www.dexteraxle.com
2900 Industrial Parkway East n Elkhart, IN 46516
Phone: 574-295-7888 n Fax: 574-295-8666

ISO 9001:2015 Certified

06/17

© 1997 - 2017 Dexter Axle Company. LIT-008-00

Introduction
This information is intended as a guide for the proper specification and application of Dexter Axle running gear,
associated components and accessories. We have attempted to define some of the terms commonly used in the
transportation industry as well as providing descriptions of the various systems used in building a wide variety of
trailers.
Dexter offers a full line of trailer axles that can be used in many different applications. When specifying any
pre-engineered components such as axles, it is the responsibility of the trailer designer to ensure compatibility with the
vehicle and all of its sub‑systems.

Important Information
The information presented is meant to assist trailer manufacturers in the specification of their running gear components.
Dexter Axle does not warrant that the information given constitutes an approved trailer design or application. Dynamic
loading, travel requirements unique to the trailer design, unusual service conditions, trailer configurations, unequal
load distribution, hitch or coupler arrangements, and towing vehicle suspension characteristics can significantly affect
the performance of any trailer axle and/or suspension systems. It remains the responsibility of the trailer manufacturer
to evaluate, specify and test their trailer/running gear combination before production and to certify it as such. While the
information presented at the time of this writing is current, it is subject to change as designs and components evolve
over time.

Disclaimer of Warranty and Limitation of Liability
All users of this product catalog acknowledge that the information presented is significantly affected by factors
within the exclusive knowledge of the user including, among other things, service conditions, trailer configurations,
load distributions, hitch and coupler arrangements and tow vehicle suspension characteristics, that the users have
independently investigated these factors and have solely relied on those investigations when using this catalog, and
that it is the responsibility of the user to adequately specify, evaluate and test its trailer/running gear combinations.
DEXTER AXLE DISCLAIMS ALL WARRANTIES, WHETHER WRITTEN, ORAL OR IMPLIED, IN FACT OR IN
LAW (INCLUDING ANY WARRANTY OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE),
ASSOCIATED WITH THE USE OF THE CATALOG AND WITH ANY INFORMATION PRESENTED BY THIS
CATALOG.
Dexter Axle shall not be liable in damages (whether compensatory, punitive, direct, indirect, special, incidental or
consequential) to any user of this catalog under contract, tort, strict liability or any other theory of liability, and any user
agrees to indemnify and hold Dexter Axle harmless from any and all claims, actions or other proceedings (including
attorney fees and court costs) arising out of the use of this catalog to the extent said claims, actions or other proceedings
do not arise out of the sole and exclusive negligence of Dexter Axle.

Load Ratings
The maximum load carrying capacity of any assembly is limited to the lowest load rating of any individual component
selected. For instance, the load rating of a pair of wheels may be lower than other axle components selected. If this
is the case, the load carrying capacity of the axle assembly is reduced accordingly. As a specific example, if a pair of
wheels is rated at 1500 pounds each and is used with other components rated at 4000 pounds per axle, the maximum
load capacity is limited to 3000 pounds. If two tires are rated at 1400 pounds each and are used on this assembly, the
maximum load carrying capacity is limited to 2800 pounds.

Table of Contents
Definition of Terms.................................................................................................................................3
Trailer Design Considerations................................................................................................................5
Running Gear Systems.........................................................................................................................9
Specifying Axles..................................................................................................................................10
Other Trailer Components....................................................................................................................11
Installation Suggestions.......................................................................................................................12
Limited Warranty..................................................................................................................................20
Dexter Video Gallery...........................................................................................................................21

Definition of Terms
Bump Clearance

GAWR - Gross Axle Weight Rating

Bump is the upward displacement of a wheel center
relative to the trailer frame (sprung mass). Clearance is
the amount of trailer frame to axle clearance necessary
to allow the axle to clear or NOT contact the trailer
frame.

The value specified by the vehicle manufacturer as
the load carrying capacity of the axles in a system, as
measured at the tire-ground interfaces. This includes the
wheels and tires.

