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At times composite (carbon) is touted as being stiffer than (aluminum) alloys as well as stiffer than steel (leaving Titanium to a different discussion).

This means that the frame will not yield (much) under a rider using the arms muscle to counter—and to strengthen—leg cranking.

At other times carbon frames are touted as having the advantage of being more flexible, gently acting as a spring or a shock absorber against road shocks, so much so that alloy frames are often fitted with a carbon fork.

How can these two aspects be reconciled?

Is it the case, for example, that the structure of the carbon mesh is built with such sophistication that it provides torsional rigidity in the frame—resisting out-of-frame movements—yet provides considerable flexibility in the plane of the frame? For this to be even feasible it would appear that the width of a frame (as seen by a rider straddling the bike) would need to be bigger, yet that doesn't seem to be the case. Can you clarify?

Update

If we are unable to determine whether it is Advanced Science and Engineering (ASE) or Pure Marketing Baloney (PMB), we could benefit from the experience of those who rode a number of different bikes and can confirm that there are indeed carbon frames that feel stiff like a rock while bending them with the arms, and yet that indeed offer a cushy ride.

In other words, even if can't understand the ASE aspects, we might at least rule out that it's PMB through empirical experiences.

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  • Al and Ti forks can be annoyingly harsh. Carbon or steel are quite 'springy'. The same applies to Al frames. The field of the question and the answer(s) to it could easily be the subject for a hefty doctoral thesis and make this forum literally explode.
    – Carel
    Jul 17 at 18:56
  • @Carel Thank you for clarifying that when we head back to browse stores in person, I shouldn't even consider bringing up this topic while discussing the merits of different bikes with the hapless freckled chap at the LBS.
    – Sam
    Jul 17 at 20:21
  • @Sam - you really need find to yourself a better LBS. Good LBS rely on repeat custom, word of mouth and reputation, so cut though the PMB and ASE to what counts - the best bike for you (which factor your riding, skill, desire and budget).
    – mattnz
    Jul 18 at 5:11
  • 2
    I read that as Advanced Research Science and Engineering.... and also I'm totally stealing "PMB"
    – Criggie
    Jul 18 at 5:47
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Looking at the stress-strain curves, aluminium is a very stiff material, especially when you design the frame to stay in low stress regions for good life expectancy. This usually means that you have to make tubes wide and stiff (with thin walls for low weight).

Carbon is more flexible. This means you can make carbon tubes relatively narrow and bendy. This explains the trend towards narrower seat posts on carbon road bikes which are intentionally designed to flex quite a lot. The same can be done for forks. For the frame itself you can make the down tube and bottom bracket very wide which should make it very stiff, especially in torsion so that it doesn’t flex when putting a lot of force into the pedals. At the same time you can make the seat stays narrower to make the rear triangle flex somewhat towards the seat.

For the handlebars to my understanding they can either be stiff or flexible. Since pulling and pushing on them is exactly the same direction as when you rest your weight on them.

Edit: I should add that from personal experience my carbon road bike (a Rose X-Lite Four from 2018) is much more comfortable than my aluminium cyclocross (Focus Mares from 2009, with carbon fork) with comparable seating position, tyres, saddles and handlebar tape. But of course this comparison is kind of unfair since the frame geometries and type of bike are completely different. I also think design philosophies have changed in recent years with more focus on comfort, even on “race” road bikes.

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A simple explanation I've heard is that metal tubes are isotropic - their material properties are identical in all directions (tube shape and thickness being constant). Carbon fiber can be made fundamentally anisotropic, i.e. their stiffness and strength differ in various directions.

I am not an engineer, but this brief synopsis was given by Josh Poertner on his Marginal Gains podcast. He has extensive engineering experience in the bicycle industry.

