Advertise on Bikeforums.net



User Tag List

Page 1 of 2 12 LastLast
Results 1 to 25 of 33
  1. #1
    Random Aussie
    Join Date
    Feb 2009
    My Bikes
    Cervelo RS
    Posts
    8
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)

    Too Much Seatpost Flex

    I've just got my new Cervelo RS. Came with a FSA Carbon seatpost which unfortunately flexes so much that it makes it hard to ride at high tempo. My riding group don't want to side behind me because they think it is going to break (this could be a good thing).

    I got the carbon bike because I wrecked my back and needed more comfort. I don't want to go back to a "hard as" seatpost. Can anyone suggest the "mumma bear" solution? Preferably reasonably easy to adjust too!!

  2. #2
    Guest
    Join Date
    Jul 2008
    Location
    Grid Reference, SK
    My Bikes
    I never learned to ride a bike. It is my deepest shame.
    Posts
    3,769
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Can you give some indication of your weight? If you are over ~250 lbs then you might break a seatpost just by bouncing on it... otherwise it is unlikely.

    The solution to this seems to be to smooth out your pedal stroke... practice riding at super-high rpms and concentrate on making complete circles with your feet until your bouncing is minimized... or get a set of rollers (not a fixed trainer - rollers where both wheels are free and you must balance to stay on) for indoor riding and do a similar drill at high rpms.

    Or get a fixed gear bike with a fairly small gear set and do some hilly rides.

    If you are convinced that you need a new seatpost, get a Thompson aluminum one. The carbon of your frame should soak up much more vibration than the 6" of seatpost showing. If you still find the bike hard on your back, replace your 700X23 tires for a set of 700X28 tires and run them at slightly lower pressure (like 90 or 100 psi instead of 120).

  3. #3
    mechanically sound frankenmike's Avatar
    Join Date
    Jan 2007
    Location
    Dover, NH
    My Bikes
    rigid 29er moto, s-works stumpjumper fsr,black fixie,masi roadbike, ugly old hardrock commuter
    Posts
    1,257
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Sorry- I only use thomson(on 5 bikes).

  4. #4
    Senior Member
    Join Date
    Apr 2007
    Posts
    3,518
    Mentioned
    2 Post(s)
    Tagged
    0 Thread(s)
    Do you really think a seatpost contributes to comfort? I've never noticed any difference.

  5. #5
    Random Aussie
    Join Date
    Feb 2009
    My Bikes
    Cervelo RS
    Posts
    8
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by LarDasse74 View Post
    Can you give some indication of your weight?

    The solution to this seems to be to smooth out your pedal stroke...
    Hi LarDasse,
    I'm about 200lbs.

    I can't imagine that I could smooth out my stroke enough to make much difference - particularly at times when I am applying enormous force while still on the seat (eg just before a sprint finish). I have been cycling for over 40years and I don't think there is a lot wrong with my pedal action. The bottom line is that even my bike mechanic thinks this seatpost has too much flex. He wants me to go to an alloy, I would prefer a different carbon to maintain some of the flex to protect my back.

  6. #6
    Random Aussie
    Join Date
    Feb 2009
    My Bikes
    Cervelo RS
    Posts
    8
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by Camilo View Post
    Do you really think a seatpost contributes to comfort? I've never noticed any difference.
    I had a Cervelo Soloist which is available with alloy or carbon posts (admitted both aero). Those that have used both assure me there is some difference. Given that alloy posts can be stronger, lighter and cheaper than carbon, I guess carbon posts have to have something going for them.

    Maybe I am just a victim of marketing??!!

  7. #7
    Senior Member DannoXYZ's Avatar
    Join Date
    Jul 2005
    Location
    Saratoga, CA
    Posts
    11,507
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Well considering that weight and road-bumps load the seatpost vertically along its axis, I doubt a CF post would compress any more than an alloy post (which is basically nil). The flex you're experiencing and that your teammates are seeing is lateral flex due to your weight and leg muscles pushing on the post. A seatpost is much, much more flexible laterally across its axis than along it, so a lightweight CF would probably flex more for the same lateral load.

    Yea, I'd say go with a more stout alloy post. You'd never feel the 50-100gm weight difference, but you can definitely feel the difference in lateral rigidity.

  8. #8
    Map maker cbchess's Avatar
    Join Date
    Feb 2008
    Location
    Richmond,VA
    My Bikes
    Ventana El Ciclon, Walt Works 29er, Specialized Enduro (fixed up for my son).
    Posts
    728
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    get a thomson post. If everyone is telling you to get a new post then that thing is going to break at the worst time and stab you in the last place you want to be stabbed.

    get a ti post if you want a touch of comfort

  9. #9
    DRF aka Thrifty Bill wrk101's Avatar
    Join Date
    Jan 2008
    Location
    The NC Mountains
    My Bikes
    Too many to list, all vintage
    Posts
    19,537
    Mentioned
    65 Post(s)
    Tagged
    1 Thread(s)
    +1 Just get an alloy post.

  10. #10
    ..... Jynx's Avatar
    Join Date
    Aug 2006
    Location
    Long Island
    My Bikes
    2006 Cannondale CAAD8
    Posts
    4,823
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    just curious how much of the post is sticking out of the frame (not that you can change it).

    Either way I would recomend an alloy post.

  11. #11
    Light Makes Right GV27's Avatar
    Join Date
    Apr 2005
    Location
    Green Mountain, Colorado
    My Bikes
    Gianni Motta Criterium, Dean Hardtail
    Posts
    1,521
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Man, something is definitely wrong other than just the material. I've never had a seatpost flex so much I can feel it, let alone see it!

    Nothing other than a suspension post should provide any comfort - most of the force is in the strongest direction......

  12. #12
    Older than dirt CCrew's Avatar
    Join Date
    May 2008
    Location
    Winchester, VA
    My Bikes
    Too darn many.. latest count is 11
    Posts
    5,345
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    I have to wonder how much seatpost is showing out of the tube.

  13. #13
    Senior Member
    Join Date
    Sep 2001
    Location
    Montreal
    My Bikes
    Peugeot Hybrid, Minelli Hybrid
    Posts
    6,521
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Al Post and get a saddle to provide confort for your back, elastomer built in or Ti rails

  14. #14
    Senior Member
    Join Date
    Feb 2005
    Location
    No Va but ride also in So Md
    My Bikes
    Cervelo SLC-SL, Guru Photon, Waterford, Specialized CX
    Posts
    8,681
    Mentioned
    2 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by GV27 View Post
    Man, something is definitely wrong other than just the material. I've never had a seatpost flex so much I can feel it, let alone see it!
    Yeah, I agree. I never have seen a seatpost flex like that and I've ridden behind some very big and very strong cyclists.

    Have someone ride behind you and verify it's really flexing that much
    You're just trying to start an argument to show how smart you are.

  15. #15
    Senior Member Retro Grouch's Avatar
    Join Date
    Feb 2004
    Location
    St Peters, Missouri
    My Bikes
    Rans Enduro Sport, Hase Kettweisel Tandem, Merin Bear Valley beater bike
    Posts
    23,898
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by GV27 View Post
    Man, something is definitely wrong other than just the material.
    I'd try lowering your seatpost a skosh, maybe 1/8" and see what that does. It doesn't cost anything to try and, if it doesn't smooth out your spin, you can always move it back.

  16. #16
    Great State of Varmint Panthers007's Avatar
    Join Date
    Sep 2008
    Location
    Dante's Third Ring
    Posts
    7,481
    Mentioned
    1 Post(s)
    Tagged
    0 Thread(s)
    I broke down last summer and bought a carbon seatpost for my Puch road-bike. I had my fun of being nervous that it would let go at any given moment. I found no advantage to the weight-loss. And I despised that it's cute finish/coating disintegrated within a week. A Thomson alloy is on it's way.

    Now to find a weight-weenie with poor eyesight...
    How do you keep an idiot in suspense?

  17. #17
    meb
    meb is offline
    Senior Member meb's Avatar
    Join Date
    Aug 2003
    Location
    arlington, VA
    Posts
    1,745
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by GV27 View Post
    Man, something is definitely wrong other than just the material. I've never had a seatpost flex so much I can feel it, let alone see it!

    Nothing other than a suspension post should provide any comfort - most of the force is in the strongest direction......
    I had a folder in which its original 400mm seatpost was a couple of inches short so I ran it above the insert line. Eventually arcced (or is that arched) the seat post but more significantly bent the top of the seat tube. I Re-bent the seat tube back but obviously it wasn't going to support the seatpost high. I looked for a 600mm x 25.4 seatpost to no avail. Went to Home Depot got some steel Pipe-I had originally intended to cut it at 500mm but found 30 inch pipe was only about 30 cents more so elected go longer so I could cut down later if I misjudged the proper length. I found at max insert, the 30 inch seatpost fits my inseam just about right, but with abour 450mm of steel pipe/seatpost above the seat tube, I get a couple of inches of seatpost oscilation on top of the extra suspension pogoing. Pretty fun ride and hilarious novelty appearance.

  18. #18
    meb
    meb is offline
    Senior Member meb's Avatar
    Join Date
    Aug 2003
    Location
    arlington, VA
    Posts
    1,745
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by DannoXYZ View Post
    Well considering that weight and road-bumps load the seatpost vertically along its axis, I doubt a CF post would compress any more than an alloy post (which is basically nil). The flex you're experiencing and that your teammates are seeing is lateral flex due to your weight and leg muscles pushing on the post. A seatpost is much, much more flexible laterally across its axis than along it, so a lightweight CF would probably flex more for the same lateral load.

    Yea, I'd say go with a more stout alloy post. You'd never feel the 50-100gm weight difference, but you can definitely feel the difference in lateral rigidity.

    I've never owned a CF seatpost but before your statement would've thought they would have been designed to flex in the axis of the seatpost to cushion the rider. What is the reason they design the flex in the lateral rather than longitudinal direction?

