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Seat post keeps moving and dropping, no matter how tight I make seat post clamp, HELP

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hyrepower

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Discussion starter · #1 ·
I just bought a new 2013 Cannondale Flash 29er. Love this bike, very fun to ride.

In any event I just put on a new Thudbuster seatpost and I can not get it to stay in place. No matter how tight I make the Cannondale single bolt seat clamp it moves, fairly easily. The clamp says to tighten to 5nm which I did and this was waaaaaay too lose to keep it even remotely stable. So I went further, to the point where I noticed the clamp bending, and it is firmer in place, but still moves way to easily. What could the problem be? I noticed there is lithium grease in the seat post shaft, don't know why this would be there, but could this be making not grip well enough? Is it supposed to be there, I can't imagine why as this is not an area that should need lubrication as it is never supposed to move.
 
#2 ·
"as it is never supposed to move."

My descents would prove otherwise... ;)

Maybe lose a little of the grease? I also don't know if you might have purchased
the wrong diameter post for your frame?
 
#4 ·
If you have the correct diameter seat post, then it sounds like the Thudbuster needs a case of the beer can shim's.

If I needed a shim, I'd use a New Belgium "Shift" can to make the shim, because, well it seems appropriate. Just don't cut yourself.
 
#5 ·
One of the two parts (seat tube or seat post) could be out of spec (or even not the same nominal size, there are a ton of offbeat seat post diameters that could get mixed up--no offense intended just thoroughly troubleshooting).

You can try a friction paste instead of grease, like you often see for carbon components. If that's not enough, a shim may work.
 
Discussion starter · #8 ·
GhostRing said:
"as it is never supposed to move."

My descents would prove otherwise... ;)

Maybe lose a little of the grease? I also don't know if you might have purchased
the wrong diameter post for your frame?
Click to expand...
Well according to cannondale spec sheet it is a 27.2mm size, this is the one I ordered and that is what the box said as well. I wonder if it could have been mis-labled and they sent me a 27.0mm by mistake. Would it fit snug though if it was a 27.0 in a 27.2mm frame? Or would it wiggle. I mean it stays put to not collapse with me being 215lbs. on it. I needs to ride it and jumps curbs and hit a lot of pot holes to get it move down and to the side, but it does nonetheless.
 
#10 ·
Just clean the grease off. Use a rag with degreaser then some rubbing alcohol. If that doesn't work then something is wrong.

--
Sent from my DROID Incredible 4G LTE using Tapatalk 2
 
#11 ·
From your description I'd suspect post & tube are mismatched. Thudbusters are stamped with size as are many seatposts. Shims aren't a good solution. I've encountered sliding seat posts on 2 of my bikes. On long rough rides I'd notice 5mm or so movement. A little chalk dust on the post increased the clamp bite and solved my problems.
 
Discussion starter · #12 ·
Jinxy said:
From your description I'd suspect post & tube are mismatched. Thudbusters are stamped with size as are many seatposts. Shims aren't a good solution. I've encountered sliding seat posts on 2 of my bikes. On long rough rides I'd notice 5mm or so movement. A little chalk dust on the post increased the clamp bite and solved my problems.
Click to expand...
I have checked the seatpost it does say 27.2 mm. I did remove all of the grease or white paste that came on my cannondale post From the inner part of the frame. I Torqued the seat clamp to 7 ft/lbs Which is about double what is required and the damn post still moves.

Maybe I somehow got a defective post that was machined wrong.
 
#14 ·
hyrepower said:
I have checked the seatpost it does say 27.2 mm. I did remove all of the grease or white paste that came on my cannondale post From the inner part of the frame. I Torqued the seat clamp to 7 ft/lbs Which is about double what is required and the damn post still moves.

Maybe I somehow got a defective post that was machined wrong.
Click to expand...
Is the seatpost scratched a lot? Cannondales should have the seat tube reamed before installing a post. Remove all the grease and ream it, should be fixed.
 
#17 ·
Pull off the clamp and spread open the top of the seat tube back to size. Make a beer can shim to fit the post correctly and go ride. Post probably on low side of tolerance and tube high side. I have often had to use some shimming on many of my bikes. If you move your seat up and down a lot forget the can shim..
 
Discussion starter · #18 ·
1niceride said:
Pull off the clamp and spread open the top of the seat tube back to size. Make a beer can shim to fit the post correctly and go ride. Post probably on low side of tolerance and tube high side. I have often had to use some shimming on many of my bikes. If you move your seat up and down a lot forget the can shim..
Click to expand...
What if I do move it a lot? I like different settings from street to dirt.
 
#20 ·
I'm guessing this sucker is aluminum? If that's the case, proceed. But if it's carbon, don't ream, pry or shim anything.

1. Reaming the seat tube won't help, that's for a tight fit and will only make the problem worse.

2. Shims can work, but are really a band-aid fix to a bigger problem that you shouldn't have on a brand new bike.

3. As counter-intuitive as it seems, lubricant can actually fill in tiny gaps between frame/seatpost and make the interface grip better. Plus it prevents the sucker corroding itself in place over time. Grease is normally a fine choice, but friction paste can work better (it's designed for carbon frames/posts).

4. You're a big guy riding a lightweight bike, with a tiny, lightweight seat clamp. Lightweight seat clamps usually don't have the clamp force that heavier ones do. If they did, they'd be too easy to over-torque and break. Replace the clamp with a heavier duty one and I bet your problem is solved.
 
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