Towing Physics 101
By Mark Penlerick
Engineering Team Leader
Blue Ox Towing Products
In the Spring 2003 issue of RV technician I touched on the subject of height differences between motorhome receiver hitches and baseplates on towed vehicles in an article entitled "Tow Bars wear out?"Ě Due to the importance of this issue, I feel this subject deserves a little more face time than it received in that article.
Seeing the hundreds of towing set-ups that cruise in to the Blue Ox factory for tours and visits to the Blue Ox RV park, coupled with having been to several rallies over the years, I've seen some strange hook-ups, even some dangerous ones that I've spoken to the owners about correcting. As a dealer, you should take it upon yourself to correct unsafe situations when Rver's stop by your dealership.
The angle of the tow bar is probably the easiest thing to spot, and one of the most dangerous if left unresolved. A few years ago, it was easy to convince an Rver that their ball coupler needed to be parallel with the ground to keep the coupler from prying itself off of the ball. Presently, in the days of motorhome mounted and stored tow bars...some Rver's do not feel the angle of the tow bar is as important as it used to be. When Blue Ox first brought motorhome mounted tow bars on the RV scene they were new to the industry, dealers, Rver's, and even the manufacturer. Back then the sky was the limit so to speak and severe tow bar angles were somewhat common. Now, you say; why does this matter? Well, if an Rver has a 10-inch height difference and never has to make a panic stop or slow very rapidly, they will likely not have a problem, but if they must brake hard, it could spell T-R-O-U-B-L-E.
Blue Ox's recommendation is that the receiver hitch of the motorhome should never be more than 4 inches higher than the baseplate attachment points. Four inches or less keeps the tow bar level with the ground or slightly angled up towards the coach from the car. The tow bar should never be angled "up"Ě towards the car from the coach. If an extension is added for say a bike rack it is even more imperative the tow bar not be at a severe angle. Not only does it increase the leverage, but each connection adds more slack in the whole setup. It's all geometry and physics. Please, no groans, I know you enjoyed those classes! The farther back you move the pivot point of the tow bar from the center of gravity of the coach, the more vertical movement you get and the more leverage is applied to the receiver hitch, tow bar and the baseplate. See, that wasn't so bad!
Lets visit the diagrams to get a better visual of a potential problem. First I need to be perfectly clear that each coach and towed vehicle set-up on the road today is different. They have different suspensions, weights, centers of gravity, and brakes. They are loaded differently; some have front engines, some rear. Long overhangs, short overhangs. Different wheelbases, you name it, they are all different. Even two coaches of the same make, model and year could react differently from each other. Towed vehicle suspension as well as weight and the presence or lack of a towed vehicle braking system, also play a big part in this formula. Now, having said all that, what we are about to discuss is a model, and only a model to show you the potential for problems. For the record, the coach in the model is 36 feet in length and has a 12-foot overhang past the rear axle. The towed vehicle is a small car that would weigh approximately 3,000 Lbs.
Figure 1 shows a setup where, on level ground, the receiver of the coach is six inches higher than the baseplate attachment points. Looks pretty harmless, doesn't it? You've probably seen worse things right in your own parking lot! Looking on to figure 2 you can see where light braking has caused the height difference to change from 6 inches to ten inches. Believe it or not, the drawing only reflects a one-degree change in the angle of the coach and the car. Think about that for a minute! Not only does the suspension of the coach give upward, allowing the front end of the coach to dip, but the suspension of the towed vehicle also gives downward and allows its front end to dip as well. This is now starting to take its toll on the suspension components, alignment and tires of the towed vehicle.
As the situation progresses to moderate braking, as shown in figure 3, you'll notice that since the towed vehicle is gaining leverage on the coach by pushing up more and more, the height difference increases about 6 inches, where it only rose 4 inches during light braking. Now, we're seeing a much more pronounced impact on suspension components of the towed vehicle, but that's not all. The angle of the tow bar and the weight of the motorhome pushing back down on the towed vehicle may start flexing the baseplate installation. Depending on the distance from the baseplate mounting points to the attachment points of the baseplate, you are starting to get a large spike in leverage, force and stresses on the baseplate, its hardware and the frame or unibody of the vehicle to which it is mounted. Depending on how the baseplate attaches, you may actually see the frame of the vehicle flex then return to either its original position or at least close to its original position. Repeated stops in this fashion may eventually lead to fatigue in the vehicles frame, loosening of the bolts, as well as fatigue in the baseplate itself, depending on its design characteristics. OK, I've got to get a plug in here. Over the last few years here at Blue Ox we have changed our philosophy on baseplate design to include letting the baseplate itself torsion, much like the receiver hitch on the back of the coach does, to absorb a lot of these types of stresses. However, when forced into these positions repeatedly, no baseplate or bracket design will hold up forever. Just remember, that coach can weigh up in the 20,000 to 35,000 Lbs range and gravity is looking to bring it back to earth. Gravity does not care that the coach is pushing down on the baseplate of the towed vehicle.
Just as we saw the jump from four to six inches between light to moderate braking, now with heavy braking, in figure 4, we see the drama unfolding. That six-inch difference sitting out in the parking lot has just turned into a very scary situation as our customer just stood on the brakes to avoid Bambi running across the road. Although it takes a pretty hard stop from a six-inch difference to make the towed vehicle end up under the rear of the coach, I have seen it happen before. Granted, that set up was closer to 10 to 12 inches off to start, but have you seen one in your lot that far off...probably have, and so have I.
Correcting the problem
This is the easy part. Blue Ox manufactures a complete line of drop receivers. They range in drop from 2 to 10 inches in increments of 2 inches. These drops can also be inverted in the event that the baseplate is higher than the receiver hitch on the coach. Along with the drop receivers (pictured somewhere) Blue Ox also manufactures what we call a hitch immobilizer which can be bolted to the drop receiver and the receiver hitch to keep the slack from allowing the drop to move around too much. I would encourage all dealerships to have several of these drops and immobilizers on hand at all times. They are a quick sale, and an even easier install. It sure beats dealing with issues down the road that were caused by too much height difference.