Thread: Drawbar design help

I once saw a trailer using a length of RHS as the drawbar. IN this case a second piece of RHS fitted inside the main bar and could be slid out to lengthen the drawbar to allow long lengths of material to be carried on the the trailer while remaining in the legal limits of overhang.

Pins were put trough the bars to set the desired length.

Of course the trailer wiring has to be long enough to allow the lengthening.

Originally Posted by simonl

Instead of the traditional A frame style drawbar, I want a drawbar made from a single piece of RHS (of suitable size and gauge) passing centrally underneath and passing the full length of the trailer. The reason for such a design is greater manoeuvrability when reversing as it allows greater jack knifing angles.

If you're at the point where the difference between an 'A' frame drawbar and a straight beam drawbar gives you a slightly better angle, then you're in a pretty tight situation to begin with. And I highly doubt that the extra couple of degrees will allow to manoeuvre the trailer much further than where the 'A' frame stops you.

The design also lends itself to a strong recovery hitch attached to the rear of the trailer where the drawbar would terminate.

That I do agree with.

In my head it seems like a pretty good idea but the fact that I have not seen any such designs on trailers makes me think there must be a reason why no one has gone this way. Is this a feasible design or is there an inherent design flaw with this idea?

Further information which may assist with your help; It will be registered with a total maximum weight of 1500Kg, 10" electric brakes, ALKO independent suspension system.

Thanks in advance for any advice on this.

Simon

Old Army trailers were built with a single beam drawbar...
The thing to be aware of is that 1500Kg is a fair weight to be whipping behind a non triangulated drawbar. The beam would need to be fairly thick walled, and be a decent cross section.

Thanks Alfclp and Yonnee.

Yonnee, the 1500Kg weight is a ballpark figure that I was working towards. In reality I could probably de-rate that quite significantly if I had to. If used for camping I doubt that I would carry more than 300Kg of "stuff" and so the aggregate trailer weight would be somewhere around 800Kg? The only time it would carry close to 1500Kg would be returning from a trip to the local sand and soil place with some gardening stuff. I just figured if I make a trailer designed to take 1500Kg then I figure it would be strong enough on and off road.

WRT the draw bar, was thinking of 100x50x4mm and 1500mm long. While it would be a single beam, I think I would also incorporate 2 45 degree bracing from the drawbar to where the "traditional A frame would attach to. So, more a miniature A frame with the single beam extending beyond.

Cheers,

Simon

Hi Simon, I once built a fully enclosed camping trailer that I towed behind my 1litre Suzuki, that had the single pole draw bar (50X50X3.5 RHS) fitted with a 50X5 FMS torsion bar underneath and a Treg rubber block coupling, as it was, I actually bent the towbar tongue up going through the Bungle Bungle Ranges. So it is possible and on several occasions I had to jack knife it. Jack knifing it makes it easier to get the backdoor open with out stepping over the draw bar. In reference to someone else's comment about an extendable draw bar, I also built a 6X4 to carry a dinghy using this arrangement, the pins removed and wheels chocked, Pajero was then driven forward/jack knifed so that the back door could be opened without unhooking the trailer.
When you get to build it, fit some 65X65X6 RHS on the end of the main bar, this can be used to act as a recovery point and also mount a bike rack off it if required.
Regards
Kryn

Without resorting to slide rules and steel tables, I would wager that 100x50x4 would not be strong enough. It will be just fine for the static loads on it, but the dynamic loads of a misjudged railway crossing or table drain or whatever once full of sand will almost certainly bend it.
I would go for 100x100 or at least 100x75. In this case, a round section (like the army trailers already mentioned) might make make better sense - I'd go for 3" or 3-1/2" NB.....

Hi all. Thanks for your input and advice.

Hi Joe, Yes I'm quite happy to pay the price of a larger sectional area or thickness to ensure it's sufficient in strength. I never want to have to wonder if it's strong enough when I'm in the middle of nowhere! I have seen much larger trailers with total masses of 3500Kg that have an A frame drawbar with 100x50x3.5 so I figured (rightly or wrongly) that a single 100x75x4mm would be fine with a 1500Kg trailer. I'm happy to beef it up but I don't want to make it heavier than it really needs to be either, I guess this is where knowledge and experience comes into it!

I'm assuming that the forces on the drawbar are predominently:

(1) Ball weight, approx 10% of total weight. So maybe a max of 150Kg
(2) Tension and compression with accelleration and decelleration. Neither of which would present a problem given the limited braking/ accellerating ability of a 4x4 and the inherent compressive/tensile strength of the RHS used.

There should be no tortional forces since I will be using a Treg style coupling that allows complete 360 deg rotational freedom, if it were ever to come to such a situation.

Hi Kryn,

Thanks for the advice on the recovery point and bike rake. That is a very good point. WRT jack knifing, there have been quite a few occasions where I have been forced to do a U turn after discovering that track ahead is impassible. Being able to jack knife right up ( and the use of reversing cameras as an aid) makes it much easier and safer to turn around. There have been occasions where the steep terain as made it near impossible or at least very unsafe to even think about uncoupling the trailer to aid in a turn around. If I can engineer a single beam drawbar that is as strong as an A frame then I'm definately going that way but I will not do it at the expense of strength/ruggedness, longevity or safety as it would be counter productive in the design.


Hi Yonnee, Thanks for your replys. I have seen many a thread on this suject with your valued input. I was hoping you would respond! Nearly 25 years ago I had the oppotunity to buy an Army trailer from Pickles (I think) for $750. I should have bought one! There single beam drawbar was my inspiration for my idea.

Cheers & Thanks

Simon

Originally Posted by Yonnee

Old Army trailers were built with a single beam drawbar...

I've got one. The drawbar is pipe, 100mm OD, with about 5mm wall thickness from memory. It's pretty solid. At the front end a thick flange is welded on and the ring pull is bolted to that with six or so 10mm threads. I swapped out the ring pull for an offroad hitch and just had to make sure the hitch mount frame (10mm plate thing) matched the threads on the drawbar. Easy.

It's a great design, but a little tricky as a platform to mount against the chassis and to put extra things on. E.g. if you want a jockey wheel, you effectively have to weld a piece of R/SHS onto the pipe. Not sure how much weight would be saved by having a single pipe vs. two lengths of RHS. Note that the army later moved to triangular drawbars, when Haulmark took over the contract in the 90s or so.

Pictures of the same chassis as mine, although I have the workshop variant:

Australian Military Trailers for Land-Rovers -REMLR

Originally Posted by simonl

...I'm assuming that the forces on the drawbar are predominently:
(1) Ball weight, approx 10% of total weight. So maybe a max of 150Kg
(2) Tension and compression with accelleration and decelleration. Neither of which would present a problem given the limited braking/ accellerating ability of a 4x4 and the inherent compressive/tensile strength of the RHS used.

There should be no tortional forces since I will be using a Treg style coupling that allows complete 360 deg rotational freedom, if it were ever to come to such a situation.

....

You are right about the lack of tortional forces and acceleration and deceleration forces. The 'ball weight' is the issue: that force is only static. The moment of the trailer weight and everything on it around the pivot point of the tire contact on the road (maybe 1m or more!) under braking - and more critically - going over even small bumps, may be in the order of 10 or 20 times the static load! Your car rear suspension won't bottom out as you might expect with that sort of force, because the shock absorbers do just that - convert that energy into heat. But the force is certainly transmitted by the draw bar.
So picture the trailer sitting with the coupling supported on some bricks or a stand, and say 15 tons sitting on it halfway between the tire centre and the coupling. Are you confident that your draw bar will stay straight (flexing of course at that point, but not deforming)? If so, the dimensions are OK.
The other forces the draw bar needs to accommodate are the lateral forces from one wheel being inhibited from rolling freely and being dragged over a stone, bump, wheel track or whatever. These are both 'static' and shock forces which are difficult to estimate. If you have ever moved a trailer by hand over a bump (hit by just one wheel), you will appreciate what I'm talking about.

