Torsion Axle – everything you want to know
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Torsion Axle – everything you want to know

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 top of the axle tube, 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).

First measure the diameter of tire and divide by 2 to get the radius

Measure from the ground to the top of the axle
If the measurement (Radius minus height) should be the 2.5″ or less.

In the above example R=12″; Frame to ground = 11″ (12″-11″=1″ axle is good)

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 top of the axle 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)

NOTE:  Axle weight capacity

Bigger is NOT 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 (bottom out the suspension).  If you put on an axle that has too high a capacity, like a 3500 lb axle, 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, therefore a 2000 – 2200 lb axle is the ideal range for the 13′ Boler

 

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 10° up which makes sense considering the axle orientation with the frame, 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 above illustrates the suspension arm movement over bumps

This illustrates the start angle, when replacing the stock axle you want a 10° up angle (top centre image)

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

Dexter #9: 2000 lb – 2200 lb with brakes

Bolt circle: 5×4.5″

Start angle:  10 deg up

Bracket type: Low Profile (if extra ground clearance needed use high profile brackets)

Bracket spacing (outside to outside frame):  48″

Hub face to Hub Face:  63″

NOTE: Double check all measurements on your axle

 

A final comment:  I highly recommend when you order your new axle always have brake installed.  You may not think it is needed but trust me, that one time when you least expect it those brakes will be worth every penny.  Please see the section on trailer brakes, Boler brakes need to be installed on the opposite side in relation to the standard axle installation.

12 Responses

  1. barbe s
    | Reply

    Ian, This site is so helpful – I’ve printed off the info about the axle & installation to take to the RV shop in Princetons & you indicate on the axle site to see the section on trailer brakes, however I cannot locate the section on Trailer brakes for the Boler… am I overlooking it?

    • Ian
      | Reply

      Thank you Barbe, many of the article are being written so some content is missing at this time.

  2. Clayton
    | Reply

    Hi, I’m curious as to whether or not the old axles are hub centric or lug centric. I would like to install regular car rims, not trailer wheels and everything I read talks about wheels for trailers need to be hub centric. My trailer is a 1974 and the typical trailer wheels weren’t around back then so it lends me to believe the original wheels were lug centric, so the axle should be also. Thank you in advance for your help.

    • Ian
      | Reply

      The axles and wheels are lug centric. Trailer rims usually have zero offset so if considering using automotive rims make sure the offset is compatible with work on your Boler

  3. Albert
    | Reply

    Ian,
    This was a great article to read up on. I did replace the axle on mine with one of 2500LBs. This seems to work very well. I was curious to know what your findings and recommendations would be on tire replacement. I’m currently on 13″ wheels, but I’m finding them to be somewhat unstable. I would like to maybe explore some wider tires. any suggestions?

    • Ian
      | Reply

      The tire I recommend and it is the old tire I will run on my own Boler is the Maxxis M8008ST in 175/80R13 size

  4. Harold Coish
    | Reply

    I have 1977 Boler 13 ft. trailer. The Dexter #9 axle doesn’t fit . the bracket is off by 1 inch. Why would say it’s fits?

    • Ian
      | Reply

      Harold, the Dexter axle can be built to any width and using a selection of different brackets. The one you were probably looking it was a pre-built unit that one “buys off the shelf”, it also was probably supplied with zero angle on the arms. You need to custom order the axle to specification you provide to the axle supplier, if you axle supplier says they cannot do that then find a new supplier, reference the Dexter website for authorized dealers

  5. Bob McD
    | Reply

    Hi Ian. My guess is the axle measurement doesn’t work with a 17 foot Boler. There is a leaf spring present and 14 inch tires, neither of which are in your example and photos above. My radius is 12+3/4 inches and the ground to frame is 16+1/2 inches. Each wheel drop when lifted is about one inch which is way under your “at least 1+1/2 inch” statement. Perhaps none of this applies to the 17 foot?

    • Ian
      | Reply

      Your right Bob, the information in this article is for torsion axles and do not apply to leaf spring suspension. On leaf springs inspect the springs for broken leaves, word bushings, or tire wear caused by a bent axle.

    • Geoff Squires
      | Reply

      Hi Bob. I have been running 14” mags with light truck tires (load Spec 96S) from a Ford Ranger truck on my boler 17’ and they worked very well as lug centric rims. I had to ream out the centre hole with a dremmel tool so the rims would fit over the hub ( I removed the excess alloy with a hand tool so it was not possible to make a perfctly symmetrical hole.) This year i have upgraded to 17” alloy wheels from a 2003/04 G53 Infiniti. The hub size is perfct for my axel hubs. I am running 265/70R17s radial Bridgestone Dueller H/T truck tires with road rating 110S so they will easily handle the trailer wieght and should tow smoothly. Lots of clearance to the wheel well but i think iam going to add 1.5” wheel spacers to make up for a slightly different offset.

  6. Tim MacDonald
    | Reply

    Great info. I printed it off and took my old axle into city spring in Edmonton. 1 day later and $450 I had a new axle
    Thank you

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