39 replies to this topic

[seymourglass]

Can anyone tell me specifically how a carbon fiber suspension arm is molded?

Is it manufactured in halves and then structurally glued together?
Is there some sort of complex bladder molding procedure?
Is there some sort of lost mold technique?

[JwS]

I am going from memory of threads here, but I believe that there is a very light metal tube (probably Al or similar, but maybe Ti if it has a structural role) which is then wrapped with the various layers of CF.
JwS

[seymourglass]

How do they achieve such complex shapes? The upper arm with the steering link for instance is quite 3 dimensionally complex not suitable for an extrusion.

Also, does this mean the arm is not fully composite, that the aluminum or metal is left in? Or is it possibly dissolved out?

[Greg Locock]

From the surface finish I'd guess they are made in female molds. I'd guess they use a foam or nomex core and then wrap it in prepreg. I don't think you could glue two halves together efficiently, maybe it is possible with some sort of overlap joint.

Our solar car chassis was made up by vac bagging prepreg and uniaxial over a foam core. It is fairly light. The trouble is you get a disgusting surface finish unless you apply a thick gelcoat and sand it back.

[desmo]

Would a "disgusting surface finish" be any real disadvantage on a racing car? I wouldn't assume there'd be any real aero ramifications and a thick gelcoat would likely be useless mass in a fairly critical area.

[Greg Locock]

No it wouldn't matter, but those Ferrari closeups show that the finish was excellent.

[scarbs]

From (broken) wishbones I've seen they are hollow with no foam or metal inner, I can also confirm that they are made in female moulds. It's often puzzled me how they make apparently one piece parts from these moulds.

[xflow7]

Perhaps they have some method where the lay-up is done over a form of foam or similar but cured in a female mold, and then after curing the form material is melted out leaving the hollow shell?

[MclarenF1]

Possibly RTM (resin transfer molding) where you have have a two part enclosed female mold to layup the carbon, insert a male "plug", close the mold then you "inject" resin into the mold. It creates very nice looking, continuous parts, no secondary bonding required.
http://www.psrc.usm....process/rtm.htm

[J. Edlund]

I think the suspension arms on the older F1 cars was manufactured by using CFRP on a metal inner shell. Typically titanium is used with CFRP, they have similar heat expansion and the titanium doesn't corrode (like Al does when bonded to carbon fibre).

[JwS]

Well I'm glad I wasn't totally off base then!
What about a plastic shell? maybe it is not obvious on a broken part? Of course the classic is a foam core that is melted out later.
JwS

[Ben]

I saw some tooling for a BAR wishbone earlier this year. They machine one half into a block of aluminium then machine an exact replica of the wishbone itself that sits in the femal half. They can then mould the other half of the mould in carbon. They may then mould the other half in carbon as well but I did't see this.

Also about three years ago I saw a Jaguar Racing wishbone and it appeared to be made from a spar made from wrapped or filament wound carbon and the actual outer surface made from two separate pieces bonded around the spar.

Ben

[dosco]

Originally posted by xflow7
Perhaps they have some method where the lay-up is done over a form of foam or similar but cured in a female mold, and then after curing the form material is melted out leaving the hollow shell?

If they use this method, the "lost salt" method is generally employed, wherein a salt mold is made - and then washed out with water later.

Ben's observation is probably the most current method - male and female metallic tooling, CF squeezed in between. Each shell is then bonded later (like a plastic model airplane). Much like the chassis tubs are glued together.

[LMP900]

"Perhaps they have some method where the lay-up is done over a form of foam or similar but cured in a female mold, and then after curing the form material is melted out leaving the hollow shell?"

Put a tailored vacuum bag over the foam mandrel you describe, lay up over that, pull a vacuum between the bag and the female mould and you have a process that has been used for enclosed components like wishbones.

No idea who uses this process now. I'd be interested to find out.

[Semtex]

Carbon Wishbones are made thus:

First you need an Alu Female Mould. Which is prep'd/polished so no particles are on the moulding surface. THen you lay the carbon cut outs into the mould via a lay-up diagram, which is supplied by the Design Studio. This lay-up diagram shows the orientation of each ply of carbon. Either 90deg etc. These orientation has been worked out by the stress anyalists.

Once the many plys of carbon have been laid. the mould is then lined and Bagged. The air is sucked out and thermocouples are attached. The it placed in the auotclave for cooking.

Once it has cooked, the next day it is broken out.

It is moulded in two parts, the days of internal alu wishbones are long gone.

The end fittings are machine 4/5th axis from solid Ti.

Once all parts are available, then the two halves are joined using expoxy glue on a designed jig.
The end fittings/Brake lines/sensor electrics are all placed in position during the jig stage.

THe appearence of the outer skin, comes from the polished service of the mould.

The wishbone once built then goes into R&D for testing to various programmes.

I think that explains it.

[dosco]

Originally posted by Semtex
First you need an Alu Female Mould.

I thought Invar was used as the mold material, since it has the same thermal expansion properties as CF?

No?

[Semtex]

Invar ????

The richer teams use a Graphite Pattern, to produce a CFRP Mould from.

