Carbon Fibre Space Frame !

[Chairman Roger]

Cranfield University’s composites manufacturing research centre and Lotus Engineering are developing a low cost passenger car based on carbon fibre composites. This is part of a government transport programme named Foresight.

 

The difference with this project is that the car will be based round a space frame with panels attached rather than the monocoque approach of current carbon fibre cars.

 

Now we know a car made on the space frame principle don’t we ?

 

The technology to produce the frame is being developed through an Industry, Engineering and Physical Sciences Research Council funded project named Fast -Frames.

 

It was announced in the February trade journal Reinforced Plastics that Caterham Cars and Ford have now joined as project partners.

 

Food for thought .....

 

Roger

[Jonathan Ratty]

A plastic Seven? Won't they have to code-name it Project W*******d?

 

 

Jonathan

[Mike Bees]

Copying a 7-style spaceframe chassis with commercially-available carbon fibre tube wouldn't be that hard (the suspension attachment points would be a lot harder to do in fibre than in metal).

 

Usually, copying a metal structure with a similar cf structure isn't the best way to make use of the material. The frame would be considerably lighter, although my fully panelled & painted chassis complete with windscreen, wiper gear, pedals, brake pipes & master cylinder, wiring loom, dashboard, switches, dials, battery, ECU, charcoal cannister etc. only weighed 119kgs - so as a proportion of the all-up weight the bare chassis frame is pretty small.

 

Still sounds like an interesting project though!

 

Mike

[Chairman Roger]

The photo that accompanied the article showed an unclothed mini mpv type vehicle. The frame is not carbon fibre looking in the normal sense and is apparently made up of an array of foam cores with a carbon fibre sleeve round each such assemblies then being infused with epoxy. It actualy looks more like a Morgan Ash frame assembly !

 

The engine is to be a Honda 660cc petrol engine.

 

I assume that this is just a "project" and will not be offered for sale.

 

One of the advantages offered is that it is easier to put doors and windows into a space frame than a monocoque

[Number Six]

"This is part of a government transport programme named Foresight."

 

Foreskin would be more appropriate for most Gubbmint projects or anything handled by 2-jags ...

 

It sounds like a good idea but I don't know much about CF - is it recycleable, where does it come from (petroleum products?), does it cost more in "green" energy terms to produce than steel or ali?

 

And Mike is right, the current chassis is incredibly light especially if you go the JPE route and eliminate the wussy things - heater, screen, wipers+motor, catalyst (which is NOT energy efficient) etc.

 

A Caterham CF chassis would bring big benefits in terms of chassis stiffness and weight reduction so I'd be tempted depending on price and answers to the above.

 

 

 

Andy

[R2D2]

The Young's Modulus of epoxy based carbon fibre (approx. 150GPa) is lower than mild steel (Typically 207GPa) so if a Seven chassis was reproduced using filament wound tubes of identical sizes the mild steel chassis would be about 30% stiffer. The carbon fibre cassis would be lighter but the overall weight change would be small.

 

The Young's Modulus will vary depending on the volume fraction of carbon fibre and fibre orientation and lay up.

 

The tensile strength of epoxy/carbon is very high ( approx 1800MPa) compared to mild steel (300MPa) so structures are very strong. I think on potential problem is that carbon fibre does fail in a brittle-elastic manner and it, therfore, absorbs virtually no energy in the process. If you do crash a chassis made of carbon fibre when it does fail all of the energy stored during the accident will be released with fairly catastrophic results. I guess this could be more important for road cars that may be run over by trucks.

 

 

 

If the wall thckness of the carbon fibre tubes were inceased so that the overall weight was the same as the mild steel unit, the chassis may be stiffer but not by a fantastic amount.

 

If the tube diameters were increased or the sections changed then the stiffness for a given weight could be changed significantly.

 

Obviously an F1 tub has extremely high stiffness and strength but these are produced in fighter aircraft quality materials and use every sophisticated autoclaves to obtain these results.

 

I would worry about low volume cost concious manufacture using sophisticated materials as batch variation could be enormous.

 

I imagine that maintenance/repair costs could be very high, particularly accident damage.

 

I guess it will make a great science project but I don't think that it would be too practical.

 

I think that one of the great things about Sevens has been their "survivability". You only have to look on John Watson's web site to see how many Series 1 & 2 cars are still around and more are still turning up.

 

In general the more exotic basic construction materials become the more limited will be the market and the worse the re-sale value (2 year old Audi A8's for less than £20K with a new list price of about £47k) and I like the Seven's ability to hold value.

