Dinghy Speed Limits

May 25, 2017
Filed under RV Tech Q&A

 

Q:

Nearly all manuals included with dinghy vehicles include the warning not to exceed 65 mph. Why? Is it a matter of control, or can damage be done to the vehicle? I consistently try to put my RV on cruise control for 65 mph; however, being human there are times when I go faster (usually when passing). Is it OK to go over the limit for short periods of time? What is the consequence of ignoring the recommended limits (besides a ticket in some locations)?

Bill Rostron | Middle Island, New York

A:

When vehicle manufacturers provide speed limitations along with their recommendations for towing behind a motorhome, they don’t include an explanation of how they came to that decision. In general, the process is for the engineering department to say which, if any, drivetrain combinations are capable of being towed long distances on all four wheels. Then, extensive tests are performed in a variety of weather and road conditions to monitor vehicle tracking and temperatures of lubricants, etc. Maneuvers such as abrupt lane changes and stability at speed tests are also performed.

Typically these must be approved (or vetoed) by a number of departments, including marketing, engineering and legal. The faster a vehicle is towed, the more difficult it is to stop the combo in an emergency and/or keep under control if swaying occurs, etc. Higher speeds lead to more serious crashes, and the manufacturer’s name is on the vehicle. Some manufacturers appear to be more risk averse than others, and some of the speed limitations may be due to concerns for liability, as much as mechanical concerns affected by heat and wear.

It’s best not to put your motorhome, dinghy vehicle, family, yourself and other road users at risk.


 

Tire Replacement

My motorhome is a 2006 Coachmen Mirada on a Ford F-53 chassis, and the tires are Samson brand 225R70/19.5 load range F. Last year I replaced all six tires on my motorhome. I was very lucky as I had picked up a nail in a rear tire and when they pulled it off they discovered extensive cracking on the sidewalls all around the rim on the inside of all four rear wheels. That’s an area I still haven’t found a good way to check without pulling a rear wheel (any suggestions would be appreciated).

Now to my question: Prior to the new tires, the motorhome didn’t exhibit any front-end shimmy, but after installation I started to experience a shimmy beginning around 50 mph. I later had balance beads put in. I’m still getting shimmy on the front end, but it is puzzling as it is not always there and it varies at what speed I’m traveling. Sometimes it shimmies at 50 mph, other times at 55 or 60. I’ve thought of replacing front tires even though there are only a few thousand miles on them.
Butch Norton | Moneta, Virginia

A:

I have found that a mechanic’s inspection mirror, which extends on a telescopic handle, works well for looking between dual tires to inspect sidewalls. If those were the original tires, they were about 10 years old, which is beyond their recommended replacement date. I have tried balancing beads to stop tire shake several times without success. The variation of speeds where it starts to shimmy may be due to different pavement surfaces. Before you replace the tires, I suggest you find a shop that can balance the tires on the motorhome. This compensates for imbalance of rotating brake parts, etc. Also look for bent rims, which often occur from curb and pothole impacts.


 

Automatic Temperature Controls

Many of us own motorhomes that came from the factory equipped with sophisticated control systems for everything from holding-tank-level systems to battery-charge monitoring systems, etc. So why is it that the cab temperature control system is nothing more than a blend door or hot-water valve controlled by a knob on the dash? Fan speed is controlled with another knob that provides high, medium, low and maybe off speed controls. In short, you, the driver, must act as the thermostat. If you want it warmer, you turn the temperature knob to warmer; for colder, you turn the knob in the opposite direction. I have found that there is no such thing as a sweet spot. Whenever you set the discharge temperature with this knob, the temperature will eventually over or undershoot the so-called set point.
I wonder if anyone out there has ever attempted to automate this archaic temperature control system. There must be junkyards galore with cars that contain factory-equipped automatic temperature systems one could salvage and incorporate into a motorhome. All one would need is a shopping list of which components to strip out of the junked vehicle. This could become a great resource for many do-it-yourselfers. At the very least, descriptions and drawings of a manual temperature control system versus an automatic temperature control system would be helpful.
Frank Harrison | Via email

A:

While I understand your plight, it pretty much comes down to manufacturing cost. Chassis manufacturers are under strong pressure to keep their prices down, and many of the chassis are not only used for motorhomes, but similar configurations also find their way into commercial vehicles, where price is very competitive in large fleet purchases. Simply stated, automatic systems cost more than manual ones.

The task of trying to develop a hybrid temperature control system, which combines new components with used salvage yard parts (which may or may not work when you get them), along with trying to design circuitry and vacuum-line routing that would interconnect these systems, is not practical. Then you have to do this for every make and model motorhome out there, and you have a potentially lifelong task. When finished, only a handful of people would be likely to do it because of the cost and complexity. However, if any readers out there have done something like this, we’d like to hear from them.


 

Power Programmers

I installed a Banks Power Pack System on my 2013 Forest River Lexington Class C motorhome, and also installed a K&N intake system. The Banks system is 50-state legal. In my opinion, the only way to increase the power and torque of the Ford V-10 motor is by increasing the intake and exhaust. As we have all heard many times, the motor is just an air pump: The more air in and the more air out, the more power it can produce. Besides the horsepower and torque increases, the fuel efficiency is better (I can get consistently more than 14 mpg on the freeway at 65 mph, without heavy wind, of course). Installing the Banks AutoMind software helped with the transmission shift points. I can now go up most hills in Utah without going below the speed limits.

Have a tech question?We recently completed a 4,450-mile trip to Vermont pulling a 6,650-pound car and aluminum trailer, and averaged more than 10 mpg. I was not in a rush and kept the speed at 65 mph. I am not a representative or related to anyone at Banks, but I have used its products for all my Ford diesel trucks (four and counting). The payback for installing a Banks system on my motorhome in fuel costs would take me forever to realize, but not having the frustration of going up hills at 40 mph is priceless.

Kim Wardle | Taylorsville, Utah

A:

Thanks for sharing your experience. We have published results of testing various Banks systems over the years, and have had positive results with all of them. While we have not seen 14 mpg, we have seen fuel-economy improvements when driving in higher elevations on flat roads with fuel-injected gas engines. The results are likely a combination of having slightly less wind resistance due to lower air density, combined with the lower air density reducing the amount of air available in the engine. The fuel injection in turn (via the mass airflow sensor) reduces the amount of fuel going to the injectors, resulting in lower fuel consumption and better mileage.

One of the keys to success in getting more power is to use parts designed to work together for a balanced result. It’s important to not only be able to get more air through the engine, but to have proper fuel mixtures and spark advance curves to match the conditions. The California Air Resources Board (CARB) scrutinizes products to ensure they maintain low emissions, which goes hand in hand with efficiency and durability, so an Executive Order (EO) number indicates a system has passed this battery of tests.


 

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