carbon seatstays + aluminum frame - Page 4

Your theory of a rear triangle being a truss and therefore not inclined to flex would assume the members involved were able to resist the loads applied to them without flexing, but only compressing prefectly and hence result in very little to no vercle give.

When a verticle impluse load is applied to the rear wheel, the chain stays are put into tension, but also a bending moment. The seat stays are put into compresion, and a bending moment since the loads are perfectly applied along its axis (welded structure not a pin joined frame). Long thin bent or deformed tubes are not good at taking these comprssive loads, specially if combined with a bending moment.

Bikes these days that are being designed to have verticle compliance designed in have shape that won't take the loads well and bend. Cannondale CAAD frames have S bends and flattened sections (to give a reportedly smooth ride), many bikes of all materials have seat stays with curvature when viewed from the side, roubaix bikes have "joggles" (with rubber inserts), and some bikes have pensil stays. All of these designs will bend more than a large diameter striaght tubed compact frame like a Klein for a given load.

The point of these designs is to flex a small amount by not resisting the compressive and bending loads, so any impulse from hitting a sharp bump are transferred more slowly to the rider. So you get a big thud not a sharp BANG

I would agree that tyres/pressure, fork, and seat post probably have a greater effect, but I don't agree that the diamond frame structure can not be designed to allow an increase in verticle compliance. Ride a mid 90s Klein then a CAAD 9 or Roubaix and tell me there isn't a difference in verticle compliance.

PS: I'm a mechanical engineer, I know what fatigue strength is, how to calculate stiffness and stresses, and that longer tube elements of the same size and material tend to bend more than shorter ones......

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Precision Sports

I'm not trying to patronize, but as a part-time scientist I like to go with the evidence. I'm not saying that there is no movement of a truss, just no meaningful movement. If you double or triple a miniscule movement, it is still miniscule. The deformation of tyres, seat cushion and bum soft tissue has got to be several orders of magnitude greater than the movement of a rear triangle and will completely swamp trivial differences between frames. As Alienator has informed, even the seatpost bend dwarfs rear triangle movement. I'm still happy to make a confident call of marketing BS with regard to so-called vertical compliance of a frame.

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"All that we see and seem is but a dream, within a dream..."

Question is, what degree of movement is required for the human body to perceive a difference? Surprisingly, often very very small.

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Morphed Bianchi Camaleonte IV 2006, Ridley Damocles 2006, Garmin, Mac

True, but the motion from tyres is much bigger. Can the body tell the difference between tyre and frame deformation?

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"All that we see and seem is but a dream, within a dream..."

Question is, what degree of movement is required for the human body to perceive a difference? Surprisingly, often very very small.

Exactly.



...but as a part-time scientist I like to go with the evidence...I'm still happy to make a confident call of marketing BS with regard to so-called vertical compliance of a frame.

As a full-time cyclist and racer with 35 years of expirience I'm still happy to call bull**** on your 'evidence'. All I have to do is sit you on one of my Pinarellos, my titanium Litespeed, my carbon Douglas, one of my track bikes and whatever else you want to sample out of my collection...all running the same basic wheelset (Sewup or clinchers...you call it. I've got a 32/3x for the trackie), same saddle (I've got enough NOS sella San Marco Concors to outfit them all) and we'll stick a Campy aluminum seatpost in everythring for you. Sound fair enough?

Then I'll set you off on 25 miles of Ohio's crappy chip 'n seal county roads. If after that you tell me there's no difference in the way those frames, and their rear ends, handle bumps, potholes and vibrations...well, all we can say is thank god there ARE instruments for some folks to rely on for their sensory input in life.

Yup, pencil seat stays are just as stout as beefy crit stays. A fastback cluster, English wrapover, socketed low-mount, big assed monostay, Hetchin's curly, curved/arc...all ride the same according to science.

This...