 

I think its a great science project but I'm not sure if it is a great idea commercially.

 

[EFA]

Just wait 'till you prang it....... It'll cost a fortune to fix - A new Seven chassis costs around £1500 from Arch if you have a wrecked one!!

 

Arnie Webb

The Fat Bloke blush.gif in a Slow Old Vauxhall wink.gif

[Number Six]

I've often wondered why Arch never tried Titanium tubing... not mega expensive these days, Ti is difficult to weld but not insurmountable and it has many advantages in terms of strength, lightness, stiffness, flex, no rust, not sure how it would bond with the ali skin re: ali corrosion.

 

Andy

[PACR]

Why not try environmentally friendly 'Carbon Fibre' like Marcos and Morgan? A real challenge would be designing a wooden 'Seven type' car - a strong central tub, engine / front suspension carrier and de Dion rear end off the back of the tub.

 

Woodworm might be a problem though.....

 

Piers

[Number Six]

Balsa wood is light!

 

I think it's all pointing to the fact that Chapman had it pretty right from day one!!!

 

Andy

[Red SLR]

LOL!

 

X777CAT

[Mike Bees]

Chris - you can get carbon in a variety of moduli - the higher the modulus the higher the cost (it used to be 2 x modulus == 4 x cost - don't know if it still is).

 

Mike

[R2D2]

DELIVERY: 15 days from receipt of order

 

VALIDITY: 90 days from the date of this

Quotation

 

PAYMENT: 30 days from delivery, net monthly

 

 

 

 

WARRANTY: 12 months from installation or 15

months from despatch, whichever is

soonest

 

SHIPPING: Delivery to Alstom Power Rugby is

included.

 

VAT: VAT is not included in these prices

and will be added to invoices at the

relevant ruling rate.

 

For Torquemeters Limited

Andy,

 

The Young's Modulus of Titanium is much lower than that of mild steel and is in fact lower than a high grade epoxy carbon fibre composite. If a spacframe were made in Titanium using the same size tubes as the stndard Caterham it would be quite "bendy".

Titanium is quite strong depending on the alloy being used so it does have a very high resiliance.

 

I think that material properties are usually not well explained and the

modulus/density ratio is quite often quoted. Titanium is also quite good in this respect.

 

I tend to agree with Mike that changing materials and keeping designs identical is not a great idea, you really need to design for specific material properties and behaviour.

 

think that a well designed mild steel tubular spaceframe with brazed/nickel bronze welded joints takes a graet deal of beating from a cost, weight, stiffness and ease of repair point of view.

 

Mike,

I don't know the current cost of carbon fibre composites as I only have experience of fighter aircraft materials which are completely insane and I imagine unaffordable.

I am not surprised that cost is the square of modulus.

 

 

 

 

 

 

 

 

 

 

 

 

 

[R2D2]

Sorry about the "cock up" at the start of the last post. Copied something from my clipboard by mistake!

[Number Six]

Thanks Chris,

 

"If a spacframe were made in Titanium using the same size tubes as the stndard Caterham it would be quite "bendy"."

 

My only experience is with high-end mountain bikes. My all aluminium framed bike was *much* stiffer (no flex - but needed enormous oval tubes to maintain the equivalent strength of say a steel frame) than my current Ti bike which is like a Rolls Royce in comfort terms as the frame, as you pointed out, bends (and is designed to. It's a hard-tail, no rear suspension other than fram "give".) Using oval Ti tubes of larger diam. might overcome the bendiness, but then why bother? I was assuming the cross-bracing of the Arch frame would compensate for "bendiness" or "flex". Thanks for the info!

 

 

Andy

[R2D2]

 

I have been "digging" in my memory and the modulus/density ratio of steel, aluminium and titanium are all about identical but the strength greatly influences the resiliance of the structure. Aluminium isn't too strong so you can't let it flex too much or it will yield and the structure will cripple, I think this is where titanium is very good because it can flex elastically.

 

I think, and I hesitate to do the sums because of the work, that for an identical structure,even when triangulated, the deflections would be double for titanium compared to steel. (E for Titanium is typically 105GPa)

 

If I am right this would mean that the torsional rigidity of the chassis would be 50%. Clearly the weight of the chassis would be 50% as the density of titanium is about 50% that of steel and I am sure that most of the rigidity could be recovered by adding triangulation and still have a net weight saving.