I also suspect he was simplifying for a non-engineering audience. For one, we are not limited to round and only round metal tubes of equal thickness throughout. For quite a long time now, we have had butted metal tubes, i.e. the tubes are thicker at the ends where you do need more strength because you're welding that tube to others, but they're thinner towards the tube center; for spokes the butting is external, but for frame tubes it's internal so you can't see from outside. For another, you can shape the tubes. For example, I know that at least some metal down tubes are oval where they join the bottom bracket - the greater lateral cross section makes for a stiffer bike. More complex shaping than that is available, as the comment below alludes to. Within material limits, you can change the diameter of the tubes as well, where larger and thinner walled tubes are stiffer than smaller and thicker ones. Somewhat related to that, variations in metal alloys can enable slightly different material properties. For example, 6/4 vs 3/2.5 titanium, steel with niobium in the alloy mix may have enabled very thin-walled (and thus large diameter) tubes that are stiffer, various alloys of aluminum. So, metal may be isotropic if you hold the tube dimensions constant, but there are still numerous ways to adjust the frame properties of metal bikes.

For another, it may be more correct to say that most carbon fiber structures can be quasi-isotropic, although you can also make them close to anisotropic - it might be more technically correct to talk about degree of isotropy. You can, I believe, make an isotropic carbon structure if desired.

Regardless of the specifics, it appears you can vary the material properties of a carbon structure more, and I believe much more, than a metal structure of the same shape. With carbon, you're not limited to changing the shape and wall thickness of the tubes involved. There are more degrees of freedom to vary the properties in.

One last bit as to the notion that carbon offers a more compliant ride. It is true that carbon can damp vibrations. However, a lot of your vibration damping comes from your tires. Even performance road bikes these days can take relatively large tires. You can safely run larger tires at significantly lower pressures than we all used to, and you are likely to have less rolling resistance, not greater (this is actually a complex topic, and that statement is an oversimplification). Also, steel and titanium have long been regarded as comfortable rides; they may feel qualitatively different than a carbon frame, but none of those materials may be clearly superior in overall subjective comfort. For that matter, aluminum frames have improved considerably since the 2000s, even though that material has been overshadowed by carbon. It's possible to make a comfortable aluminum bike as well. It is not pure marketing to state that carbon can be made not isotropic, so a carbon fork can be more compliant vertically (i.e. it damps vibrations) than it is laterally (i.e. it doesn't flex side to side when you pedal). It is not marketing to state that carbon bikes can be designed that way as well.

For fork material, it does appear that carbon has come to dominate. Steel forks can also be comfortable, but they do have a weight penalty. While the amount of weight in question actually only has a minor impact on performance in most cases, it is a pretty visible parameter, and anyway steel forks don't seem to be a mass production thing for performance bikes. (FWIW, I have a custom steel bike with a steel fork.) I'm pretty sure aluminum forks were always harsh. I know that titanium forks never made headway, in part due to the cost, and possibly because some of the earlier ones had failures. I'm not sure if that was a quality control issue or something inherent to material properties.

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  • 3
    Most alloy bikes these days are not made from simple tubes, the alloy frame is made using hydroforming techniques that go a long way to making the gap between alloy and carbon smaller. While carbon can in theory be laid up to provide flex and stiffness exactly where its needed in directions its needed, mass manufacturing quickly gets into diminishing returns for effort (therefore cost). Therefore when reading this stuff, you need to factor in age of the information and price point being targeted. A modern alloy frame will compare favorably to a modern carbon frame at the same price point.
    – mattnz
    Jul 18 at 0:17
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    I've rarely ridden carbon but certainly find my steel forks give a better ride than the chunky aluminium ones on another bike. I know a few people with (endurance) Ti frames. I know at least one has steel forks, not sure about the others
    – Chris H
    Jul 19 at 6:21
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Depending on how the carbon fibres are laid up, the flexibility can be altered by a huge amount. Different fibres such as kevlar and glass can be added to the mixture to increase flexibility in areas and specific directions where it is wanted.

In this way a carbon frame can be both stiffer and more compliant, but in different areas of the frame depending on where the designer would like these attributes. It is not possible to do this to such an extent with other materials.