For what it's worth, middle of last year I accidentally overloaded my very strong 6x4 trailer with small cross section ironbark timber and just before getting home broke my tried and true A-frame draw bar right off! Wasn't funny at the time..... but taught me a lesson....

"Thanks for the advice on the recovery point and bike rake. That is a very good point. WRT jack knifing, there have been quite a few occasions where I have been forced to do a U turn after discovering that track ahead is impassible. Being able to jack knife right up ( and the use of reversing cameras as an aid) makes it much easier and safer to turn around. There have been occasions where the steep terain as made it near impossible or at least very unsafe to even think about uncoupling the trailer to aid in a turn around. If I can engineer a single beam drawbar that is as strong as an A frame then I'm definately going that way but I will not do it at the expense of strength/ruggedness, longevity or safety as it would be counter productive in the design"
(Part quote)

I have seen where the single pole drawbar has had an A frame of 75X50 come from the front corners to about 2/3 of the main drawbar. If using 100X75X5 as the main drawbar, the 65X65X6 should slide inside to give the recovery/towbar point, keeping it to the bottom so that there will be clearance for the tailgate.
A camper trailer manufacturer that I use to work for, fits them under the rear beam and connects to the next beam back. Sometimes 2 are fitted so a tinny can be mounted on top of a frame fitted to these points.
Kryn

Ahh yes Army trailers. Swapping out the ring pull definetely a good idea.

Used to hate backing the buggers. Fortunately for me didnt have to tow one too often. Seemed to be pretty strong, but they have to be.

Cool as my rover was gutless enough with the weight of a fibreglass canopy + gear in the back.

Thanks so much guys for all your help.

I'm thinking I'll measure the distance from the vehicles tow hitch to the left and right of the bumper. Once the drawbar extends past this distance, the A frame can be joined off the main bar and not affect reversing angles. So I'm thinking the A frame will terminate into a single beam drawbar maybe 600 - 800mm from the coupling. While still achieving my outcome, it will still be inherently stronger than just a single drawbar. It will also facilitate a (SMALL) gear box in front of the trailer on the drawbar.

Hi Kryn, I didn't even think of allowing for another toe hitch at the rear. If I'm going to make a recovery point, it may as well be designed to take a hayman Reece!

Hi Joe, Thanks for the explanation. I understand what you mean now. Yes the uneven shock forces would be quite unforgiving on an under engineered trailer. Pushing/pulling a trailer by hand is good until one wheel hits a bump!

You guys have all given me much more to think about. My eventual trailer design will no doubt be better for it. THANKS!

Simon

I know this is a bit of a stale thread...but a few thaughts.

The standards specify the required drawbar strength....have alook at the VSB1 link above.

Have you thaught about a two bob each way drawbar.

make it an A frame drawbar, but narrow the A right down..instead of projecting the base of the A at the spring hangers, project it right thru to the rear corners...please yourself if you want a plain A or want to project a single beam forward from the centre.

This would handily tie the whole trailer together....then fit a pair of recovery points on the corners...this gives the potion of pulling from a corner or from the centre using an equaliser strap.

I am pretty disapointed with drawbars in general....very little imagination and inovation.

One of the biggest problems with off road trailers is bottoming the drawbar in ramp over situations.
I have seen one or two drawbars that do not come from under the tray.....but higher up the body...more like gooseknecks.

It does complicate the issue .....the front and the forward portion of the sides have to be built like a truss to bear the stresses.

Another more complicated thaught is to not build the draw bar as a single beam, but as a truss.

Most trailer engineering is by simple weight of metal and size of section.....a bit of imagination could produce some very interesting ideas..particularly if you have the engineering smarts available to back it up.

just some thaughts.

cheers

If you do run a single drawbar the full length I would triangulate from the spring hangers to the drawbar attaching about 750mm up the drawbar (given a 1500mm drawbar length).

Have fun!

While looking into off road trailers a few years ago I saved these pictures I saw on ebay.

Rob