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Old 05-04-2006, 02:18 AM   #1
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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?" 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" 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.

Light Braking

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.
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Old 02-05-2007, 02:12 PM   #2
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Ok. I'm going to have to flat tow my jeep wrangler behind my MH. The hitch on the MH is around 20". The bumper on my jeep is 27". Would there be a problem using an 8" drop receiver and turning it upside down?
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Old 02-05-2007, 02:29 PM   #3
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Mike:

I use a drop receiver turned upside down to level my hitch....4" in my case. I checked with Blue Ox and they confirmed it was OK to do this.
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Old 02-05-2007, 05:14 PM   #4
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Ok, thanks for the reply but I see the answer in the origional article. Guess I didn't read close enough.

My next question is If I have a 7" difference do I go with an 8" and have the RV 1" higher or a 6" and have the RV 1" lower?
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Old 02-06-2007, 03:09 PM   #5
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Mike, you want the RV end higher by 0-4". The idea is that you want the toad to push under the RV, not leapfrog over the top of the hitch. Of course this would only happen in an emergency stop.

The reason that 0" is acceptable is because as the RV brakes the nose will squat which will cause the hitch ball to rise. The reason for the 4" recommended maximum drop from the RV to the toad is to keep from popping the tow bar off of the ball (not a problem with the self storing tow bars). The 4" drop recommendation also helps reduce stress on the toad suspension and tires during normal braking (as the RV stops there can be significant downward pressure on the toad front suspension if there is a lot of RV to toad drop).
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Old 02-06-2007, 04:56 PM   #6
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Thanks for the response. As I'll be picking up the RV on thursday if the weather cooperates.

I'll be using a uhaul hitch for now as the blue ox base plate wont work on my jeep without removing the steering box skid plate.
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Old 02-07-2007, 03:11 AM   #7
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To; ALL, re; Drop Receivers,

IN many earlier postings I told what happened to my toad baseplate on the way to a Noreaster rally in Verona, NY.

My moho was 8 inches higher than the toad , but I wasn't aware of any problem until arriving at campground and starting to unhook tow bar I found the base-plate juet about falling off toad. If I had traveled any further, the toad would have been unhooked, and on it's own. Yes, the toad would have stopped, because of the brake buddy, but would the vehicles behind it safely stop? This could have been a disaster.

With the help of PAR_THE COURSE, Mike, we found a welding shop and had base-plate welded to toad.

The point of above info is; the moho and toad should be as close to level as possible.

Toad too low ; when braking, front of moho dives, rear of moho lifts, front of toad dives resulting in toad trying to go under moho placing a lot of stress on base-plate

Toad too high; moho lifts, toad dives, same stress and base-plate will eventually be ripped off.

I hope no-one has to experience the consequences stated above.

AS LEVEL AS POSSIBLE, but no more than 4 inches lower or higher.

Hope this info helps.

BE SAFE-- BE HEALTHY--

=== AIME=== AJBJRVERS===
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Old 02-07-2007, 06:55 AM   #8
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Whats your thought on installing the brackets on a winch plate? It's 3/16" plate and is strong enough to hold a 15K lb winch. I could also weld it.
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Old 02-07-2007, 03:24 PM   #9
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Quote:
Originally posted by Mike (KC9IPV):
Whats your thought on installing the brackets on a winch plate? It's 3/16" plate and is strong enough to hold a 15K lb winch. I could also weld it.
Me thinks the winch plate is too narrow. My winch plate is only as wide as the frame rails. Memory says (could be wrong) that the tow bar brackets are outside the frame rails.

There has been some discussion as to whether the Wrangler stock bumper is strong enough for a tow bar. It would be marginal with backing plates. The alternative that I like, and use, is to replace the stock bumper with a stronger one and use it as the basis for tow system. Check the Tomken Machine website

Towbar

Universal Tow Brackets

Bumper with brush guard There are other bumpers without brush guards and are wider.
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Old 02-08-2007, 03:28 PM   #10
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For this one trip to pick up the MH I am using the stock bumper with reinforcing pieces on the back. I'm checking everything every 50 miles or so and keeping the speed down to no more then 55mph. All looks good so far about 175 miles to go.
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Old 04-06-2007, 11:17 AM   #11
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Just found this thread and am getting ready to tow very soon.

I have a couple of questions.

The discussion above talks about the toad diving below the MH in a panic stop. My question is: why would the toad run under the MH if the toad supplemental brakes are applied at the same time as the coach? My feeble mind thinks that the car being of lesser mass would stop faster than the MH and would not be able to dive under the MH since the toad would be stopping faster than the MH and since the toad would be actually pulling on the towbar.