Camber
The angular relationship of the wheel to the road
surface in the vertical plane. Axles are typically built with
a pre-determined bend in the tube that compensates
for the expected deflection under load. Ideally, the tire
footprint will contact the road evenly across the width of
the tread.
LOAD

LOAD

For example:
An axle beam may be rated for 6000 lbs., the springs
rated for 2500 lbs. each, the wheels rated for 3500 lbs.
each, and the tires rated for 3042 lbs. each. The GAWR
for this example will be 5000 lbs., limited by the capacity
of the springs.

Hitch Weight
The portion of the weight of a trailer that is carried on
the towing vehicle through the connection point.

AXLE CAMBER

Center Of Gravity
The point at which the entire weight of a body (vehicle)
may be considered to be concentrated so that if the
vehicle were supported at that point, it would remain in
equilibrium. For any vehicle, both the longitudinal and
transverse center of gravity must be considered when
placing the running gear to establish proper weight
distribution.

HITCH
WEIGHT

GAWR

GCWR - Gross Combined Weight Rating
For motorized vehicles, the manufacturer rates the
vehicle for towing capability by defining the Gross
Vehicle Weight of the vehicle and combining that with
the GVW of the vehicle to be towed.

Ride Performance
The term 'ride' is a general one referring to the vehicle
motions of the sprung and unsprung masses caused by
the longitudinal road profile.

GCWR

3

Definition of Terms
GVWR - Gross Vehicle Weight Rating

Polar Inertia

The value specified by the manufacturer as the loaded
weight of a single vehicle.

The property that can cause a trailer to swing from
side to side during operation. One of the causes of this
phenomenon can be demonstrated with the following
example:

GVWR

GVWR

Dog-Tracking
The behavior a vehicle will exhibit when the body
of the vehicle is skewed relative to the line of travel
during operation. This happens when the axles have
been mounted to the frame incorrectly, that is, not
perpendicular to the centerline of the vehicle. When the
unit is towed, the running gear will align itself to the line
of travel but the wheel track will be offset from the track
of the tow vehicle and the body or frame will appear to
be tracking out of line.

4

A large travel trailer has the kitchen located at the
rear of the coach. The owner has loaded supplies
and personal belongings in the rear of the unit. Side
forces such as high winds or the bow wave from a
passing vehicle may start the swaying. As the rear end
swings to the right, the trailer pivots on its running gear
and the front end swings to the left, causing the rear
end of the tow vehicle to be pulled left. If the driver
overcompensates by steering left, the front of the trailer
will be forced right which reverses the direction of the
trailers' rear end. Too much mass at the ends of the
trailer will accentuate the swing of the vehicle and may
set up the conditions for side to side sway that gets
greater and greater as the driver attempts to get control
of the vehicle.

Toe Angle
The plane of rotation of a wheel relative to the centerline
of the vehicle.

Trailer Design Considerations
Trailer design is an extremely broad subject. Covering all the guidelines for the design of every towable vehicle is
beyond the scope of this manual. Instead, we have attempted to present some of the more common rules of thumb
that will, in most cases, provide a reasonable result when employed in a trailer design.
The following list describes many of the typical designs, grouped according to their intended use.

Recreational Vehicles

Fold Down Camper

Travel Trailer

Fifth Wheel Trailer

Utility Trailers

Snowmobile Trailer

Enclosed Cargo Trailer

Flatbed Cargo Trailer

Livestock Trailer

Portable Equipment Trailer

Water Craft Trailer

Manufactured Housing

Single Width Home
Double Width Half Unit
Park Model Home
Modular Carrier

5

Trailer Design Considerations
Things To Consider When Designing Your Trailer

6

•

The load distribution between the hitch and the running gear is determined by placement of the axles in relation
to the center of gravity.

•

The hitch weight for conventional, bumper type hitches should be 10% to 14% of the gross weight of the vehicle.
The remaining 86 to 90% of the load will be carried on the running gear, so make sure that the axles, wheels and
tires are properly matched and have sufficient capacity rating to support this load.

