lateral movement on 'rail on rail'

[dermal fauna]

Hi, I guess this information is probably buried in the forum, I read somewhere that when you have plates restricting lateral movement of the floating end of a rail on rail subframe, there needs to be a gap between the plate and the chassis rail of a few millimetres, I wondered if this makes sense to anyone? How big does the gap need to be?
Could you lubricate the gap, and or fill it with a sacrificial layer of polyurethane or wood or ally..???
Does that make sense?
Also I'm hoping to use sprung U-bolts, does anyone have a design or ideas. It's for my Bedford RL .
Thanks loads for any advice.

 

[broncobowsher]

Typical from what I have seen, the U-bolts are what limit lateral movement. Will you get some movement? Yes. Is it a problem? No. Just plan on getting a little movement and don't put anything in a place that will be damaged when the movement happens.

 

[Ramdough]

I have seen it done where they used an extra long U bolt with springs under the nuts to allow separation. The U bolts were used to restrain lateral movement. At one end, the u bolts were tightened down to create a “fixed” end. They also had the camper blocked at the back so the camper could never slide forward if they got in a wreck.

For the type of design that has plates on either side, there needs to be enough room to allow the frame to twist without binding on the straight subframe. Basically, when the frames are twisted relative to each other, the subframe overall width is greater due to the twist. So, you need to allow for the room.


I hope this helps.


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[Pinnacle Campers]

The term I have heard used to describe them is "Fish plates" Click for local search results.

 

[Ramdough]

The term I have heard used to describe them is "Fish plates" Click for local search results.

Fish plates are something else. They are used to join two beams together and often are welded.


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[Pinnacle Campers]

Fish plates are something else. They are used to join two beams together and often are welded.


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Oh?
Take a look at Page 7 fig. 7, https://www.nhvr.gov.au/files/201901-0631-vsb6-section-j-body-mounting.pdf
Wait, is that a weld?
Page 10 figure 17 is what the OP is talking about, no names given....
If you look at the results in my first post, most are talking about fish plates in reference to the OP's application.
"Fish plates are actually an ADR requirement here, I didn't read the Truck Body ADR's until I'd almost finished building my box - I think they are really more to keep the subframe from dislocating in the event of an accident, but good insurance incase of a spring bolt fail. I have teflon (kitchen chopping board) on the sides of my fish plates incase of rubbing - but all good so far."
Even if some use that term loosely the OP can still find info using it.

 

[Ramdough]

Oh?
Take a look at Page 7 fig. 7, https://www.nhvr.gov.au/files/201901-0631-vsb6-section-j-body-mounting.pdf
Wait, is that a weld?
Page 10 figure 17 is what the OP is talking about, no names given....
If you look at the results in my first post, most are talking about fish plates in reference to the OP's application.
"Fish plates are actually an ADR requirement here, I didn't read the Truck Body ADR's until I'd almost finished building my box - I think they are really more to keep the subframe from dislocating in the event of an accident, but good insurance incase of a spring bolt fail. I have teflon (kitchen chopping board) on the sides of my fish plates incase of rubbing - but all good so far."
Even if some use that term loosely the OP can still find info using it.

So page 7 figure 7 shows a fish plate that is welded on the top and bolted on the bottom.

That is a fish plate. They are used to rigidly joint two structural pieces of steel. One end of the subframe is often secured with a fishplate, but I have seen other methods to fix one end.

Page 10 figure 17 is something else…. But that is what I assumed the OP was referring to. They are used to prevent the subframe from slewing right and left. Something that performs that function is required.

My original post explained that there must be some gap to allow the two frames to twist independently without binding.


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[dermal fauna]

Hi, thanks loads for the replies, how much gap would you think? It'll be for a 14' frame with springs to allow maybe 70mm distortion at most. I'll probably use 10mm plate and tap a hole for a grease nipple or 2.

 

[Ramdough]

Hi, thanks loads for the replies, how much gap would you think? It'll be for a 14' frame with springs to allow maybe 70mm distortion at most. I'll probably use 10mm plate and tap a hole for a grease nipple or 2.

If you know the max displacement of your frame at the end, take that and the width of your subframe to make an isosceles triangle. One leg is the total max displacement of your springs, the other two are the widths of your subframe. Take the small angle that is left. Then take that small angle and use the height of your subframe beam as the hypotenuse to find the small leg of the triangle that is created when your subframe rotates relative to its original position. That small distance is roughly how much space you need.

I can give an example maybe tomorrow if my description is hard to follow. I need to do this still for my truck.


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[dermal fauna]

Thanks for the info, it's given me a stroke. I'll have to get a piece of paper.