Also, does the pivot ball on the Aventa II help to eliminate the problem with having the MH too high relate to the toad by providing an additional pivot point between the receiver hitch attachment and the toad? The pivot ball would seem to allow the towbar to "bend" in the middle and offset much of the height differences in either direction (high or low relative to the two vehicles).

Comments?
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Old 04-09-2007, 09:31 AM   #12
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Quote:
Originally posted by SteveR:
The discussion above talks about the toad diving below the MH in a panic stop. My question is: why would the toad run under the MH if the toad supplemental brakes are applied at the same time as the coach? My feeble mind thinks that the car being of lesser mass would stop faster than the MH and would not be able to dive under the MH since the toad would be stopping faster than the MH and since the toad would be actually pulling on the tow bar.
You are not feeble, you are asking a relavent question.

There would be no problem if the braking between the RV and the toad could be perfectly synchronized so that the toad brakes slightly more than the RV. This same principle would also help all automobiles....ideally the rear brakes will be slowing slightly harder than the front brakes. The reality is that we can't get it right on 4 wheeled cars, its nowhere near right on toad cars without anti-lock brakes.

Anti-lock brakes try to resolve this issue by relaxing the brake on any wheel that is spinning slower than the other wheels (in a nutshell this may not be right, but bear with me). Many tests have shown that professional drivers (whatever that means) can stop faster without anti-lock brakes, but the vast majority of us are better off with anti-lock brakes. Problem is, no one has implemented a coupled anti-lock brake system between the RV and the toad.

Two stopping scenarios are possible; 1) the toad is going to stop faster than the RV, 2) the toad is going to stop slower than the RV. Most auxiliary brake systems are going to fall into the 2) category. They will be set up for normal braking, so that the toad tires never smoke during a slow to middle braking maneuver. I doubt that anyone ever tests their toad braking system under a full lockup panic stop.

Personally, I would be perfectly happy to smoke the toad tires in a panic stop, but I have experienced ,nor heard of, any brake systems that will do this. I have, however, seen pictures of jack-knifed toads because the tow bar was tilted up from the RV to the toad.

Bottom line, given how little control we have over the relative braking characteristics between the RV and toad, it is wise to plan for the worst case. Hence the recommendation that the tow bar be almost level or slightly sloping down from the RV to the toad.
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Old 04-10-2007, 07:43 AM   #13
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Thanks for the reply. Futher measurements of center to center from the toad bracket tow bar attachment shafts to the raised receiver show a 5 inch increase above dead level with the RV side being the high end. I had to use a Blue Ox drop receiver in the inverted position to raise the RV receiver height to be above the toad height. Since it only comes in 2" length increments, I am opting to be a little high on the coach end vs spending another $100 for an other receiver that would drop the height by 2 inches.

The toad has a proportional braking system (SMI Silent Partner) which I will use in "Real Time" mode. This means that when the coach brakes come on and the coach is slowing down, the toad's brakes will come on. I have adjusted it were it is pretty sensitive on the early application of the toad brakes. Also, since the SMI creates a vacuum in the brake booster, the effort required to apply the toad brakes is very near "power brake" levels and should provide for much improved toad braking both in response time and in the amount of force applied.

I believe with all the above being considered I should be OK.
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Old 07-02-2007, 03:08 PM   #14
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Here is another line of thought to tune up your brain.

The tow bar angle, in my mind, is less of an issue with a proper tow brake. By that I mean a good system with no built is surge suppressor or technology that determines if you are going down hill or stopping. These systems must think and in a panic, you nor your brake should think, it should just do it. Reading through the posts here it seems that the concern is that the car might try to climb in to the bed or some how sneak under it. IF, and I say IF you have a proper brake, then how could it?

Proper angle does reduce sway and tows a bit better. But those drop receivers add at least two pin connections which adds slop in the tow set up. Blue OX makes an extra part to help with this on one pin but rather than that, I still believe that a proper brake will eliminate the major part of this issue. Slop in the tow set up will add to sway in the tow which can add to more wear on the front tires and cupping. I do know a customer (a customer now but she used to have a competitors product) that folded a motorcycle carrier up into the back of her coach and then onto the hood of her truck. The angle was right but the stop was so fast that her brake could not keep up. At least that is what she told me. At any rate the brake failed her and she had 1000's of dollars of damage. A proper brake would have been much less than of the repair.

Food for thought.

Pete
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