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Framebuilders Thinking about a custom frame? Lugged vs Fillet Brazed. Different Frame materials? Newvex or Pacenti Lugs? why get a custom Road, Mountain, or Track Frame? Got a question about framebuilding? Lets discuss framebuilding at it's finest.

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Old 10-11-17, 12:24 PM   #1
Lakerat
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Frame flex questions

Please explain how frames flex that allow a smoother ride. I assume some is chainstays moving up since I've seen some seatstays on carbon frames that have what appear to be flexible features on them. If seat stays are a controlling feature of shock absorption, possibly they curve to flex and seat stay diameter and wall thickness could be a large factor in steel frames.

Does the main triangle move much? Seems if the seatstays push the top tube forward flexing the head tube that smaller frames where the top and down tubes are almost touching wouldn't hardly move. If main frame tubes don't really have to move in relation to other tubes in order for bumps to be absorbed, then the tubes themselves are flexing to absorb impacts, which would be by bending.

If the main triangle moving acted as a suspension, it seems that frames with a second top tube such some Rivendells would have a poor ride, and they likely don't. That they exist might indicate that the main frame doesn't do much shock absorption by flexing at tube junctions.

Some steel forks have bent tubes and some are straight. It appears straight forks absorb bumps by bending along their length, and bent ones more in the lower section. Does the head tube rotate towards the seat during bumps softening the ride? How do you build a frame that allows this vs one that doesn't?

When frames are said to be stiff and responsive in climbs and sprints, where are they mainly stiffer than whippy frames? Is it mainly bottom brackets that don't twist or stiff rear triangles and forks?

Also why on steel frames do the chainstays need a bridge when they supported so solidly by the bottom bracket just an inch ahead of it. The hub would appear to prevent the chainstays from spreading anyway. Maybe it serves little purpose besides being a fender mounting location and is just a tradition and still added to frames that will never have fenders.

Last edited by Lakerat; 10-11-17 at 12:39 PM.
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Old 10-11-17, 01:52 PM   #2
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A lot of frame characteristics are speculative and downright contradictory. For instance a slim set of seatstays can't really allow flex if the chainstays are stiff enough not to.
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Old 10-11-17, 10:04 PM   #3
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yeah, those elastomer fittings in seat stays don't seem to make any sense on a bike with giant carbon fiber chain stays. It's really hard to say with a bike like that though, maybe they came up with a secret that is more than just marketing.
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Old 10-12-17, 05:04 AM   #4
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They very likely absorb vibration but not through active flex.
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Old 10-12-17, 07:57 AM   #5
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Here is Waterford's statement on seatstays and ride: "Exquisite Seatstays: We taper our seatstays at both ends of our road racing tubesets. The small diameter at each end improves rider comfort which the stiffer center section of the stays insure more positive braking."
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Old 10-12-17, 08:29 AM   #6
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the bicycle industry is fond of marketing b.s. Waterford has likely never done any analysis of their frames at all. That just sounded good and nobody is going to disagree with them. At least you know that Trek and Spesh have done FEA modeling of their bikes, so their marketing b.s. at least has some possibility of being true.
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Old 10-12-17, 08:44 AM   #7
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Originally Posted by unterhausen View Post
the bicycle industry is fond of marketing b.s. Waterford has likely never done any analysis of their frames at all. That just sounded good and nobody is going to disagree with them. At least you know that Trek and Spesh have done FEA modeling of their bikes, so their marketing b.s. at least has some possibility of being true.

"Possibility" being the key word here. Andy
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Old 10-12-17, 08:49 AM   #8
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right, I'm just saying that Waterford (or really, any steel frame company) making statements about frame flexibility is really only backed up by confirmation bias. "we put pencil thin seat stays on this frame because then you will believe it is more comfortable" "Helping you exploit your confirmation bias to get what you want is our specialty."
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Old 10-12-17, 03:12 PM   #9
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Maybe calling them "Exquisite" makes them ride better.
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Old 10-12-17, 03:43 PM   #10
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If you list what you want, your custom frame to be a builder can adapt their materials selection to try to meet your desires..


Some builders of steel frames, notably Hetchens Bent the seat and chainstays.. that certainly, softens the rear end a bit..




I have 2 bikes without chainstay bridges .... that makes mudguard fitting difficult.. ..







.....

Last edited by fietsbob; 10-12-17 at 03:48 PM.
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