Tramp Truck Build

Vehicle prep and projects

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Rusape
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Re: Tramp Truck Build

Postby Rusape » Sun Sep 30, 2018 9:59 am

Ivan C wrote:I've deliberated long and hard over the body mounting, to allow rail seperation and have decided against using springs. My (possibly flawed) logic is that springs are for bearing weight, the chassis will do that, what I want is the movement of the subframe and body damped, rather than restricted.
Comments welcomed.


Ivan,

You've obviously researched this first, but from my understanding, (I'm not and engineer) the movement of the subframe is non-linear, and using a single mounted shock absorber (viscous damper)to try and counter this may not be particularly effective. I may be wrong, but it would need the damper to work in more than just the vertical plane. A spring does just that.
I'd also consider adding check straps to stop the shock pulling out on severe articulation or twist/lift.

Ivan C
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Re: Tramp Truck Build

Postby Ivan C » Sun Sep 30, 2018 11:46 am

Thanks for the input, Neil and Laurence.
I'll add a few clarifications and some explanation.
I have no engineering qualifications, which I suppose is why I threw this out for comments in the first place.
I'm planning two anti-slew mounts at the front to keep the body from moving sideways, this makes the movement of each subframe rail, relative to the chassis linear. (it's really an arc, but at 4000mm radius and 100mm travel minimal)
This set-up would work without springs or dampers in that it isolates the subframe and body from the twisting of the chassis.
The only potential issue is that of " box flop" which is why springs are used. Springs however will restrict the relative movement of the two as they compress, which will transfer stress to the box and subframe.
I don't want to restrict the movement, just dampen it. It is self limiting because at the limit of chassis twist a wheel will lift. As long as the dampers (one on each side) have more travel than the maximum separation, all is good.
I hope this will clarify my original explanation, which wasn't really clear.
Ivan C

crinklystarfish
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Re: Tramp Truck Build

Postby crinklystarfish » Mon Oct 01, 2018 9:54 am

Ivan C wrote:... I've deliberated long and hard over the body mounting, to allow rail seperation and have decided against using springs. My (possibly flawed) logic is that springs are for bearing weight, the chassis will do that, what I want is the movement of the subframe and body damped, rather than restricted.
I've roughed up a couple of brackets to allow me to do this, how it will work in practice remains to be seen. Comments welcomed.
Ivan C


Ivan C wrote:... I'm planning two anti-slew mounts at the front to keep the body from moving sideways, this makes the movement of each subframe rail, relative to the chassis linear. (it's really an arc, but at 4000mm radius and 100mm travel minimal).

This set-up would work without springs or dampers in that it isolates the subframe and body from the twisting of the chassis.

The only potential issue is that of " box flop" which is why springs are used. Springs however will restrict the relative movement of the two as they compress, which will transfer stress to the box and subframe.

I don't want to restrict the movement, just dampen it. It is self limiting because at the limit of chassis twist a wheel will lift. As long as the dampers (one on each side) have more travel than the maximum separation, all is good...
Ivan C


Comments welcome eh?:

1. I've seen dampers used to restrict 'box flop' in pivoting-subframe systems - where box flop can be dramatic - but never in a rail-on-rail system.

2. When the full mass of the completed hab box is bearing down on the rails - even at maximum articulation - I can't see the separation at the leading edge of the subframe being that great so it's unlikely any kind of opposing-spring system would actually restrict articulation (unless the springs were very short)

3. Box flop generally isn't something that rail-on-rail systems suffer from.

4. I personally reckon slew plates alone would get the job done just fine.

5. In rail-on-rail systems springs essentially exist to counteract the forces imparted during separation - and so reduce the rate of separation. They also help control slew.

6. Dampers would counteract the forces imparted during separation too, but would introduce the prospect of a more controlled closing of the separation.

7. Ideally, it would seem logical if the dampers were set up to have minimal rebound damping (so as not to restrict articulation) but a good deal of compression damping (to take advantage of the 'soft close' benefit).

8. Using dampers would possibly replace the benefits conferred by a sacrificial hardwood (or similar) strip between chassis rails and subframe.

