Good Trailer Design – Trailer Strength
Page 2: Trailer Strength
Factors in Determining a Sufficient Design
Trailer Strength is the first aspect of “Good” trailer design. Adequate strength allows them to carry loads over less than perfect roads. This is especially important for utility trailers because they are often overloaded or loaded unevenly. They are for utility — for a million uses — and serve in numerous environments, for years, no matter what the requirement. This page of the article is about factors relating to strength along with ways of evaluating them.
The maximum load, or the capacity determines much about the strength. It 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 one 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 frame is bolted together, joints will act differently than if it is welded. Look for gusseting, triangulation, and other such techniques to indicate solid construction.
Look for things that indicate the trailer design is well thought through. This will give reason to believe there is strength and care in the design.
In some areas bolts are desirable — especially for options, and when things are removable or adjustable. So, with bolts, look for methods of security like lock nuts, keys, tabs, etc. which keep the bolts from vibrating loose.
What to Look For:
Examine the trailer to determine if strength is in the trailer frame, or if it is really in the sides. A trend in light duty trailers is 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.)
The real problem comes when the trailer “feels” strong (because steel is stronger than your hands), but the dynamics of a large load will make it flex.
On The Trailer Frame:
Look at welded joints and be comfortable that welds are sufficient. Like many of these suggestions, this is subjective. Small short welds can indicate short-cuts or weakness or cheapness.
Examine bolted joints for size, number and separation of bolts, then see that the members will support the bolt loading. Thin or weak members (such as bolts through tubes) will crush by tightening bolts. Also look for lock nuts or other retainers to keep things tight.
Look for gussets. (See the image.) Gussets offer strength and rigidity to the frame. Also read this article on strengthening a trailer frame if you’re into the DIY of making changes yourself.
Keep in mind the trailer use too. A light trailer requires less than something heavy — like a dump trailer.
At Axle Mounting:
Evaluate reinforcement in areas where the axles mount. Since these points of attachment are high stress areas, reinforcement or double layers of material are always good to see.
Torsion axles 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 accommodates the added loads. However, if the frame is not reinforced where a torsion axle mounts, it’s a sure sign of weakness. Please see this article for an FEA comparison showing axle types and the resulting stresses on the frame.
For existing trailers, a quick test of construction is to hook it up to the tow vehicle, then jump up and down on each corner. The trailer should move up and down with you, but the trailer frame should not twist noticeably. If twisting is noticeable, the design is likely weak.
See 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 trailer 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 like the main beams. See that their size and thickness (relative to the load capacity) will handle the things you wish to carry.
Look at the frame overall. Is the trailer repurposed from something else? Like from a mobile home for instance? If so, did the fabricator take time to modify things so it is ready for your purpose? If not, run away. (As a side note, I’ve seen several repurposed trailers that look good initially, but fail miserably on closer inspection. Here’s an example. Just use caution.)
Flooring materials and spacing between flooring support members is important. This is an area often overlooked, but very important. If you haul an even distribution of boxes, the floor needs less strength, than if your primary use is motorcycles, a tractor or something else where the weight concentrates at just a few points. The even distribution of weight lessens the required floor strength as compared to point loading. Be sure there are enough and strong enough cross members under the flooring to support the expected loads.
Examine the frame 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.
Basic Trailer Frame Layout:
The basic layout of the trailer can be a good estimation of trailer strength and will also affect stability in operation. Here are a few things to note:
- Length of the Tongue. The trailer 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, including back up, turning radius and jack-knife susceptibility. A longer tongue is also nice for extending axle separation (distance from the tow vehicle rear axle to the trailer front axle) for stability and bounce.
- Axle Location. The axle is 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 is noticeably behind the center of the bed. It is always best to calculate axle position for perfect placement. For maximum versatility allow axle position adjustability.
- Strong Rear Member. The back most “bumper” of the trailer should be strong. Usually loading and unloading occurs over this member, so it is disproportionately larger (stronger) than other cross members. This is also the bumper, so if there is ever a wreck, it is strong enough to protect against intrusion on the load. Also, make sure it is attached to the main trailer frame members securely.
There are so many trailer parts that contribute to strength or weakness, and are 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 match the trailer capacity. Each has a capacity marked on it. That load capacity should be at least 15% of the total trailer capability, and preferably much more.
- The ball receiver fits a ball of adequate size. In general, a ball of at least 1 7/8″ for trailers of 2000# capacity; a ball of at least 2″ for trailers of 3500# capacity; and larger sizes for trailers of 6000# load and up. A little big is not a bad thing. Note: Be sure the trailer hitch on the vehicle is the same or greater, too.
- Trailer axle(s) match the 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 1: For multi-axle trailers, the capacity is NOT the sum of the axle ratings because they do not share the load perfectly. In operation, axle load sharing will be close, but not exact, but it depends largely on the mounting configuration. Note 2: We do NOT recommend torsion axles for use in Tandem or Triple because they do not load share. When you drive over a bump or through a dip in the road, one axle is disproportionately loaded. This can pop tires or damage equipment.
- Axle springs match the trailer. Again, this is not usually a problem. However, with leaf springs, longer springs are generally indications of a better design. Longer springs help with the ride and load distribution on the frame. With used trailers, watch out for old automotive springs kludged in.
- Wheels and Tires are key to trailer strength on several levels. They are also worth a bit of a separate discussion to cover all the points.
- The use of trailer brakes is largely dependent on the tow vehicle and the trailer loading. Oh, and applicable laws. State or Province regulations apply. In general, if your tow vehicle is large with respect to the trailer and expected load, then trailer brakes are less important. If you have questions, it is generally better to fail to the conservative side and install brakes. When choosing brakes, there are several things to consider. First, 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 are in place around the trailer. If these are missing, it is an indication of shortcuts. Check that wiring for the lights (brakes, etc.) is an appropriate size, and mounting is secure. The wiring connection to the tow vehicle should be protected and perhaps encased to avoid damage in handling and in operation. (Lights don’t affect trailer strength, but the application gives clues about other aspects.)
- Sides are discussed 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 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 thorough evaluation is probably not necessary, but a good “gut feel” is.
- If sides get damaged (bent or deformed) will it compromise overall trailer strength?
- Are the sides adequate (tall enough, strong enough, etc.) for the job you require for the trailer?
- 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. Is the connection solid to the trailer frame?
If a load pushes against the side, will it deflect out? (Or in?) Will deflection of the sides also twist the frame members?
Look at the supports and spacing between them. If you carry a load that presses against the sides, be sure they are adequately strong.
Examine how the sides attach to the tailgate (if present). Be sure that a load against one of the sides will not impact the ability to open or close or latch the tailgate.
Concluding Thoughts …
Appropriate trailer strength, especially for a utility trailer is very important. There are many things to look for as clues in all areas that indicate “cheapness” in the design. Primary methods of evaluation include judgment with respect to the specified load rating, and evaluation of trailer parts with an eye to expected use.
Looking for shortcuts in trailer frame design (and other places), then using a good “gut feel” is a valid and valuable method of evaluation. It should have strength sufficient for it’s need. It does not have to be heavy duty if it’s a light trailer. A strong trailer that meets your needs, certainly increases the pleasure of ownership.
Do you need trailer plans designed for strength? Check out the plans at MechanicalElements.com for both light duty and heavy duty trailers.
Good Trailer Design:
Build a Great Trailer with Engineered Blueprints that have the design principles.
These trailer plans were designed here, at Synthesis, and are sold through MechanicalElements.com