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



## hyrepower (Sep 7, 2009)

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.


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## GhostRing (Feb 29, 2012)

*"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?


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## rangeriderdave (Aug 29, 2008)

Are you sure that you have the correct diameter post? There many sizes of posts ,sometimes you need to use shims for the correct fit. Check with you LBS.


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## Spec7 (May 3, 2000)

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.


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## Snfoilhat (May 3, 2010)

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.


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## vqdriver (May 8, 2009)

1 - confirm the seatpost diameter is correct

2 - lose the grease and try again. can try carbon (friction) paste instead

3 - replace the clamp. uneven clamp pressure has caused this more often than not for me.


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## Glide the Clyde (Nov 12, 2009)

Friction paste then a new clamp, in that order.


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## hyrepower (Sep 7, 2009)

GhostRing said:


> *"as it is never supposed to move."*
> 
> My descents would prove otherwise...
> 
> ...


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.


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## hyrepower (Sep 7, 2009)

Thanks guys.


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## SpyderPride (Jul 22, 2008)

Just clean the grease off. Use a rag with degreaser then some rubbing alcohol. If that doesn't work then something is wrong.

--
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## Jinxy (Aug 1, 2010)

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.


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## hyrepower (Sep 7, 2009)

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.


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.


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## mbmb65 (Jan 13, 2004)

Measure the post to make sure it's within tolerance.


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## GTR2ebike (May 3, 2010)

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.


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.


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## theMeat (Jan 5, 2010)

Malibu412 said:


> Friction paste then a new clamp, in that order.


This, and make sure it's the right size post for your frame.


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## hyrepower (Sep 7, 2009)

GTR2ebike said:


> 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.


Wouldn't reaming the post make it smaller, it would need to be bigger to make it tighter.


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## 1niceride (Jan 30, 2004)

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..


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## hyrepower (Sep 7, 2009)

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..


What if I do move it a lot? I like different settings from street to dirt.


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## 1niceride (Jan 30, 2004)

hyrepower said:


> What if I do move it a lot? I like different settings from street to dirt.


If you are lucky the shim might stay in place. Otherwise it is a pain if the shim ends up down inside the seat tube and you have to dig it out and its all grease. I'm used to it but ive been doing this for years.


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## Berkley (May 21, 2007)

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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## jeffj (Jan 13, 2004)

I had this issue with a new Super V several years back. After trying several different clamps that did not solve the proble, a new Salsa clamp was the only one I found that worked and it did so for the seven years I kept the frame.


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## hyrepower (Sep 7, 2009)

Berkley said:


> 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.
> 
> ...


Who in your opinion makes the best seat post clamp?


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## Spec7 (May 3, 2000)

hyrepower said:


> Who in your opinion makes the best seat post clamp?


First clamps that come to mind are:
Salsa
Hope
Thomson


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## Slash5 (Nov 27, 2011)

There are double height seat clamps available for just this problem. Depends on your seat tube diameter if they make one to fit.
You should measure the seat post diameter - if it's out of spec you could make a warranty claim. After all, this isn't a $10 seat post you are looking at.


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## zebrahum (Jun 29, 2005)

Those stock Cdale clamps are useless. The first clamps that I would suggest are: Chromag, Salsa, Hope.


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## rhjohn (Nov 6, 2012)

I would get a new tube clamp, remove the grease and add friction powder. Make sure you remove the grease from inside the frame tube.


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## zebrahum (Jun 29, 2005)

Don't remove the grease, that's how frames die. If you absolutely can't get the post to stop slipping by improving the clamp then you add a friction compound.


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## Noclutch (Jun 20, 2010)

My Rumblefish did this until I replaced the OEM chunky flip lever clamp with a Salsa lip lock-> prob solved YMMV


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## Bokchoicowboy (Aug 7, 2007)

Had this problem with a frame that came with a post...always had slip problems. I figured the frame was probably the culprit as the seatpost measured at what was stamped on it. I went up to the next decimal increment size and got a taller clamp that was also non-quick release. Problem solved.


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## SpyderPride (Jul 22, 2008)

zebrahum said:


> Don't remove the grease, that's how frames die. If you absolutely can't get the post to stop slipping by improving the clamp then you add a friction compound.


