This post is a question from an engineering student wondering about trailer frame materials selection and safety factor. His analysis shows a material thickness that “does the job” is much less than his welder associates recommend. Wondering about why, he asks: “Am I missing something critical in the stress simulations?”
That’s a super good question because stress analysis for things with vast operational conditions is not plug-n-play. Judgement and experience play a significant role. There is also the practical side of engineering.
Practical Design Considerations
There is often a big difference between theoretical design and practical design. In theory, Aluminum foil works for many areas of a bicycle frame, but in practice, that’s a disaster. In a trailer frame analysis, no matter how hard you try, it just does not include everything. Since the nature of the trailer is one of surprises, every bump, every corner, every swerve and every load is different. Even if you limit it to worst case conditions.
Load cases also present an interesting challenge. Yes, the trailer will haul stuff down the road, but how will it be loaded? Does the load come on and off the back bumper? If so, did you analyze the case where a huge load applies only on the back bumper?
When loading a trailer, they say “put heavier items at the front.” Is that how you setup an analysis?
Then, don’t forget about abuse. Think of things like a load shifting around a corner, or bouncing a wheel up on a curb. How about backing up a little too far and bumping the pole behind you. This is all part of choosing trailer frame materials.
The dynamics of motion and impact are just one area of concern. In an engineering project years ago, we used a 12 ton hydraulic press to “smash” small metal pieces to the right thickness. Interestingly, we then did the same thing with a small slide hammer dropped just a few inches. Don’t ever underestimate the effects of impact. For this discussion, a pothole, or rock or a washboard road.
Dynamics are very hard to accurately analyze because every dip, every turn, every payload and every washboard road is different. The real question: What conditions do you set up for an analysis?
Material Properties in Analysis
Material properties are easy to assign in CAD models for FEA, and there is great info easily available. MatWeb is one of our favorite, because they have a huge database of materials. Yet, even with good representative numbers, there are limitations for analysis because of conditions. For instance, does your analysis include heat distress and annealing near each weld joint?
The areas around welds are the weakest part of the weldment — usually. Yes, if welded properly, in a test the parent material will break before the weld, but often breaks occur at the fringes of heat distress or other stress risers. The thinner the material, the more more important it is.
To help, we suggest practical measures like this discussion on mounting suspension.
Good design and practical experience are keys to success, because the analysis won’t automatically fix things to avoid welds in areas of high stress. It also puts an exclamation point on practical design bits like using Gussets. This idea of compensating for welds is especially true when modifying a trailer frame to add strength, or lengthen a trailer, and even when you want to widen the deck.
Trailer Frame Materials Selection
Commercial materials are available only in designated increments. And, within those increments, not all are accessible. 16 gage might be readily available, but 15 gage not so much. 11 gage is quite common as (almost) 1/8″ and it’s easy to get. How much heavier is the trailer if built in 1/8″ over 1/16″? Again, this is a practical matter. If on a smallish trailer the difference is only 100 pounds, then does it really matter? Maybe a lot? Maybe not much? That’s an engineering judgement call.
Adding 100 pounds to your bicycle trailer can mean the difference between making it usable or not. On the other hand, for a utility trailer, if a small change in material keeps a minor mishap from ending up with a damaged (weakened) beam, then it’s worth considering.
And on the topic of weakening beams, our post on engineering for welding on the main beam is worth a read.
There is always a “Weight / Cost / Strength” balance with trailer frame materials.
Safety Factors In Trailer Design
Finally, safety factors are a big deal. I do not know of a specific safety factor established for trailer frame materials selection. In many cases only bad experience will teach you what’s right.
For components, like axles and couplers, I trust the proper safety factors are already in the design. If it says 3500#, I trust it’s good for normal wear and tear up to 3500#.
That said, we also know that pushing anything at the limit is much more likely to cause problems. So, especially for items like tires on the front lines of action, I like to specify them over the requirement. Perhaps 5% – 10% or even more.
A safety factor is a way of compensating for anomalies, transitions, and things the design simulation does not include. So what is the right number? That’s a great question. As humans, we love it when things can boil down to a single number. I wish it did.
Safety factors and human analysis of the computer design analysis is where good engineering judgement and experience really come into play. It’s balancing the potential load conditions — with the practicality of using the trailer — with trailer frame materials selection — with results of the analysis — with use cases — with experience. I’d love to give a simple number and say that’s it. Unfortunately, it’s not that simple.
What Does The Analysis Really Mean?
Remember the old adage for computers? Garbage In = Garbage Out. It’s still true, and it has a special place in simulation. Just because we put in loads and assign trailer frame materials does not mean it’s correct. The pretty pictures don’t mean it’s right, or even that we constrained the models accurately. Computers do a great job of theoretical analysis with what they’re given, but they can’t yet think through it all for us. We supply the input.
As a matter of course, the analysis we do at Synthesis for the trailer plans on Mechanical Elements is multi-dimensional. We do various loading cases and think through abuse situations. It’s not black and white, just shades of gray. Importantly, the simulation changes for each type of trailer. Tiny house requirements are different than those of a utility trailer. Then, when decisions come, it’s our opinion that ending on the side of a little extra strength is the better choice.
Finally, any analysis assumes a lot of things — like the welds are good, for instance. Who will weld it, and with what kind of welder? If you’re making your own, and if you don’t have a lot of skill with your new welder, then maybe you should bias the safety factor up some. Remember, the safety factor as we use it is really just a compensation for trailer frame materials, processes, and analysis that might not be as perfect as we think. There’s a lot to consider.
Wrapping It Up
In practice, we know a safety factor of 1.0 won’t work. In engineering school one professor said anything with a safety factor less than 2.0 is a bad design. (This is not an endorsement, just a perspective.) For things in dynamic situations some say to use a safety factor of at least 3.0 — and 4.0 is better. Some books give a formula like: Se=ka*kb*kc*kd*ke*kf*S’e for calculating endurance limits. (McGraw-Hill Series in Mechanical Engineering.) Each “k” is a factor modifying the material strength. Also, impact loading can easily reach 10 or 10,000 (depending on the impact), so let’s throw that into the mix.
So where does that leave us in choosing trailer frame materials? The more we know about all the conditions, the more confidence we have in the design. So, because there are so many things that happen to trailers, generally a higher safety factor is better. Think through the design and the weird possibilities. Consider the beam shapes in the design and where in the trailer frame each is used. Consider the possibility and issues with failure, then use your best reasoning and fall to the side of safety.
Another great approach is to buy plans that fit, or almost fit your need. This buys you a proven design, then you can customize the plans to meet your specific application.
At the risk of offending, I’ll throw in one more example of thinking beyond the box. Consider the fat guy sitting down on the end of an already full trailer. On a 1000# trailer, a 350# dynamic addition to the back bumper is significant. Him stepping up onto the trailer or sitting down hard on the bumper will momentarily add far more than 350#.
Food for thought. Good luck with your project.