Fentuz

Hi all,

I have been riding my carbon frameset for 3yrs in mullet (650b with MTB XC Tyres and MTB XC transmission) and cyclocross (700c with 33 CX Tyres and gravell transmission) configurations. When purchased during covid shortage, I got a frame a little too big (I'm in between small and medium) so, I have a short seatpost protrusion and a 20mm shorter stem in line with bike fitting measurement. That said, I could do with a top tube 1" lower, especially when jumping on the bike after hurdles during CX race.
So, I was wondering about swapping the carbon medium frame for a Ti small frame and ,other than the stem, transfer everything across (inc the carbon fork). The weight would go up by ~0.5kg from Med to Med frame so going for a small might offset the weight increase a little. On of the appeal to me is the slickness of the frame, Ti and Steel frames seem to use smaller diameter tubing than carbon.

I know many claims about Ti is great for comfort, others said it is not much different than Aluminium because of close density/Young modulus etc. So although it is very subjective, what is people real life experience between Carbon and Ti?

thanks

tFUnK

Not gravel-specific but I have (or had) several carbon road bikes dating from 2005-2015, and one titanium road bike from 1998. The carbon bikes are lighter, smoother, stiffer, and more comfortable. The Ti bike is fun to look at and does great on roads that have recently gotten some tax dollars put into it. But it rides just fine, goes as fast as its rider can take it, and I don't even notice the skinny straight gauge Ti tubes feeling flexy. But also when I'm on it I don't ride with the same aggression as when I'm riding my carbon bikes - it's more of a leisure vibe.

chaadster

My Ti gravel bike is extremely sweet, and just handles perfectly in the rough stuff. I haven’t ridden carbon gravel, but the Ti is much better than the older aluminum gravel in terms of handling and comfort, but Al bike was not gravel specific. I do love my carbon roadie, even compared to my steel roadies of both modern and vintage designs. So while my “apples to apples” experience is only on road, not gravel, I can still say that probably carbon fiber can do anything, and if I wanted a “quiver killer,” i.e. one bike to rule them all, I’d go with carbon fiber. Other than that, though, there are so many variables that, even though Ti, aluminum and steel don’t have the absolute range of CF, any of those materials can be used to make a really great bike, so as long as it’s choosen well, I don’t fuss about frame material much.

TC1

There is no difference in comfort between any rigid double-triangle bicycle frames. Anyone who claims that there is, is selling you snake-oil, or repeating the snake-oil pitches that they've heard. All of the other meaningful components on your bicycle will flex before the frame will, in the vertical plane. There is no subjectivity to the question, either, it is a simple matter of physical properties -- a double-triangle frame will not deflect vertically before:

* your tires collapse onto the wheels and probably debead
* your seatpost deflects
* your saddle rails deflect
* your handlebar deflects
* your fork deflects
* your wheel deflects, probably to the point of permanent deformation
* all of the above ( not necessarily in that order )

This was all proven decades ago, but amazingly, poorly-informed cyclists keep repeating the same nonsense.

Trakhak

Thanks for the link. I just wasted 20 minutes reading the comments. So many of the people posting those comments "knew" that steel frames are categorically more comfortable than aluminum frames, serenely ignoring the data from tests of vertical compliance, of over 1,000 frames of different materials and geometries.

The results showed conclusively that the differences between those frames with respect to vertical compliance cannot be perceptible to riders. Setting aside confirmation bias, of course

One Wheel

In musical instruments, the distance of deflection is "volume" and variety of overtones and undertones produced (vibrations other than the fundamental note) is either richness (of concordant) or harshness (if discordant). If you made carbon fiber, aluminum, steel, and titanium tuning forks, and rang them all with the same energy, the carbon fork would ring the shortest (if at all), then aluminum, steel, and the titanium fork would ring the longest. If the same energy went into the forks to start, then more of that energy was converted into other frequencies in the carbon tuning fork, and the least into other frequencies in the titanium fork. Since bikes are not specifically designed for their acoustic properties, the other frequencies are almost always going to be "harsh." There are obviously a lot of variables, and parts hung on a bike frame that can damp vibrations in unpredictable ways, but comfort of the ride isn't necessarily only explained by a spring diagram.

I don't know if this explanation is right, and I'm sure it's not the whole explanation, but it's a reasonable, physics-based explanation that describes, rather than dismisses, thousands of subjective observations.

TC1

It's not right, nor particularly physics-based, for two principle reasons. First, a cyclist never touches their frame while riding. So this theory requires the existence of an entire catalog of components to be developed with acoustic fidelity as a priority, and they'd need to achieve the level of audio speakers, in order to faithfully reproduce the alleged differences in material. No such catalog exists, and in fact, the opposite is the case.

