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Carbon seatpost.. [JeffJ] [ In reply to ]
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I have actually measured SP deflection on a number of bikes, and it can be quite significant, and there is pretty clearly a log relationship between SP length and deflection amount (visualize a seatpost curving evenly along it's length, and you can see why.) That said, generalizing that CF posts (more accurately, composite posts) deflect more or less than other materials is just silly - this flexibility is much more a function of design parameters than materials used.

Having said all that, where a composite does differ from ferrous seatpost materials is in it's damping characteristics. Vibrations propogate through materials at differing rates (wavelengths,) and combining dissimilar materials in a transmission medium can serve to disrupt (or dissipate) the energy transmitted in these waves (how/if this happens is again subject to design, and yes, you can use composites to increase the ability of a structure to efficiently transport energy.. ) The resonant properties of metallic frame/bike parts are generally immutable, but with composites a good designer can tailor these properties to their liking.

As a gross generalization, composite structures in bikes, which combine many different layers (and often materials) in their composition, AND are very dissimilar from the metal tubes of the bike, CAN be quite efficient at damping high frequency energy transmission - which results in less "road chatter" reaching the rider.



As an aside, by introducing carbon and other high stiffness/weight elements to a bike frame, one can also work to adjust the resonant frequency of the structure such that it is outside of the normal conditions encountered by the bike ("speed wobble" is quite often/mostly the effect of road vibrations equaling the resonant frequency of the bicycle, causing the entire unit to resonate/oscillate "sympathetically" at that frequency.)



MH

- building musical instruments since the early eighties...

Tech writer/support on this here site. FIST school instructor and certified bike fitter. Formerly at Diamondback Bikes, LeMond Fitness, FSA, TiCycles, etc.
Coaching and bike fit - http://source-e.net/ Cyclocross blog - https://crosssports.net/ BJJ instruction - https://ballardbjj.com/
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Re: Carbon seatpost.. [fredly] [ In reply to ]
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in my simple mind, I think I've extracted that there is very little difference in deflection between materials, however, they "dampen" the transfer of energy as it gets to your butt. Am I correct in assuming that this actually does not "absorb" shock any more that an alum post since there is no actual deflection difference ? It just feels "different?"

I recently had a very stiff alum bike and switched everything over to a new carbon fiber frame (down to the same tire tubes at the same psi.) I really don't think the materials are that big of a deal for triangle bicycle frames, and all things relative, the actual deflection difference in the frame tubes would seems to nearly be inconsequential when compared to the deflection in the tires for example. My bikes do "feel" different but I really don't feel comfortable concluding that the different frame materials contibute to the different feel because one absorbs more (especially since they are both triangled frames).

If the same amount of harshness/energy is transmitted to two different frames (of different materials) because they are driving over the same road, can you assume all the energy ends up in the same place (my butt) but with a different "feel " or do the different materials dissipate this energy somewhere along the line before it gets to my butt?
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