•

The hitch weight for fifth wheel and gooseneck type hitches should be 15% to 20% of the gross weight of the
vehicle. The remaining 80% to 85% of the load will be carried on the running gear, so make sure that the axles,
wheels and tires are properly matched and have sufficient capacity rating to support this load.

•

Trailer handling may be adversely affected if the load(s) are concentrated at the ends of the vehicle. This
condition can occur even when the hitch weight is within the recommended proportion of vehicle weight. Probable
causes for this phenomenon may be excessive frame flexure and/or polar inertia.

•

Polar inertia and frame flex can impose dynamic loading on the axles and suspension system which may exceed
the design loads and result in bending or fatigue failure.

•

Excessive frame flexure can affect ride if the natural frequency of the vehicle's structure matches the frequency
of the suspension. Once the flex of the frame is in phase with the suspension's vertical movement, the dynamic
load input to the suspension will cause it to deflect more than it would under static load conditions. This greater
loading of the suspension results in greater rebound which causes greater frame flexing. Now the larger degree
of frame flexure is imposed on the suspension which causes an even greater vertical travel, and so on. If this
condition exists, damage to the vehicle's structure can occur. Either the structure should be stiffened or the
suspension characteristics should be altered to prevent this ‘in phase’ behavior.

•

Uneven side to side loading of a trailer can cause dog-tracking. For double eyed leaf spring and single slipper
type springs, the front end of the spring is anchored to the vehicle frame. As the load increases, the spring arch
flattens, resulting in a lengthening of the spring. Since the axle is attached near the mid-point of the spring, it will
move rearward as the spring deflects. If the springs are unevenly loaded, the axle will be skewed relative to the
vehicle centerline and may cause tracking problems.

•

A trailer designed to carry a load with a high center of gravity should have a wide enough axle track to prevent or
diminish the tendency for the vehicle to tip over on curves or turns with little or no banking of the road surface.

•

Trailers equipped with Torflex® axles must be towed in a level attitude to ensure even loading of the axles. Outof-level towing results in higher loads being imposed on the axle at the low portion of the frame and less load on
the axle(s) at the high end. This uneven load distribution may cause excessive stress concentrations on the frame
structure. Uneven loading of non-equalized suspensions can also affect the ride characteristics by altering the
natural frequency of the structure.

•

The wheel and tire diameter should be large enough to provide sufficient ground clearance when used with drop
spindle type axles. Insufficient clearance may result in the axle components dragging the ground in the event of a
flat tire.

•

Axles should be spaced to allow at least one (1) inch of clearance between the tires under any loading condition.
To determine the proper spacing, find the manufacturers maximum diameter for the tire and add one inch
or more. The result will be the axle center to center dimension. If tire chain clearance is desired, additional
clearance may be necessary.

•

When designing the attachment system for Torflex® axles on aluminum trailer frames, it is important to
understand the compressive stresses imposed by the fasteners against the aluminum surfaces. Yielding in these
areas can lead to loosening of the axles and could result in fatigue failure of the axle bracket and tube structure
and/or the frame members. Most combinations of aluminum and steel do not result in conditions that lead to
galvanic corrosion. However, if non-metallic materials are to be used between the mating surfaces to prevent
galvanic corrosion, the designer must consider the stability of these materials under the high clamp loads.
Extrusion of these materials under load may also lead to loosening of the axle attachment.

Trailer Design Considerations
•

Spread axle mounting will lend added support to frame structures but will result in more tire wear and impose
higher stresses on the axle components and axle mountings. Increased tire wear usually results from the added
side scrubbing that occurs when negotiating sharp turns or corners.

•

Wide-spread Torflex® axles will be subjected to higher stresses at the bracket/tube interfaces as a result of frame
racking. Racking occurs when the vehicle travels over uneven surfaces and the loads imposed at each wheel
are substantially different. If the torsional stiffness of the vehicle structure is relatively low, the areas where the
cross members are joined to the main frame rails and the axle bracket/tube welds must withstand the twisting
that occurs in these critical regions. Excessive flexing may result in fatigue failures. To reduce the potential for
problems due to racking, position the axles closer together.

