5th wheel assembly

[Robo McDuff]

I am making a tractor out of our dump truck. I have all the stuff ready. Unexpected question and maybe problem now.

The standard 5-ton M39 army tractors have the 5th wheel plate connected directly to the top flange (right word?) of the frame. They also have a spacer plate between the assembly and the frame rail to cover for the rivets sticking out (I presume, see two M39 pictures)).

Most European and probably also US civilian trucks have the fifth wheel attached to a sub-rails that connects to the vertical part of the frame (see picture). Basically the Dutch MoT does not like it when people drill holes in the top part of the frame because it would weaken the entire construction too much.

My idea is to follow the US original and drill holes next to the rivets through the top flange of the frame rails. The dam-wand plate used as spacer in my Jost 5th wheel assembly fits exactly around the rivets, so I do not need a separate spacer with holes for the rivets.

Question 1) With the M39 tractors, the holes for the bolts for the 5th wheel assembly are drilled next to the rivets or do they replace the rivets in some places?

Question 2) The 5th wheel is located in the area of the frame where there also is this big X cross-member between the two frame rails. Does that gives-leaves enough strength in the frame after drilling the holes for the assembly or is there another strengthening plate below the frame flange, or does the original spacer rail also provide this extra strength?

 

[KsM715]

What truck is the 5th wheel coming off of? On my M818 there is a flange rivited to the side of the frame rails, then that spacer block is bolted on top of that and the 5th wheel is bolted to the spacer.

Sorry for the poor quality of the pics, I just ran out and snapped a few in the dark. Hopefully you can see them well enough to understand what Im saying.

 

[Robo McDuff]

Thanks, great pics. What is clear here also is that the side rail actually is not very high and the bolts or rivets connecting side rail to the frame are close to the top of the vertical body of the frame -see your second picture. That would be needed in my case as well: too many other attachments to the frame already there.

One question though. The bolts connecting the 5th wheel assembly to the spacer seems to be positions straight above the real frame. Are they just screwed into the spacer plate without bolt (not likely) or do they run through both the spacer and the real frame?

If the latter, it means that your 5th wheel is connected with 10 bolts on each side to the frame: 4 directly - vertically - through both spacer and frame, and 6 indirectly bolting the spacer vertically to the sub-rail, with the sub-rail horizontally connected to the vertical body of the frame. That would be a much stronger construction.

Below an overview and a detail picture of my 5th wheel on the original DAF truck. Here also connected to a side-rail that is riveted to the main frame rails. The actual 5th wheel assembly is bolted with 12 bolts (3 on each corner) to the dam-wand profile spacer plate. First pic, in the lower right corner you can see the first of the three bolts and just below the coupling plate part of the second bolt. The third bolt is more to the middle of the truck. The spacer plate is bolted to the side-rail with 6 bolts. 2 next to each other on each outer side and two times one in the two middle profile dips: details in 2nd pic. The side rail is riveted with numerous rivets to the main frame rail. You can see that the side-rail is a bit raised above the main frame.

 

[zout]

Figure out what 5th wheel (top of head laid horizontal) height you want as well. Most top of plate height is 48".

The frame rails to mount the 5th wheel assembly on get placed in those rivet holes - you MIGHT have to drill a couple extra and that is acceptable.
use Grade 8 FRAME BOLT - these are a special bolt.

Then you plate assembly will bolt onto these rails.

The 5th wheel assembly will have diferent rise heights - this is where the plate height comes from.

Acutally looking at your second set of pics - you should be able to use the existing rails - bolting the new 5th wheel to that - you might have to weld stops onto the slider.

 

[Robo McDuff]

Figure out what 5th wheel (top of head laid horizontal) height you want as well. Most top of plate height is 48"..

If I remember correct, bolting the assembly with this dam-wand profile spacer plate straight on top of the frame would give me around 48''.

The frame rails to mount the 5th wheel assembly on get placed in those rivet holes - you MIGHT have to drill a couple extra and that is acceptable

Let me get this correct. I should cut and remove the rivets in the top flange of the frame rails and use those holes to bolt the spacer plate onto?

I just figured out how to leave the rivets in place and fit the dam-wand profile of the spacer plate around it, drilling 6 new holes. Not sure the profile will fit so snug using the existing rivet holes.

Actually looking at your second set of pics - you should be able to use the existing rails - bolting the new 5th wheel to that - you might have to weld stops onto the slider.

The second set of pics is of the DAF truck with my 5th wheel still mounted onto it. I only got the 5th wheel with spacer plate, not the side-rails. These are riveted with a lot of rivets to the frame. Stitched would be a better word to describe it. Nobody wanted to take the side-rail off, so I would have to buy and prepare that separately.

Next problem is that the chassis of the donor DAF is a a fraction smaller than the 5-ton. That means that using the existing holes on the spacer plate would mean holes in the side rail that are close to the bend in the side rails. Also, the existing attachments would force me to connect the side rails rather high in the frame body. Doable, but also not a perfect solution. I would have to sit next to the truck with a micrometer and a piece of side rails to see if it would work.

All in all, just drilling six new holes in the top flange of the frame and in the spacer plate of my assembly and bolt it on directly to the frame is the shortest, easiest, and cheapest way. Not sure though if that would not mean too much wakening of the frame rails. Even if not, quite possible the Dutch MoT does not agree with it, have to discuss this with them and a truck builder. Making a complete new spacer plate to fit the existing rivet holes might be the next alternative.

 

[KsM715]

One question though. The bolts connecting the 5th wheel assembly to the spacer seems to be positions straight above the real frame. Are they just screwed into the spacer plate without bolt (not likely) or do they run through both the spacer and the real frame?

This morning I went out just to see what I coould find. I found out the bolts holding the 5th wheel to the spacer plate are threaded into the spacer plate. They do not go all the way through the frame. I would guess that would be the reason for the spacer, so there wouldnt be any more hole in flange part of the frame rail.

 

[KsM715]

The funny part of this thread is Im sitting here thinking of going the exact opposite direction. I have an M818 and I know where there are 4 complete 5ton dump beds, hoist, pump, res. and all.

 

[Robo McDuff]

Actually, mine will hopefully be both.