How To Setup A Trailer Frame Build

Just how do you setup for building a trailer frame?  Seems simple enough, just layout the frame members and start welding, Right?  Not Quite.  Here are some tips for Success.

If you want your trailer frame to be straight, flat and square, then you must start that way.  Below are 5 setup tips (and one hopefully obvious) to help make your trailer frame turn out great.

0.  The Pre-Tip

Cut frame pieces accurately.  Make the cuts square, and all at correct length.  The more accurate, the pieces, the easier the setup.  This is a non-tip because I hope it’s a statement of the obvious.  See metal cutting options and the trailer material example in the previous post.

1.  Start With A Qualified Surface

Just like building a house, you need a good foundation before you start building.

Is my garage floor good enough?  I have to admit, my garage floor looks pretty flat, because the contractor did a great job.  I almost convinced myself to just build the trailer frame right on the concrete — because it’s easy.  However, I set a length of steel across the floor and look what I found.  Oh my!  It’s way farther out that I thought.  Nearly half an inch from high point to low point.

And, I don’t want a trailer frame with that deviation right from the start!

So, How Do I Make A Flat Surface?

Let’s go back to high school geometry and think about plane theorems.  No, it’s not that complicated.  The theorem I’m thinking about says two non-parallel, intersecting lines create a plane.  Simple.  Just set up two lines that intersect, and you have a plane (flat surface to build on).

So, extrapolating from theoretical to the practical, it means two beams in a “V” create a plane.  If I set 2 beams down and connect them at one end with a flat plate (since they are not actually lines), then the top of them is actually very close to a flat planar surface.

For ease, place a 2×4 piece under the beams at 3 points.  (Again, from geometry, 3 non-linear points define a plane.)  It does not matter if it is level, it just matters that it is a plane, and you can work on it.

You can then clamp the trailer frame members onto those first two setup beams and know they are resting on a flat surface.  It may not be super perfect like building on a huge granite surface plate, but it’s much flatter than the garage floor.  And, it gives something to clamp to and hold it all together while measuring and welding.

Trailer Frame Setup Mistakes

On YouTube there are videos of people making trailer frames supported by saw horses or other separated supports.  While it’s convenient, there is nothing ensuring the two (or 3) are parallel.  It’s easier, I agree, but not smarter.  Such methods are used frequently, and maybe if you can’t see the misalignment it won’t bother you?  Or, maybe I’m a little OCD?  (That’s possible.)  But, if a flat starting surface is as easy as two intersecting beams, why not?

To me, you should avoid the obvious problem areas when possible so the end project turns out good.  The trailer frame is the base for everything that will go on it, so construct it the best you can.

2.  Jig & Clamp For Perpendicularity

Perpendicularity or squareness of the frame is a big deal for performance.  On the other hand, it can be a pain to really measure how square beams really are to one another.  A carpenter’s square can go a long way to checking it, but Is there a better way?  Trailer frames are pretty big, so it would be nice to have a really big right angle to measure with.

What do you know that’s pretty big, and is almost always really square?  Answer:  Plywood.  The store-bought sides of plywood are almost always square, so buy a sheet of plywood and use it as a huge square.  (Don’t use a warped board.)

Setting square can also set width.  For this trailer, the space between the main frame rails is 50 inches, so 1 sheet of plywood (48″x 96″) plus a piece of 2″x 2″ steel tube define the inner width.  There are 2 pipe clamps and one ratchet strap holding the frame members parallel.  Note:  Both members are also clamped vertically (see next tip).  Measuring verifies both parallelism and squareness.

3.  Drill Holes Before Welding

This may seem simple, but it’s easier to drill holes with a drill press than a hand drill.  And, it’s easier to put a single piece on the drill press than a whole trailer frame.  Where possible, drill holes for bolts and lights and other stuff first.

Here are some more tips about drilling — and some tips about fixturing.

4.  Don’t Ever Rely On Cut Ends

You have probably heard this before.  However, let’s go one more step and say “Don’t rely on beam sides to be flat or square either.”

Well, if you can’t rely on the ends, and you can’t rely on the sides, what can you rely on?  Good question.  I personally rely on fixture blocks to force square.  You can see the (blue) braces in these photos.  These fixture blocks clamp to the surface defining beam “V” and to the frame members to hold the frame members vertical.

Look at the photos.  There are two fixture blocks, one on each main trailer frame beam.  Both fixture blocks are attached to the setup “V” beams.

Of course this does not guarantee perfection.  Make sure you sight the beam to be sure it does not twist.  When you start with things pretty square, you set yourself up for success.

5.  Secure Trailer Frame Members

Before starting to weld, clamp on as many trailer frame members as practical.  Lock down everything possible, as much as possible so that nothing moves when you start to weld.  When it’s all secure, measure everything again.  Yeah, this is belts and suspenders, but it’s so much easier to adjust things before you weld.

One issue with welding is the way a weld can pull.  If frame members are not secured, they can move slightly while welding.  I recommend you use all the clamps you have to lock everything down, and leave it all clamped (as much as possible) until the welding is done.  Obviously some of the clamps will be removed to access areas to finish welding, but keep as many on as possible.  It will help assure a nice finished product.

6.  Tack, Tack, Tack, Tack

As noted above, weld pull is one hiccup with welding.  It comes from heat distortion as the molten metal cools.  There are lots of ways to work around weld pull on your trailer frame, and one of them is tacking — maybe to excess.

After setting everything up and securing the trailer frame members, start tacking.  Move around the trailer tacking one, then across the trailer to tack another, then back and forth until you’ve tacked several places on each intersection.  Tack both sides at least for each joint.  Tacking like this all around minimizes movement and secures each joint.

Don’t forget the angle braces or gussets.  Those are critically important to making the frame rigid and strong.  Tack them in as well before welding.

Finally, measure everything again.  Now you’re ready to complete the welds.  Again, though it’s not as important, I recommend moving around the trailer to complete the welds.  Start at the center and move out.

Good Luck with Your Trailer Frame!

Hopefully the tips above as well as our other Mechanic’s Tips help.  These tips and the photos are part of our new trailer build which you can read more about with these direct links:

– The Economics Of DIY Projects
– Trailer Build – Project Progress
– A Better Folding Trailer Tongue
– Twin Torsion Suspension Design

More to come  . . .

Share the Article: