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

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.
Guidelines for Outset Single Wide Base Wheels

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

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 wheelmounting 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 wheelend component suppliers.

Load point for outset wide base wheels Load point for standard dual application 

Wide base wheel outset Standard duals shown in gray 

Outset wide base wheel 
Guidelines for Outset Single Wide Base Wheels

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

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. 
Guidelines for Outset Single Wide Base Wheels

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

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”