SAFE TRUCK for Pocket PC
(requires Pocket Excel)
©2004 by William B. Trescott
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download SAFE TRUCK for Pocket PC
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Dear user,

SAFE TRUCK for Pocket PC calculates lateral stability ratings for combination vehicles using axle weights.  It is designed to operate on personal data devices equipped with Microsoft Pocket Excel.


Because SAFE TRUCK is an Excel database and not an executable program, you need only move it into an appropriate folder on your pocket PC such as My Documents.  Select it with your stylus or cursor button and Excel will open automatically.  If your device does not have Excel, visit to download it.  SAFE TRUCK has not been tested on any other database programs.


Click on the SafeTruck.xls icon and Excel will display a table listing various measurements of a single trailer truck.  If your truck has only a single semi trailer, you can use your up and down thumb button to change the numbers in the boxes to better describe your type of truck. (select "view>vertical scroll bar" if your Pocket PC does not have a cursor control button): If your truck has more than just a single semi-trailer, use your left and right thumb button to select from the following types of trailers (select "view>horizontal scroll bar" if your Pocket PC does not have a cursor control button):

Single trailer (18 wheeler), stability rating—20%;
Double trailer (two pups connected by a converter dolly), stability rating—7.3%;
Triple trailer (three pups connected by two converter dollys), stability rating—3.8%;
Turnpike double (two full size trailers connected by one converter dolly), stability rating—6%;
Rocky Mountain double (18 wheeler pulling a converter dolly and pup), stability rating—8.8%;
B-Train (A fifth wheel mounted on the first trailer supports the nose of the second trailer without a converter dolly), stability rating—4.3%;
Truck-Trailer (A straight truck pulling a trailer using a converter dolly), stability rating—18.1%;
Stinger steered (A truck pulling a trailer without a converter dolly in the same manner as a car pulls a trailer, such as a dump truck pulling an excavator on a vehicle trailer, or a car hauler where the 5th wheel is located behind the drive axles, or an inverted gooseneck trailer.), stability rating—9.4%;
Tridem (single three axle trailer), stability rating—12.5%;
Spread Axle (a single trailer with the axles spread 8 feet apart), stability rating—7.5%;
Euro truck (a full size three axle trailer with super single tires designed to be pulled by a single axle tractor), stability rating—5%;
Super Single Double (a set of pups with super single tires), stability rating—16.5%;
Super Single Rocky Mountain Double (18 wheeler pulling a converter dolly and pup with super single tires), stability rating—13.8%;
Unibody truck (A future type of truck having two steering axles, two middle axles, and two steerable rear axles on an articulated trailer similar to the rear of an articulated bus.), stability rating—16.3%;
Quad Axle (a four axle tractor pulling a four axle trailer), stability rating—8.6%;
Jeeps (a single trailer truck with a special type of dolly called a "jeep" between the tractor and trailer to reduce the amount of weight borne by the tractor.  A second jeep, called a "booster" is attached to the rear of the trailer to reduce the amount of weight on the trailer axles), stability rating—3.4%;
Truck-Trailers (A straight truck pulling two trailers using two converter dollys), stability rating—8.4%;
30' Dump (a short semi trailer with several axles), stability rating—6.3%;
Centipede (a full size trailer with eight axles), stability rating—2%;
Michigan Train (a 30' dump with a congear and pup), stability rating—1.4%;
Transportation Research Board recommendation (two 33 foot tandem axle trailers connected by a tandem axle dolly), stability rating—1.5%;
1950's era tractor trailer (a 30 foot single axle trailer with single tires), stability rating—100%.


Once you select the column that best describes your type of truck, you can enter new data into the boxes to more accurately calculate your vehicle's stability rating.  The variable truck measurements are:

Tractor Axle Weight—the weight of all rear axles added together, but not including steering and self steering axles;
Trailer Axle Weight—the weight of all rear axles added together, but not including self steering or dolly axles;
Dolly Axle Weight—the weight of all dolly axles added together (Note that on a set of doubles, dolly 1 is between trailer 1 and trailer 2, while on a jeep or truck trailer, dolly 1 is between the tractor or truck and trailer 1.);
Tractor Wheelbase—measured between the center of the steering axle(s) and the center of the non-steering rear axle(s);
5th Wheel setting—the distance between the 5th wheel turntable pivot (or pintle hook on stinger steered trucks) and the center of the non steering rear axle(s) (Note that if the 5th wheel is behind the center of the rear axles as in a stinger steered trailer, then the 5th wheel setting is a negative number.  If the 5th wheel is forward of the center of the non-steering rear axle(s), then the number is positive.  A positive setting improves vehicle stability while a negative setting reduces lateral stability.);
Trailer Wheelbase—measured from the kingpin to the center of the non-steering rear axle(s);
Trailer Number of Axles—does not include self steering or dolly axles;
Axle Spacing—the average spacing between the axles on the trailer (Note:  If there is more than one trailer and the axle spacing is different on each trailer, enter the axle spacing of the dolly for the average axle spacing and scroll down to the boxes labeled Trailer 1 Axle Spacing, Trailer 2 Axle Spacing, etc. and enter different numbers in the boxes.  The greater the axle spacing, the less stable a truck is.);
Dual Tire Spacing—the average spacing between the center of the tread of the inner tires and the center of the tread of the outer tires on each trailer (Note:  If the truck has single tires, use the one half the tread width for the dual tire spacing.  If there is more than one trailer and the tire spacing is different on each trailer, such as when one trailer has dual tires and another has single tires, scroll down to the boxes labeled Trailer 1 Tire Spacing, Dolly 1 Tire Spacing, Trailer 2 Tire Spacing, etc. and enter different numbers in the boxes.  The greater the dual tire spacing, or the wider the tread on single tires, the less stable a truck is.  A truck with single tires will generally be more stable than one with dual tires.);
Payload—your cargo weight.


SAFE TRUCK provides four kinds of output:

Stability Rating—is a percentage of the lateral stability of an ideal truck.  A 1950's era 30 foot single axle trailer with single tires scores 100%.  This is considered to be ideal.  A similar trailer with dual wheels would score only 50% and an 18 wheeler with tandem axles as well as dual wheels scores only 20%.  Scores below 10% are not considered acceptable and such vehicles must be considered unsafe.  Lateral Stability can be improved by reducing the number of axles or switching from dual to super single tires.  A set of doubles, which scores only 7.3%, will improve to 11.2% just by changing the dolly alone to super single tires.  If all the tires on a set of doubles are single tires, the stability improves to 16.5%.
Highway Damage—is a probability of causing highway damage compared to a 1950's era 30 foot single axle trailer with single tires, which scores 100%.  Of course, the actual amount of highway damage caused by any particular truck is determined by the driver.  A skilled professional is much less likely to damage the highway than a recent truck driving school graduate, but with drivers of equal skill, some trucks are more likely to damage the roads than others.  Probabilities of damaging the highway above 1,000% are considered unacceptable.  A good determinant of driver skill is the number of days between accidents.  As a rule of thumb, the likelihood of a particular driver in a particular truck damaging the highways on any given day can be estimated by dividing the truck's probability of highway damage by the driver's average number of days between crashes—probabilities above 1% are unacceptable.
Trailer Sway—Multiplied by the coefficient of friction between tire and pavement, this number will indicate the maximum amount of lateral force applied to the kingpin or pintle ring of a trailer or dolly when negotiating a tight turn.  Because most truckers do not have a way of measuring coefficient of friction, a 1:1 coefficient of friction is assumed because the actual coefficient of friction is not needed to calculate stability ratings.  In its simplest form, a SAFE TRUCK stability rating is the weight on the rear axles of the tow vehicle divided by the lateral force applied to the 5th wheel or pintle hook.  It is expressed as a percentage only in comparison with the ideal truck.
Adjusted Highway Damage—is the probability of highway damage factored by the payload weight.  Some trucks can legally carry more cargo than others.  If a truck can carry twice as much cargo as another truck with an equivalent stability rating, then the amount of highway damage per pound of cargo will be half.  Probabilities above 500% are considered unacceptable.


SAFE TRUCK calculates lateral stability ratings based on "rearward amplification" forces.  Whenever a truck with dual wheels turns even slightly, the outer tires must travel a different distance than the inner tires.  If a truck has tandem axles, the tires on one or both of the axles must also be forced to skid sideways as the truck turns.  Before skidding, however, radial tires store energy like springs, making the steering seem sluggish until the stored up energy is suddenly released, either in an oscillating snake like tail wagging motion called "sway", or in a rapid change in direction that can cause an inexperienced driver to lose control.  This storing of energy followed by a rapid change in direction is called "rearward amplification" because energy stored in the rear tires of the truck amplify or increase the steering input.  Rearward Amplification makes trucks with low stability ratings difficult to drive and increases the highway damage due to sway because their weight is continually shifting from one side to the other, overloading the tires on one side of the vehicle and then the other causing a scrubbing action on the pavement surface.  Concrete and asphalt are very strong  to resist compression forces under the weight of trucks, but most paving materials lack the tensile strength to resist the side to side scrubbing action caused by a truck weaving in its lane.  Pavement under tension can crack, allowing water to infiltrate into the soil underneath—forming pot holes.  If only 10% of a vehicle's weight shifts from one side to the other, highway damage will be increased by 50%.  A 20% weight shift doubles the highway damage.


The stability rating obtained from SAFE TRUCK is expressed in terms of a percentage of the ideal.  Without knowing the coefficient of friction of the highway surface, it is not possible to calculate absolute stability ratings, so SAFE TRUCK considers a fully loaded 30' single axle semi with single tires on its trailer to be the ideal type of tractor trailer with a stability rating of 100% and compares other trucks to that.  A car with a single axle trailer has a stability rating of only 50%.  This means it is only half as stable as an ideal tractor trailer.  Stability Ratings above 10% are considered good, but such trucks have ten times the rearward amplification of the ideal tractor trailer.  Trucks with ratings below 10% are unsafe because they may not be stable enough to maneuver to avoid a crash, or they may lose control and roll over as a result of making an evasive maneuver.  Stability ratings above 100% are possible for partially loaded trucks.  Stability ratings for vehicles without trailers cannot be calculated because they have no measurable amounts of rearward amplification.  As far as lateral stability is concerned, vehicles without trailers are considered infinitely stable.  The understeer or oversteer characteristics of single unit vehicles are not a threat to the safety of the skilled professional driver in the same way that the unpredictable oscillations caused by rearward amplification in long combination vehicles are.


If you discover that your truck has a low stability rating, there are several things you can do to improve it.  First, adding cargo to the trailers in front and lightening the trailers in back will improve stability.  The most common cause of instability is a rear trailer that is heavier than the one in front of it.  Replacing dual wheels with single tires or switching to a longer converter dolly or reducing the number of axles will often double the stability rating.  Replacing a set of doubles with a single trailer will reduce both highway damage and driver fatigue.  Perhaps the most useful feature of SAFE TRUCK is the ability to test the stability of various truck designs without having to modify your vehicle.  In the future, crash victims might use Safe Truck to sue trucking companies if trucks are poorly designed or unsafely loaded.  States may use the program to require truckers to pay for the actual amount of highway damage they do, rather than assessing road use taxes on the basis of fuel economy as is done today.