TMC   Recommended Practice



RP 6XX	VMRS XXX

Guidelines for Outset Wide Base Wheels for 

Drive, Trailer and Auxiliary Axle Applications

DRAFT FOR TASK FORCE REVIEW – 9/28/09

Text in blue has been added or changed from the February ’09 draft

This draft includes changes discussed at 9/14/09 Task Force meeting in
Raleigh

In the load tables, changed from 1.0” outset to 1.13”

Below the tables, simplified the statement on “Effects on other
components”

PREFACE

The following Recommended Practice is subject to the Disclaimer at the
front of TMC’s Recommended Maintenance Practices Manual.  Users are
urged to read the Disclaimer before considering adoption of any portion
of this Recommended Practice.

PURPOSE 

The purpose of this Recommended Practice (RP) is to provide guidelines
for selection and specification of wide base tire/wheel applications for
drive axles and trailer axles.

SCOPE

The scope of this RP is limited to single wide base outset wheels on
drive, trailer, and auxiliary (e.g, dolly, tag, or pusher) axle
applications. Singled inset wheels are not included in the scope of this
RP.

REFERENCES/GLOSSARY

RP-222 Users Guide to Wheels and Rims

INTRODUCTION

Replacement of standard dual wheels on drive axle and trailer
applications with single tire/wheel applications can adversely affect
the loading on the axle end. This can alter the stresses in the
bearings, spindle, hub, axle housing, and reduce the load carrying
capability. The anticipated life of the axle end may also be affected. 

In some applications, it is permissible to operate an outset wide base
wheel on the wheel end without significantly affecting the anticipated
life, if the maximum load is reduced. The amount of load reduction
depends on the wheel outset.



For wide base wheels, either outset or inset is used to describe wheel
geometry. Both are calculated as the distance from the centerline of the
rim to the wheel-mounting surface.  A wheel is inset when the centerline
of the rim is inboard of the wheel-mounting surface.  A wheel is outset
when the centerline of the rim is outboard of the wheel-mounting surface
as shown in the figure.  The axle end load ratings are based on this 
outset condition, which increases the bending moment (load) on the axle
end and correspondingly lowers the rating as compared to dual wheel
applications.  Some trailer axle manufacturers may recommend a P Type
spindle for single outset tire/wheel applications.

MAXIMUM LOAD GUIDELINES

The tables below show recommended maximum axle load ratings for various
wheel outset values on common axle configurations.   These
recommendations are based on maintaining original fatigue life of hubs,
bearings, and axles.  

Wheel outset may be verified by contacting your wheel manufacturer.  

Note that R-Series drive axles are available in three “basic” load
capacities: 20,000 lb, 23,000 lb and 26,000 lb.   

Additional restrictions may apply, depending on factors such as spring
center distance and suspension type.  

For consideration of wheels with outset greater than 2.0”, consult
your axle and wheel-end component suppliers.

Maximum Loads for Outset Wide Base Wheels – Drive Axles

Wheel Outset  	R Type Axle 

with 9.5 mm Wall Thickness 	R Type Axle

with 11 mm Wall

Thickness 	R Type Axle

with 12.7 mm Wall

Thickness	R Type Axle 

 	Axle Load Rating 

(lb)	Axle Load Rating 

(lb)	Axle Load Rating 

(lb)	Axle Load Rating 

(lb)

Duals	20,000	20,000	23,000	26,000

1.13” maximum	17,000	20,000	20,000	22,000

1.14 to 2” 	Consult Axle Manufacturer	17,000	17,000	19,000



Maximum Loads for Outset Wide Base Wheels – Trailer Axles

Wheel Outset  	P Type Axle

(Parallel Trailer)	N Type Axle* (Tapered Trailer)

 	Axle Load Rating 

(lb)	Axle Load Rating 

(lb)

Duals	25,000	23,000

1.13” maximum	20,000	19,500 *

1.14 to 2” 	19,000	18,000 *



* Note - some axle manufacturers do not recommend N type axles for use
with outset wheels – confirm your application with the manufacturer.

EFFECTS ON LIFE OF OTHER COMPONENTS

In addition to the primary issue of  hub / bearing / axle fatigue life
addressed above, other axle-end components may be affected by use of
wide single tires.  To assure successful operation, suitable components
should be fitted as recommended by the vehicle manufacturer. 

EFFECT OF WIDE BASE  WHEELS ON VEHICLE TRACK AND WIDTH

Changing from dual wheels to wide base single wheels can affect
significant dimensions such as overall width, inside width, and track.

The effect on these dimensions that will result from a wheel/tire change
can be calculated as follows:

New Track = 

Original Track (Dual Tires) – (2 X Dual Wheel Thickness) + (2 X Wide
Wheel Outset)

New Overall Width = 

New Track +  Wide-Base Tire Width

New Inside Width = 

New Track – Wide-Base Tire Width

EXAMPLE CALCULATION

Original Track as measured:  76.8”

Dual Wheel Thickness as measured at wheel disc:  0.42”

Wide wheel outset from Wheel Mfr (Catalog):  2.0”

Wide-Base Tire Width from Tire Mfr (Data Book):  17.2”

Then New Track = 76.8 – (2 X .42) + (2 X 2.0)  =  79.96”

New Overall Width = 79.96 +17.2 =  97.16”

New Inside Width =  79.96 – 17.2 =  62.76”

Guidelines for Outset Single Wide Base Wheels

	RP XXX Guidelines for Outset Single Wheels Rev Sept 29, 2008

	RP XXX Guidelines for Outset Single Wheels Rev Feb 10, 2006

Wide base wheel outset                       

Load point for outset wide base wheels                       

Standard duals shown in gray                      

Outset wide base wheel                      

Load point for standard dual application                      

