Trailer Frame Materials And Safety Factor

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

Safety Factor In Trailer Frame MaterialsMaterial 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.

Engineering Design Analysis On Trailer Frame Materials

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.

Safety Factor in a Tiny House Foundation

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.

No one has share their thoughts...be the first!

No one has share their thoughts...be the first!

Leave a Comment

We Found These For You . . .

Article
What Tires For My Trailer?
There are a million tires out there with a ton of classifications and designations.  What tires do I choose for my trailer?  That is not a silly question at all.

Read The Article

Article
Understanding Bolt Choice
This is Page 2, continuing the Bolts 101 article.  Here we discuss choices for bolts for an application.  The previous post, Page 1 of Bolts 101, gives a ton of background info, so we recommend reading that first,

Read The Article

Article
DIY Steel Cutting Options
So, you need to cut steel for a DIY project (like a crane or trailer).  Easy, Right?  What’s the best, easiest, cheapest, fastest way of cutting steel?  There must be dozens of ways, so, the better question might be

Read The Article

Article
Which Trailer Plans To Buy?
I want to build a trailer, but there are so many possibilities — and some are mutually exclusive.  How do I narrow my needs, and how do I choose which Trailer Plans to buy?

Read The Article

Article
Don't Look at the Mig Welding
I grew up with a fascination for metals.  My father was a woodworker, and taught me young, so I have always built stuff from wood.  Yet, metal is different, and welding beats glue in the fun department any day!  And…

Read The Article

Article
Wide and Wider Tiny House Trailer Plans
What if . . . You want to live Tiny, but not “that” tiny?  Sure, there are a lot of attractive elements with living Tiny, but to some, the 8 foot width is just too tight.  If that’s you, go…

Read The Article

Article
Aligning Misaligned Holes
Sometimes, despite your best efforts, holes you drill for a project don’t quite align with those in the mating parts.  It’s maddening, and sometimes you feel dumb because it’s messed up, but there are ways to fix

Read The Article

Article
Trailer Tire Comparison
The topic of trailer tires has come up a hundred times.  Why can’t I use automotive tires on my trailer?  That’s a fair question, so let’s explore it.

Read The Article

Article
All The Steel Cut And Ready For The Trailer Build
Update on building a new design specialty utility trailer.  The trailer build is going well, and we thought you might have interest in following the progress.  Especially for some of the experimental new concept features.

Read The Article

Article
How We Make The Plans
What goes into the engineering of Do It Yourself Plans?  How do we make them?  When you purchase and download project plans from Mechanical Elements, you get a TON of work and information, much more than first meets the eye.

Read The Article

Article
DIY Drilling Steel
Need a hole?  Many of our plans require drilled holes, so here are some Tips about drilling in metal, like steel, as well as in wood, and other materials.

Read The Article

Article
20ft Deck Tiny House Foundation Trailer Plans
There’s an old song comparing the wise man and the foolish man.  It might just be talking about a tiny house foundation!  Goes something like this:

Read The Article

Product
8.5' x 16' 7000 lbs Deck Over Trailer Plans

With a full flat deck, these plans show how to build a workhorse of a deck-over trailer.  Options are plentiful, including ramp sizes, deck types, sides options, rub rails, front rise rail and more.

Article
As a simplistic introduction to beam loading, this article discusses the complications in calculating stresses and deflection.  While it’s pretty easy to calculate beam loading for simple theoretical cases, very few beams are actually simple.  A trailer frame, for instance,

Read The Article

Product
Walking Beam Suspension Plans 8K Max

Put a walking beam style tandem axle suspension under your 5000 - 8000 lbs capacity trailer. Plans are engineered for the benefits of torsion rubber axles in a load sharing combination for a better ride.

Article
Walking Beam Trailer Suspension With Twin Torsion Axles
Plans are now available for the Twin Torsion Axle Walking Beam Trailer Suspension.  This was introduced on a build early in 2018, with posts about the trailer including a video of the suspension in action.

Read The Article

Article
Spiraling the DIY Project Plans Back Online
January 7, 2016.  Mechanical Elements is back online with a new look, a new logo, new functionality, and a better shopping experience. — With — an expanded offering of the Same Great DIY Project Plans and so much more.

Read The Article

Product
Heavy Duty Gantry Crane Plans

The big one, actually the big many.  These Gantry Crane Plans include Engineering graphs to choose the beam and legs from 2000 lbs - 6000 lbs lifting capacity.  Free Standing, Large or Small, Wide or Narrow. Build to fit your…

Product
4'x6' Off Road Trailer

Build a great little DIY Off Road Trailer with these Plans.  The 4 x 6 is a nice adventure buggy size to use with a Jeep or any other 4-Wheel-Drive.  And, customize with options in the plans.

Article
Winner of the Visa Gift Card
We have a winner of the $100 Visa Gift Card!  We sent out the Photo request, and you responded.  Now the drawing, and the winner.

Read The Article