Would you like some examples? The GT grade in carbon was an interesting bike on launch with 4cm ''virtual suspension'' travel in the rear yet an excellent power transfer.

http://s3.amazonaws.com/gtvendors/gorogue/img/content-pic-triple-triangle.png

From the website:

The solid core provides incredible rigidity, and the carbon outer layer acts as added vibration dampening. In the end, you have a bike that gives the rider a lot more control and a lot less fatigue on the ride.

Bianchi use kevlar fibre for the Infinito CV seat stays to get a similar if less dramatic effet.

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  • If a frame is capable of giving for a span of 4cm, it would appear that the manufacturer should produce a sales video demonstrating this. That frame would sell like hot cakes. But conceivably one could also have too much of a good thing. Consider riding in a car with just springs for shock absorbers. The ride would be smooth all right, but soon enough all people in the vehicle would get sick. Riding on 700mm (or so) wheels is certainly better than the typical 15"-19" rims on cars, but springiness would still get exhausting.
    – Sam
    Jul 17 at 20:25
  • It's not a spring but 4cm of damping effect. It's not like a 120cm travel mtb where you can see the movement easily. Don't make comparisons with springs as it's not the same effect.
    – JoeK
    Jul 17 at 20:31
  • I see. You were not actually describing the existence of a frame that deforms by 4cm. It is equivalent, in some sense, to 4cm of damping.
    – Sam
    Jul 17 at 20:33
  • having talked to the designer of this bike when it was released, my understanding is that the rear wheel can move a maximum of 4cm on a big hit in the same way that a wheel moves relative to frame with sprung suspension.
    – JoeK
    Jul 17 at 20:38
  • 1
    ''Micro suspension'' is maybe a better term
    – JoeK
    Jul 17 at 20:39
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How can these two aspects be reconciled?

Easy! It's marketing. Marketing need not be true. It only needs to be something a stupid buyer will believe.

Even a high-pressure narrow road tire has many centimeters of flexing. Compare that to the flexing of the frame or fork, probably measured in millimeters, you see that the flexing of a frame is nowhere near the flexing of a tire. If you buy a frame or fork that supposedly offers a "harsh" ride, and put 32mm tires on, and compare that to a frame or fork that supposedly offers a "soft" ride, and put 23mm tires on, you'll see the bike with 32mm tires is more comfortable -- assuming you didn't put equal pressure in the tires but rather adjusted the pressure appropriately according to the tire width.

The only design criteria of a frame is that it must be stiff. It is achieved in the rear by using a tetrahedron comprised of the rear wheel axle, two seat stays, two chainstays and the seat tube. In the front, it is achieved by using a triangle comprised of the seat tube, the top tube and the down tube. Because a tetrahedron is a three-dimensional structure offering stiffness in all directions, but a triangle is a two-dimensional structure lacking stiffness in the third dimension, the triangle has large diameter tubes, far larger than those used in seat stays or chainstays.

About the only location in a bicycle that can flex somewhat in addition to the tires is the saddle. The reason is that the saddle has far thinner rails than the diameter of the frame tubes, so the rails flex, and also the plastic in the saddle can flex too.

Frames and forks however can affect the feel of the ride, not through flexing, but through their geometry. For example longer chainstays offer a comfortable and stable ride (unfortunately the trend today is that long chainstays are not fashionable whereas chainstays so short that the seat tube must be specifically shaped to make room for a 23mm tire behind it are fashionable -- and don't try fitting a 25mm tire in there!). The head tube angle and fork rake also affect ride feel.

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    Re: "Even a high-pressure narrow road tire has many centimeters of flexing", I'd love to believe that, but how does it work? The tire itself has only a few millimeters of give, if pressurized enough to avoid pinches. Is it established then that the rim itself flexes that much?
    – Sam
    Jul 17 at 23:02
  • See frame flex in action youtube.com/watch?v=BH_AL4rxrp8 Find video of some speed wobble to see centimetres of flex.
    – Vladimir F
    Jul 18 at 17:45

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