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Article Pages:

1. - Introduction

2. - Trailer Strength

3. - Stability

4. - Versatility

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  Engineering Articles Technical Articles Trailer Design
Page 2:  Trailer Strength
Factors in Determining a Sufficient Design


Trailer Strength is the first aspect of "Good" trailer design.  Especially with utility trailers, strength is important because they are frequently overloaded or loaded unevenly.  By their nature they are for utility -- used in so many different ways -- the trailer must be sufficiently strong so it can serve well no matter what the requirement.  Some factors relating to strength are discussed below along with ideas about how to evaluate them:
Tandem Axles
Tandem Axles for Capacity

Load Capacity:

The maximum load for which the trailer is designed to carry will determine much about the strength of the trailer.  It will also determine how strong the trailer should be.  If a trailer is designed for a capacity of 1000 pounds (short-hand 1000#) it will obviously be of lighter construction than a trailer designed for 6000#, but they should perform equally well at their rated capacity.  Judge trailer strength with respect to its intended capacity.

Construction (the way it is built):

Construction is a big factor in strength.  If the trailer is bolted together, joints will act differently than if it is welded.  Look for gusseting, triangulation, and other such techniques to indicate sound construction.  Look for ways that things are generally thought through to indicate strength and care in the design.

Weak Construction

In general, we recommend staying away from trailers that are bolted together.  This is not to say that bolts are inherently inferior, we have just seen too many poorly bolted applications.  Again, in general, frames and structural parts of the trailer should probably be welded.

In some areas it is desirable to use bolts -- especially with options, and when things need to be removed or adjusted.  So, when bolts are used they should include methods like lock nuts, keys, tabs, etc. to keep the bolts from vibrating loose.

What to look for:
Examine the trailer to determine if the strength is in the frame, or if it is really the sides.  A trend in light duty trailers is to make a light frame, then rely on the sides for stiffness.  This is typical for trailers made mostly of angle iron -- frame and sides are angle iron.  (see image below)  The design is generally stiff and certainly lighter, but if a side is damaged, integrity of the whole trailer is compromised.  (Not all trailers built this way are weak, but look closely and decide for that individual trailer.)

Angle Iron Trailer Frame Construction  Angle iron construction can indicate a weak design.  In this trailer example, the weak frame relies on construction of the sides for strength.  Note that one upright is damaged, so the whole trailer strength is compromised.

The real problem comes when the trailer "feels" strong (because steel is stronger than your hands), but when a large load is carried, the dynamics can make that steel flex all over.

Examine welded joints and be comfortable that welds are sufficient.  This is subjective, but small short welds may be short-cuts causing weakness or cheapness.

Examine bolted joints and be comfortable with the size, number and separation of bolts -- and be comfortable that the members will support the bolt loading.  Thin or weak members can be crushed by tightening bolts.  Also look for lock nuts or other retainers to keep things tight.
Trailer Frame Gusset
Welded gusset on frame.

Look for gussets.  See the figure at right.  Gussets offer strength and rigidity to the frame.

Look for reinforcement or additional material in the areas where axle springs mount.  Since these points of attachment are high stress areas, reinforcement or double layers are good to see.

Torsion axles in particular add a lot of extra stress on the frame at the attachment point.  (Load is both vertical and torsional.)  It's OK, as long as the trailer builder accommodated these added loads.  If the frame is not reinforced where a torsion axles mounts, it's a sure sign of weakness.

For existing trailers, a quick test of construction is to hook-up the trailer then jump up and down on each corner.  The trailer should move up and down with you, but the frame should not twist noticeably.  If twisting is noticeable, the design may be weak.


Be comfortable that materials and construction techniques are adequate.  (Each design is different and it is hard to give a visual judgment, but think about the size and relative strength of the materials, then make a judgment for the effect on strength.)  It is always a trade-off for strength versus weight, so give it the good "gut feel."  Most trailers will indeed carry the specified load, however, if the weight is not distributed well, it can seriously damage the trailer and/or the towing vehicle.

What to look for:
Examine load carrying members and be comfortable with their size and thickness relative to the load capacity and the things you wish to carry.

Examine flooring materials and spacing between flooring support members.  This is an area often overlooked, but very important.  If you need to carry evenly distributed boxes, the floor needs less strength than if your primary use is to carry motorcycles, a tractor or something else where the weight is concentrated at just a few points.  The even distribution of weight lessens the required strength as compared to point loading.

Examine the main beams and cross members for rust.  Excessive rust will reduce capacity.  This is also an indication of a lack of care or shortcuts in finishing after construction.

