dozracing
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| posted on 10/12/03 at 10:13 PM |
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Carbon Chassis
Hi all,
On the yahoo list they have had a thread about carbon fibre for locost tpye car builds. One of the guys asked me about F1 monocoque construction.
Whilst not exactly relevant to the manufacture of a low cost carbon chassis, i thought it might be of some use to go through some of the techniques we
use when moulding carbon and maybe they will prove useful in some way.
If you want to post comments and replies, can you temporarily post them to a new topic, as i want to add other sections to this and i think it would
be easier to read if they all follow one another. I'll try and reply to any questions and comments in the other topic.
I'm not writing this as a this is the way you have to do composite work, but, more a simple description of how and why we do it like this in F1
and you can make your own judgements on how you can borrow these techniques for you own use.
I have been thinking about the best way to describe the construction of a F1 chassis and i think as its such a involved process i'll break it
down into various parts. If you dont get the next part in a couple of days or less then hassle me for it, i'm dreadful for remembering to do
things. Email me at darren@gtstuning.co.uk to ask questions or chase me for the next instalment. Check out the website www.gtstuning.co.uk for some
useful more Locost related stuff.
Firstly i think a description of what carbon fibre (US guys note the spelling!) means to an F1 engineer, as it means subtly different things to
different industries.
Carbon used in F1 is exclusively pre-preg. That means its PRE-imPREGnated with resin, so it comes as a sheet of tacky carbon material. This differs
from the normal proccess of wet layup that you associate with glass fibre moulding and cheap decorative carbon fibre, in that you dont start with a
dry material and apply wet resin to it with a bucket and brush, but, that someone has under very controlled conditions pre-applied the resin to the
carbon fibres, so that the amount and penetration of the resin is accurately controlled. This is important to get the required properties from the
material.
There are two basic types of carbon fibre materials. Cloth and UD. Cloth would be recognised by everyone, as its the one with the classis carbon look,
its basically a woven fabric of carbon fibres. UD stands for Uni-Directional, and all the individual fibres run in the same direction, so you dont get
the weave look you associate with carbon. They are used in different ways, but, basically you are always looking to put the load down the length of
the fibre, and so with UD you can get a very high concentration of working fibres in this load direction for a lot less bulk than with cloth. Cloth is
very good though at tying in UD and controlling loads that are multi-directional. As a rule fibres running in orientations off the load axis are
treated as only having 10% of their down axis properties. So you can see from this that the direction that you lay the fibres in a structure are
critical to making it work as you hope.
There are many different types of fibre, all having different strength/stiffness properties. The Ferrari and Prost chassis' were amde from a
majority of M46J fibre which is a high stiffness fibre to give torsional stiffness. If its a particularly high strength application we might use T800
fibre. To break down these codes simply the higher the T number the stronger it is and the higher the MJ number the stiffer it is. There is a trade
off, high stiffness fibres are not very strong and vice versa. The cheapest fibre type we use and is what F3 chassis are made from is T300 and its
priced at about £20 per metre sqaure.
Cloth is made from strands of carbon weaved together like another textile fabric. Each strand (called a tow) is made of thousands of individual carbon
filaments. It comes in rolls about 800mm wide and with in reason as long as you like, say 25m on average.
Cloth weaves are very complex and there are many of them to choose from. The most commonly used in F1, and again is the classis carbon look is 2x2
twill. This designation describes how the tows of carbon are weaved, basically your tow goes over two other tows, and then under two others. Your
fibres are laid at 90 degrees to one another. If you lay one set of fibres down the axis of a load, then you are said to have laid the cloth a 0/90
which describes that one set of fibres is 0 degrees off load axis and the other set are 90 degrees off axis. The opposite of this is +/- 45 where one
set of fibres run -45 degrees tot he load axis and the other set being 90 degrees to these must therefore be +45 degrees to the load axis.
As a guide line, most of the carbon cloths i use daily in F1 are around 0.23mm thick per sheet (called a ply). UD plies on the other hand are around
0.14mm thick. A monocoque wall usually consists of a skin of carbon of around 2mm thickness, then 12mm of honeycomb material, then another skin of 2mm
of carbon. This means that in each skin you have around 9 to 10 plies of carbon. When you lay a ply down in the mould you are looking to make sure
that you maintain the fibre directions as you butt various sections of cloth down and around forms in the mould. When you lay another ply ontop, you
are looking to stagger any joints so that you dont layup a weak spot. You would want to look to stagger joints as far apart as possible, but, on small
components 10-15mm is about as practical as you can get. Usually on a chassis you can stagger joints but big distances, as you have the room.
In the next mail i'll describe how the moulds are made and maybe get round to describing some of the features required for mounting things onto
the monocoque. In the last mail i'll describe the curing and assembly processes.
Kind regards,
Darren
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