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Old 02-19-09, 07:35 PM   #1
Psychel
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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!!
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Old 02-19-09, 08:23 PM   #2
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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).
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Old 02-19-09, 08:24 PM   #3
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Sorry- I only use thomson(on 5 bikes).
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Old 02-19-09, 11:52 PM   #4
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Do you really think a seatpost contributes to comfort? I've never noticed any difference.
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Old 02-20-09, 12:04 AM   #5
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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.
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Old 02-20-09, 12:12 AM   #6
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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??!!
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Old 02-20-09, 03:16 AM   #7
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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.
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Old 02-20-09, 10:05 AM   #8
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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
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Old 02-20-09, 10:11 AM   #9
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+1 Just get an alloy post.
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Old 02-20-09, 07:37 PM   #10
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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.
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Old 02-20-09, 08:46 PM   #11
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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......
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Old 02-20-09, 08:50 PM   #12
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I have to wonder how much seatpost is showing out of the tube.
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Old 02-20-09, 09:40 PM   #13
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Al Post and get a saddle to provide confort for your back, elastomer built in or Ti rails
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Old 02-20-09, 10:58 PM   #14
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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
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Old 02-20-09, 11:06 PM   #15
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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.
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Old 02-21-09, 02:20 AM   #16
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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...
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Old 02-21-09, 02:50 AM   #17
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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.
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Old 02-21-09, 02:55 AM   #18
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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?
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Old 02-21-09, 08:03 AM   #19
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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.
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Old 02-21-09, 09:19 AM   #20
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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.
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Old 02-23-09, 05:26 PM   #21
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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?
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Old 02-23-09, 05:39 PM   #22
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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)
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Old 02-24-09, 12:23 AM   #23
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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.
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Old 02-24-09, 12:58 AM   #24
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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!
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Old 02-24-09, 06:12 AM   #25
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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.
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