  19. #19
    Senior Member DannoXYZ's Avatar
    Join Date
    Jul 2005
    Location
    Saratoga, CA
    Posts
    11,507
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by meb View Post
    I've never owned a CF seatpost but before your statement would've thought they would have been designed to flex in the axis of the seatpost to cushion the rider. What is the reason they design the flex in the lateral rather than longitudinal direction?
    It has to do with the 3D shape of the post and how much material is in-line with the direction of the force. Vertically, you're talking about 200-250mm of material. Laterally, you've got a thin 27.2mm ring. Take a broomstick and bend it sideways, easy huh? Now try to compress it lengthwise and see how much you can shorten it.

  20. #20
    Senior Member Road Fan's Avatar
    Join Date
    Apr 2005
    Location
    Ann Arbor, MI
    My Bikes
    Terraferma 650b, Mondonico SL and ELOS, Masi Gran Criterium, Trek 610, Breezer Liberty, Georgena Terry Classic
    Posts
    11,144
    Mentioned
    6 Post(s)
    Tagged
    0 Thread(s)
    Thomson posts are thinner on the sides than on the front and back, to stiffen it against in-plane bending caused by rider weight and road bumps. I don't know if this is done in carbon posts.

    A high saddle causes pelvic rocking, which is in opposition to frame torque due to pedaling. With a very stiff BB and seat tube and a high saddle, I could see this net seat tube/seat post torque resulting in seatpost bending. I could see a lower saddle helping.

  21. #21
    Random Aussie
    Join Date
    Feb 2009
    My Bikes
    Cervelo RS
    Posts
    8
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Sorry Guys - somehow notification got turned off. To answer a few of the questions..

    I have about 170mm of seatpost showing (clamp to top of carbon).

    I have a Stratos seat - personally think its the most comfortable seat around (of reasonable weight).

    I don't think posts compress much - its the flex that's the problem - sideways and back/front.
    The guys riding behind me are getting worried its about to break - one said its moving an inch.

    I will try lowering the seat but I have been using this seat height now for about 5 years and haven't had any problems. Mind you, the old Soloist had an aero seatpost which is about as stiff as they come.

    For all the Thompson fans - Is it worth getting the Masterpiece or just the Elite?

  22. #22
    mechanically sound frankenmike's Avatar
    Join Date
    Jan 2007
    Location
    Dover, NH
    My Bikes
    rigid 29er moto, s-works stumpjumper fsr,black fixie,masi roadbike, ugly old hardrock commuter
    Posts
    1,257
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by Psychel View Post
    For all the Thompson fans - Is it worth getting the Masterpiece or just the Elite?
    If you can afford the masterpiece, it's a bit lighter. (I would if I could afford it-which i can't)

  23. #23
    Senior Member DannoXYZ's Avatar
    Join Date
    Jul 2005
    Location
    Saratoga, CA
    Posts
    11,507
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by A.Winthrop View Post
    Hi,
    .
    I read somewhere about a guy who wanted to beef up his
    seatpost for reasons I can't remember and he took it to
    his hardware store, found a lenght of wooden dowel that fit
    snuggly into it, greased it so he could get it out some day
    and tapped it in with a mallet. Can't remember what kind
    of seatpost he had but maybe this is possible to do with
    your carbon seatpost? Cheap and you wouldn't add that much
    weight, it seems to me.
    .
    Or you could go aluminum, which might be heavier than the
    carbon\dowel "solution".
    .
    Nor sure if beefing up a tube on the inside would help much as its the outer skin that takes most of the load (stiffness goes up to the 4th-power of diameter). I know a couple guys who needed beefy seatposts for their track bikes. I took a 25.0mm American Classic post and epoxied it inside a chromoly steel tube with 25.0mm ID and 27.2mm OD. Only added about 75gm to the post and easily made twice as stiff as a 27.2mm alloy post.

  24. #24
    Senior Member bigvegan's Avatar
    Join Date
    Feb 2009
    Posts
    659
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    http://www.bikesonline.com/index.php...mm-Silseatpost

    For the love of god, please get a cheap alloy post while you're waiting for your Thomson so we don't have to hear horror stories about you getting carbon shards surgically removed from your perineum!

  25. #25
    Senior Member Road Fan's Avatar
    Join Date
    Apr 2005
    Location
    Ann Arbor, MI
    My Bikes
    Terraferma 650b, Mondonico SL and ELOS, Masi Gran Criterium, Trek 610, Breezer Liberty, Georgena Terry Classic
    Posts
    11,144
    Mentioned
    6 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by bigvegan View Post
    http://www.bikesonline.com/index.php...mm-Silseatpost

    For the love of god, please get a cheap alloy post while you're waiting for your Thomson so we don't have to hear horror stories about you getting carbon shards surgically removed from your perineum!
    Actually, this raises the possibility that your Cervelo post is just plain broken!

    For my money, the Elite is adequate.

    What setback do you need? If your old one has no setback, get a standard straight Thomson. If it has the usual 15 mm setback, get a setback Thomson. Also get the Thomson in at least the same length as your old post.

    Setback Thomsons are harder to find.

Page 1 of 2 12 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •