ExpeditionGuru: Because Independence Counts

I.                                Longitudinal Weight Distribution 

Longitudinal weight distribution refers to the balance of weight as it is applied to the tongue and the axel respectively. A trailer will support a portion of its weight on its axel(s) and the other portion is supported by the tow vehicle at the connection point to the tow vehicle

1)                             Static Tongue Weight

The first aspect to consider is the distribution of the weight: axel weight and tongue weight. A general rule of thumb for on highway bumper towed trailers is the tongue weight should be about 15% of the weight of the trailer. This means that approximately 85% of the trailer weight is on the axel. It stands to reason that the tongue weight of a trailer should be engineered to suit the trailers intended purpose. Fifth wheel trailers have significantly more tongue weight as their connection point to the tow vehicle is above the axel. Semi trailers with their axels often 40 feet behind the connection point have a significantly higher percentage of the overall trailer weight applied to the tongue.

How one loads one’s trailer also determines the tongue weight. Too much tongue weight overloads the rear suspension and unloads the front suspension. Most of your braking power comes from the front brakes so when the front suspension is unloaded your vehicle braking is severely compromised. You steer with your front wheels and if the front suspension is unloaded, your steering is also severely affected. Not enough tongue weight, or a negative tongue weight, will unload your rear suspension causing your vehicle to jack-knife as the rear wheels have insufficient traction on the pavement.

Take these one ton dually trucks that we all see hauling big trailers down the highway. The suspension travel of these trucks that are built for hauling heavy trailers on highways is minimal compared with a typical suspension system of an off-highway vehicle. Vehicles that are modified to go off-highway are generally lifted to improve ground clearance and have significantly more travel in their softer suspension systems so that they can go over rough terrain and maintain vehicle stability.

I’m sure we have all seen an empty tow dolly bouncing around behind a tow vehicle. Not only are the tires at a pressure suitable for carrying the appropriate load, but the suspension system is essentially nonexistent – hence the bouncing about. Imagine towing an empty tow dolly off-road, its wheels would spend more time in the air bouncing from bump to bump, even at slow speeds. If a vehicle’s suspension was that hard imagine how difficult and uncomfortable it would be to drive off highway – hence the softer and greater travel suspension systems used on off-highway vehicles.

2)                             Dynamic Tongue Weight

This is a much overlooked concept in designing off-highway trailers. Perhaps the easiest way to describe this is as follows: imagine a golf player swinging a club and you had a choice of when you were going to get hit by the club on the down swing. Naturally the impact would be the worst at the bottom of the swing when the club is intended to hit the ball. So the best place to get hit where the pain would be minimal is immediately after the swing started. This way the club has only traveled say 1” instead of 8 feet and so does not have time to build up that momentum that causes pain on that little white ball.

The greater the distance a suspension system can travel, the greater the impact of the dynamic tongue weight. Dynamic tongue weight is that influence on the vehicle suspension as the vehicle and trailer together go over an uneven surface, commonly found in off-highway conditions.

Let’s add another aspect into the golf swing example – speed of the swing. If the swing was really slow, the relative pain difference between being hit at to top of the swing or at the bottom of the swing still stands. Now have the golfer swing his club much faster. I’m sure you can appreciate that the faster swing will cause that much more pain. Vehicle speed over uneven terrain has a similar impact with regard to dynamic tongue weight. The faster one drives the greater the influence of dynamic tongue weight on the vehicle. The result, as you can imagine, is not good.

So how does one engineer an off-road trailer to mitigate against the negative consequences of dynamic tongue weight without compromising on highway performance?

Engineer the tongue weight so that the trailer is very close to being balanced when properly attached to the tow vehicle. A properly attached balanced off-highway trailer should have a slight nose down angle when attached. This slight nose down angle imposes a positive tongue weight on the vehicle, which is required for high speed highway driving, but does not as negatively impact the tow vehicle's performance and traction when driving off highway.

I like to engineer our trailers to have about a 7% tongue weight relative to the trailer weight. This results in the axel carrying more than 90% of the weight.

One can take a 3500 lb. trailer with a 7% tongue weight, which is about 240 lbs. and that tongue weight is about the same as a trailer weighing 1500 lbs with a 15% tongue weight.

II.                              Vertical Weight Distribution

Vertical weight distribution is similar to the Center of Gravity or Center of Mass. The higher the center of mass with respect to the area of influence the less stable and visa versa.

1)                             Area of influence

Simply put the area of influence is a function of the axel track, which is the width of the axel. The wider the axel the greater the area of influence and the more stable the trailer will be. However, wide axels on an off-highway trailer limit where you can take the trailer. The area of influence on a single axel trailer is greatest at the axel where the wheels touch the ground. The area of influence narrows and consequently decreases forward and aft of the axel. The fact that it narrows forward of the axel is of significance with regard to the proportionate tongue weight engineered into the trailer. The greater the proportionate tongue weight, the further forward of the axel is the center of mass and consequently the less stable that trailer will be.  Another good reason for minimizing the proportionate tongue weight of an off-highway trailer.

2)                             Center of Mass.

For purposes of this discussion I am going to use the term Center of Mass. It is obvious that the higher the center of mass the less stable an off-highway trailer with a narrow axel track will be. The instability results when one is off-highway on side slopes but more importantly when traveling at speed on highway. It is this second aspect that is all too often over looked.

We all know that SUV’s are less stable than low slung sports cars when cornering at high speed for obvious reasons. The same applies to off-highway trailers with big wheels and gobs of ground clearance. So the how does one engineer an off-road trailer for optimum stability both on road and off highway?

Let’s first take a look at ground clearance. Some manufacturers of off-highway trailers market ground clearance as a supreme benefit. The question is how much ground clearance is required?

Take a standard 4X4 SUV or pick-up truck. Many have independent front suspension systems and even independent rear suspension systems. However, a solid rear axle is more the standard. A reasonable assumption is that the lowest point under a tow vehicle is the differential of the rear solid axel.

If one was to draw a line through the center of the axel from one wheel to the wheel on the other side and then measure how far below that center line the bottom of the differential was, one would arrive at a number not less than about 4”. For larger axels the number is greater. For example the bottom point of the differential on a Dana 44 is about 6” below this center line. None the less lets be generous and use 4” in this example.

Assume the tow vehicle is equipped with 35” tires, the ground clearance under the differential would be 13 ½”. Now assume your trailer has a solid axel with a tube diameter of 3”. This results in 1 ½” below the trailer axel center line. In order to have 13 ½” of ground clearance under the trailer axel we would need a minimum of 30” tires as compared with the tow vehicle’s 35” tires.

Granted there are many other advantages of more ground clearance but more ground clearance comes at the price of stability. Personally I’d rather have a stable trailer with the same under axel ground clearance as my tow vehicle than a trailer with more ground clearance than my vehicle. After all I’m using my vehicle to pull my trailer and if my vehicle is going to get hung up, what point is it if my trailer can clear the obstacle?

Instead, not only do I have better stability on my trailer but loading and unloading my trailer is easier too. When I tow my trailer off-highway, I want to know that it can handle greater side slopes than my tow vehicle. This way when I start getting nervous, I know my trailer is still safe. The other way round makes no sense at all because now my vehicle can go where my trailer can’t.

Again, one can take a trailer with a 7% tongue weight on 30” tires with a ground clearance of 13½”, an area of influence of 72” with a center of mass at 30” and it should theoretically not flip if positioned on a 50° side slope. Remember that driving on a 30° side slope in a vehicle is REALLY SCARY.

The same trailer but now on 35” tires and a center of mass at 32 ½” will have a limit of 48°. We may say that 2° is no big deal but remember this is just a theoretical example considering one aspect only. Bigger tires need more clearance too so although I have only increased the center of mass height by half the proportional increase in tire size, this is most generous as the clearance above the bigger tires would need to be proportionately increased too and in so doing raising the center of mass by more than 2½”, in reality probably closer to 3½” at least. Never the less we will use 2½” for our calculations.

The design of the trailer with respect to where the heavier items [water tanks, batteries, tents, etc. are located] has a huge influence of the center of mass that can be far greater than the influence of putting on bigger tires and thereby raising the whole trailer. A well engineered off-highway trailer will have these heavy components as low as possible.

The point to remember is that the center of mass really does not have a whole lot to do with how much the overall trailer weighs with respect to the vertical distance above the ground the center of mass is. As already discussed it does however have an influence as to the longitudinal distance forward of the axel that the center of mass is located. This factor is directly related to the proportionate tongue weight. The more weight that is proportionally placed on the tongue of the trailer the further forward in front of the axel the center of mass is and consequently the less stable the trailer becomes. This is another good reason for minimizing the tongue weight of an off highway trailer.

For example, if we had a trailer with a 15% tongue weight on 35” tires, its slope would now be reduced to 45° compared with a trailer of 7% tongue weight on 30” tires that can cope with a slope that is at 50°.

Now take suspension into consideration, more especially suspension travel. We all want our off-highway vehicles to have a lot of suspension travel and so it follows that our off-highway trailers should too. But, suspension travel can really hurt you in certain conditions as a result of body roll and high speed cornering and side slopes are perfect examples.

The impact of body roll shifts the center of mass downhill. The greater the body roll as a result of greater suspension travel the greater the shift to the downhill side of the center of mass. The same concept applies to high speed cornering. The greater the body roll, the less stability one has when cornering at high speeds.

Looking at it in practical terms, because body roll tilts the vehicle downhill, the slope one can traverse is proportionately reduced. So let’s take our trailer with a 15% tongue weight on 35” tires and let’s factor in 8” of suspension travel and compare this with 4” of suspension travel on our trailer with a 7% tongue weight on 30” tires. The 15% tongue weight trailer is now down to 42° while the 7% tongue weight trailer is down to 49°.

Now if we were to raise the center of mass to a point where the allowable slope was 30°, where would that point be on each of our trailers? With the 15% tongue weight trailer that height is about 48” or 4 feet of the ground. This is only about a foot above the top of the 35” tire! Our 7% tongue weight trailer would have to have its center of mass at 60” or 5 feet off the ground. This is 2 ½ feet above the top of the 30” tire!

I would encourage you to go out there and look at some typical off highway trailers and imagine where the center of mass actually is. Hopefully you will have a better understanding of weight and its overall importance to trailer performance. 

One particular off-highway trailer manufacturer apparently trains people towing their off-highway trailers on a side slope to attach a rope to the trailer and hang on - that way preventing the trailer from rolling onto its side. This may be something competition rock crawling professionals do but is certainly not something I would recommend at all. In my opinion this unnecessarily introduces the risk of potential injury and even death. Soft tissue injuries such as sprains, dislocations, back injuries may be caused from straining against the rope. If the trailer does roll and you are not quick enough to let go the rope, or the rope gets caught around you foot, now you are catapulted unceremoniously over the trailer to almost certain severe injury. Even though this might prevent the trailer from rolling over, one cannot use the "rope trick" when driving at 70 mph down the highway and having to make an emergency maneuver to avoid some idiot driver not paying attention and cutting you off.  It seems to me that a far more sensible solution is to design the trailer for optimal safety and performance. Life is too precious to risk on stupidity, even if it looks really cool and tough! 

Leaving the most obvious weight related aspect to the end. Naturally pulling a trailer weighing 3500 lbs will take more effort from the tow vehicle’s engine than pulling a trailer weighing 1500 lbs. However if one man can pull a Jumbo Jet how much effort does it really actually require compared with safety and performance with regard to correctly engineered weight distribution and overall balance? 

III.                                Trailer Axle Articulation

For me this is perhaps the more interesting of myths in that it is actually so obviously misrepresented. Of course I am referring to single axle trailers purpose built for on and off-highway use.

Let’s examine a 4WD vehicle for a moment. Imagine driving your 4WD vehicle off-highway and your front right wheel goes into a deep hole. Now if the hole is deeper than the amount of downward travel the suspension has, the front right wheel will be suspended off the ground as it is being held from lowering further by the suspension system. So you now have no traction on your front right wheel because it is suspended in the air. Without lockers all the drive from your front axle goes to the suspended right front wheel, which spins helplessly. You are left with trying to push your self out of the hole with your rear axle. Of course you are hoping that your vehicle has not pitched forward too far into the hole thereby also lifting your back left wheel off the ground too. Should this unfortunate even occur your back left wheel and your front right wheel spin helplessly and you go nowhere.

Four wheel drive suspension systems with more articulation [suspension travel] would allow the front right wheel to reach the bottom of the hole and ensure that the back left wheel also stays on terra firma. This way you can drive through the hole as you still have all four wheels planted on the ground. This is obviously a significant advantage off-highway.

Now let’s look at a single axle trailer in the same situation. First off there is only one axle and it does not have drive power to the wheels either. Now if the right wheel goes into the hole, unless the trailer rolls, then the left wheel will be on the ground. The trailer will pivot on a longitudinal axis running from the back of the tow vehicle receiver through to the back of the trailer and the trailer will lean to the right because the right wheel is in the hole.

Now let’s examine the notion of sprung weight and un-sprung weight. Sprung weight is as the name describes, the weight of everything being supported [sprung] by the trailer suspension – so everything above the springs. Un-sprung weight is the springs and everything below the springs like the wheels. The same applies to a 4WD vehicle. As you can imagine sprung weight is usually many times greater than the un-sprung weight. The trailer and its contents will usually weigh more than the springs, axle and wheels.

Getting back to a 4WD vehicle with a front and a rear axle. If a suspension system has a great deal of articulation [suspension travel] then one gets a phenomenon called “body roll”. This is when you drive around a corner and the whole vehicle leans away from the corner. So if you are turning right, the body [sprung weight] leans left. Go too fast and the shift in the center of mass of the greater sprung weight extends outside the area of influence [boundaries defined by the wheel contact point of the road surface] and the consequence is an undignified roll. Similarly if you have very little articulation then body roll is also minimized as are the precarious consequences of rolling as is evident with low slung sports cars.

Take a Jeep for example. It has “anti-sway bars” on the front and rear axle for the specific purpose of controlling body roll in on highway high speed driving. One can buy “anti-sway bar disconnects” that are disconnected when driving off-highway as disconnecting these anti-sway bars offers more articulation, an obvious advantage off-highway at slower speeds, while at the same time a significant disadvantage on highway at higher speeds.

Getting back to our single axle trailer. Because it only has one axle, articulation is essentially a mute point that helps little overall off-highway but instead could actually hurt dramatically both on and off-highway. Perhaps the only help that articulation [suspension travel] may offer off-highway is allowing the right trailer axle wheel to travel down enough to the bottom of the whole and thereby prevent the trailer from bottoming out on its axle. However, there is a limit to this as the axle is connected to the wheel so with a 35” tire, after about 16” [assuming a 3” axle outer diameter] the axle will bottom out anyway. More suspension travel won’t change that. However, remember that the increased articulation that would allow just what I have described, will also allow the trailer body to roll a great deal more. So the end result is that you may not have bottomed out but instead you rolled. Personally I would rather bottom my trailer out on its axle than have it roll and I would certainly not want to drive around a corner at highway speeds and have my trailer roll due to excessive body roll.

Articulation, as with many things, is a compromise we have to accommodate with 4WD vehicles as they have two axles. On the other hand, articulation with a single axle trailer pivoting on a longitudinal axis, which they do, is more of a hindrance and a hazard than anything else.

It is exactly this myth of trailers requiring articulation [suspension travel] that encourages certain people to “train” individuals towing trailers to attach a rope to a trailer in an off-camber situation and hang on much like competition 4WD vehicles – who seldom do so anymore anyway. Be cautious about “training” that requires you to risk life and limb from preventing a trailer rolling due to poor suspension design or for any reason. Potential shoulder, back, knee, and ankle injuries from straining against a rope are not worth it and will ruin an adventure. Pray you can let go the rope fast enough should your trailer roll, otherwise you become a projectile and far more serious injury and even death could be the unfortunate result.