Good Trailer Design – Trailer Stability
Good Trailer Design – Trailer Stability
Page 3: Trailer Stability
Why are some trailers stable while others wander all over the road?
There are dozens of factors in trailer stability while towing. No one factor is an absolute for making a trailer stable, or not, but they all have an effect. Here are a few of the most common things to look for:
This is the most common cause of trailer stability concerns — and the easiest to correct.
First, the trailer load should always be reasonably distributed from side to side. Don’t put a lot more weight on one side or the other. Center the load on the trailer as much as possible.
Secondly, load the trailer front to back with at least 10% of the trailer weight on the hitch. With typical trailers, more weight on the hitch is better for stability. 15% is usually a good number.
To illustrate, think of 5th wheel trailers. They have much more weight on the hitch, and they tend to be pretty stable. The extreme example is over-the-road trucks. They carry up to 60% of the load at the front.
Third, (hopefully this is common sense) secure the load properly. If the load shifts, the load distribution changes, probably in a way that is not helpful. Always make sure the load is secure.
More On Load Distribution
This topic extends beyond just how a load is put in a trailer. On the road, the trailer and the tow vehicle become a system, and the dynamics of the system make it stable (or not).
For the tow vehicle, “Motor Boating” is a big stability concern. A large weight on the hitch — at the rear of the vehicle — can make the back of the tow vehicle sag, and the front rise. When this condition is significant, it adversely affects stability, driveability, and control because weight lifts off the steering wheels.
This may be an issue of the tow vehicle being too small for the trailer weight, or it may be loading incorrectly inside the trailer. To correct this problem, load distributing hitches can be used to level the trailer with respect to the tow vehicle. An example is in the image below. These hitches can make a big difference — especially when trailer size and/or weight is near or exceeds that of the tow vehicle.
Note: Before using a load distributing hitch, make sure the tow vehicle, hitch and coupler can handle it.
Usually if a Damper (like a big shock absorber between the trailer and the tow vehicle) is needed there are other, perhaps fundamental, issues that should be addressed first. A trailer properly configured and properly loaded should not need a damper — though in a pinch, they can help. (Note: There is one in the photo above.)
Length with respect to width is another stability factor. Longer trailers typically tow better — or more accurately, a greater separation between the rear most wheels of the tow vehicle and the fore most wheels of the trailer. A long tongue (as discussed in the Strength section) can facilitate such separation.
Evaluate trailer length in the context of width. Look at the width of the trailer with respect to the tow vehicle and with respect to length (or more accurately, with respect to the separation of the axles). In general, the wider the trailer, the more it will benefit from length.
Another length factor is the distance between the rear wheels of the tow vehicle and the hitch point. When this distance is short, the trailer can’t “wag” the tow vehicle as much, nor do the “bumps” of the tow vehicle affect the trailer as much. Fifth wheel trailers are a good example.
Finally, a hitch at the back of a really long overhang (like the back of a school bus) will have a negative effect on stability. Extend-a-hitch bars that push the hitch point back with respect to the rear most wheels of the tow vehicle are a similar bad example.
A Couple of Axle and Wheel Items:
First, use a trailer axle – an axle made for a trailer. The wheels on a proper trailer axle have just a little camber to them — which greatly enhances tracking.
Next, Axle alignment with respect to the pull direction is very important. The axle(s) should be perpendicular to the direction of pull — and perhaps more important, if more than one axle is used, they must be parallel.
(This condition is easy to see from behind the trailer on the highway — because the trailer follows a little to the side of the tow vehicle.)
You can also see Axle alignment issues in trailer tire wear, but that takes a while to show up. Better to get it right at first, so trailer stability and fuel economy don’t suffer.
Finally, Wheels and tires should be in good condition. I can’t tell you how many wobbly (bent) rims I have seen on trailers going down the highway. The wobble may come from a bent rim, or from a wheel that is not attached properly. Either way, it causes problems with stability — and safety.
Proper tire pressure is a common trailer stability concern. Inflation pressure will vary depending on conditions, tire type, load, etc.. When the trailer is to be pulled a long distance empty, or without significant load, tire pressure should be reduced — perhaps as low as 10 or 15 psi. This allows the tires to soak up more of the road inconsistencies, and the tires become a damper to keep the trailer from bouncing around. When the trailer is full, the inflation pressure increases per the tire manufacturer recommendations.
Though not a specific stability concern (until there is a blow-out) Dry Rot on Trailer Tires is one of the most common causes of trailer tire failures – and when a tire blows, it’s a BIG stability AND SAFETY concern!!
A blow-out is a horrible way to interrupt your trip, so carefully check the tires before traveling.
