Thread: Help with draw Bar opinions

Originally Posted by loveit

....
one thing i noticed is when i had the outer poles in too far it was less effective then when i had them right out closer to the sides (ends?)

also i noticed the pull on the tops of the uprights in the outwards directions as shown by arrows

what do you think of this design....where the top of the uprights are trying to pull the diagonals rather then push it

I'm not sure of your truss modelling. The forces in the truss members are dependent on where the loads are applied and where the points of support (the spring shackles) are located. Refer to my previous posting, second drawing.

The truss is designed so that the top horizontal member (your bow) is in tension, the vertical posts (your poles) are in compression, and all the diagonals in tension. If the top member is in tension, then it will behave as you have observed. It might be handy if you measured the length of the deck and located the positions of the spring shackles. This would help in making a sensible design. The wheelbase on the Monaro is also an important dimension.

The simplest analogy I can think of is a coat hanger, where the vertical upward support is the group of 3 shackles, and the downward loads are at the "shoulders" at each end. With this image you might see why the diagonals should slope downwards and outwards from the centre.

Styx

hi styx

please see dimensions...and wheel base....i also changed the drawing to show you exactly how it is set up and where the hangers are located

last thing to mention is the wheel arches are 180 high

drawbar 4.JPG

regarding materials.......i could probably use 50x25 laid on its side.....and use the 50x25 on the uprights as well.....ill do some more measuring tonight....i might have enough room for 50x25 laid flat over the top of the wheel arch....and still be able to open the door.....MAYBE

do you prefer the square or the rectangle rhs

180 mm high sides is not much. Higher sides and climbing through window would be a stronger option.
If it is difficult to fit 50 mm wide truss members, then car fit on trailer must be very tight.
This suggests a slightly wider trailer, and stronger, would be better overall.

The longer drawbar may have been an attempt to make it tow better, as it seems the axles are central
and not biased towards rear, as they should be for safer towing.

However, if 180 mm is all you can fit, then you need to make the truss members as strong as possible.
The deeper the truss, the lower the forces in the members.

Attached drawing shows suggested compromise.

Styx

i was planning on running everything down the sides of the side rail.....there is absolutely no room on the deck at all....the car takes up the total width.....it is a wide trailer....as wide as is legal.....its just the car with its racing wheels is very very wide.....absolutely no room on deck ..the truss can only be joined down the sides....is this still ok?

the plan was to run the truss down the outside of main rails and between that and the guard (50x25 laid vertical)....or i can go over the guard is its 50x25 laid on its side.

if between the guard and deck.....i could use 50x25 on its side and then 50x25 as all the uprights and diagonals

if on its side going over the top of the guard (a big ugly) then i could use 50x50 uprights i guess

as for loading...i do use supports under ramps....but it also has rocker springs which let the trailer squat untill almost touching the ground at the rear



( ill take some more measurements tonight......i might be able to cut the guard and put the 50x50 through it....as long as it has clearance from tyre)

Not sure I understand your proposed options.

As the truss is only 180 mm high, the posts will be short and stocky, so 50 x 25 x 2.5 should be OK.

Diagonals in the mudguard section would need to be "flat" otherwise they will interfere with the tyres.
Even putting a 25 mm section may cause a problem.

The diagonals outside the mudguards (front and rear sections of truss) could be fitted otherway, if you get what I mean.
It is always preferable to have all sections in the same plane to avoid eccentric loads which could cause buckling.

Seems messy to cut the mudguards along the top, (see end view 2), so maybe you could fit the top rail inside the mudguard.
Probably easier to remove the mudguards, then make the truss, then cut the guards and re-fit.
Good idea to have all joints welded all around.

Styx

Hi styx.... How was your day?

If i use 50x25 all mounted vertically for the whole setup i can fit it in between rail and guard.... No guard mods needed

if i go over the top of the guard then yeh ill need to cut a spot for upright

now you mention it diagonals in the wheel wells can JUST be done but will need to be inch thick maximum

So i guess options are
1 - use 50x25 everywhere all mounted vertically over the whole setup
2 - use 50x25 mounted flat for top rail... Use 50x50 uprights and 50x25 diags
3 - use 30x30 everywhere and 25x25 for diags

one last measurment to add is door height is 24cms

Well if door height is 240 mm, then truss can be higher and above guard.

