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 Post subject: Diagonal bracing
PostPosted: March 16, 2018, 1:19 pm 
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Joined: March 15, 2018, 6:03 am
Posts: 89
This is my first post here, after long months of working on my car design, and after weeks of ploughing delightedly through so many of the valuable contributions on this site, so I would like to ask a (relatively) simple question:
Having designed a chassis where all the stress points are joined together (as simply as possible - this is my first attempt) I have a design where all the 'spaces' are 4-sided. Do I now just add diagonals, presumably across the shortest diagonal, so that all the spaces are 3-sided...?
Is it really as simple as this or is there something important I am missing...?

Many thanks for any simple tips you can pass on - MangPong.


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 Post subject: Re: Diagonal bracing
PostPosted: March 16, 2018, 2:18 pm 
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Joined: July 29, 2006, 9:10 pm
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Location: Oregon, usually
MangPong wrote:
Do I now just add diagonals, presumably across the shortest diagonal, so that all the spaces are 3-sided...?
Is it really as simple as this or is there something important I am missing...?
The (relatively) simple two-part answer to your two-part question is Yes and Maybe. It's an essential step for chassis of this nature, but may not be everything you're missing. After your next post, you can attach pictures.

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 Post subject: Re: Diagonal bracing
PostPosted: March 16, 2018, 3:46 pm 
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Joined: June 5, 2016, 7:03 am
Posts: 215
Location: ontario
MangPong wrote:
This is my first post here, after long months of working on my car design, and after weeks of ploughing delightedly through so many of the valuable contributions on this site, so I would like to ask a (relatively) simple question:
Having designed a chassis where all the stress points are joined together (as simply as possible - this is my first attempt) I have a design where all the 'spaces' are 4-sided. Do I now just add diagonals, presumably across the shortest diagonal, so that all the spaces are 3-sided...?
Is it really as simple as this or is there something important I am missing...?

Many thanks for any simple tips you can pass on - MangPong.


Welcome Mang,

Could you post a drawing illustrating your questions. I am one of these people who always need to draw as part of my thinking process.


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 Post subject: Re: Diagonal bracing
PostPosted: March 17, 2018, 2:08 am 
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Joined: March 15, 2018, 6:03 am
Posts: 89
Wow... you guys are not only incredibly helpful to newcomers but also to each other (I’ve noticed after several weeks of checking your forums, one of which ran to 100 pages) but you’re also darned quick...

I'm all for drawings as well (I've got lots, and I have a bit of a model as well) but I wasn’t sure if I could post graphics yet - I hope they work, but at least mine are very small files...

Please note: pretty well all the chassis tubes are already ‘diagonals’ - there are very few 90º joints. The rear engine section will follow the same principles but as i’ve not sourced an engine yet I'm not getting too detailed there - walk before run...
The suspension mounting points are marked by red circles. With the two points above each other (upper front & lower rear) I'm thinking of a double tube for additional strength. I'm slightly concerned about the rear/upper point and wonder whether to include the diagonal marked by the dotted line. It certainly can’t do any harm but it somehow ‘looks wrong’ to my eye - like an afterthought, which it is.
There are no transverse links to support the upper points, because my legs are going there (lol) but there will be upper transverse links matching the lower section.

I am thinking of 1” sq. 16 swg. tubing throughout but from what I've seen online maybe I can consider 3/4” for the diagonals.

I have another ‘simple’ question about the suspension mounting points but don’t know where best to post it.
Many thanks, Mangpong

[Phil - thanks for the welcome. My nickname is ‘mangpong’ - one word. It’s Thai for scorpion, and is the name of the car. My mistake to capitalise the ‘p’...]
_______________


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 Post subject: Re: Diagonal bracing
PostPosted: March 17, 2018, 9:26 am 
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Joined: July 17, 2008, 9:11 am
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Location: West Chicago,IL
Welcome MangPong, drawings are great. Don't forget a balsa wood model will help you determine which x-members add rigidity and which are simply adding weight. A lot can be learned with a few pieces of balsa and some glue. You will need some reinforcing across the top of the space frame too. Possibly a sheer panel behind the driver's back.

