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Trailer Design
for Strength
Links for further reading:
1. Introduction.
2. Trailer Strength.
3. Stability.
4. Versatility.
Choosing the Right
Utility Trailer.
Trailer Towing Tips.
More Articles . . . |
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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 strong and rigid so it will serve well no matter what the requirement.
Some factors relating to strength are discussed below along with some ideas about how to evaluate them:
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 1000# it will obviously be of
much 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, the joints will act quite
differently than if it is welded. Gusseting and other construction techniques along with ways things are generally
thought through can indicate strength and care in the trailer design.
In general, we recommend staying away from trailers that are bolted together.
This is not to say that they are inherently inferior, we have just seen too many really poor bolted applications.
In some areas it is desirable to use bolts -- especially with options. When bolts are used they should also
include methods like lock nuts to keep the bolts from vibrating loose.
| What to look for: |
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Examine the trailer to determine if the strength is in the frame, or if it is really in the sides.
A trend in light duty trailers is to have a very light frame, then rely on the sides for stiffness. This is typical for
trailers made mostly from angle iron -- where the sides are welded angle iron up about a foot from the bed. (see image below)
The design is generally stiff and certainly light, but if you damage a side, the integrity of the trailer is compromised.
(Not all trailers built this way are weak, but Look closely and decide for the individual trailer.)
The real problem comes when the trailer "feels" strong (because the 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 the welds are sufficient for the application. This is
subjective, but small short welds may be an indication of weakness or cheapness.
Examine bolted joints and be comfortable with the size and number of bolts -- and be comfortable that the
members will support the bolt loading. Also look for lock nuts or other retainers to keep things tight.
Welded gusset on frame.
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Look for gussets in the design. See the figure at right. Gussets offer strength and
rigidity to the trailer.
Look for reinforcement or additional material in the areas where the axle springs mount. Since the
points of spring attachment to the frame are usually the highest stress points on the trailer, these should be adequate.
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. |
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Materials:
Be comfortable that the materials used and the construction techniques are adequate with respect to the materials.
(Obviously each design will be different and it is hard to give a visual judgment, but think about the size and
relative strength of the materials and make a judgment as to the effect on strength.) It is always a trade-off
with 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: |
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Examine the main 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 big 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. |
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| A Note About Aluminum: |
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Aluminum is a popular material for light weight trailers. It can have an expensive feel and a nice allure.
I've seen lots of really nice trailers made from aluminum. However, it can also be deceiving. Aluminum, in general,
is more prone to the propagation of cracks, and because aluminum is so pretty unfinished, it may also be more susceptible to
corrosion from road salts, etc.. Secondly, when judging a trailer visually, you must note that typical aluminum is not
as strong as typical steel, so the beams of an aluminum trailer must be larger than beams of steel for an equivalent capacity.
Another consideration with aluminum is the flexibility. Aluminum has a modulus of elasticity that is significantly lower
than steel. This means that for the same strength, aluminum will bend more.
Aluminum can make a very nice trailer, and because they are much more expensive to build (aluminum costs more), they are usually
done well. However, this note is just a reminder to look at aluminum trailers with a little different perspective.
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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:
- Length of the tongue.
Trailer with a very short 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 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 than the other cross members.
This is also your bumper so, if there is ever a wreck, it needs to be strong.
Components:
There are many components used on a trailer that will contribute to its strength or weakness - or be indications of
short cuts or cheapness in the design. Componentry and options will be discussed again in the section on
Versatility. Here are a few things to look at:
- The 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 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 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.
- The 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 from the frame. With used trailers, watch out for old automotive springs kluged in.
- Wheels and tires need to match the trailer load capacity. For wheels, the best indication is the number of
lugs. 1000# and 2000# axles often have 4 lugs, 3500# axles usually have 5 or 6, and 6000# + axles have 6 or 8 in
a larger pattern. It is not necessary to use "trailer" specific wheels and tires, but the load rating must be
appropriate. Often automotive wheels and tires can be used to give a wider range of options and a better ride.
Again, just make sure the load ratings are sufficient.
Interestingly, I have had several comments about this statement. Yes, trailer manufacturers want you to
use trailer specific tires; and yes, they do have some advantages in load capacity for the size. Trailer specific
tires are also better at resisting damage, neglect and abuse (because they're built tough). On the other hand,
automotive and light truck (LT) tires typically have a much 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,
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.
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 the
dynamics better. Trailer tires won't do that, and rightfully so -- trailers just don't see the extremes of dynamic
loading and weight transfer like vehicle tires. Remember, this discussion is encapsulated in the assumption of
similar load capacity and some smarts with respect to neglect and abuse.
- The use of trailer brakes is largely dependent on the tow vehicle and the 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. Also, check your local regulations about surge type brakes. There have been rumblings about
banning such brakes in some areas. In general, I don't recommend surge brakes.
- Tail and 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:
- Most importantly, are the sides part of the strength of the frame? 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.
- If the sides get damaged (bent or deformed) will it compromise the overall strength of the trailer?
- Are the sides adequate (tall enough, strong enough, etc.) for the job you require for the trailer?
| What to look for: |
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Examine the attachment of the sides to the frame. Are they well attached?
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. |
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Summary:
Appropriate strength in a trailer, especially a utility trailer is important. There are many things to
look for in evaluating strength, and there are also clues in other 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,
looking for shortcuts in the design and using a good "gut feel" with respect to the intended use. Having a
trailer sufficiently strong to handle the required needs will increase the pleasure of ownership.
 Next: Stability
- What makes some trailers stable?
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