Carbon Fibre pushrod, a closer look

[xpensive]

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 ...

I'm not so sure if I get the numbers for carbon fiber in that table, are those for the fibers alone without the matrix?

[marcush.]

it should not matter -as the matrix is at least in theory -not there for carrying the loads.It´s just there to support the single fibres.
Any excess resin is just obsolete weight.One reason why wet layups of Carbon are never ever useful apart from looks..

[xpensive]

I beg to differ, the values in the table are given in Pascals, which is N/m^2, while the matrix of course also has an area?

Anyway, I guess the reason for the bother with carbon fiber suspension parts with molded metallic inserts is weight?

[marcush.]

I beg to differ, the values in the table are given in Pascals, which is N/m^2, while the matrix of course also has an area?

Anyway, I guess the reason for the bother with carbon fiber suspension parts with molded metallic inserts is weight?

I´d think the numbers are derived from the number of fibres and do not take into account the resin .But I couöld be wrong...
The company claims to use only prepreg material for their work so no excess resin to take into consideration.

[riff_raff]

it should not matter -as the matrix is at least in theory -not there for carrying the loads.It´s just there to support the single fibres.
Any excess resin is just obsolete weight.One reason why wet layups of Carbon are never ever useful apart from looks..

marcush-

Maybe it's just semantics, but the resin matrix does more than simply "support" the fibers. In a laminate the resin also transfers shear forces between plies, which is a critical function of the laminate's load capacity.

Regards,
riff_raff

[thisisatest]

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..?

ti bicycle frames are in the neighborhood of 1300g for the lighter ones. a light aluminum frame weighs about 1150g. a carbon frame has to weigh under 1kg to even be considered light. 800g frames that do everything right can be bought at your local bike shop. sub-5kg bikes are attainable.
http://ax-lightness.de/en/cycling/bike-components/
http://www.cannondale.com/2012/bikes/ro ... o-ultimate

[flyboy2160]

...One reason why wet layups of Carbon are never ever useful apart from looks..

somebody else covered the shear carrying function of the matrix. the quote above isn't correct. wet layups can be only slighly heavier in resin if the fabric and resin are weighed. and they can be very useful - the early and mid career rutan designs were wet layup. (i don't know what Scaled does now.)

carbon fiber polymer matrix composites show the most advantage over aluminum when the loads are in one direction so that the layup can be something like 70/15/15 or 80/10/10 0/90/+-45, thus giving a load direction modulus much higher than aluminum even given the modulus knockdown for the off-angle fibers and for the matrix (the charts cited above are for the fiber alone.)

for a driveshaft dominated by torsion, the layup would be mostly 45s.

if the pushrods loads are primarily compression, then the layup could be predominantly 0 degree uni as above. since the buckling depends on the modulus of the material, i suspect that the buckling stiffness is more important an advantage for the pushrods than the weight savings.

for the ultimate compression resistance, those pushrods should use the pultruded rods with slightly pretensioned fibers pioneered by Graphlite (but now made by others also) for the 0 uni direction.

[xpensive]

Anyway, if I was asked to design an F1 car, something which obviously never will happen, without knowing the competition any more than from a picture, I would have gone with an Aluminium pushrod instead of tinkering with carbon fiber and inserts.

But that's me of course.

[hardingfv32]

instead of tinkering with carbon fiber and inserts.

The pushrod I have only has inserts on the end with the strain gauge and shim adjustment assembly.

Brian

[humble sabot]

It's interesting you guys are suggesting aluminium. Back in the pre carbon suspension days, and in Indycar, 4130 or similar steel alloy tubes are used. Aerodynamic cross section and a couple welds. It would seem a false compromise going for alu. The inserts on carbon links aren't made of such a soft material for the most part, with only a couple cm of suspension travel you can't really afford to have your links wear and develop play. There are harder alu alloys though like 7068, which is comprable in hardness to less fancy steels but you'd have to figure a way to build such a long member without a weld in a light fashion.
I wonder if f1 suspension relies at all on the self damping properties of a composite structure? Probably not.

