Boler Axle

The original Boler came equipped with an Ingersoll Rub-R-Ride rubber torsion axle and later was replaced with the Dexter Torflex. Rubber torsion axles are a good system that requires little servicing other than the usual bearing repacking and maintenance. One very unusual aspect is that the axle in the Boler is actually installed backwards. These axles are designed to be installed so that the suspension arm trail behind the axle tube, the wheels are pulled behind the axle, but the Boler designers saw the opportunity to install the axle tube behind the step in the kitchen leading up to the dinette area, but to locate the wheels in the correct location for proper trailer balance the suspension arms need to point forward (called a leading axle), the wheels are essentially being pushed down the road ahead of the axle.

There does not appear to be agreement on what the original Boler used as the start angle, I believe the angle they used was a 0° or 10° up angle but this I cannot confirm. An initial setting of zero degrees or a slight up angle will maximize the suspension geometry and provide the smoothest ride. The reason for this is that when the arm is positioning upward and being pushed down the road the impact or compression of the suspension arms arcs toward the rear of the trailer following the direction of travel, if the suspension arms are is positioned at a down angle any compression actually forces the wheel forward against the direction of travel which results in a harsher ride.

Replacing your Axle

Over time the axle will wear out, the rubber elastomer will deteriorate with use and age.  As the elastomer deteriorates the axle will start to sag, causing the trailer to sit lower, there can also be movement laterally in the torsion tube which will cause the wheels to toe out, this can cause slight swaying and tire wear. The most noticeable result will be a harsh ride, often noticed by seat cushions flying everywhere, cupboard doors open and even bouncing of the trailer even when the road appears smooth.  As a general rule of thumb an axle that is 15-20 years old probably needs replacing.

How do I check my axle for wear?

According to the Dexter engineers “Dexter would consider any Torflex arm that has moved more than ten degrees from the original build angle as weak or losing suspension”.  But how would you know if your axle has sagged 10° or more when you don’t know what the start angle was?

The first test is to jack up each side of the trailer and watch to see if the wheel moves down as weight is removed from that side, if on either side the wheel does not drop at least 1½” the axle needs replacing.  Second, measure the diameter of your tire (as an example ST175/80D13 trailer tires have a diameter of 24″) and divide this number by 2 (in this case 12″).  Now with the trailer sitting on level ground measure the distance from the ground to the underside of the street side or drivers side frame rail as close to the axle as you can, either just ahead or behind the axle mounting brackets.  Take the tire radius and subtract the frame to ground measurement, if the result is greater than 2½” your axle needs replacing (the above calculations are based on a fully loaded axle with an initial start angle of 0° to 10° up angle).

As an example: your trailer has ST175/80D13  tires which have a diameter of 24″, dividing the diameter by 2 gives you the radius of 12″.

The distance measured from the ground to the underside of the frame behind the axle mounting bracket measures 7″

12″-7″=5″  The axle arm has dropped more than 2″ and should be replaced

How do I order an Axle?

When replacing your axle there are a number of items to consider, these include:

  • Axle weight capacity
  • Axle arm start angle
  • Axle bracket spacing
  • Hub face to face measurement
  • Brakes
  • Axle type or manufacturer
  • Axle orientation (leading arm vs trailing arm)
Axle weight capacity

Bigger is NOT always better.  The axle capacity is determined by the weight of the trailer, you want the suspension to work within the range it was designed to carry, if you put an axle on that is rated too low then the suspension will be at or near the maximum compressed range, when you hit a bump the axle will not absorb the bump and send the shock through the trailer and frame.  If you put on an axle that has too high a capacity the suspension will resist moving and again any bump will be sent through the trailer and frame.  Ideally you want the capacity so that the suspension is slightly compressed when the trailer is just sitting, when a bump is hit the suspension compresses and absorbs the impact rather that transferring it to the trailer and frame.

Reviewing real world trailer weights the originally advertised dry weight for a Boler of 900-1000 lbs is not realistic.  The average weight identified by owners is in the 1400-2000 lb range.

 

Axle Arm Start Angle

The axle arm start angle is one of the primary factors that determines the ground clearance or ride height of your Boler.  In most instances you are replacing your axle because it is worn out, as the elastomer deteriorates the ride height goes down and ground clearance becomes less, so don’t just guess at the start angle, read on.

Some things to consider when deciding on the final ride height and ground clearance of your Boler.

  • The original axle used on the Boler was either a 0° or 10° up, so use those measurements in the table as a starting reference, NOT the measurements from your Boler with its worn out and sagging axle
  • The roads or terrain you will be traveling on, rougher or rutted roads need more ground clearance than paved highways. As a reference the average pick-up truck has 8”-10” of ground clearance.
  • The height of the step-up, the average step riser is about 7”-8”, more can be awkward, again the original 0° or 10° up results in the ideal set-up height
  • Since the standard Boler axle is installed as a leading arm, a 0° or 10° up is beneficial because suspension movement arches in the direction of travel which provides a smoother ride, a down angle will result in a rougher ride because the arm arches against the direction of travel.
NOTE: The following measures are based on using 175/80R13 tires with a diameter of 24”. Ground clearance is calculated from the ground to the bottom of the lowered kitchen floor which is 6” lower than the top of the frame rails.
The following table provides the ground clearance measurements based on two factors:

  1. Axle Start Angle: the angle of the torsion arm relative to the horizontal plane, stated in the unloaded condition. This angle is a factor in determining the frame height. (quote from Dexter definitions)
  2. Mounting Bracket Height: two bracket heights are available, the Low Profile Bracket which is the standard mounting bracket, and the High Profile Bracket which will add 0.88” additional ground clearance

The measurements in the following table indicate the average ride height for a Dexter #9 axle, the ± provide the range from no load to full load.

 

Axle
Start Angle

22.5° down

10° down

10° up

22.5° up

Ground clearance
Low Profile Bracket

10.57” ±1.15”

9.29” ±1.17”

8.27” ±1.15”

7.29” ±1.09”

6.15” ±0.97”

Ground clearance
High Profile Bracket

11.45” ±1.15”

10.17” ±1.17”

9.15” ±1.15”

8.17” ±1.09”

7.03” ±0.97”

 

Replacement Axle Specifications

Bolt circle: 5×4.5″

Start angle:  10 deg up

Bracket spacing (outside to outside frame):  48″

Hub face to Hub Face:  63″

NOTE: Double check all measurements on your axle

 

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