Trailer brakes, and calibrating the brakes to the tow vehicle is worth a long discussion (which we won’t give here). Just note that the brakes and application of the brakes need to be appropriate for the load and the trailer size. This is less of an issue with electric brakes than with surge type, but it applies to both. Numerous issues with setting and calibrating surge brakes have caused several states to consider banning their use. Check local regulations before buying a trailer with surge type brakes.
One idea (please be careful in how you might apply this) is to connect and use the trailer axle brakes when the trailer is loaded, then disconnect the brakes when the trailer is empty. This works only if the empty trailer is much lighter than the tow vehicle. It will not work if the trailer is large or significant with respect to the tow vehicle. (Note: this idea addresses concerns when the brake calibration for a loaded trailer causes the trailer brakes to “skid” when the trailer is empty.)
Sources of Dynamic Loads vary, but they can have a significant effect on stability. For instance, if a large, heavy pipe is loose in a trailer, it will roll from side to side or front to back with motion of the road. This constantly changing load makes the trailer move. If the dynamic load is small compared to the weight of the trailer, it may not be important, but if the moving load is a large, the effect is significant. Live loads like large animals can create this concern. In general, the load is secured as much as possible to accommodate stability.
Center of Gravity
Trailer CG is another important consideration. In general, a lower center of gravity helps trailer stability. I once saw a trailer tip over around a corner because the CG was too high for the conditions. Where possible, load the trailer with the heavy items low. Of course, this is related to the discussion above about load distribution.
The topic of Aerodynamic effects on trailers, and specifically on trailer stability, deserves a page by itself. There are hundreds of studies on the subject, mostly on large, over-the-road trucks, but they are applicable to trailers of all sorts. It’s a fascinating subject, but I won’t delve too deep here.
When discussing aerodynamics, the answer is almost always … “Minimize the effect of wind on the trailer, and it will help with all sorts of concerns – including stability, fuel economy, driveability, etc..”
Aerodynamics are more than just wind drag. Effects from side winds or other passing vehicles can contribute to stability issues. Typically high profile and larger trailers like campers suffer more, but be aware of aerodynamic loading on utility trailers too. Things like high profile objects sticking up, tarps that flap relentlessly in the wind, even sharp corners on the trailer contribute (some in a minor way) to aerodynamic stability.
There are all sorts of ways to improve aerodynamics. Some trailers have a “V” nose; others use full bulbous fronts; some use trailing spoilers; yet others have full fish tails. All the above will help, but not all are necessary. The largest “bang for the buck” is keeping the trailer directly behind and in the slip stream of the tow vehicle. If the trailer has a larger profile than the tow vehicle (taller or wider), the biggest “bang for the buck” is round corners. It does not have to be dramatic like the U-Haul trailer pictured above in order to have an effect. A little goes a long way like the picture below.
Aerodynamic Side Note:
From a fuel consumption perspective, the best aerodynamic improvements are at the rear of the trailer, not the front. The “V” nose is popular as an “aerodynamic” feature, but it’s effect is not as significant as the salesman would have you believe. To achieve big changes in aerodynamic efficiency, you have to address air flow between the tow vehicle and the trailer, around the wheels and fenders, underneath, and most importantly, the exit beyond the end of the trailer. The back is often the hardest because of access, length, storage, etc., but it’s worth mentioning because it is often overlooked.
In context of trailer stability, aerodynamics that increase fuel economy the most don’t necessarily increase stability the most. Without going into detail, just understand from a stability perspective, rounding the corners and reducing profile are the big hitters.
Along with aerodynamics, trailer attitude has an effect. Trailer attitude is the relationship of the trailer with respect to the ground and tow vehicle — basically, the trailer should be somewhat level (not with the butt dragging the ground or the tongue extra low). Attitude has a smaller effect on trailer stability, unless combined with other things. If the trailer acts unstable because of other things, poor attitude makes it worse.
Attitude is also part of aerodynamics. A level trailer moves less air and creates less turbulence.
Strength and Rigidity
One final item is the strength or rigidity of the trailer itself. If the trailer is flimsy it will twist and bend with the dynamics — especially if the load is also dynamic. Just one more reason to choose a sufficiently strong trailer as discussed on Page 2: Trailer Strength.
Concluding Thoughts …
There are numerous things that affect safe towing and stability for a trailer – all need consideration, but a few are most important. Items of construction include wheel and axle alignment, axle placement, brakes, calibration, and overall strength.
Effects to consider with each use include (first and foremost) distribution of load in the trailer and load securing. Other things (also very important) are trailer attitude (with respect to the ground and the tow vehicle), tire condition, and tire inflation pressure.
By considering, adjusting and making sure these things are all correct for your situation, the trailer can tow in a stable, predictable and enjoyable manner.
Build a Great Trailer with engineered Plans that incorporate these design principles.
These trailer plans were designed at Synthesis, and are sold through MechanicalElements.com