[dosco]

Originally posted by Semtex
Invar ????

The richer teams use a Graphite Pattern, to produce a CFRP Mould from.

Yes, Invar.

It is my understanding that Invar is a nickel alloy that has the same thermal expansion characteristics as CF. Thus when using it as a mold material in an autoclave, it expands with the CF rather than having differential thermal expansion......

[LMP900]

Originally posted by Semtex
Invar ????

The richer teams use a Graphite Pattern, to produce a CFRP Mould from.

Graphite pattern?

Please explain. Graphite in what form?

[Melbourne Park]

Why would one join a structural part? ie make it in halves? The join would be weak. If the join was fibre re-inforced, it would be heavier than a one piece structure.

[Semtex]

In my experience the graphite pattern was made from Graphite IP13, it was always a solid Graphite Pattern.

Invar..... Well I have come across it and I cannot comment about the technical charteristics of the material.

Joining is quite simple. Using expoy glue (High Temp) the halves are butted together, just like an Airfix part, except there are no locating holes. The strength is calculated in through the weave orientation and the glue.

[dosco]

Originally posted by Semtex
Invar..... Well I have come across it and I cannot comment about the technical charteristics of the material.

I'm not saying Invar as some "piece that goes inside the arm." I'm saying Invar makes up the female mold. Instead of Aluminum.

[dosco]

Originally posted by Melbourne Park
Why would one join a structural part? ie make it in halves? The join would be weak. If the join was fibre re-inforced, it would be heavier than a one piece structure.

No.

The high-strength epoxies that are used are rather....um, strong.

This is exactly how the "tub" is made, glued up halves. Like a plastic model airplane.

[Semtex]

Dosco.... You may be right, I should have said I have never come across it though.

[MclarenF1]

In Secondary bonds, you typically design such that if there is a failure, it is on the substrate. Very rarely do you see the actual bond fail. Even with a questionable bond prep, the adhesive is stronger then the interlaminar shear strength of the laminate.

[Melbourne Park]

Originally posted by dosco
No.

The high-strength epoxies that are used are rather....um, strong.

This is exactly how the "tub" is made, glued up halves. Like a plastic model airplane.

So I presume the stresses transfer across the glue to the carbon fibre ... the cars would not be as strong if made from glue.

[Semtex]

Spot on MclarenF1.

Most Wishbone failures are in fact shearing of the end fittings from the laminate, unless there is a restraint screw/lock in place.

The best example I can think of is Ricardo Zonta at Silverstone Test, when he vaulted the perimeter fence, all do to a failure of the bond between the end fitting and the Wishbone.

[red300zx99]

So how often is a screw/lock in place?

[Semtex]

In todays F1, they are I believe always in place, to stop the end fitting from shearing away from the laminate under dive-plane effect.

It used to be that, they where not used at all, but the understanding of CFRP Wishbone Technology has come on leaps and bounds over the last 4 years.

[dosco]

Originally posted by Melbourne Park
So I presume the stresses transfer across the glue to the carbon fibre

Yes, but the idea is the make each half very precisely, so the actual interface between parts is very very small.

It is a bad idea to have glue "fill in " large gaps between parts.

... the cars would not be as strong if made from glue.

Not sure what you mean by this. To a certain extent, the cars are made of glue in that the glue is what holds all the CF cloth together.

AS I said, though, the idea is to keep the gaps between parts (which includes laminate layers) very small.

[desmo]

http://www.tc.faa.go...pt/ar95-109.pdf

This paper, that MacF1 referred me to, from the Tech Paper PDFs "bookshelf" might be of interest. Pretty "techy" though!

[MclarenF1]

Originally posted by dosco


Yes, but the idea is the make each half very precisely, so the actual interface between parts is very very small.

It is a bad idea to have glue "fill in " large gaps between parts.



Not sure what you mean by this. To a certain extent, the cars are made of glue in that the glue is what holds all the CF cloth together.

AS I said, though, the idea is to keep the gaps between parts (which includes laminate layers) very small.

Just to expand on this a bit further.
In a lap joint there will always be induced peel stresses in the bond. This is due to the eccentricity of the loading (i.e. single shear vs. double shear). Therefore, the failures in a lap shear test are caused primarily by this peel mode which is why it is so important to keep bond gaps as small as possible.
Calculation of these peel stresses is a very complicated subject worthy of a PhD. Several attempts have been made but none (that I have found) match well to test data. There are so many variables which will effect the strength of a bond. One of the biggest influences is the stiffness of the laminate. By having a laminate that is stiffer, the peel stress increases. Which is one reason why in ideal world, all bonded joints would be tapered off (scarfed) at the edges.

[Greg Locock]

Has anybody got photographic evidence of these screws? The reason I ask is that the aerospace lads always seem to try and combine fasteners with composite layups, and they are typically the point of failure, since it is very difficult to transfer the load from a single fastener into the layup.

15 years ago the automotive boys followed aerospace practice, but fortunately having more sense and smaller budgets, they developed the techniques we see today, with large tapered shear plates bonded into the layup, and with no fasteners through the weave.

The aerospace people of course, continue to know better.