How does that kill the frame?

--
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## wookie (Jan 24, 2007)

Use minimal grease (light film). Too much causes slippage and too little will cause the post to become stuck in the frame.


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## zebrahum (Jun 29, 2005)

SpyderPride said:


> How does that kill the frame?
> 
> --
> Sent from my DROID Incredible 4G LTE using Tapatalk 2


Grease stops corrosion. Corrosion kills frames. Anyone who's worked at a bike shop long enough has put a seatpost into a vice or attacked one with a blowtorch and a gallon of PB Blaster because someone's let it sit in the frame with no lube on it.

There is absolutely no reason to remove grease from a seatpost, if you are having issue with slipping seatposts then you can't clamp hard enough or you have a tolerancing issue with the parts you are trying to fit.


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## SpyderPride (Jul 22, 2008)

I suppose, if its getting wet and left in there for years. I'd think if you kept it clean and dry there wouldn't be any significant corrosion since its aluminum. Non-lubricating corrosion inhibitor sounds like a good idea if you are worried about it.

Heavier rider + QR clamp + greasy seatpost = unacceptable movement IME


--
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## zebrahum (Jun 29, 2005)

SpyderPride said:


> I suppose, if its getting wet and left in there for years. I'd think if you kept it clean and dry there wouldn't be any significant corrosion since its aluminum. Non-lubricating corrosion inhibitor sounds like a good idea if you are worried about it.
> 
> Heavier rider + QR clamp + greasy seatpost = unacceptable movement IME
> 
> ...


You can get corrosion from water intrusion from riding conditions, you can get corrosion from sweat getting into your post, and you can get galvanic corrosion from dissimilar metals contacting. I would never recommend riding a dry seatpost but everyone gets to make their own decisions.


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## SpyderPride (Jul 22, 2008)

zebrahum said:


> You can get corrosion from water intrusion from riding conditions, you can get corrosion from sweat getting into your post, and you can get galvanic corrosion from dissimilar metals contacting. I would never recommend riding a dry seatpost but everyone gets to make their own decisions.


Everything you say here is absolutely true. But I'd rather pull the post out and clean the seat tube once a month than have things sliding around as I ride.

--
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## zebrahum (Jun 29, 2005)

SpyderPride said:


> Everything you say here is absolutely true. But I'd rather pull the post out and clean the seat tube once a month than have things sliding around as I ride.
> 
> --
> Sent from my DROID Incredible 4G LTE using Tapatalk 2


Unfortunately based on my experience of seeing a lot of bikes, most people are not like yourself and they won't pull things out to clean and check. So to stay on the safe side, I stick with my recommendation but there are always more than one way to tackle an issue and your method is correct so long as a little care is taken.


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## car bone (Apr 15, 2011)

SpyderPride said:


> I suppose, if its getting wet and left in there for years. I'd think if you kept it clean and dry there wouldn't be any significant corrosion since its aluminum. Non-lubricating corrosion inhibitor sounds like a good idea if you are worried about it.
> 
> Heavier rider + QR clamp + greasy seatpost = unacceptable movement IME
> 
> ...


this is bs imo, alu dont rust per se but it definitely corrodes. Water in an uncoated alu cooking pan will corrode in less than 2 weeks. Turns white, also severe pitting. Duie to unknown to me mechanisms, maybe hydrolysis or whatever, but it does corrode.

When steel rusts the iron oxide molecules gets bigger then iron itself thats why it flakes off, it tazkes up more space as an oxide, but alu is not like that, alu oxide don't grow and it wont flake off. But stiull you have corrosion and possible problems, thats whay you coat seatpost and tube with grease. Also galvanic potential can speed this up incredibly fast. You definitely have a galvanic potentuial between different alu alloys and their oxides/anidize finishes. May be small though. But still present. So something has to act as the sacrificial anode. whats it gonna be?


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## hyrepower (Sep 7, 2009)

Well I am going to try getting a new seatpost clamp. However I do not know what size to get. I have a 2013 Cannondale F29 2, with a 27.2mm seatpost that it came with. So what size clamp would I get? 

I see Salsa offers a 28.6mm, 30mm, 30.6mm and so on. Which one should I get, I do not own a micrometer so I can not measure.