9. I'm not entirely convinced that a problem that requires this solution exists...

10. ...but I can see some potential advantages anyway.

11. I like the stress-relief profile at the leading edge of the subframe.

12. There are some stonking trucks being built at the moment.

13. I'm definitely and officially not an engineer.

Big Monkey
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Re: Tramp Truck Build

Postby Big Monkey » Mon Oct 01, 2018 10:17 am

Hi Ivan,

I think that your proposal to use dampers is a great idea. As Steve has suggested, you would benefit from a more controlled closing event. There is enough compliance in the rubber bushings at either end to cope with the 4m radius that the box chassis describes.

Are they classic Mini front dampers by any chance?

You are an engineer. I'm an engineer, with a degree, but I also work with a lot of "engineers" who have a 50% chance of hurting themselves whilst using a screwdriver. One of the most gifted engineers I know doesn't have any engineering qualifications.

Right, I look forward to further updates.

Ivan C
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Re: Tramp Truck Build

Postby Ivan C » Mon Oct 01, 2018 6:43 pm

Well I asked for comments.......... :D
1 the only time I've seen them used was also on a pivoted subframe

2 Around 100mm or 4 inches, as measured by Phil.

3 agreed, probably the worst you would get would be more box drop than flop

4 probably

5 & 6 agreed

7 good point

8 maybe, though with a steel chassis and aluminium subframe I think some isolation would be good. I have some nice rubber strip eyed out for the job

9 & 10 not so much solving a problem as refining a solution?

11 I read about it in a book somewhere......

13 nor I., though I agree with Darren, sometimes intelligence and common sense can seem to be mutually exclusive.

Darren, they are Ducato rears - they have the right amount of travel, and they were to hand.
I have a couple of koni adjustables coming so I can pretend I'm fine tuning it :D
Ivan C

crinklystarfish
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Re: Tramp Truck Build

Postby crinklystarfish » Tue Oct 02, 2018 7:58 am

Ivan C wrote:2 Around 100mm or 4 inches, as measured by Phil.

8 maybe, though with a steel chassis and aluminium subframe I think some isolation would be good. I have some nice rubber strip eyed out for the job

9 & 10 not so much solving a problem as refining a solution?

13 nor I., though I agree with Darren, sometimes intelligence and common sense can seem to be mutually exclusive.


2. With a bare subframe or with 2+ tonnes of fully built hab box / subframe unit bearing down?

8. Agreed.

9&10. And kudos for the innovation.

13. It's probably fair to say that when it comes to creating and using self-built overlanders, people very publicly wear their engineering prowess on their sleeve...

Ivan C
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Re: Tramp Truck Build

Postby Ivan C » Tue Oct 02, 2018 9:21 am

Agree with all of the above,
it's inevitable I suppose, that pride in your work continues into pride in what you have produced - something sadly lacking in today's world.
I freely admit that I enjoy the building and use of the vehicle equally, in fact I have to make a concious effort to keep them in proportion!

2 unless either chassis or box are going to deform with the weight then the distance should remain similar?

Ivan C

crinklystarfish
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Re: Tramp Truck Build

Postby crinklystarfish » Tue Oct 02, 2018 10:10 am

Ivan C wrote:
2 unless either chassis or box are going to deform with the weight then the distance should remain similar?

Ivan C


I'm not an engineer and existing builds demonstrate that it doesn't hurt to work with the max separation dimension encountered when cross-axled with a bare subframe.

But; if I were an engineer and wanting to tune some dampers, I'd definitely advocate measuring that same separation with a full-weight box in situ.

Let us know the difference... ;)

Ivan C
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Re: Tramp Truck Build

Postby Ivan C » Tue Oct 02, 2018 11:24 am

Agree 100%. - there's nothing to beat a real world measurement to test a theory :D
Watch this space........

Ivan C

crinklystarfish
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Re: Tramp Truck Build

Postby crinklystarfish » Tue Oct 02, 2018 11:32 am

I'd be surprised if you weren't surprised...


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