[MclarenF1]

The "chicken" fasteners you are referring to are essentially peel stoppers. This goes back to what I was saying about peel being the primary mode of failure. In practice, tapered joints are very difficult to manufacture and control. But are obviously the best method of bonding lap joints. But in complex load situations (pressurization of a fuselage combined with bending for example) even scarfing is not guarantee of a structurally sound joint. And since fatigue of composites is not much of a concern (a topic for another day) fasteners are an easy, effective way of preventing a bond from unzipping.

And I take offence to your insinuation that aerospace companies have more money then common sense....you obviously have not worked for a start-up aircraft company.

[Melbourne Park]

Originally posted by MclarenF1
The "chicken" fasteners you are referring to are essentially peel stoppers. This goes back to what I was saying about peel being the primary mode of failure. In practice, tapered joints are very difficult to manufacture and control. But are obviously the best method of bonding lap joints. But in complex load situations (pressurization of a fuselage combined with bending for example) even scarfing is not guarantee of a structurally sound joint. And since fatigue of composites is not much of a concern (a topic for another day) fasteners are an easy, effective way of preventing a bond from unzipping.

And I take offence to your insinuation that aerospace companies have more money then common sense....you obviously have not worked for a start-up aircraft company.

My conclusion from this and the other great comments is that it would be better to make a suspension arm in a single piece: considering the cost of F1 equipment, I am surprised they do not.

Re aircraft and composites, I went through Hawker de-Havilland's plant in Sydney almost 10 years ago. They had acres of parquetry flooring, to prevent alloy being dinted. The autoclaves there were huge there: they had a contract with Boeing for making wings etc., they were one pice I recall. They tested their finished surface by streaming water over them.

[dosco]

Originally posted by Greg Locock
Has anybody got photographic evidence of these screws? The reason I ask is that the aerospace lads always seem to try and combine fasteners with composite layups, and they are typically the point of failure, since it is very difficult to transfer the load from a single fastener into the layup.

15 years ago the automotive boys followed aerospace practice, but fortunately having more sense and smaller budgets, they developed the techniques we see today, with large tapered shear plates bonded into the layup, and with no fasteners through the weave.

The aerospace people of course, continue to know better.

LOL.

Don't know about fasteners in F1, but I did interview with an aerospace company back in 1997 who patented a new method for integrating fasteners with composite layup. They were a startup firm whose goal was to fabricate a 4 seat light GA aircraft (very much like a Cessna 172) but for a sale price of $80,000 (new Cessna 172 goes for like $160,000).

Anyways, they produced a center wing box and had it tested to failure. No fasteners pulled through the layup (which was fiberglass). Apparently the FAA guys were very happy with the result.

The company folded in 2002 (IIRC) b/c they never found a place to start a factory. Too bad.

[dosco]

Originally posted by Melbourne Park
My conclusion from this and the other great comments is that it would be better to make a suspension arm in a single piece: considering the cost of F1 equipment, I am surprised they do not.

Well, the F1 guys are looking for optimal fiber compaction...a problem with 1 piece construction. With 2 piece they get the high compaction, but the "price" is a glue joint. I don't think this is necessarily a problem in that F1 tubs are multi-piece units that are glued together.

[Engineguy]

Since each leg of an A-arm functions more or less in pure tension and pure compression, I don’t think the bond line sees great loads. The upper half helps prevent the lower half from buckling and the lower half helps prevent the upper half from buckling. I would think there would be little in the way peel stresses… for the leg to flex (considering vertically only; the horizontal cross-section is much greater), the entire bond line has to slide axially (shear)… and the bond lines are very long in that direction.

I never liked composite suspension arms... a ductile material makes more sense.

[dosco]

Originally posted by Engineguy
I never liked composite suspension arms... a ductile material makes more sense.

Well, considering on an F1 car they are the first things to break in a crash, the problem I have is all the razor-sharp CF shrapnel that is generated. If there were a way to mitigate that then I think I'd have less of a problem.

[Melbourne Park]

Originally posted by dosco


LOL.

Don't know about fasteners in F1, but I did interview with an aerospace company back in 1997 who patented a new method for integrating fasteners with composite layup. They were a startup firm whose goal was to fabricate a 4 seat light GA aircraft (very much like a Cessna 172) but for a sale price of $80,000 (new Cessna 172 goes for like $160,000).

Anyways, they produced a center wing box and had it tested to failure. No fasteners pulled through the layup (which was fiberglass). Apparently the FAA guys were very happy with the result.

The company folded in 2002 (IIRC) b/c they never found a place to start a factory. Too bad.

I consulted (once) to Eagle Aircraft in Australia - they had a two place with plans for a 4 place, composite plane; they spent too much on their factory, not enough on getting into production, and Malaysian equity took it over, and moved the plant to Malaysia. Originally targetting in 1987 a price of around $US30,000, the two place now sells for about $US120,000, with full instrumentation. It was a John Ranche (I have forgotten how to spell his name) designed canard front engine. That company would likely be well able to build that plane in their plant in Malaysia. I don't think they build a four place. The old owners have a quick cure process for composites which they are trying to float ...