Carbonfiberboy 

The nice thing then about carbon is that when you tap it, it doesn't ring, it just goes Thunk. Doesn't matter what you tap it with, just Thunk.

guy153

Good question. It's not just the grades but also how shonkily they're planning on building it.

I heard an interview with the guy who makes Starling MTBs (fillet brazed steel in his shed) and his day job is an aerospace engineer. He said most bike grade CF was over-specified because it's full of voids and other jank. Some of the high end frames will be better no doubt.

Certainly it used to be the case that very cheap CF frames were actually no lighter than good aluminium frames. I don't know if this is still true.

bruce19

This is interesting. I have a very short experience with a mid-range Bianch CF bike. And, it was that "thunk" that I interpreted as dead and hollow. I suspect, but don't know, that I would get used to that over time. I do like some of the CF designs. Aside from the "thunk" the thing that has kept off a CF bike is the cost.

big john

Oh, no. Material threads are a constant on the forum, although the disk/rim brake argument has taken over lately.

big john

Funny, I have a titanium framed bike and it is quite stiff, bordering on brutal on rough roads. My steel framed bike rides smoother. I rode a friend's titanium Moots and it was springy and soft.
The stiffest, most jarring ride I ever had was steel. The softest, most noodly ride I ever had was also steel.

It's not the material as much as it is how it's built.

big john

They're cheap now, in line with other materials.

bruce19

I don't think top level CF is as inexpensive as equivalent AL. But I'm happy to be wrong. Which CF frames did you have in mind?

facorsig

A friend was looking at a titanium Lynskey Helix and it was very stiff. My bike is a titanium VaMoots Moots. In addition to material of construction design would be important.

What I dont understand about carbon frame bikes is frame repairs. I really struggle with repairs restoring original properties. Load transfer through the fibers simply cant be the same when there is a discontinuity of matrix. I see people citing expert repairs. It just cant be the same.

asgelle

https://podcasts.apple.com/gb/podcas...=1000474359104

big john

It's tough to say what the "equivalent" CF frame is to a given aluminum frame, but there are CF bikes now that seem to be a pretty good deal. $2200 for a 105 bike.
Viathon Bicycles | R.1 Road Bike | Shimano 105 Equipped

I was looking at a BMC with 105 for about $2700. I suppose a CAAD12 is cheaper, but I'm sure there are other CF bikes out there in a similar price range.

Here is one for $1600.
2020 Polygon Strattos S7 Road Bike - Size: Large | Bikes Online (USA)

Here is one for $1100.
Save Up To 60% Off Shimano NEW Carbon Aero Road Bikes Motobecane Immortal Spirit with Shimano 22 Speed (bikesdirect.com)

WhyFi

I don't think that top level CF has an AL equivalent.

big john

It may not be exactly the same, but, depending on the repair, it's doubtful anyone could feel the difference. And if they could, the repair is probably preferable to tossing the frame into the trash. I also think frame repairs are rare. Most of the people I know who have cracked frames, CF or other materials, have had their frames replaced under warranty. I've broken 2 steel frames and 2 aluminum frames.

Trakhak

I take that line of thinking further: I don't believe that there are significant differences in shock absorption or vibration damping among steel, carbon, and aluminum forks. I have bikes with all three, and all ride exactly the way I expect a bike with the given wheelbase to ride. On the other hand, I understand that there are those who believe they can detect meaningful shock absorption in aluminum or steel bikes built with a section of carbon fiber in the seat stays. Puzzling.

In any event, that particular design feature disappeared quickly for some reason, just as dark anodized rims had, a decade earlier.

Carbonfiberboy 

The reason I brought that up is that the Thunk is what makes carbon special - vibration damping. You can get the same stiffness in metal, but without the vibration damping. That's a product of the fiber/resin interface. A metal bike rings like a steel guitar string. Carbon is like you plucked household twine - thwunk - and for a similar reason, because of the fibers. Vibration is turned into heat energy. That's not a great analogy, but you get the idea.

guachi

I supposed that's why you'll find carbon forks on otherwise aluminum bikes? For the vibration dampening?

big john

I think aluminum is a bad material for a fork.

Seattle Forrest

Girlfriend says she used to hear something a lot like that.

Trakhak

Of the two of my bikes with aluminum forks, I bought one in 1995 and the other in 2005. Many tens of thousands of miles on both, and they're two of my favorite bikes. I suppose it's possible that the vibration-damping characteristics of aluminum forks, being between steel and carbon in that regard, might account for the enjoyable ride of those bikes, but I've always found tire pressure and wheelbase to be by far the major determinants of how bikes ride, nearly to the exclusion of other factors such as frame or fork material. (Although, come to think of it, I always disliked the way heavily loaded long-wheelbase steel touring frames tended to wallow around when standing and climbing.)

bruce19

I think this would depend on your definition of "equivalent" but point taken. As soon as I win Powerball I will give an in-depth report to BF. Should be any day now.

WhyFi

It's certainly the case from any of the major manufacturers. I wouldn't be surprised if you could spend more on a bespoke frame from a builder, but I'd question if they were able to meaningfully improve upon the performance characteristics of the leading AL manufacturers with their hydroforming and alloys, etc.

woodcraft

I'd say it's because it's much easier to mold CF into a tapered shape than it is to form aluminum that way.

DangerousDanR

I started out in Mechanical Engineering probably a few years after you did, back when drawings were made with a drafting machine and a pencil. And mine were just plain ugly. But what I remember best was the "Materials and Processes" class where the instructor spent one day telling us why welding was a bad idea. Soon after I took that class I switched majors.

I discovered that there were people doing some really fun stuff using software to control machines, and that I didn't have to study or anything even remotely like it; just be clever. I have had to use vision systems to correct for the (very small) changes in shape of welded aluminum tooling fixtures over time in one very high precision manufacturing system. They should have been carved from a forged billet but even custom high dollar automation systems have a budget.

So, for the most part I agree with you 100% about metal. In spite of that, most of my bicycles are made of welded metal. However, not all of them are made from constant wall thickness drawn tubing. I believe that Lynskey forms some of their frame elements from sheets of Titanium, and I suspect that if they want to they can use variable thicknesses by working the sheet before it is welded into the frame element.

Another example in the bicycle world, the Finnish company Pole is machining some of their frames from a billet which allows them to use variable element thickness and an alloy which is not weldable, but those bikes cost as much as carbon fiber. They machine the main frame assembly in two pieces, then use an adhesive to bond them into the final assembly. I believe they do the same thing for the swing arm. I suspect that they chose this process because it was easier than low volume carbon fiber manufacturing in Finland.

Also, there are now processes which get lumped into the category of "additive manufacturing" which allow the mechanical designer to specify variable element thickness. Again they are not the cheapest. Atherton (formerly Robot) Bicycles is using additive technology to build custom frame lugs to which they bond carbon fiber tubes. My guess is that they could use steel or Titanium for the tubes if they wanted and that the use of carbon fiber tubes was to reduce weight and have a vibration damping material. Or maybe it was marketing.

mstateglfr 

Seems like a very wasteful process.

Carbonfiberboy 

Still, metal is an isotropic material and so will always be at a disadvantage w/r to anisotropic materials when trying to create an object with very specific properties.

DangerousDanR

Again, in the normal case I completely agree. Carbon fiber has lots of great properties and I don't understand why people disparage "plastic" as a material.