Second, it is myth that carbon fiber is some magical sink into which vibrations disappear. The proof of which can be found in many research papers, but most clearly in the market for carbon fiber musical instruments.

One Wheel

Most concert-goers never touch a violin while they're listening to an orchestra, either. The parts of the bike that you do touch are not perfectly damping, so vibrations still matter.

I never suggested that carbon fiber was a magical vibration-damping material, but that it propagates a wider variety of frequencies than metals, and less of the fundamental frequency. Acoustically it's fairly similar to wood in that way, and most instruments are made of wood. I don't know if there would be a noticeable difference between a bike frame that propagates concordant vs discordant harmonics, but there is a clear difference in musical instruments. Instruments that are made of wood (and carbon fiber) are intended to propagate more harmonics and therefore less of the fundamental note. If someone were to design a bike from the ground up for its acoustic properties it wouldn't surprise me if some very interesting ride properties with regard to vibration could be attained, but it's likely that other properties such as stiffness and lightness would suffer. Instruments that are intended to propagate a more pure fundamental note, like a trumpet or a flute, are typically made of metal rather than wood or carbon fiber.

You don't need high-fidelity audio levels of vibration propagation for vibrations to affect comfort.

base2 

The engineering merits of the double triangle diamond frame leaves little room for "vertical compliance." Lateral and tortional stiffness, however is another matter.

Titanium is very strong. It is also very springy. If a titanium frame were designed only as strong as necessary and only where necessary like a carbon frame tends to be designed, it would be so noodley so as to be dangerous. Consequently, all the supposed weight savings is eaten up by additional material added to address the absence of tortional and lateral stiffness. The end goal/result is often a ride comparable to a steel bike with some weight savings but not as much as *could* be. The trade-off is often cracking sometime far along the life cycle as manufacturing can be difficult to execute and the elasticity of the welds is different than the tube that was welded. (On a general level.)

Carbon, on the other hand, is not so "springy" and it can be very light weight with enough strength to do the job. So, there is a lot of freedom to design around an intended use. Larger diameter down tubes, head tubes, larger bottom bracket areas and thicker chainstays can be done with very little weight penalty. (Conspicuously absent from this tube growth phenomonon is top tubes and seat tubes.)

For "vertical compliance" ie "comfort" in the traditional sense, tires (bigger softer "air springs") and seatpost will yield much more fruit. A longer seat post will offer much more deflection than a short one. You can see this in the recent trend of lower and lower seatpost collars and ever more sloping top tubes in more recent designs.

I think that what a lot of people mean when they repeat: "Steel is real" (& by extension, titanium) is that the frame twists, the quill stem flexes, the side of the handlebar that had weight on it flexed, the bottom bracket deflected, etc...All give the impression of a smoother ride. Whereas a carbon frame by nature of desing is intended to resist those forces so can be perceived as "wooden" by comparison.

Designing flex into a carbon bike has been the latest trend with IsoSpeed, IsoShock, Zertz, decoupled seatposts, etc...

All that is to say, the smaller frame will be smoother not because it is Titanium, but because all the bits that stick out will have room to be flexier.

TC1

So you are alleging that carbon fiber bicycle frames sound more comfortable?

You do if one is claiming that the small differences in vibration significantly effect rider comfort.

Not only does the rider not touch the frame, the rider does not touch any component that touches the frame -- so there's (at least) two levels of isolation between any part of the human rider's body, and the frame of the bicycle. Claiming that vibrations are faithfully reproduced across those interfaces is a claim of significant magic. Especially as regards the front end of the bicycle, where the frame is not even in the path which vibrations travel -- which is tire, rim, spoke, hub, axle, fork, stem, handlebar.

Fentuz

yes, this was my initial thoughts; it is all linked to the young modulus of the given material and then the frame design. While I was asking about Ti vs Carbon, I "knew" that the answer was not binary as for example 2 carbon design can be very different depending of the fiber quality and filers but also composite design (I have not done mechanical engineer of these material for 20 years...).

So basically, as I thought, as long as the frame is better fitted to my size, I will get better extrusion of various beams allowing better deflexion and therefore more confort. Beam calc 101, 1 week of mechanical engineering degree

TC1

That's an incorrect statement. The Young's modulus of the frame's material is irrelevant, so long as the frame is a double triangle arrangement. Any double-triangle frame made from material strong enough to be a bicycle will not flex in the vertical plane before all of the other components do -- and in fact, it won't flex vertically before several of those components deform to the point of permanent failure.

There are reasons to buy titanium frames, and reasons to carbon fiber frames, and reasons to buy aluminum frames, and so forth, but one of those reasons is not, and cannot be, rider comfort, because it is physically impossible for a double-triangle frame's material to have any effect at all on that front.