•

Certain Torflex® axles should not be used in situations requiring more than two axles. Triple axle assemblies are
not recommended for #9, #10, #10F, #10L, #11, #12, #12V, #13, and #14 axles. These axles are non-equalized
and may experience momentary overload when traversing uneven operating surfaces such as driveway entries
or speed bumps. Torflex® axles can take this momentary overload in a tandem set, however it is not reasonable to
expect one axle to carry the load of three or more axles even in a momentary situation.

•

Torflex® axles that have been designed for triple applications however, are #12T, #13T, and #14T.

•

Torflex® axle ride performance is at its best when the torsion arm is at or nearest to horizontal when the vehicle
is at its rated load. This is due to the geometric relationship of the arm to the direction of loading. Torsion arms
operating above the horizontal tend to exhibit a stiffer ride. As an example, for a 3000 lb. wheel load acting
perpendicular to a 6" long arm, the torque input to the suspension system is 18,000 inch pounds. For the same
wheel load imposed on a 6" long arm at 45 degrees, the torque input to the suspension drops to 12,727 inch
pounds. (Torque = 6(.707) x 3000, since the sine of 45 is .707).

•

Axle capacity will be reduced by at least 50% when used without a suspension system (axles or stubs attached
directly to vehicle frame). This is NOT a recommended configuration as it transfers load directly to the frame rails
and is a very harsh ride.

•

Oil lubrication systems for wheel bearings should not be used in applications in cases where the vehicle will be
stationary for long periods of time. The oil will drain down to the bottom of the cavity and leave the exposed parts
of the bearings subject to corrosion.

•

Dual wheels cannot be used as singles unless they are used on hubs that have been specifically designed for
that application. The large offset of a dual wheel shifts the load line too far from the hub face or intended load line
of most hubs. This condition will result in a serious degradation of the bearing life.

•

Dexter recommends that all axles be equipped with brakes. For trailers used in commerce, the trailer axle(s)
must be equipped with brakes unless the GAWR of the trailer axle is less than 3000 pounds and the hitch load
imposed on the towing vehicle does not exceed 40% of the towing vehicles GVWR. For other details concerning
commercial applications, refer to the Federal Motor Carrier Safety Regulations published by the U.S. Department
of Transportation.

7

Trailer Design Considerations
Determining Dimensional Requirements

Center of Gravity

A
B

Center of Axle Set

Note: “Center of Axle” on a Torflex® axle is defined as the center of spindle.
1. Measure the distance from the center of the hitch to the center of gravity (dim. A).
2. Divide this value by the percentage of the load to be carried by the running gear.
3. The result will be the distance from the center of the hitch back to the center of the axle set (dim. B).

Brake Requirements
Commercial trailers must comply with the requirements of the Federal Motor Carrier Safety Regulations as
prescribed by the U.S. Department of Transportation which calls for brakes on each wheel for most applications.
Consult the regulations that pertain to the type of trailer being built.
The recommended practice for any trailer design would be to use brakes on all axles. The use of trailer brakes can
help prolong the life of the tow vehicle brakes as well as provide for safer operation.

Wheels And Tires
The wheels and tires should be matched in capacity to the axle whenever possible. The Gross Axle Weight Rating of
the running gear will be based on the lowest rated component.
Tires are designed to be mounted on specific rim sizes and contours as defined by "The Tire and Rim Association".
Mismatching of these vital components is dangerous and can result in serious injuries, catastrophic failure or poor
performance and reduced service life.
Tires of greater capacity should never be mounted on wheels of a lower capacity since most end-users will inflate
and load them to the rating embossed in the tire. This practice can result in dangerous failure of the wheel which may
lead to an accident. Wheels must also be matched to the particular hub and mounting system being used. Wheels
are designed to be either hub piloted or stud piloted.
Hub piloted wheels have the center hole machined to a close tolerance and are intended to mate with a hub having a
properly sized pilot.
The bolt holes will be bored or stamped straight through the center disc which is designed to be fastened with either
flanged nuts or a clamp ring using cone nuts.
Stud piloted wheels have a center hole which provides clearance to the hub nose. The bolt holes feature a tapered
seat designed for clamping with properly matched cone nuts. The cone angle of the nut MUST match the cone angle
around the bolt hole. Failure to properly match these components will result in catastrophic wheel loss.