A Note About Aluminum:
Aluminum is popular for light weight trailers.  It can have an expensive feel and a nice allure.  There are lots of really nice trailers made from aluminum.  However, because of the properties of aluminum, special consideration must be made in construction and in care.

Here are some things to think about:

  • Aluminum is more prone to propagation of cracks, so when injured, extra care must be given.
  • Aluminum is more prone to fatigue -- and life as a trailer can be stressful -- so the design must be right.
  • Aluminum is very pretty unfinished, it is often left unfinished, making it more susceptible to corrosion from road salts, etc..
  • When judging a trailer visually, note that aluminum is not as strong as steel, so the beams of an aluminum trailer must be larger than beams of steel for an equivalent capacity.
  • Another consideration with aluminum is flexibility.  Aluminum has a modulus of elasticity significantly lower than steel.  This means that for the same strength, aluminum will bend more.
  • Aluminum is quite a bit more expensive than steel.  And, because it is not as strong, more aluminum (volume) is needed.  The added volume is compensated, in part because aluminum is 1/3 the mass (by volume) than steel, so the overall trailer is lighter.  The nice thing, because they are more expensive to build, they are usually built well.
  • Finally, aluminum is not as durable and does not take abuse as well as steel.  Choose the material for the purpose.

Aluminum can make a very nice trailer.  When used in the right application it is wonderful, but choose carefully how it is used.  In context of a utility trailer, it may not be the best choice just because of how utility trailers are typically used.

This note is not to discourage aluminum, it is just a reminder that it's not just about weight.  Look at aluminum for trailers with a little different perspective.

Basic Trailer Layout:

The basic layout of the trailer can be a good estimation of strength and will also affect stability in operation.  Here are a few things to note:
Short Trailer Tongue
Trailer with a very short tongue.

  • Length of the tongue.  The tongue should be long enough to allow a reasonable turning radius and clearance to the vehicle when backing.  If the tongue is short, it may be an indication of weakness or shortcuts in the design.  Short tongues affect both driveability and stability -- basically back up, turning radius and jack-knife susceptibility.  A longer tongue is also nice for extending the axle separation (distance from the tow vehicle axle to the trailer axle) for stability and bounce.
  • Axle location.  The axle should be behind the load center for stability.  Often trailers have the axle at or just barely behind the center of the bed -- assuming (I guess) that the load will be heaviest at the front.  This is short sighted with respect to versatility, but it is also an indication of weakness.  For most applications, the axle should be noticeably behind the center of the bed, and for maximum versatility, allow the axle position to be adjusted.
  • Strong rear member.  The back most "bumper" of the trailer should be strong.  Usually the loading and unloading occurs over this member, so it should be disproportionately larger (stronger) than the other cross members.  This is also your bumper so, if there is ever a wreck, it needs to be strong enough to protect against intrusion on the load.


There are many components of a trailer that contribute to strength or weakness - or be indications of short-cuts or cheapness in the design.  Although components and options will be discussed more in Versatility, here are a few things to look at:

  • Ball receiver and hitch should be appropriately sized for the trailer capacity.  Each should have a capacity stamped on it giving the rated load capacity.  That capacity should be at least 15% of the trailer capacity, and preferably much more.
  • The ball receiver should be made for a ball of adequate size.  In general, the ball should be at least 1 7/8" for trailers of 2000# capacity, 2" for trailers of 3500# capacity and larger sizes should be considered for trailers of 6000# and up.  Being a little big is not a bad thing.  Note:  Be sure the trailer hitch on the vehicle also matches the same use.
  • The axle(s) should be sized for the trailer load capacity.  Usually this is not a problem because the capacity of the trailer is a function of the capacity of the axle(s).  Just make sure it is.  Note:  for multi-axle trailers, the capacity is not the sum of the axle ratings because they do not share the load perfectly even.  In operation, the axle loading will be close, but not exact -- and that will depend largely on the mounting configuration.  Note 2:  We do NOT recommend torsion axles for use in Tandem or Triple.  They do not load share, so when you drive over a bump or through a dip in the road, one axle can become drastically overloaded -- popping tires or damaging equipment.
  • Axle springs should match the trailer.  Again, this is not usually a problem.  However, when leaf springs are used, longer springs are generally indications of a better design.  Longer springs will help with the ride and load distribution on the frame.  With used trailers, watch out for old automotive springs kludged in.

