SNAIL

Harley · Posted: Sun Jan 25, 2009 4:53 pm Post subject: reinforcing Chassis

just wanted to get some opinions on strenghthening a standard 2cv chassis...
i know of these techniques:
http://www.cats-citroen.net/citroen_atypes/po2x2.html

this car will be used for raids etc.

Harley
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backfire · Posted: Sun Jan 25, 2009 5:19 pm Post subject:

The techniques on Jeroens site are the original modifications from Citroen, they used to supply a booklet to modify 2cv for raids.
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acadamié · Posted: Sun Jan 25, 2009 6:28 pm Post subject:

This Citroen Book is intituled "Ici commence l'aventure" :
here
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EirikJ · Snailer Joined: 23 Jan 2008 Posts: 36 Location: NOrWAY

In english here:

http://www.scribd.com/doc/8569197/2CV-Raid-Handbook

bleu comme le ciel · Posted: Mon Jan 26, 2009 9:43 am Post subject:

In english with a working link:
http://www.2cvtransahara.nl/images/stories/aventure/ici_commence_l_aventure.pdf
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Lionel · Posted: Mon Jan 26, 2009 1:11 pm Post subject:

Hello Harley,
Basicly, the "here start the adventure" method is still right (it's citroen recommandation for the raid afrique 73).

If you have access to one easely, use an AMI chassis. Stronger springs, you can use same shock absorber at front and rear, bigger fuel tank anf often less rust!

Anyway, change your front chassis shock absorber axesby AMI or ACADYANE rear ones, so you can use rear AMI (8 or Super) shock absorbers in front by using late model ACADYANE shock absorber plates.
Create reinforcement between central chassis and front fixation point of suspension pot. It exists between back fixation point and central chassis, but not always on front, exept on PO chassis.

Citroen front skid in fun looking but heavy as can be, and something I heard every time I am on a raid is:
"Weight is THE enemy"
Something more basic would be a 3 or 4mm aluminium sheet skid.
Idem under fuel tank and make it pass under chassis half moon lip so that it won't be caught by a coming obstacle and be torn away!

The reinforcement of the front axle bolt retainers (reinforcement B) may be replace by a U shape reinforcement welded upside down with the bolt retainer. Again easier to do, and you keep a better access to the bolt.

I'd rather use Citroen arm reinforcement than the method Jeroen describe. for it's far easier, and more than enough.
Another way is to weld half arm (cut along the welding seams) along the botom half of each arm.
don't forget to reinforce the knife edge pin brackets on arm so that they don't "open" under contraint.

Use rear axle bolts on both front and rear, they are longer, and if they turn loose, you will have a longer time to realise.

I will come later if I remember something else...
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Harley · Posted: Wed Feb 04, 2009 6:02 pm Post subject:

Hi Lionel,

Thanks for the reply.
Ami Chassis are hard to come by in OZ, maybe a bit to heavy for my needs...
What gauge plate do you recommend using for the reinforcments?

Harley
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Lionel · Posted: Thu Feb 05, 2009 10:46 am Post subject:

Bart · Posted: Thu Feb 05, 2009 12:28 pm Post subject:

Lionel · Posted: Fri Feb 06, 2009 8:59 am Post subject:

JuanNavarro · Posted: Fri Feb 06, 2009 12:06 pm Post subject:

Here is a page where you cand find the real reinforcement parts:

http://www.pp-2cv.be/FR/News/4%20x4%20pp.htm

(take this link for pics reference, is not my intention to promote any shop)

dyanut · Posted: Sat Feb 07, 2009 12:24 am Post subject:

Bartje,
not too sure I'd agree with that...
A very common mode of failure of an OE Citroen chassis is in the area just behind the front axle, which bends or buckles. This is usually triggered by corrosion of the internal stiffeners, throwing extra stress onto the outer webs.

Citroen's 'patch' for this problem, as used on 'P.O. chassis was to add those 'U' section plates to the outer webs, spanning from the front axle mounting points to the first transverse tube.

Perfect Product's item, replicating Citroen's original, recognises that the bending strength of a 'U' section benefits to a much greater extent than one might expect from a small amount of additional metal concentrated at its outer edges.

