Trailer Drawbar Design & Function

Perhaps the most important and overlooked part of the trailer design is its drawbar. Have a look at a couple of trailers around your area, and you will see a multitude of materials and designs.

The drawbar does a couple of important jobs apart from being the part that keeps your trailer attached and at a distance from your tow vehicle. The drawbar keeps your trailer in balance when towing and assists in keep weight on the tow bar which is required for controlled towing. It also adds stiffness to the chassis and depending on the axle and load placement will assist in keeping the trailer running true and stable.

A drawbar needs to have good vertical and sideways strength and depending on the size of your trailer and the loads you wish to carry, this will determine the size of the material required and the design that you should use.

The drawbar does a lot of work while towing. Every little bump in the road and every turn you make transfers stress through the drawbar and compresses, twists and stretches the drawbar material constantly. If the trailer has been built with an undersized drawbar or the trailer is constantly overloaded or unbalanced, this repeated loading and unloading (cycling) of stresses on the drawbar can create microscopic cracks within the grain structure of the drawbar material.

Over time a microscopic crack can grow and eventually reach a critical size, where the drawbar may suddenly fracture and ultimately fail.
This can also occur on a drawbar that has been damaged or distorted through normal use.

The amount of cycles that a drawbar can take before failing is dependent on a couple of factors -

  • The drawbar design (see below)
  • The drawbar material size - bigger is always better to a point
  • How the drawbar is attached to the chassis
  • Drawbar length
  • How the trailer is loaded and towed

Drawbar Design

There are 3 main styles of drawbars -

The straight drawbar -
The Compound drawbar -
And the “A” frame drawbar

Straight Drawbar

Straight_drawbar.pngThis design harks back to the old horse and cart where you had a horse tethered to either side of the drawbar. It is a bit old fashioned but still has its uses particularly on dinghy and agricultural trailers.
Whilst this design has less chance of getting jacknifed when reversing, if the drawbar does get damaged it creates a highly stressed section which can rapidly cause internal cracking or failure.

Because straight drawbars are centrally mounted, they do tend to come under a lot of vertical stress particularly where the drawbar and chassis meet. This stress increases if the load is incorrectly balanced over the axles.
Length should be kept to a minimum as the longer the drawbar, the less load is able to be carried and the weaker the drawbar becomes.

The strongest design for a single drawbar is to have 2x RHS (Rectangular Hollow Section) pieces laminated together (stitch welded) and fitted to the trailer with the longest edge vertical. The drawbar Straight_drawbar_gussets.pngneeds to go under the chassis a minimum of half its protruding length and attached to at least the first intermediate cross-member.

Triangular gussets, bracing the drawbar to the chassis front and intermediate cross members should be fitted.

Composite Drawbar
Composite_Drawbar.png

The composite drawbar is a variation of the single drawbar design with lateral supports to both strengthen the drawbar and provide support to the chassis.
This style enables the drawbar to be slightly longer as well as not so heavy a section of steel.
Composite drawbars give the benefits of good clearance to the tow vehicle when reversing around sharply and give more support to the sides of the trailer.
You will find a lot of single drawbar trailers retro fitted with the composite angle frame to stabilise and control the trailer better, as well as adding strength.
Ideally the composite angle frames should go under the chassis at least to the first cross member.

Boat trailers commonly use a compound drawbar using the angle sections as part of the chassis frame. The strength in this comes from the bow post which supports the front keel of the boat. When the boat is locked into the bow post and secured to the trailer the boat effectively becomes part of the chassis/drawbar.

Composite_Drawbar_2.png

"A" Frame Drawbar

This style drawbar is one of the most common due to the strength the full angle frame gives to the trailer. It supports the outer edges of the trailer and prevents the chassis from twisting when poorly loaded.
A_Frame_Drawbar.pngThe length of the drawbar can be increased as long as the material section is increased in size or bracing (see below) is added to ensure drawbar stiffness and rigidity.
The longer a drawbar can be (to a point where the drawbar weight becomes excessive) the more smoothly and stable the trailer will travel. It will also be more responsive and forgiving when reversing. Short drawbar trailers are prone to rapid and sometimes unexpected turning, slight movements of the towing vehicle can put the trailer into a jacknife in a moment of inattention.
Out of the three drawbar designs, the “A” frame is structurally superior as it offers excellent horizontal stiffness and good vertical support. Any stress’s on the drawbar are transferred to the chassis outer edges through the drawbar and effectively share the load.
One disadvantage is the greater chance of the drawbar being damaged when jacknifed while reversing.

Drawbar Length

Drawbar length is the distance measured from the centre of the trailer front cross member to the centre of the tow ball and can be determined from a number of considerations. As well as having the choice of straight, composite, and “A” frame drawbar, there are also the options of having a fixed length, extendable, and tilting drawbar. Each of these is discussed below.

Fixed length drawbars are the most common and account for probably 95% of all trailers built. Most trailer manufacturers have their own standard length for a given size of trailer based on experience, trailer stability, turning and reversing responsiveness, axle position and the trailers maximum load capacity.

Fixed Drawbars

Too short a drawbar, and you run the risk of clipping the front of the trailer with your tow vehicle when turning corners and whilst the trailer will follow the tow vehicle nicely, when reversing, the trailer will be oversensitive. As noted above, short drawbar trailers are prone to rapid and sometimes unexpected turning. Slight movements of the tow vehicle can put the trailer into a jackknife in a moment of inattention.

Short drawbars are the domain of farm trailers and serious off road trailers due to their better tracking when taking tight corners (less chance of the trailer hitting fence/gate posts or trees when turning) and reduces the occurrence of the drawbar bottoming out when going over and through steep undulating ground or sand dunes.

Drawbars need to be at least as long as half the tow vehicles width. Most cars and SUVs’ range between 1.6m to 1.9m so the minimum length of drawbar needs to be half of this plus a little bit more (say 200-300mm) to gain a margin of clearance when turning and to add stability to the trailer.

Standard drawbar lengths range between 1.2m to 1.8m depending on the tow vehicle, what is going to be mounted on the drawbar (toolboxes, spare wheel, bicycle rack, etc..), length of the load when carting over length goods (timber, pipes, kayaks/canoes and similarly long items) and the position of the axles in regard to the chassis.

For general cartage on a standard trailer with nothing mounted on the drawbar apart from maybe a spare wheel and axle(s) mounted rear of the deck centre, I would recommend a drawbar length of 1.2 to 1.4m. This is a good compromise on length for stable towing and turning circle without adding unnecessary weight to the trailer.

If a full width toolbox or similarly wide items (bicycles, lawnmower deck, stone guard, etc) are to be mounted on the drawbar, the drawbar needs to be measured forward of these items to ensure sufficient turning clearance.

Another thing to be aware of is if you have a side hinged or top hinged rear door on your tow vehicle. Make sure that the door can open without hitting the front of the trailer or any accessories permanently fitted to the trailer.

Drawbars over 1.5m, depending on the material used, should be braced to give additional stiffness and prevent vertical flexing. Click here for information on "Drawbar Bracing/Strengthening".

See also "Tilting Drawbars" & "Extendable Drawbars"