Carbon Fibre pushrod, a closer look

[xpensive]

- Round that off to 600 sq.mm area and estimate the force to 4000 N/sin 30, that's just a 13 Mpa compression stress.

- When Aluminium has a modulus of 70 GPa, that's a compression of only 0.10 mm for a 500 mm rod, is that significant?

- A 500 mm Aluminium rod with a 600 sq.mm cross-section would weigh 800 g, how much do you save with carbon fiber?

- As for the buckling force, you need to know the Area moment of inertia, which I have no means of estimating,
but a 55*15 mm rectangular profile, 500 mm long and free at both ends, would take 39 000 N to buckle.

Conclusively: What do you really gain with the carbon fiber rod, stiffness, weight or buckling-strength?

[marcush.]

- Round that off to 600 sq.mm area and estimate the force to 4000 N/sin 30, that's just a 13 Mpa compression stress.

- When Aluminium has a modulus of 70 GPa, that's a compression of only 0.10 mm for a 500 mm rod, is that significant?

- A 500 mm Aluminium rod with a 600 sq.mm cross-section would weigh 800 g, how much do you save with carbon fiber?

- As for the buckling force, you need to know the Area moment of inertia, which I have no means of estimating,
but a 55*15 mm rectangular profile, 500 mm long and free at both ends, would take 39 000 N to buckle.

Conclusively: What do you really gain with the carbon fiber rod, stiffness, weight or buckling-strength?

the word back then was 4-5 kg in terms of weight savings per car...BUT I have never seen much activity in terms of confication of tubes etc to optimise weight per stiffness /buckling strength ..so I´d think this is a rather otimistic view .

In bicycle frames ,I´m not sure if cf frames are already significantly lighter than the very best fabricated ones made from Ti or steel..?

We have yet to see the first pushrod to show any sign of design to buckling strength -someone would expect more crosssection in the middle of the rod - and as we have seen they also do not work with material thickness change to account for this.....If weight is critical you would optimise something like that ,don´t you?

[xpensive]

The only area where I can see a gain with carbon fiber vs Alu in the push-rod is on weight, but if an Alu-rod is 800 g, the total savings can never be more than 3.2 kg, right?

But it is indeed unsprung mass, why I guess everything counts, or is it just a simple case of follow-the-leader, even with the manufacturing complications we've seen above?

[marcush.]

weight savings was for a complete car set ,that´s:

8 wishbones
4Pushrods
4 trackrods

so 312grammes per suspension member roughly-coming back to your 800grammes starting point it´s a significant gain but in absolute numbers ?

as for the claim unsprung weight...not really as any suspension link is sprung weight as well so it´s not so clearcut in terms of how much these 5 kg would be able to help unsprung .

[xpensive]

Right marcush, a 300 g saving, 40%, on the pushrod when going from solid Alu to hollow carbon fiber sounds reasonable.

But if those 300 g is the only benefit with carbon fiber, is the bother of the complicated production above really worth it,
I mean the Aluminium-rod could be machined in one piece from 6063-T6, 170 MPa yield, and anodized any color you like?

I don't really get it.

[mep]

Well if you have in mind that carbon fibre has a modulus of 170-700GPa and a density of 1.85 g/cm³ + very good resistance to corrosion and fatigue then it should be clearly the better material.

However in the may issue of racecar engineering there is article saying that Mclaren has build a full aluminium front wing in 2009!
Also they say that the current wing has a 2-3kg aluminium spar milled from one 160kg solid aluminium block.
Milling away 158kg of aluminium seems to be the absolute waste of material and time for me.

[marcush.]

Well if you have in mind that carbon fibre has a modulus of 170-700GPa and a density of 1.85 g/cm³ + very good resistance to corrosion and fatigue then it should be clearly the better material.