If this is the case, then I would make the truss deeper than 180 mm, say 230 mm.

Now in the wheel wells, because of space limits, the diagonals need to be 25 mm and 50 x 25 would be OK.

The upright/post in the middle of the wheel well, at the centre shackle position, would clear the guard if no more than 25 mm thick.
Possibly the uprights at each end of the guard would also need to be 25 mm max.

So in the wheel well area you might be limited to using 50 x 25 on the "flat" for both diagonals and uprights.

Outside the wheel well you could use the 50 x 25 diagonals and uprights the other way round, as you have the space.

The top rail, if it goes over the top of the guard, could be not on the "flat", if you get what I mean.

See drawing below. Hope that this has been helpful.

Styx

Im just trying to figure out how to make it look pleasing as well .... Changing directiom may look a bit dodgy...

If it has downward pull on the uprights putting the 50x25 vertical on the SIDE of the 50x25 horizontal would cause twisting wouldnt it?

Did tou not like the idea of putting everything on the vertical? I can still go 230mm high

Ideally the truss should be directly on top of the side rail, this avoids any eccentric forces which can cause buckling.

However, as this is not possible due to the width constraints, a side attached truss is the next best option.
In this case you don't want to make it less stable by having a "thin" truss, which is offset from the side rail.

Two measures which can help overcome the problem:
1. Make the truss as "thick" as possible, in this case using the 50 mm dimension horizontally (green) as much as possible.
2. Take the uprights and diagonals down the side of the 75 mm side rail, which makes them stronger against sideways
buckling.

Now you can't do item #1 in the wheel well area, but you can do it in the front and rear, which is what I have suggested.
The eccentric forces where the members change plane, (from blue to green), will be countered by the more rigid "thicker" truss
at each end.

Any truss is going to better than no truss, as at the moment. The truss will virtually prevent any bending of the side rail. Making the truss members all in the same plane (blue all over) would be OK I think, but not as strong as what I've suggested.

As for appearances, the "flat" sections (blue) will mostly be within the mudguards and not so noticeable. Remember the top rail does not and should not change orientation along its length. This is the most important member and the most noticeable.

I've just suggested a way to make best use of the materials, to give a safer and higher load capacity trailer. You don't have to follow my advice, and other contributors might have better suggestions. You haven't confirmed whether or not the axles are centrally placed along the deck, i.e. without a rear bias, which I think is a potential safety problem. I have seen some hire trailers with a welded sign saying engine must be to front of trailer, specifically to overcome tail wagging problem.

Styx

hi styx....almost friday!

ive towed a lot over the past 10 yrs.....and ive never come across a trailer with the wheels futher back.....ive seen them in pics etc....but never actually used one....all the trailers ive used have central wheels and i thought it was common knowledge to load the car forward ?

your right....if i want to fix the dam thing and fix it once i may as well hack the guard apart and use big rhs

if you are happy with the 50x25 laid flat over guard.....then ill use 50x50 uprights and 50x25 diags mounted vertical ( i just wasnt sure if the 50x25 laid flat would have the strength needed) ( also didnt want to use 50x25 uprights mounted across ways on the top strap as it would have a sharp "punch through" effect on the top rail....rather then spread out the load)

or other option.....40 x40 everywhere except the 2 diags in gaurds (and ill have to cut the 40x40 into the guard as i want at least a few cms clearance from door incase i have to put stock wheels on it one day)

one last thing to mention.....is it important to have all diags on 45' angles? or just be even?
and do the uprights have to be DIRECTLY over hangers
the side rail area is 360 long....with 5 uprights that is 60cms long sections which puts the middle upright over the hanger but the uprights either side about 20cms inside of their target....but doing it this way makes everything even inc diags etc

thanks again for your help....definitely want to do it once do it right
sorry for 50 million questions....i just like to understand things before i do them.....it just looks drawn out cause we are on a forum lol this prob would have been a 10 min convo around the trailer hahaha

Re axles position, there has been quite a lot of discussion on this forum about the axle position. Having the engine loaded first probably deals with the problem, but maybe not if the trailer was used for general cartage with a uniform spread load.