[img]http://www.desperadocycles.com/S3/St...at_Cluster.jpg[/img]

will ride the same as this...



and the same as this...



same as this...



no different than this...



same as this...



of corse, the same as this...



and the same as this...



same as this...



same as this...




yup. it's all just marketing designed to separate our dollars from our wallets. s.

When it comes to vertical deflection of the rear triangle of the frame... Yes. If you doubt it, why don't we come up with an experiment or two that tries to map actual frame deflection to what you claim to feel.

Have someone independently measure the vertical stiffness of a few frames (the more frames we test, the more significant the results are), but keep the results from you. Then ride around and give your impression of vertical compliance from 1 to 10.

Throw the numbers on a graph of perception vs. measured stiffness and see what you can get. You might be surprised! I'm currently working on creating a human perception model for something at work and so far there have been a few counter-intuitive results, and *way* more noise than I first thought there would be. Apparently, human detection of a signal gets muzzy because of physical and neural noise. For any given input, there's only a probability that it will be detected.

Anyways. Your claims do not match what would be expected from the amount of deflection in the frame and system. This means that an experiment is in order!

John Swanson
www.bikephysics.com

Wouldn't the deflections be proportional to the loads? What loads are you going to apply to the frame to get the next to non exsistant deflection?

You put me at 100kg on a bike, 65kg of that probably on the rear wheel. The high pressure tyre deflects the most (more than I could ever push with hands), frame loads up but probably doesn't deflect anything like enough to measure without a laser.
I then ride at 30kph and hit a square edged object and that hard to deflect any further tyre deflects considerably for a split second.
How do you calculate the static load into the bike to then measure the deflection in the system. I'd say that peak load the system experiences is pretty high, and not all of it is experienced by the rider thanks to tyre and frame flex.

We aren't talking suspension plush rides here, even soft tail mountain bike are far from rear suspension. If you can get 1 1/2 inch of movement in a soft tail mountain bike, surely you could get a few extra mm out of a verticle soft triangle if correctly designed. That would be enough to help take the edge of larger impulse loads by spreading the time over which the load is applied to the rider.

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Precision Sports

My inclination is to think the rear triangle likely deflects less than a few mm's. That square edged bump compresses the tires a lot with it's large impulse. We've all felt that when we've hit such bumps, and it's seemed as if the bump actually hit the rim....you know, you hear/feel that "thunk." The system response to that impulse is non-linear, for sure, and the response of the frame is likely second order at best.

How much you can compress a tire with your hands bears no real significance on how much a large impulse will compress the tires.

Don't know. But there would be a frequency difference b/n the two. Is the body able to detect it? I don't know. But sometimes the body is a strange instrument.

Of course, it's a separate question whether the difference is worth the money.

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Morphed Bianchi Camaleonte IV 2006, Ridley Damocles 2006, Garmin, Mac

This is your same old schtick, my friend. Your **** outranks any other level of evidence. No point arguing.
By the way, I'm not talking about "ride". I'm talking about bump transmission to your rear end.

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"All that we see and seem is but a dream, within a dream..."

Whatever you want to buy, baby, it's your cash to squander. And if the blurb mentions "vertical compliance", offer them double. Your **** is worth it.

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"All that we see and seem is but a dream, within a dream..."

Your **** outranks any other level of evidence.

'What' evidence?

By pushing on the tyres I was referring to how much extra force would be required to fully compress a high pressure tyre to the rim. The spring rate probably would not be progressive as the compression relative to the complete air volume is small, but it is still a stiff spring rate. Hence the impluse load must be pretty large, and that would be transferred to the frame.

I do agree the rear triangle would only deflect a few millimetres, and that pales compared to the tyre. What I'm trying to get at is the rear triangle would deflect vertically, and that different designs may deflect more than others, and it could be felt by the rider under certian loads.

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Precision Sports

I doubt it has enough extra A)flex or B)weight to have any effect on acceleration. I doubt even Campybob's Hetchins would bob.