts, it’s not for exact results.
The scale at the right side shows a bunch of colors. They represent stress levels in the frame. Blue is near Zero stress, and Red is at the design stress of the material. (A36, rating at 36,000 psi.) The Bright Pink shows areas that exceed the material strength. That means failure. Sure, actual materials can be a little stronger, but it varies from batch to batch, so we design under the minimum. Read this article about material and safety factors.
One more important point. This analysis is Static (non-moving). In practice, we almost never have true static loads. While it’s nice for simplicity, trailers are all about dynamics with bumps, potholes, and dips. The car on the trailer will jostle up and down which momentarily increases load. For example, read about bending the tongue on this trailer. We compensate by designing with safety factors because we can’t possibly guess all the conditions it will experience.
Exploring Other Beam Options
So what happens when we change it to 5″ Channel? This is the result with 5000 lbs at 157.2″ on 5″ C-Channel.
For the 5000 lb. pickup, this is a marginal design. According to these results, it is just below the failure threshold. (Note the Max stress label.) So, this is possible, but don’t take it on a rough road.
The Suburban is heavier, but shorter, so let’s try it on 5″ C-Channel.
For all practical purposes, the stresses are the same. That shows us that wheelbase makes a big difference. The lighter vehicle with a longer wheelbase results in nearly the same tilt deck stress.
Next, let’s change the frame to 6″ C-Channel. This first image is, again, the 5000 lb F-150.
This 2nd image is the 5700 lb Suburban.
Again, the results are nearly the same for the 2 vehicles. Notice that the high stresses are now about half the minimum material strength. It’s a big difference from 5″ to 6″ channel.
We had to go to 6″ Channel to find something acceptable for these vehicles. Acceptable, to me, means not near the limit of failure. BUT, on most trailers the 6″ Channel doesn’t appear until the 10,000 lbs or 14,000 lbs trailers. Hmmmm.
14,000 Lbs Car Hauler Design
Let’s take it one more step. If a 14K Car Hauler weighs something less than 4000 lbs, then we can put a 10,000 tractor on it. Right? Let’s see what happens. Here is the 10,000 lbs tractor on a 6″ C-Channel tilt deck. Wheelbase set at 12′. What do we see?
Oops. The analysis shows the 3″ C-Channel cross bars fail. That’s what we have in the CAD model because that’s what the trailer manufacturers say they use. OK, we’ll give them the benefit of the doubt since most large vehicles will have a wheel stance that will put at least part of the wheel on the main beam instead of completely on the cross member as in our model. Well, for the sake of simplicity, we will change to 4″ and rerun the analysis.
This shows the 14,000 lbs car hauler design is right on the limit if you actually put 10,000 lbs on it. Over the limit, technically. If we’re lucky and this batch of steel is 3% stronger than the minimum, then it lives? If the wheelbase is longer, the beams will fail. Ouch.
We ran a lot of tests with fun images, but what what does it mean?
Basically, when comparing what trailer manufacturers are saying, when we put a these specific vehicles on, the tilt bed is either at the limit, or ready to fail. All of these are beyond the design limits that I am comfortable with.
Now in fairness, this is a simple analysis with some hand-waving to do. The analysis does not take into account extra structure that is present on some trailers. Some manufacturers have made accommodations for more of this load, but it’s not as complete as I would like to see.
Also, on the practical side, once the vehicle is on the trailer, it will move forward, past the tilt point. This will reduce the distance to the overhanging back load. However, no matter how you justify it, these numbers are painfully close to disaster.
Another Way To Look At It
If we do the analysis again, but this time use an evenly distributed load, we see something entirely different.
These look much better. Maybe that’s how big trailer manufacturers design? But, the data is irrelevant, because that’s not how a vehicle sets. Compare this to the F-150 on 5″ C-Channel frame. The truck puts weight in 4 locations. I don’t really know what they do, but it’s fun to experiment. If load ratings come from analysis like this, it’s a problem for something sold as a “Car Hauler” design.
One conflict for trailer makers is material choice. They want a light trailer to save money, so they go to the limits. But what about the failures? Just search “Bent Trailer Frame” on Google or YouTube. You’ll find them. Unfortunately, most people blame themselves, rather than the weak design.
From a personal note, I wish manufacturers would make robust trailers. Maybe it’s just my career path, but I get to see failures. People contact me about them, and I’ve made some good money consulting to get them fixed. That’s not how I want to make money. I prefer to work with happy situations rather than stressful ones. And, maybe that makes me skeptical.
Anyway, you get to benefit from the experience with plans here at Mechanical Elements. We do the full engineering, so build one that will perform.
Of course, building is not for everyone. If you are buying, get it one weight class heavier than you need. Then, label it down.
Examining Trailer Length
Another trend I see too often for Car Hauler and Equipment Transport trailer design is length choices. I think it’s wonderful to offer the various lengths, but it’s not what it seems. As we saw above, distance between the load points matters a lot. So, a chart like this showing trailers ranging in length from 16′ to 24′ (50% more length) using the same material is a little scary.
Of course, capacity is the same, and the longer trailer obviously weighs more, so the load you can put on it is less. And, it has the ability to spread out more. But, if you load something on the front (like pallets of bricks), then you need to push the skid loader back for proper tongue weight. That is opposite what we said above. That’s why, 50% longer with the same material is kind of scary.
The above analysis we did is based on a 16′ tilt deck. What happens when you put 2 Jeeps on a 24’er?
What Are We Learning?
The above quick analysis tells me our new design must consider the real loads. Failures in the field, and issues with weak trailers (also seen on YouTube and in Forums like this one) tell me I’m not wrong.
There are a lot of these trailers around, and they have a lot of history, so it can’t be that bad. Certainly, we can’t argue with historical success, but I also can’t ignore the “it’s so close to the edge” engineering. I think we’re learning once again about opting for cost effective instead of solid quality. They sure talk about quality, but I’m not seeing it where it counts. I think we’re learning about setting (and meeting) appropriate trailer design goals.
So, if you have a failure, a true warranty claim will they deny it? I’ve seen that in a some videos and forums. So, we might be learning the wrong things. Are we learning again: buyer beware?
There is a statistical element here too. I’d like to know if it’s real, but I wonder how much the manufacturers look at numbers. Do they think a certain number of failures are OK if they justify the lower cost? It’s hard to say.
Anyway, after doing the analysis above, I won’t be pushing the limits of any of those trailers.
How Would I Design a Car Hauler?
OK, I’m blabbing pretty negative in these last few sections. It’s much easier to critique than to create, so you might just say “Hey, put your money where your mouth is!”
Stay tuned, because we are answering the question with complete plans you can build. The first is a fixed deck at 20′, and 10,400 lbs. More Car Hauler Trailer designs are in process — made for DIY — with full engineering. Hang tight. Tilt Decks will come in a few months. (This is how we make the plans.)
There are a lot of things I really like about the trailers in the research we did. Some features we will include, some not, some will be in the plans as an option. While some features are similar to existing trailers, we are definitely putting our spin on them. Especially when it comes to strength. Our car hauler trailer(s) are different, and stronger than what we see from the big boys. Here’s the article on Car Trailer Philosophy.