Funkychicken

Some like Hopper have probably already seen this, but here's a review/update on the Lahar carbon DH for the non-Aussies on the forums.

Jason222

Maybe Carbon fibre is the future, but I sure hope the future bikes arn't as ugly as that one

dminor 

Alex Morgan of BCD has been doing such things for years. I will add that the future of DH is definitely gearboxes or at least multi-speed hubs. Gearboxes mean less unsprung weight, though.

WannaGetGood

Wouldn't that be weaker. Becuase of the frame style, and becuase it is made of carbon fiber?

dminor 

It's actually mega-strong. The trick is to get good bonding at the junctions and, as with fiberglas, get any metal pieces (pivots, etc.) solidly embedded. I've seen Alex's creations in person and they are indeed burly. We're not talking foo-foo weight-weenie CF here.

WannaGetGood

Ahh, I have a better idea of it now. Thanks.

myenzo

I just do not see that as working I mean if you were to hit a rock just right or something like that, bam your frame is a piece of junk. No doubt is is strong it's just carbon sort of breaks easily when something like that happens.

dminor 

You obviously do not understand laminated composites. Imagine fiberglas with a much tougher stranded fiber cloth even than glass.

Van

That rear swingarm/linkage unit is so thin looking! I wonder how durable it can really be? Plus, it's one size fits all, I can't imagine how that works.

EJ123

I saw that site a week ago, and that's cool they have a bike with two rear shocks. Ive always wondered if that was possible.

Falanx

You obviously do not understand the meaning of the word 'tough'. Glass is not tough. Not even 'toughened' glass. And carbon fibres have a crack propagation velocity three times that of dynamaite's compression wave.

A KIC value of under ten is by no stretch of the imagination 'tough'. By the commonly applied materials science standards, most aluminium alloys are not 'tough'.

Why carbon fibre composites, and indeed, all other fibre-reinforced-matrix work is due to another mechanism entirely. Friction.

What happens is that your strong fibre is not intended never to break. It is understood and expected to fracture. But it will never fracture along the line of the crack. For the same reason a screw thread always fractures three turns into the thread when fully tightened and overloaded, the fibre breaks embedded within the matrix. Then the friction of the 'sizing' layer against the fibre stops pull-out of the fibre, and bridges the crack, preventing or slowing its propagation. That's what gives you your illusion of toughness.

samster143

I like Metal!

dminor 

I love it when I get flamed by a materials scientist. Keeps me honest with my limited 'shade-tree' engineering knowledge . My bad choice of phrasing, Falanx. I might have said 'more robust' maybe?

I know what you're getting at about "tough" versus other kinds of strength - - for the same reason mild steel tubing is preferred over chromoly in race car roll cages.

Correct me if I'm wrong, but it's my understanding that one advantage of Cf over fiberglas is that the 'cloth' is composed of longer strands, hence less of a tendency for the fibers to separate under stress? Anyway, the material fascinates me, especially in its potential to go beyond weight-weenie appllications.

Flak

All i know about carbon fibre is that im investing some pennys in the industry.

Falanx

That's cool, I just like to pass the flame on

Robust, yes, is probably a better word. While, indeed, CF is actually tougher by conventional testing result, the kind of statistical scatter you get makes the argument very contentious. They're both essentially brittle, but E-glass, or indeed S-Glass, has a theoretically lower KIC (That's plane-strain fracture toughness. There are all kinds of fracture toughness measures, some more applicable to certain failure modes and service regimes than others).

And you, know, I've never understood that whole mild-steel over chromoly thing. Chromolys tend to be a lot tougher, in all tests than mild, except dead milds, and they are very low on the strength scale. You see, over in the EU, an alloy refered to as 'T5' which means absolutely nothing is the alloy of choice for roll cages. It's a chromoly with extra molybdenum and manganese. Sort of like 531 alloyed with straight 501. We don't have any issues with high-speed impacts. Low alloy steesl are all tough enough at ambient temperatures, and up to about 0.75 Mo, the more Molybdenum the better.

You want to look into CF? There are some spectacular materials out there, like carbon fibres bonded with more carbon. They pyrolize a pitch-like resin, in much the same way some carbon fibres are made themselves, once it has been pre-pregged into carbonfibre weave mat. Its amazingly robust at high temperatures, despite being made purely of carbon.

Just don't place too much faith in this fools errand that presenlty researched nanoscale materials are. It insults me that materials engineers payed much, much more than me at place like NASA scored so low in their exams that they are unaware of the concept of natural lattice defects....

I've just always been somewhat mistrustful of fibre-reinforced composites because of the very nature of their failure mechanism. You want them to break to be tough.... Even metal-matrix composites suffer from some dirty, underhand fracture mechanics....

Maelstrom 

Agreed on both counts. bCD is ahead of his time in regards to usage of carbon combined with, I hope to dear god in a non denominational way, the beauty of gearboxes.

Personally, still not a fan. Not sure why.

Serendipper 

Don't know about a full carbon fiber frame, but I have a carbon seatpost and bars on my rigid bike, and it takes shocks superbly, while putting up with all sorts of abuse.

I suppose you would call me a born-again CF convert.

I would compare the bash resistance of carbon fiber to bamboo, where it's strong because it flexes, but when the limit is reached, it will splinter.

As opposed to metal, which will just break, and transfers stress through it like a conduit.

This is just my unscientific opinion, of course.

free_pizza

I cant wait for Kuhon to see this thread

dminor 

Threads like these are why I love the Forums. Every now and then someone pipes up who REALLY knows what they're talking about (instead of people like me who only think they do ) I enjoy reading the real in-depth technical stuff and learning something new along the way.

Darn, I wish I had my copy of Ron Fournier's Fabricator's Handbook - Fabrication Techniques for Race, Custom and Restoration Use here at work. I really wanted to be able to quote him accurately. Basically, his contention is that a mild-steel tube roll cage (provided there is good penetration on the welds) will absorb energy of a crash better because it will stretch and deform much farther before it reaches a breaking point (like taffy as opposed to a candy cane - bad example). In fact NHRA (National Hot Rod Assoc.) rules require that roll cages for cars quick enough to require them be nade of mild steel tubing.

Glad to get a metallurgist's take on things; thanks Falanx.

Falanx

I have to assume that Ron is a mechanical engineer, not a materials scientist...? Because, on his part, unless he's been persuaded of that by someone else, that's a very bad piece of MEing.

See, the energy absorbed during deformation is not wholely, or even majorly based on the actual percentage deformation. A piece of 1018 (mild steel) one centimetre in diameter bends 180 degrees and absorbs x amount of energy based on

a.) rate of strain
b.) work hardening coefficient
c.) grain size
d.) initial microstructure (ferrite + pearlite/ferrite + martensite + carbide/bainite etc)
e.) total deformation
f.) yield strength
etc....

But, for example, if I have a steel with twice the work-hardening coefficient, or twice the yield strength, or half the grain size and all other things equal, then it would absorb more energy on collapse. The biggest controlling factor is the work hardening rate, followed by a very low yield: UTS ratio. Bainites tend to be perfect for this, especially lower carbon ones. They typically have yield strengths between 50 and 70% of the UTS and big WH rates due to dislocation debris throughout the microstructure from the transformation to bainite.

The long and the short of it is 4130 with upper limit chromium and molybdenum, and lower limit carbon is perfect for this, much moreso than welded mild steel.

If you want to learn a lot about really tough steels dirt cheap, go to https://www.msm.cam.ac.uk/phase-trans/index.html. Those guys are pretty good at what they do.