Any engineers on board? Questions about my foray into modern carbon fiber... - Page 2

911pcars · 02-05-2010, 12:18 AM

You guys in So. Cal. looking for a challenging ride a few weeks from now?

Year of the Tiger Bike Rides. This one is about 45 mi.:
Firecracker - Maps - 2010 45 Mile Bike Route Map

Sherwood

dd74 · 02-05-2010, 07:58 AM

That ride sounds fun, Sherwood. When is it and what time does it start?

911pcars · 02-05-2010, 08:45 AM

Quote:

Originally Posted by dd74

That ride sounds fun, Sherwood. When is it and what time does it start?

Sat., Feb. 27, 9am.
Stick around for a complimentary Carbo-Load meal and entertainment after the event. There is no lack of Chinese food in the area.

A big run/walk event is on Sunday.
More info here: Firecracker

Sherwood

mistertate · mistertate's Garage

According to Dr. Scott Beckwith, SAMPE Technical Director, in his November 2009 SAMPE Journal article, carbon bicycle forks are the most common source of lawsuits in the entire composites industry. People making the forks don't realize that on average, the compressive strength of carbon is about 30-40% of it's tensile strength. They make the forks leading edge (compression) as thin as the trailing edge (tension) and they break, launching the rider.

dd74 · 02-05-2010, 09:59 AM

Quote:

Originally Posted by mistertate

According to Dr. Scott Beckwith, SAMPE Technical Director, in his November 2009 SAMPE Journal article, carbon bicycle forks are the most common source of lawsuits in the entire composites industry. People making the forks don't realize that on average, the compressive strength of carbon is about 30-40% of it's tensile strength. They make the forks leading edge (compression) as thin as the trailing edge (tension) and they break, launching the rider.

Well, I think I can counter that argument with real-world observation. A stem "froze" inside my Ti bike's head tube - why is up to analysis once the stem is extracted.

Nonetheless, the carbon forks that came standard on the Serotta-built/Schwinn-spec Titanium Paramount, which this bike is, was used as a lever against first a pipe wrench, then secondly a vice grip, all in effort to remove the frozen stem. A mechanic who can easily bench press 300 lbs as well as a second guy who is a very strong Cat. 2 racer, inserted a mallet handle and then a piece of pipe between the forks to counteract the vice grip's efforts.

In short, if the fork was going to break, these guys would have done so. I'd expect the headtube on the Ti frame to compress first.

Of course I'll be royally pissed off, but that's another story...

island911 · 02-05-2010, 10:41 AM

Shumi had a great write up.

I'll add that stiffness is not born from materials alone. The geometry of the structure can often out-pace the benefits of material stiffness. (consider a Canondale aluminum down-tube vs a 1" dia CF down-tube)

Quote:

Originally Posted by mistertate

According to Dr. Scott Beckwith, SAMPE Technical Director, in his November 2009 SAMPE Journal article, carbon bicycle forks are the most common source of lawsuits in the entire composites industry. People making the forks don't realize that on average, the compressive strength of carbon is about 30-40% of it's tensile strength. They make the forks leading edge (compression) as thin as the trailing edge (tension) and they break, launching the rider.

Interesting. Tho' to split a hair, it's not really that the fiber (CF) has compression strength of only 30-40% of it's tensile strength; but rather that the lamina has only 30-40% of it's tensile strength. (buckling, shear zones..)

But that does get to a very important point; failure mode. When steel forks 'fail' they are bent, but still attached. When CF fails, it's usually completely gone.

Of course CF forks can be made strong enough, that no mater how hard you hit something, the wheel will break, and the rider will launch. (...something about a chain being as strong as it's weakest link.)

dd74 · 02-05-2010, 10:48 AM

True. Geometry is the makings of stiffness on any frame. This is why riders on track bikes are rarely seen on the street -- if ever - too twitchy and a ride that will break backs.

Angles on the Look are steep and the chainstays short. Somehow, though, it dampens road imperfections (bumps). I'm very curious as to how this is done. More carbon weaves, less -- I'm not sure yet.

Composite wheels is probably the most readily available place of carbon failure on a carbon road bike. In other words - "...what pothole...doh!"

island911 · 02-05-2010, 10:59 AM

Have you ever noticed the weave properties of a neck-tie? --the material is set on the 45° bias. This allows flexing in the long direction.

Same goes for composites. --Fiber orientation makes a big difference.

look 171 · 02-05-2010, 11:24 AM

Quote:

Originally Posted by dd74

True. Geometry is the makings of stiffness on any frame. This is why riders on track bikes are rarely seen on the street -- if ever - too twitchy and a ride that will break backs.

Angles on the Look are steep and the chainstays short. Somehow, though, it dampens road imperfections (bumps). I'm very curious as to how this is done. More carbon weaves, less -- I'm not sure yet.

Composite wheels is probably the most readily available place of carbon failure on a carbon road bike. In other words - "...what pothole...doh!"

They really absorb vibration well. Better then steel frames, IMO. I have had many steel frames over the years, and nothing comes close. As least to my look. I did not like my ICE. Feels like a log. I do not ride a big frame, so flex is not that much of an issue. Plus, I am not racing anymore.

dd74 · 02-05-2010, 11:41 AM

I don't race either, at least in some sort of sponsored, have coach and team and travel environment.

(Incidentally, I met a pro racer at the shop whom I never heard of. Gosh! He was bone thin, very young, cocky but made only $10K a year riding).

What is important to me is pedal efficiency. I'd hate to think some approachable amount of my wattage is being wasted on a soft BB or a frame with a pliable geometry.

As to carbon - I hear inefficiency is truly nonexistent.

island911 · 02-05-2010, 12:37 PM

meh. steel or CF, your body is the place where big losses (damping) occur.

Think of a mass on a steel spring . .. bouncing up and down. Those deflections are large compared to those of a bike frame, yet the steel spring is damped almost entirely by air over the system. (not by internal friction of the material)

OTOH, your body, is a bag of water covering a skeletal linkage system of mushy joints.

dd74 · 02-05-2010, 01:01 PM

Yes, I understand that. As well, dampening can be accomplished in much easier methods and adjustments, starting first with air pressure in the tires. It's old school, and free, but the marketing department down the hall isn't going to tell you that.

What I'm more interested in is how well these bikes propel themselves with nary a pedal stroke from the rider.

Now, I have to say I picked up late last fall an el cheapo 1987 Columbus SL steel hand built frame, slapped on some very simple parts to make it very period correct, and low and behold, the bike is a rocket. Each pedal stroke makes it go in a sort of turbo-diesel way - not explosive, but before you know it, you're staying with traffic.

At this point, I think it's the fit and geometry of the old frame that makes it accelerate as it does. But still I'm curious to see what the C/F frame has in propulsion that this frame lacks.

Weight, btw, I believe to be B.S. If a rider can't handle 2 add'l pounds, then they're not trying.