However in the may issue of racecar engineering there is article saying that Mclaren has build a full aluminium front wing in 2009!
Also they say that the current wing has a 2-3kg aluminium spar milled from one 160kg solid aluminium block.
Milling away 158kg of aluminium seems to be the absolute waste of material and time for me.

the question is could you get the same result with a casting? and how much would it cost?
Audis R18 front diffusser is an alumnium piece with some CF inserts -so this seems to be best practise ...
But sure one could make a mold and produce a cf spar weighing less ?

and with a high speed mill machining time is surprisingly short and the chips can be recycled ..

[xpensive]

My point is an Alu-pushrod would be machining-friendly, stiff enough with 70 GPa, buckle-safe up to some 30 kN and probably
only 300 g heavier than a carbon fiber one, where you have to consider matrix and fiber orientation to get the true stiffness.

[hardingfv32]

This data came from a solar car design report at Yale. How does this compare to the values you used for your calculations? It would seem that the numbers for a carbon structure might be hard to come-by.

Brian

Hexcel Carbon Fiber 8552 AS4 Prepreg
Quasi-Isotropic Tensile Strength: 107,000 psi (Ply Orientation: 25/50/25
0º Tensile Strength: 320,000 psi
0º Tensile Modulus: 20.5 msi
90º Tensile Strength: 11700 psi
90º Tensile Modulus: 1.39 msi
0º Compressive Strength: 222,000 psi
0º Compressive Modulus: 18.6 msi

6061-T6 Aluminum
Tensile Yield Strength: 39,900 psi
Tensile Ultimate Strength: 45,000 psi

7075-T6 Aluminum
Tensile Yield Strength: 67,000 psi
Tensile Ultimate Strength: 76,000 psi

[marcush.]

but what about compression ? I thoght carbon was not a good thing in terms of compression capabilities..

[xpensive]

but what about compression ? I thoght carbon was not a good thing in terms of compression capabilities..

Fagedid marcush, the above just some copy-and-past stuff, not even in SI-units, for a true comparison with an Aluminium rod, you need to know the fiber-orientation and matrix of the pushrod as completed.

[mep]

The article also says that the spar can be bend by hand so it must be quite fragile. I wonder if it is possible to have high cutting speeds for that. Aluminium tends flutter a lot on the milling machine. Anyways manufacturing costs are probably not a big concern for F1 teams.
But still a CF spar should be stiffer and lighter than a aluminium one. I am quite surprised that they use aluminium for the wing but it might be an answers for the flexible wing case as it is quite soft.
What do you think about a full aluminium wing?
That sounds just insane. Maybe it is just a mistake from the journalist.

A pushrod is very simple in terms of fibre orientation. UD (uni directional) fibres along its length. A weight reduction of 300g is quite significant and worth the effort.

[riff_raff]

This discussion of the relative merits of composite vs metal for pushrods is very interesting. The F1 pushrod example presents a unique case. While it is a very high performance component, it also has a very limited life requirement and is used in a closely controlled environment. So it can be designed using much smaller factors than one might use for other applications.

Carbon composites can give good compressive performance. The trick is keeping the carbon fibers properly aligned. Once the uni fibers lose alignment and experience micro-buckling, the only thing left to carry the load is the resin matrix itself. A careful layup and a structural shape that minimizes changes in orientation of the uni fibers helps greatly.

There are some applications where metals can still outperform composites. A good example is the fan blades of large turbofan engines. Rolls-Royce still uses titanium because they can make titanium blades thinner and more aerodynamically efficient than using composite.

Interesting topic.
riff_raff

[xpensive]

This discussion of the relative merits of composite vs metal for pushrods is very interesting.
...
There are some applications where metals can still outperform composites. A good example is the fan blades of large turbofan engines. Rolls-Royce still uses titanium because they can make titanium blades thinner and more aerodynamically efficient than using composite.

Interesting topic.
riff_raff

It is indeed, I spoke to a friend at Saab Aerospace, building the Gripen jet-fighters, when he explained that with modern
CNC-milling machinery, they often make more efficient components, weight vs strength/stiffness, from 7075-Alu than CFRP.

[marcush.]

http://carbonfibretubes.co.uk/technology.html

I´d say on this table you can quite easily see what you get using steel ,aluminium or CF ...