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Author Topic: Stevenson design analysis  (Read 681 times)

StefanN

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Stevenson design analysis
« on: 22 May 2020, 01:54:33 pm »
Warning - if you're not interested in theoretical pondering then stop reading.

I've been thinking about the Stevenson plywood box approach and wondering how light it could be.  So, out of interest, and because its something new to learn, I drew a simple Stevenson box with no formers in the middle and used the analysis tools in Fusion 360 to optimise the shape to those areas that carried the load in two scenarios:
1) 120kg person sat in the cart
2) A torsional load on one corner.

I thought the results were interesting.


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Ron

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Re: Stevenson design analysis
« Reply #1 on: 22 May 2020, 02:56:15 pm »
I love all this analysis stuff, one thought, does the software assume that the sides are connected seamlessly along each edge as if fibreglassed together or can it be worked to pinpoint the load to represent screws and brackets? It's a great piece of software !

TheGiantTribble

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Re: Stevenson design analysis
« Reply #2 on: 22 May 2020, 10:05:15 pm »
OK, why has the top one got a blue dildo where the driver sits...mine doesn't have that  ;D

More importantly, what thickness ply have you considered?
And what happens wen you have a semi circular top, rather than the flat you show?
I'll also believe the number of bulkheads and their position would make a big difference...possibly

StefanN

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Re: Stevenson design analysis
« Reply #3 on: 23 May 2020, 01:28:05 am »
What thickness ply have you considered?
And what happens wen you have a semi circular top, rather than the flat you show?
I'll also believe the number of bulkheads and their position would make a big difference...possibly

My idea is to use 6mm ply, doubled-up just in the areas carrying high load.

It’ll be interesting to see what different adding a former in the dashboard area will make.

A number of the Stevenson cyclekarts had a cosmetic curved bonnet and I’m thinking that’s the approach take

Chris Brown

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Re: Stevenson design analysis
« Reply #4 on: 23 May 2020, 09:07:22 am »
Thanks Stefan that's interesting, I have been debating (with myself, it's a self employed thing) a 6mm ply monocoque. Not quite Stevenson, as the chassis members would be ply, integral with the body structure. I was thinking of the layer structure being: outer chassis member; body side with lightening holes in chassis member area; chassis member with lightening holes (extended to full height in cockpit area in view of this info); inner chassis member. All to be constructed from 6mm birch or other 5 ply sheet, as the more plies the stiffer the ply.

Chris

Ron

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Re: Stevenson design analysis
« Reply #5 on: 23 May 2020, 11:14:41 am »
That's an interesting thought Chris to maybe skin it in thin ply and thicken up at the stress points, it 'wood' be ultralight Haha and as you say with enough plies be very rigid.
Thinking ahead what's the usual method with joints, some sort of epoxy glues,self tappers?
« Last Edit: 23 May 2020, 11:16:22 am by Ron »

RhysN

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Re: Stevenson design analysis
« Reply #6 on: 23 May 2020, 01:32:29 pm »
I love the analysis, would it be fair to say that without comparing to something else it doesn't tell a lot?
The ones I have built with the plywood box were built with 8 mm for the first, and lighter subsequently. Right around the cockpit edge I added a piece 150 mm deep of 8 mm in each one (because it was left over from the first).
All corners were joined with a least one layer of double bias glass tape and epoxy, additional in the cockpit area as that's the bay with the "missing" face, otherwise you can't get in :)
 My dash bulkhead has always been strong and more like a wooden version of the diaphragm bulkhead found in most of the Lotus space frame designs. Reason is to help that missing face, and to cope with loads of steering input (I hate scuttle shake). The horizontal frame in front of the dash goinfg to the front I have cut out so that it's another diaphragm and allows for getting at the pedals when the bonnet is removed.
The seat back bulkhead is built very strongly, you use it to lever yourself in and mostly out, and the ERA broke that out early in the piece.
I took most of my build clues from the ocean racing multihulls I was involved with. In those the mechanical fasteners were only used until the bonding had cured then removed, apart form very high load points. All about distributing/spreading loads to minimise the point loads. The boats (what you call sailing dinghies) I built for myself at World Champs had no screws, bolts or nails except for mast loads. And no failures.
The engine bay creates some interesting considerations, it seems to me so far that a separate subframe of some sort is the best way to spread the loads, although having a deeper "box" see the Bentley on Gittreville has had no issues in about 6 years of thrashing round with a "large" driver.
Weight of the one hanging in the rafters here with only the tub and vestigial panels is a bit under 25 kg ready to bolt mechanicals.
Thanks for raising this Stefan

RhysN

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Re: Stevenson design analysis
« Reply #7 on: 23 May 2020, 01:52:49 pm »
I have had a bit more of a look at the analysis drawings.
Please not a criticism, and I am just asking to further my understanding.
With the load on one corner of the open box is it not fair to expect what the designers call "lozenging"?? 
Is it possible to model in where the actual loads would be, ie the points where loads are applied, ie front suspension and rear axle attachments. All the chassis texts I had to use for my studies define a chassis as the "best solution to join the major loads. In full size cars, suspension, engine weight and torgue and driver about 5th in the path. Because ours are so much lighter the driver is a much more significant "lump" to deal with.

Ron

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Re: Stevenson design analysis
« Reply #8 on: 23 May 2020, 04:49:11 pm »
I did some basic calculations for torsional load in a cross axle fashion given an estimated rear axle load of 133kg and 44kg
36kg front formed from some estimations of engine and driver positioning and I reckon 157 foot lbs is about right as a static load with potentially that being way more dynamically say if one front wheel ran out of spring travel, or indeed had no articulation.

Dropped a clanger there with the calcs , its 127.6 Lb/ft !!
« Last Edit: 23 May 2020, 05:02:58 pm by Ron »