Folding the edges into flanges also helps to resist buckling failure of the web. Some rough calculations I've just done suggest that adding 2mm thick x 12mm wide flanges to a 100mm x 2mm flat section adds about 50% to its resistance to bending, but only adds about 16% to the total amount of metal.

Enough thinking for Friday evening, off through the snow to the pub... Wink

Ken.

Bart · Posted: Sat Feb 07, 2009 9:20 pm Post subject:

This is what the thinking I did a while ago, please correct me if I'm wrong.

First of all we actually talk about a C section rather than a U. x axis is horizontal to the screen, y is vertical

Generally Kens thinking is right, many chassis designers weld metal strips on the top and bottum of the C to increase bending resitance, mainly because it is the easy option. Morover the flaps definatly improve buckling(bending around the y axis) Ken mentions.
However when the reinforcement is welded in a good condition chassis and the welding in the corners is done such that it melds together with the original three folded layer of steel (together around 2,5mm)... My opinion is that it should be good enough and adding another 2mm, makes it around 4,5mm (or more) of C section flaps, is over the top. Anyway.

Than on the calculation:
Bending tension until plastic deformation:
Sigma(around x-axis)= (M*L)/(E*I)
and or
Sigma(around x-axis)= (F*L^2)/(2*E*I)

M or F=torque or force load on the chassis through the load on the front axle.
L = leaverage distance to first transverse/cross tube for sping mounting.
E = material constant.
I = what is called in dutch surface inertia moment, basically the parameter which defines the resistance of the contrustion or profile agains bending.
I around x-axis:
Ixx =1/12*Xdim*Ydim^3 when considering a square surface

The force or torque conditions are open for discussion, nevertheless what ever happens the resistance agains bending is defined by the I.

Definition of I for bending around the x-axis of a C must be done on three parts, main "leg" and top and bottum flaps.

Ixxmainleg=1/12*2mm(=thinkness mat.)*100mm(=highth)^3=166.667 mm^4

Ixx flap =1/12*Xdim*Ydim^3 + Xdim*Ydim*distance to centreline (=middle of C), this second part to correct for the distance to the centre of the C in y direction.
Ixxflaps=2*(1/12*12mm*2mm^3 + 12*2*51)=2*(3888+1224)=10.224mm^4

So according to this calculation the flaps only ad 1/17 to 1/18 which is about 6% to the bending strength of the P.O. reinforcement, this is why I said the flaps hardly add strength.

edit: this purely based on bending around the x-axis, when considering bending around teh y axis/buckling than the flaps obviously add a lot because for that condition the flaps should be calculated like the main leg. But as said I think that I good chassis and good welding job should be adecuate (?spelling) for that.

edit2:
For everybody who doesn't understand this lot, I understand, but have a practical trick to explane.
Take a magazine or such and hold it flat in front of you and try to bend it.
It bends easily.
Now hold it straight up try to bend it again in the same direction as previously.
Now it is much harder to bend (and will fold outwards).

In the same manner the flaps on the C don't add much to the bending stifnnes (as far as I'm aware and can see).
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dyanut · Posted: Sat Feb 07, 2009 11:56 pm Post subject:

Hi Bartje,
I think there's just one difference between our calculations, since my understanding is that the correction for the offset of the flanges from the neutral axis of the complete section is accomplished using a factor of (offset distance)^2.
That would seem to be correct, since I is always expressed as mm^4...

If that is indeed the case, then this calculation...
Ixxflaps=2*(1/12*12mm*2mm^3 + 12*2*51)=2*(3888+1224) =10.224mm^4
would become like this...
Ixxflaps=2*(1/12*12mm*2mm^3 + 12*2*51*51) = 2*(3888+62.424)
= 132.624mm^4

Apologies, but when I started studying structural engineering, the university's computers were still operated using stacks of punched cards.
I've had to look around in the attic to find my old textbooks and refresh my rusty memory. Sad

Ken.

Bart · Posted: Sun Feb 08, 2009 12:29 am Post subject:

Oeps.

.. you are right!!! I had to look in my book as well, as I didn't do any of this kind of calculation for about 6 years and I missed the square....
The worst bit is that I should have realised that 3 times a mm dimension is mm^3 rather than mm^4....

So main thing is, get the flaps cause they help!!!

p.s. luckilly I don't have to bring any of my degrees back, and please don't tell any of the companies where I'm trying to get a job at the mo Laughing

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