I do wonder a little why they use the carbon, your frontal area penalty is not insignificant. But then again it seems there's no limit to how deep they're willing to go with the chord and thickness so maybe the net aero penalty is not so great.

As far as the bike thing goes, at the bleeding edge you can find sub 700g carbon frames, and even sub-kilo steel frames. The limiting factor in a bicycle is not the frame, more the wheels.

[xpensive]

Naah, I don't think that anyone considered ball-joints from aluminium, more like steel joints press fit in an Alu-rod.

Anyway, when comparing different metals for stiffness-dependent applications such as a a pushrod,
taking Young's modulus over Density is usually a good stiffness-to-mass index;

Steel: 2.1 E11 / 7800 = 27 E6

Aluminium: 0.7 E11 / 2700 = 26 E6

Titanium: 1.1 E11 / 4500 = 24.5 E6

Look at that, almost the same index, you didn't xpect that now, did you fellow members?

[marcush.]

It's interesting you guys are suggesting aluminium. Back in the pre carbon suspension days, and in Indycar, 4130 or similar steel alloy tubes are used. Aerodynamic cross section and a couple welds. It would seem a false compromise going for alu. The inserts on carbon links aren't made of such a soft material for the most part, with only a couple cm of suspension travel you can't really afford to have your links wear and develop play. There are harder alu alloys though like 7068, which is comprable in hardness to less fancy steels but you'd have to figure a way to build such a long member without a weld in a light fashion.
I wonder if f1 suspension relies at all on the self damping properties of a composite structure? Probably not.

I do wonder a little why they use the carbon, your frontal area penalty is not insignificant. But then again it seems there's no limit to how deep they're willing to go with the chord and thickness so maybe the net aero penalty is not so great.

As far as the bike thing goes, at the bleeding edge you can find sub 700g carbon frames, and even sub-kilo steel frames. The limiting factor in a bicycle is not the frame, more the wheels.

hm I was sure the limiting factor was the (human )spacer...

[humble sabot]

Well stated! You don't go faster without improving the motor! hahah

Considering that in non carbon but still racing applications it's always fabricated steel and a good weld is much more reliable than a pressfit... I wasn't suggesting we were talking about making the actual bearing joints out of alu, more that there isn't all that much point mixing materials since you almost invariably end up with a less efficient structure. Which is why after this dissection and thought exercise i'm more puzzled than before.
I've had my hand in on the deep end of designing bicycle frames and the application is pretty comparable to a control arm. You want to balance stiffness across the spans with toughness and precision at the bearing locations, and once you go through the permutations the advantage of one material over the other is more practical than performance related. The crappy thing is that all the disadvantages of relatively soft aluminium threads, bearing seats and fork ends are seen in carbon applications. And the one you can safely ignore in an f1 car - prolonged use leading to galvanic corrosion of the aluminium, moves the material somewhat down on the shortlist in my book.

[xpensive]

I don't think welded 4130 steel tubes, 25 CrMo 4 in my book, for suspension parts have been seen in F1 since the 70s, why I believe that machined Alu or Ti was the way to go before the carbon age. I can remember Gordon Murray already in the seventies xpressing his dislike for all sorts of welded components and John Barnard would probably have had a heart failure at the proposition.

[Richard]

Surely the consideration is based on a combination of least weight and shallowest profile? A machined alloy might be easier but not if it results in a bigger cross section in the air flow? I'm assuming the carbon construction allows a more slender profile than a machined alloy.

x - I like the E over density data, it does infer some fundamental constant in life about stiffness. It often confuses young engineers who think stronger materials is always the answer for deflection critical applications (inc buckling), as opposed to stress critical. You end up adding a lot of material to combat geometric problems and slenderness.

ps - we also need to consider the anisotropic nature of CF, the strength or stiffness is in placed in the desired direction so you utilise every last bit of it unlike isotropic materials.