The 75 Percent Rule of Thumb

The 75 Percent Rule of Thumb

By Stephen Talmadge

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One often-debated topic regarding pop-up trailering pertains to appropriate “weight ratings” for tow vehicles and trailers. The core question is: What weight trailer can I tow both safely and successfully? At first, the answer seems obvious—tow anything that weighs up to the tow-rating weight specified by the tow vehicle’s manufacturer. That is one answer and, under ideal conditions, may be correct and safe.

However, as actual operating conditions diverge from ideal conditions, the appropriate answer depends upon complications that can arise from several sources:

  1. Many campers are using pop-up trailers for the first time, moving “up” from tents, and are not conditioned to think about “rigs,” weight ratings or other towing stuff. Tossing a nylon tent, cooler and some clothes into the family station wagon is a far cry from hauling a 3,000 lb. trailer through the Vail Pass in Colorado.
  2. Tow ratings assigned by tow vehicle manufacturers are based upon ideal conditions: sea level operation, perfect day and a flat, dry surface. Changing that scenario reduces the effective towing capacity of the tow vehicle.
  3. Selection of tow vehicles is frequently a compromise. Many families use minivans because, 90+ percent of the time, they don’t tow with them; and one result they want is better gas mileage when not towing. However, problems can loom because typical minivans do not make very good tow vehicles.
  4. As much as we may not want to admit it, some of us are unwilling to identify with the “realities” of towing. We may buy into “macho” images projected by tow vehicle manufacturers or perhaps assume that, because a bad thing hasn’t happened to us yet, it never will.

First, Some Definitions

To understand and deal with these issues, we first need some towing weight definitions:

  • UVW (Unloaded/Dry Vehicle Weight): The weight of the vehicle as built at the factory. In the case of a trailer, the UVW does not include cargo, fresh water, LP gas, installed or optional accessories.
  • NCC (Net Carrying Capacity): The weight of all allowable, additional goods placed in or on the vehicle/camping trailer and/or trailer hitch while in tow.
  • GVWR (Gross Vehicle Weight Rating): The maximum permissible total weight exerted on all wheels and, if appropriate, the hitch. The UVW plus the NCC should not exceed the GVWR.
  • GVW (Gross Vehicle Weight): The actual, total weight exerted on all wheels on either the tow vehicle or the trailer. The GVW should never exceed the GVWR — either on the tow vehicle or the trailer.
  • GCWR (Gross Combined Vehicle Weight Rating): The value specified by the tow vehicle manufacturer as the maximum allowable total loaded weights of both the tow vehicle and towed trailer.

“The Rule”

A simple, relatively consistent Rule of Thumb has evolved: Multiply tow vehicle GVWR and GCWR ratings by 75 percent to calculate a generally safe, accurate estimate of the tow vehicle’s capacity.

“The Rule” is designed to provide a quick, simple and practical adjustment to published tow vehicle ratings that will serve well in almost all towing situations. It is derived from three general issues: performance decreases in normally aspirated gasoline engines, variances in towing/trailer equipment, and relative operator expertise. If one or more of these issues has been addressed, then its relative impact can be discounted when applying The Rule.

Issue One: Engine Performance Degradation
Degradation in engine performance is exemplified by statements such as this one from Ford Motor Company: “Since gasoline engines lose power at a rate of three percent to four percent per 1,000 ft. elevation,” (above mean sea level) “a reduction in gross vehicle weights and gross combination weights of two percent per 1,000 ft. elevation is recommended to maintain performance.” (Source: Ford Motor Company 2001 RV & Trailer Towing Guide, North American Fleet, Lease and Remarketing Operations, Copyright August 2000.)

For example, suppose I intend to take Canadian Highway 93 between Banff and Jasper, Alberta, or US Interstate Highway 70 between Denver and Grand Junction, Colorado (where elevations can easily exceed 10,000 feet above mean sea level). I have to plan for the available horsepower of the engine being reduced by 40 percent and for both the GCWR and the tow vehicle’s GVWR to be reduced by 20 percent at the summit(s) to “comfortably” make either of those runs.

Typical tow vehicle: a 2000 Ford Explorer 4×4 with 5-speed automatic/overdrive, powered by the 210 horsepower SOHC 6-cylinder engine, equipped with the 3.73 final drive – part of the factory towing package—and a Class III hitch. This tow vehicle has a UVW of about 4,110 lbs., GVWR of 5,340 lbs., rated towing capacity of 5,740 lbs., and a GCWR of 10,000 lbs.

Trailer: a 2000 Coleman Sedona that goes out the driveway right at 2,000 lbs. – well below it’s GVWR of 2,500 lbs.—and total road weight split 15 percent (300 lbs.) on the tongue and 85 percent (1,700 lbs.) on the trailer’s wheels.

Applying the Ford Guidelines: At 10,000 feet elevation, the 10,000 lb. “sea level” GCWR for the Explorer, is reduced by 20 percent to 8,000 lbs. The GVWR for the Explorer is also reduced by 20 percent to 4,242 lbs. The towable weight at that elevation is 3,278 lbs. (8,000 lbs. minus 4,242 lbs.) and since the Sedona’s weight rating does not change with elevation—at first glance everything looks OK.

However, I see a yellow flag: The Explorer’s UVW is 4,110 lbs. and its GVWR at this elevation is 4,242 lbs.—a difference (if I want to maintain sea-level performance) of only 142 lb. for the Explorer’s total carried load: people, personal stuff, fuel, trailer tongue weight, etc. Realistically, carried load with full fuel adds up to about 1,055 lbs.—making the Explorer’s adjusted gross weight 5,165 lbs. or 923 lbs. over its elevation-adjusted GVWR.