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 Post subject: Re: Diagonal bracing
PostPosted: March 17, 2018, 12:33 pm 
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Hi Chuck (rx7locost) - thanks for the welcome and taking the time to comment. I aim to have the upper section of the frame braced to pretty much mimic the floor section.
I'm not sure exactly what you mean by a *sheer* panel. I was thinking of a steel sheet completely filling the area behind the seats, to protect from heat and (hopefully never) fuel/oil/water spillage.

On google-images I came across the following, which seems to me to be somewhat dictatorial if it's intended as a general rule. Would the same apply, for example if the basic outline is a longer, thinner rectangle...?

Comments - thanks, mangpong.


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 Post subject: Re: Diagonal bracing
PostPosted: March 17, 2018, 1:47 pm 
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Quote:
Do I now just add diagonals, presumably across the shortest diagonal, so that all the spaces are 3-sided...?


Basically. Some places may only need a triangle to begin with, triangles are your basic building block. Adding the second diagonal across a square or rectangle only adds a little bit more to the stiffness whereas the first diagonal adds greatly to the stiffness.

Shear ( not sheer ) describes forces that cause to things to slide compared to each other. If you look at an I-beam used to support a weight across 2 supports, the large top and bottom surfaces are referred to as flanges which are in compression or tension and the thin center section is the "shear web". The shear web keeps the top and bottom flanges from moving compared to each other. Depending on the design the shear web may have holes in it because there is little to no stress in the middle of the web or it can be replaced by diagonal tubes. Your frame may be considered a beam to resist the bending of your weight and engine weight. The top and bottom rails are flanges and the diagonal tubes are the shear web. The frame has other loads too of course and each must be considered. In our case the cars are usually overly strong, but if you do a bad job that will not be true and there could be issues and dangers....

You can use a free program called "Grape" which will allow you to analyse your frame and the stresses and strains, but it will take a good number of hours to learn. I think it's worth it.

In the picture of good and bad tube layout above, it is not possible to be so simple. I t depends on what you need to do to get what you want. You can see many examples of the "bad" design above and there are reasons for doing that sort of thing. You are right to be suspicious of simple answers to fit all cases...

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 Post subject: Re: Diagonal bracing
PostPosted: March 17, 2018, 4:52 pm 
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Joined: June 5, 2016, 7:03 am
Posts: 215
Location: ontario
Hi again Mangpong


You have already got a good deal of input from colleagues who are more qualified than myself. I will just add my 2 cents.
In the quest for structural rigidity and strength versus weight there is one theoretical factor that need to be entered in the design. It is the target degree of deflection. Meaning how much do you expect the chassis to twist. In some countries (Australia is one) home built sevens I believe are subjected to a torsion test. A good deal has been written on this in the past, in particular with respect to sevens designed to be equipped with heavier engines like V8s. I got my structural basics from Herb Adams. Chassis engineering. The frame design issues are discussed on page 85.
Here are a few quotes (some of it will of course be obvious to you and others):

The basic shape for constructing rigid structures is the triangle. Its shape and dimensions will not change much unless one of its 3 legs is broken. In contrast a square-shaped set of tubes has very little structural rigidity, in that it will bend diagonally when even a small load is applied to it...etc

Then Adams explains that dividing a rectangle in half creates two triangles achieving rigidity.

And then he talks about double diagonals (and this may be more within your question). He says double diagonals can be used (which creates four triangles) for still more rigidity, but these additional members are usually unnecessary unless very high loads are anticipated.

And back to you, this of course brings the question: how is your car going to be used, what kind of engine, etc.


As you will agree one could go on for a long time discussing chassis engineering. This is a fascinating subject. Good luck and keep us posted. :cheers:


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 Post subject: Re: Diagonal bracing
PostPosted: March 17, 2018, 11:39 pm 
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Joined: March 15, 2018, 6:03 am
Posts: 89
Hello Marcus (horizenjob)
Many thanks for your detailed response. Some of this I'm aware of (I think) but I'm not always sure of the terminology. I have an engineering background (in school & college) and have renovated/restored old houses over the past forty years. Fifteen years ago I designed, single-handed, and built my own house... BUT... I have not undertaken anything like this (I've not even worked on my own cars for nearly thirty years...!), so I regard it as a new challenge, and am wary of trying to walk before running - my car design I've been working on (and off) since the house build but personal circumstances delayed me. Now I'm back in the saddle and have been drawing, and thinking, and re-drawing, quite intensively for a few months, and this time feel more determined.

