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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.
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 with whatever the requirement.
Some factors relating to strength are discussed below along with some ideas about how to evaluate them:
Load Capacity:
The maximum load the trailer is intended to carry -- will determine much of 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 light utility 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. Look closely at the design and decide for the individual trailer.)
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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Basic Trailer Layout:
The basic layout of the trailer can be a good estimation of strength and will also effect 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 effect 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.
- The use of trailer brakes is largely dependent on the tow vehicle and the anticipated loads. State or Province
regulations can also have an effect. 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.
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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