This is a simple point that everyone who buys bicycles should understand.

base2 

There is more at play than only the in a dynamic system.

The dubious premis that comfort=vertical compliance=comfort is fallacious bordering on absurd.

This is a simple point that everyone who buys bicycles should understand.

Atlas Shrugged

So to clarify, seat stays, top tubes chain stays etc., do not flex at all or play any factor in a bikes ride?

Eric F 

The forces involved with riding a bicycle are not all in the vertical plane.

TC1

That premise (nb) is not remotely absurd, since it is precisely vertical energy input to the rider that most riders deem "uncomfortable".

You may also note that all bicycle suspensions operate in the vertical plane.

TC1

In a double-triangle frame, they do not deflect in the vertical plane, before all other components do. This has been proven decades ago, as I referenced earlier.

TC1

No one claimed they are -- but the forces involved with discomforting a rider are exerted vertically, well, not counting things like being run into by a car, but that's a different issue.

Atlas Shrugged

Your argument does not take into account that frames flex in all planes, including vertically, thus increasing ride comfort. As an extreme example if you constructed a frame with 3mm titanium seat stays they will flex in the vertical plane. This situation illustrates how excessive emphasis on academic knowledge can sometimes overshadow practical street smarts and common sense. Your argument contains significant flaws. Combining book smarts with real-world experience is vital for a comprehensive understanding of complex issues.

Fentuz

well i may misread what you are writing but yes you can add flex in the frame but altering the mounting point of each tube (manufacturer move the seat stay to add compliance).
then, there are the diameter of the tube to consider depending of the material chosen.
As I initially said in the post you replied,
depending of material properties, the frame design will choose the tube mounting point and dimensions based on ergonomic requirement and mechanical specifications they want to achieve for a given application.
so the carbon frame for gravel type a will have the same properties and Ti frame for gravel type a but their shape and dimensions may differ.

Eric F 

I would suggest that it's not just the amount of flex that affects ride comfort, but also how the tubes react to being flexed, which will vary with tube dimensions and material.

TC1

I answered your question. Here is it again, since you apparently skimmed past it the first time. In a double-triangle frame, they do not deflect in the vertical plane, before all other components do. This has been proven decades ago, as I referenced earlier.



You are confusing "comfort" and "handling", which are two dissimilar qualities. Lateral deflection of a bicycle frame does not increase ( or decrease ) rider comfort.

In reality, this situation illustrates how susceptible people are to marketing and snake-oil salesmen -- and how difficult it is for people to disabuse themselves of notions planted by same. People have been told by salesmen for decades that a new bike made from a different material will ease their sore back and bum, and they believed it, and often, spent considerable money on that new bike. This was wasted money -- in terms of improving comfort, but that's difficult for many to admit, despite, again, this having been proven decades ago.

The fact that you are not aware of those studies, and that proof, does not make them disappear -- nor does it make pneumatic tires stronger than double-triangle frames, among other components. If you doubt the proof previously demonstrated, do you own test: sit on one of your mounted tires, and see it deflect, then set your frame on the ground, and sit on it... and see it not deflect.

TC1

This does not add compliance as shown by the fact that the tires on these bikes do not rub on the seatpost, and the cables do not slacken. It's marketing.

There are a raft of reasons why different tubing diameters are selected by manufacturers, but one of them is not vertical compliance. Aesthetics is a common one, as are ease of manufacturer, material cost, deflection in other planes, durability, and, of course, marketing.

No one is suggesting anything different from this statement -- of course bicycle frames differ in shape and dimension. What they do not differ in, as long as they are composed of a rigid double-triangle is that their resistance to deflection in the vertical plane far exceeds that of all of the components which are attached. And that renders that frame's vertical compliance irrelevant to the rider's comfort.

TC1

I believe you are also conflating the concepts of "comfort" and "handling". The former is what you get when you buy a Cadillac, but the latter is what you get when you buy a Lotus. In cars, with vastly more complicated suspension systems than bicycles, the two concepts are almost diametrically-opposed. Luckily for cyclists, the same degree of opposition does not exist in simpler two-wheeled craft and it is possible to build one that is both comfortable and which handles brilliantly.

And again, as cited above and confirmed by a number of studies, double-triangle bicycle frames simply do not deflect in the vertical plane when used as-designed, so there is no reaction-to or recovery-from flexing to consider.

Eric F 

I'm probably wrong, but your statements come across to me as someone who has read a lot of books, but hasn't spent nearly as much time riding bicycles. As much as you are trying to stuff this into a simple 2-dimensional plane, riding a bicycle is not a 2-dimensional activity. The perceived smoothness of how a bike rides also happens when turning the pedals and not only riding in a straight line. Those things bring lateral and torsional factors into the game.