I'm sure you will understand how indebted people like me are to people like you guys.

As for Grape - for someone who has constantly failed to drive CAD programmes for twenty years I fear for my sanity if I try it... lol.
_______________

and Phil
Thanks for continuing with me - it is much appreciated. The small improvement between having one diagonal or two (within a 4-sided structure) is interesting, and seems logical but, without these comments from those ‘in the know’ I would be unlikely to learn for myself.

So... for me... I am doing this because... well, because I can... Like climbing Everest because it’s there. Long before I could legally drive I fell in love with a Morgan 3-wheeler which roared past me on my way home from school. I went into the nearby library and described the fantastic vehicle to the bemused librarian who sweetly told me to return the next day - when she proudly announced the result of her research. Those were the days...

See above, when my house was finished, and I had retired, I needed a new project... How was I to know that fifteen years later Morgan would be making their own replica... so I've now decided to do my own design - and here I am.

So... I want to do this because I can, because I have the time, and the money (I hope), and because it should be fun. I don’t intend to race it, or otherwise compete with it but I will investigate the possibility of running on a local track just so I can go above 50mph without endangering others. Whether I use it for more than trips to the supermarket remains to be seen - but I secretly suspect I will find other uses for such a vehicle...

I have yet to source an engine, and other parts (these things are very few and far between in my neck of the jungle), but I'm not looking for anything large - if the car has better performance than my Honda City I'll be happy. Daihatsu have a transverse 3-cylinder that is very compact and their car is very nippy - like an original Mini. I am also considering motorcycle engines but the 500+ cc models are not plentiful here.

I agree about chassis discussions. Now I need to have a similar conversation about wishbones/control arms. Can I do that here, or do I need to move to another section. (I am still learning my way around this wonderful site.)

Many thanks, all, Mangpong
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 Post subject: Re: Diagonal bracing
PostPosted: March 18, 2018, 5:27 am 
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Joined: June 5, 2016, 7:03 am
Posts: 215
Location: ontario
Mangpong,

To discuss «suspension, steering» you may want to ask your questions on a separate section.

Your idea of a Morgan three wheeler sounds quite exciting. The tube chassis could be light and simple. The «Book» (Ron Champion construction of a seven) would be a good guide .

On the engine too you may want to ask questions in the appropriate section. There are a few people who have built threewheelers (see the non conventional build section). Again good luck :cheers:


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 Post subject: Re: Diagonal bracing
PostPosted: March 19, 2018, 7:53 am 
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Joined: March 15, 2018, 6:03 am
Posts: 89
OK guys... next step. I omitted to mention before that I also have a central (transmission) tunnel in the design - I wanted to discuss principles of diagonals first. I am thinking 16 swg. steel, but I'm worrying about the panel flexing noisily. It is (currently) 13’ high, 2 1/2” wide at the top and 3 1/2” wide at the bottom, to give a little additional ‘triangulation’ - I am trying not to have too many parallel members, or right-angles. At the moment it is open at the bottom (see later) but could be easily closed as part of the floor-pan.

This feature will provide a comfy armrest; a place for a gear lever and handbrake (and hidden linkage); and ought to create a stiff ‘spine’ to the chassis. Incidentally... the car is rear-engine/rear drive, so I don’t need a transmission tunnel as such. However I can use this space to run water/oil pipes to a front radiator (I don’t want to have ‘bits’ stuck on the side of the body...!), plus brake, clutch and throttle pipes/cables. It also occurs to me that if I end up with an air-cooled engine I can funnel air from the nose through to the rear of the car - there’s novel... lol. If it isn’t enough I can add an air-scoop behind the occupants’ heads.

All (polite) comments welcome...

Many thanks, Mangpong
_______________


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