    Trailer Springs

  • Wheels (rims) must meet or exceed trailer load capacity.  For wheels, the easiest indication is the number of lugs.  1000# and 2000# axles usually have 4 lugs;  3500# axles usually have 5 or 6 lugs;  and 6000# + axles have 6 or 8 lugs in a larger bolt pattern (or BCD, Bolt Circle Diameter).  It is not necessary to use "trailer" specific wheels as long as load rating and size match.  Use trailer specific wheels with trailer specific tires because of mounting width concerns.  Use automotive wheels with automotive tires.
  • Tires are key on several levels.  Tire problems have haunted many a trailer owner, so watch what's happening with them.  Tire ratings must match trailer capacity.  It is not necessary to use trailer specific wheels and tires, but load rating must be appropriate.  Often automotive tires will give a wider range of options, a better ride, and track better.  Make sure load ratings are sufficient.

    Interestingly, I've had several comments on these statements about wheels and tires.  Yes, trailer manufacturers want you to use trailer specific tires; and Yes, they do have some advantages in load capacity for the size.  Yes, narrower profile tires (like trailer tires) do wear better when not aligned properly.  Finally, Yes, trailer specific tires are also better at resisting damage, neglect and abuse (because they're built really tough).  So, if you neglect your tires, by all means, use trailer specific wheels and tires.
    On the other hand, automotive and light truck (LT) tires typically have a better ride (for the capacity), and they're available in more sizes and styles.  Perhaps most importantly, trailer tires are not as available when you desperately need one, (get a flat on the road?) nor do they interchange with tires on your tow vehicle.  I personally try to match wheels and tires to the tow vehicle so a spare can work both places.
    Then there's the discussion of Radial vs. Bias Ply.  I'll let you research that.
    One big error ... People sometimes claim trailer tires can handle dynamic loads better ... not so.  If anything, automotive tires which are designed for high, simultaneous, steering, breaking and weight shift loads
    (think right front tire in a hard left turn while braking), will handle dynamics better.  Trailer tires won't, and rightfully so -- trailers don't see extremes of dynamic loading and weight transfer like vehicle tires.  Remember, this discussion assumes similar load capacity -- and intelligence with respect to neglect and abuse.

    Tires for Trailers
    Typical Light Truck Tire, Radial (Left).  Typical Trailer Specific Tire, Bias Ply (Right)

  • Using trailer brakes is largely dependent on the tow vehicle and anticipated loads.  State or Province regulations also apply.  In general, if your tow vehicle is large with respect to the trailer and expected load, brakes may not be required.  If you have questions, it is generally better to fail to the conservative side and put brakes on the trailer.

    In choosing the type of brakes there are several things to consider.  Again, how will the trailer be used, and what vehicle will be towing it.  Electric brakes usually require additional equipment mounted on the tow vehicle.  Check local regulations about surge-type brakes.  Some areas have threatened bans on such brakes.  In general, I don't recommend surge brakes except as a secondary or emergency measure.
  • Tail lights, running lights and reflectors should be in place around the trailer.  If these are missing, it can be an indication of shortcuts in the design.  Wiring for the lights (brakes, etc.) should be of appropriate size and be mounted securely.  The wiring connection to the tow vehicle should be protected and perhaps encased to avoid damage in handling and in operation.
  • Sides are really addressed more in the section on Versatility, however, there are several strength concerns regarding sides:
    1. Most importantly, are the sides part of the strength of the trailer frame?  (See above.)  If so, are they also adequate for carrying side loads at the same time they are carrying vertical loads?  Again, a good evaluation is probably not necessary, but a good "gut feel" is required.
    2. If sides get damaged (bent or deformed) will it compromise the overall strength of the trailer?
    3. Are the sides adequate (tall enough, strong enough, etc.) for the job you require for the trailer?
    4. Do the sides shake, rattle, or feel loose?  Will that effect the way you want to use them?

    What to look for:
    Examine the attachment.  Are sides well attached to the frame?

    If a load is placed against the sides, will they deflect out?  Will deflection of the sides also twist the frame member where the sides are attached?

    Examine supports and spacing between them.  If you carry a load that will press against the sides, be sure they are adequately strong.

    Examine how the sides attach to the tailgate (if present).  Be sure that a load on the sides will not impact the ability to open or close the tailgate.

Concluding Thoughts ...

Appropriate strength in a trailer, especially a utility trailer is important.  There are many things to look for as clues in all areas of the trailer that can indicate "cheapness" in the design.  The primary methods of evaluation include judgment with respect to the specified load rating, and expected use.  Looking for shortcuts in the frame design (and other places), then using a good "gut feel" is valid and valuable method of evaluation.
Having a trailer sufficiently strong to handle the required needs will increase the pleasure of ownership.

Need plans to build a trailer designed for strength?  Check out the ones offered at

ContinueNext Up:  Stability   -  What makes some trailers stable?

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