I think I've been a bit pedantic about the size of members, orientation, etc., as I think any sort of truss would be of benefit, and you don't need the strongest possible. So I think it is probably best you size and position sections how you think best and convenient. I will offer the following suggestions though:

1. Make the truss as high as convenient, as this makes it stronger, and you can use lighter sections as the forces are lower.
2. Top rail could be tapered to zero at each end if you want, but suggest try to keep full height over the spring shackles zone.
3. Preferably make the top rail wider than deeper, i.e. 50 x 25 with the 50 side horizontal.
4. The diagonals could be quite light if you want, even 25 x 25 but avoid the 1.6 mm thick walls as welding is more difficult.
5. Diagonals don't have to be at 45 deg, but at whatever angle suits each panel, preferably sloping downwards from the centre shackle towards each end.
6. The verticals could be either convenient orientation, preferably 50 x 25 minimum, with 35 x 35 or 40 x 40 or 50 x 50 other options.
7. Space the uprights at convenient positions, but as close as possible to each of 3 spring shackles; don't have to match cross beam positions.
8. Uprights don't have to be uniform spacing, but if there is variation, make the spacings closer at each end of the trailer.
9. Good fillet welds at all joints, and preferably on all faces of section; avoid leaving joints where water can enter and corrode.

Styx

Heres 2 designs that i can use with the length

as you can see this way meens the uprights are only 10cms away from the outer hangers .... What to do from there .... Drop down striaght away or go to ends more a have unequal spacing

going off what you were saying you prefer the bottom one as it has more uprights and stays higher for longer

can you explain why you think 50x25 laid flat on top would be best? I thought it would bend really easily downwards as the chassis bends and pulls it down with it? I thought the top bar would work better i it couldnt be bent downwards with the chassis

ALSO is it important to get rid of the bow that it has now? Or build truss even though its bowed

The bottom truss would be stronger because, as you say, the top rail is higher for longer. Personally I think I'd go for that one.

However, I suggest you try to put the uprights as near to the shackles as possible, because this is where the upwards loads come from), but it needs to be practical and not interfere with the shackles, bolts, etc. (Close is good enough. The side-rail, 75 mm deep, is quite stiff over short distances.) Personally I wouldn't worry about equal spacing of uprights, but it will be symmetrical about the centre shackle. You seem to want to have equal spacings.

Originally Posted by loveit

....

can you explain why you think 50x25 laid flat on top would be best? I thought it would bend really easily downwards as the chassis bends and pulls it down with it? I thought the top bar would work better i it couldnt be bent downwards with the chassis

I think the top rail laid flat makes the truss stronger against twisting along its length, i.e. sideways buckling, and for the same reason I would orient the uprights so a 50 mm dimension is out from the deck, i.e. try to make the truss 50 mm wide. Diagonals are not so important as they are in tension.

The top rail will hardly bend as the truss will make the side of the trailer much stiffer, and stronger, than it is now. You will be surprised I think. Anyway, the top rail is not acting as a bending member, it is a tension member, more like a cable, that's why the end welds, and any joins along its length, need to be strong. Go for heaviest 50 x 25, that is 3.0 mm wall thickness, as it will be better to weld and much stronger in tension. The 50 x 25 x 3.0 mm section has a design tension strength of 149 kN (i.e. about 33,500 lbs force), whereas the 2.0 mm section 105 kN. (25 x 25 x 2.5 mm section has a design tension of 79.9 kN.)

The existing side rail at the moment is a beam over the axles with cantilevers at each end. This is why it is so flexible and deflects considerably, but it is probably strong enough (i.e. not to deform permanently.) Putting a frame on top of the existing side-rail makes the beam much stiffer, though it will act as a beam to some extent. The side trusses are actually quite a complicated structure, combining a beam and a frame. the deeper the truss, the more it acts as a frame, and less as a beam. That's why I think you should make it as deep as practical.

Styx

awesome info again...

i was thinking of using 40x40x3 for everything except diagonals in guards (50x25 on its side to clear wheels)

but can still use the 50x25 if you like that better

i will put the uprights over the shackles...or as close as possible.....unfortunately the front and rear guard mounts are right above the shackles and directly in the way of me putting a decent upright there( thats why they were 10cms off in my drawings)....i might be able to cut them in around the mount....not sure whether it would be strong enough but might be able to work something out.