Presuming everything is working properly, will the Explorer make it through the mountains? Yes—primarily because of the large “reserve” in the elevation-adjusted GCWR. Will there be a performance hit towing to and around 10,000 feet elevation? Definitely. How will I compensate for the hit? Lower gears, slower driving, probably using the right lane a lot, and close attention to the engine gauges. But, if I proceed prudently, most likely I won’t be surprised or disappointed.

However, if both the elevation-adjusted GCWR and tow vehicle’s GVWR were below their respective actuals—and a 3,300 lb. trailer on the back of the Explorer would do it—that would be a different matter and I should be looking for a way around, not over, those 10,000 foot mountains.

The example above pertains to operation at a 10,000 foot elevation. If the operating elevation were maxed at 4,000 feet, then the tow vehicle’s GCWR and GVWR should only be reduced by only eight percent and the remaining calculations adjusted accordingly.

Issue Two: Common Factors That Limit Weight Ratings
Front-wheel drive and/or short-wheelbase tow vehicles frequently have both a lower tow rating and lower GCWR if they do not use a weight distributing hitch. These same vehicles almost always need a sway control device when towing trailers with a light tongue weight ratio. Catch-22: Some pop-up trailer manufacturers void their warranties if either weight distributing hitches or sway controls are used when pulling their products.
Tow vehicles equipped with a manual transmission generally have a lower tow rating than the same vehicle equipped with an automatic transmission.

Most tow vehicles carry a higher tow rating if the trailer has brakes—preferably electric brakes.

Trailer Brakes: Now that you’ve got yourself going…can you stop?

Let’s go back to our perfect day with a level, dry surface and assume that we’re using a production tow vehicle with properly functioning brakes. A fixed amount of total braking energy will stop a 5,000 lb. tow vehicle from 60 mph. If we want to shorten the stopping distance we must apply that same amount of energy over a shorter time.

It is important to remember that tow vehicle brakes are designed to stop the tow vehicle, not the tow-vehicle/trailer rig. If we want to stop something that we tow, we should add that part ourselves.

Attach a 2,000 lb. trailer with no trailer brakes to that 5,000 lb. tow vehicle. It will now take 40 percent more energy to stop that combination from the same speed under the same conditions. Mass is mass, the energy to decelerate the mass to a stop has to come from somewhere.

If we do not have trailer brakes, we must get 40 percent more energy to stop the rig from the tow vehicle’s brakes, or take 40 percent longer distance to stop, or some combination thereof. If we try to stop over the same distance, we will try to transmit 40 percent additional energy as friction (the stopping mechanism) to the tow vehicle’s brakes. Of course, just because we press the brakes harder, there is no guarantee that the rig will actually stop more quickly or in a shorter distance. The tow vehicle’s braking system isn’t designed to do all of that extra work and can quickly be overloaded.

Typical things that happen to overloaded braking systems are:

  1. The tow vehicle’s brake pads overheat, then glaze— rendering them polished and less effective, stretching out the stopping distance. In extreme situations, the brake pads may smolder and smoke as they cook.
  2. The brake rotors overheat and (after scorching) may warp, rendering them incapable of uniform contact with pads, therefore both ineffective and dangerous.
  3. Assuming (optimistically) that neither of the above happens, the tow vehicle’s brake pads will wear out up to 40 percent faster.

The Downside
Descending the back side of the Rockies (or hills or other smaller mountains) we need a greater amount of stopping power (due to the incline) to stop in the same linear road distance or same amount of time. On an downward incline without trailer brakes the choices are more stopping distance, more heat or more wear.
Can we compensate by using a lower gear? Sure. Those diamond-shaped, yellow “Use Lower Gear” signs at the top of a long, downhill run are put up by highway engineers who know we’ll need extra help slowing down. Question is: do we want our tow vehicle’s transmission to be our only option? If we don’t have trailer brakes and we toast our tow vehicle’s brakes that’s pretty much the hand we’re holding.

Other situations when ‘routine’ stopping can quickly become a critical and scary maneuver:

Towing in rain, snow or wind—or some combination thereof.
Being passed by someone on a two-lane highway who misjudges that ours is the only vehicle in front of them, then tries to make a hole for their car in that rapidly-shrinking space between our front bumper and the car in front of us.
A sudden tire failure at highway speeds.
Being cut off at 65 mph as someone swerves in front of us across four lanes of Interstate traffic to make an exit that they didn’t see in time.

What’s my point?
When the trailer weighs more than 1,000 lbs. the absence of trailer brakes should reduce the GCWR of any rig by at least 25 percent. Adding trailer brakes increases the stopping ability of the rig by compensating for the added mass of the trailer. If the trailer brakes are the correct type and set up properly, stopping distances for the rig will correspond to those of the tow vehicle alone and no downward adjustment of the weight ratings need be made to compensate for inadequacies in the rig’s braking system.

If the tow vehicle has an Antilock Braking System (and most newer ones do) inertial trailer brake controllers can be set to engage a bit more aggressively to keep the trailer straight behind the tow vehicle– compensating for the stuttering lock-and-release operation of the ABS in panic stops. When the braking trailer remains in line behind the tow vehicle, the rig works with you, not against you – even in emergency situations.

Lastly, A Margin of Safety

Even if we put all of the correct safety equipment on appropriate tow vehicles and trailers, those of us who are (relatively) new to pop-up trailering need an extra margin of safety. The newer among us need an ‘experience buffer’ to learn how to operate our equipment properly so that we can encounter the unexpected more calmly and decisively.

Even on flat terrain at sea level on a perfect day, The 75 Percent Rule of Thumb provides a quick and effective hedge against the unknowns that we will encounter. We could all benefit from that extra margin of safety.

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