8

Running Gear Systems
Axle Types

Brake Types

Torflex – The Torflex axle is designed as a completely
self-contained axle and suspension system. This trailing
arm type torsion axle employs natural rubber cords
supporting heat treated inner bars of solid, medium
carbon steel. Press-fitted and welded to the ends of
these independently floating bars are the high strength
steel torsion arm/spindle assemblies. These arms can
be specified to a range of starting angles, which allow
the designer to tailor the running height of the vehicle.
®

®

Leaf spring – These axles utilize high strength steel
spindles welded to high strength tubing to form an
axle beam. The spindles are usually available in either
a straight or drop design to help designers establish
the desired frame height or ground clearance. Leaf
springs are attached to the axle using u-bolts and can
be positioned either under or over the tube. Use under
mounted springs (underslung) to lower the frame height
and over mounted springs (overslung) to raise the
frame. The designer can chose stamped steel hangers
of varying heights to allow additional control of the
vehicle height.

Electric and hydraulic, shoe/drum – Electric
brakes are similar to the hydraulic drum brakes used
on automobiles and trucks. While those brakes are
actuated by hydraulic pressure, generated by the
master cylinder to expand the wheel cylinder, electric
brakes function by the action of an electromagnet inside
the brake drum. When a voltage is sent by the brake
controller to the electromagnets, they are attracted to
the rotating armature surface of the drum. The sliding
friction of the magnets against the armature surface
actuates a lever which in turn expands the brake shoes
out against the drum surface. This is much like the
action that occurs within a hydraulic brake when the
wheel cylinder expands. The braking effort is modulated
by varying the amount of voltage supplied to the
magnets whereas, hydraulic brakes are controlled by
the output pressure of the master cylinder.
Retractor Spring

Primary Shoe

Secondary Shoe

Actuating Lever

Shoe Hold
Down Spring

Front of Brake
Magnet

Adjuster
Adjuster Spring

Anchor Post
Retractor
Springs
Backing
Plate

Axle Attachment Systems
Hangers and attaching parts – Most hangers are
channel shaped steel stampings, designed to be
welded to the underside of the frame rails to provide the
attaching points for the leaf springs and their associated
parts. The attaching parts are normally provided in kits,
which contain all the necessary hardware to properly
install the running gear. In the case of multiple axle
installations, devices called equalizers are often used to
help transfer load from one axle to the other(s) as the
vehicle travels over uneven terrain. Not all combinations
of equalizers and hangers are suitable. If the equalizer
hangers are too short, the spring eye may contact the
frame during articulation of the suspension and may
result in overloading of the spring.

Hydraulic Wheel
Cylinder
Actuating Pin
Hold Down
Spring

Secondary
Shoe

Primary Shoe

Adjuster Spring
Adjuster Assembly

9

Running Gear Systems
Air brakes, drum type – This type of brake operates by
rotating an 's' shaped cam between the non-anchored
ends of the brake shoes, causing them to expand
outward and exert pressure against the drum surface.
The rotation of the cam shaft is accomplished using an
air cylinder called a brake chamber, acting on a lever,
(the slack adjuster) which also provides a means of
adjusting the clearance between the brake shoes and
the drum surface. This adjustment compensates for
wear and can be done manually or automatically.
Cam-Shaft Bushing

Disc brakes – Disc brakes employ a component called
a rotor, which takes the place of the drum in a shoe/
drum brake. The rotor or disc as it is sometimes called
is generally machined from a special grade of cast iron
and has integrally cast fins to help dissipate heat. The
braking force comes from a clamshell-like structure
called the brake caliper, which is attached to the axle.
The caliper is positioned to straddle the rotating disc.
When hydraulic fluid extends the piston, the caliper
grips the rotor to generate the braking effort.

Automatic Slack Adjuster

Caliper Pistons