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## ShinDiggity (Mar 29, 2010)

hyrepower said:


> Well I am going to try getting a new seatpost clamp. However I do not know what size to get. I have a 2013 Cannondale F29 2, with a 27.2mm seatpost that it came with. So what size clamp would I get?
> 
> I see Salsa offers a 28.6mm, 30mm, 30.6mm and so on. Which one should I get, I do not own a micrometer so I can not measure.


Take the bike back to the shop you bought it from and have them help you. Have them check the tolerances. Check the seat tube diameter and have them physically measure the new seatpost to make sure it is a 27.2.

Did the OEM seatpost move as well? If not I would guess the Thudbuster is out of spec.

If the frame is carbon, which it sounds like it is, use friction paste. The stuff really works.

A new seat clamp is quite possibly a good idea regardless. OEM clamps are notoriously junk. The seat clamp size is measured on the frame seat tube where the clamp slides over the opening. Any shop can measure this.

One or multiple of these should solve your problem.


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## car bone (Apr 15, 2011)

MEASURE IT! seriously you have to get a "vernier caliper", otherwise you are only guessing, and we are too. You need to know the dimensions. Look up on youtube how to use a vernier caliper. the cheapest **** on earth except plastic will do for you. for good stuff it needs to say mitutoyo on it. I only have mitutoyo. and they only cost like 30-40 bux.

You need to measure the outside diameter of the frame where the post goes in. that affects your purchase of a seatpost clamp. 0.1mm to large means it won't work. ok.


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## hyrepower (Sep 7, 2009)

car bone said:


> MEASURE IT! seriously you have to get a "vernier caliper", otherwise you are only guessing, and we are too. You need to know the dimensions. Look up on youtube how to use a vernier caliper. the cheapest **** on earth except plastic will do for you. for good stuff it needs to say mitutoyo on it. I only have mitutoyo. and they only cost like 30-40 bux.
> 
> You need to measure the outside diameter of the frame where the post goes in. that affects your purchase of a seatpost clamp. 0.1mm to large means it won't work. ok.


Sounds good. I'll have it measured before I buy a new Salsa Clamp. I would imagine I should buy the next size down not up, if it does not measure exactly to Salsas Specs? So lets say the frame measured 30.4mm, I should buy a 30mm seat clamp, not a 30.6mm


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## ShinDiggity (Mar 29, 2010)

Did you try friction paste?

Did you have the Thudbuster measured?

Did you have the dealer help you?

A new seatpost collar might not solve the problem.

Try the friction paste, measure the Thudbuster/frame and as far as the dealer goes ....


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## wahday (Mar 23, 2012)

I have had this problem with a bike that was notorious for it. I used several "quick fixes" in the field, including toothpaste with baking soda (the grit helps). I wasn't comfortable with that as a permanent fix, so ultimately I used some friction paste and cleaned out my QR clamp and that did the trick. Others with this same make and model replaced the seatpost. Apparently the original was milled a tad too narrow and that was the problem (Bontrager Select, 2003).

Your LBS should be able to measure the tube and post for you instead of buying calipers (unless that's a tool you want to own).

As far as the corrosion discussion goes, I am not a bike mechanic, but I can tell you without question that ALL metals have a tendency to oxidize.The relative strength of this tendency is measured by the Galvanic Scale. *Zebrahum* knows what he is talking about!


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## hyrepower (Sep 7, 2009)

The cannondale seatpost measures 27.3mm-27.33mm, the micrometer is an off brand. 

The Thudbuster measures 27.3..-27.36mm using the same tool. 

The cannondale frame measures 27.23mm Inside diameter 31.13mm outside diameter.

so would I get the the 30.6mm or the 32mm Salsa clamp?


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## hyrepower (Sep 7, 2009)

I somehow knew I would be right in the middle, lol.


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## ShinDiggity (Mar 29, 2010)

I guess having questions answered is out of the question for some unknown reason. I actually was curious to know if you tried the friction paste. Was also kind of curious why one wouldn't rely on the LBS to help. Oh well. 

You now seem fixated on getting a new clamp to solve the problem. Hope that works. Good luck.


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## car bone (Apr 15, 2011)

the 30.6 is closer. you can spread that. I would at least


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## hyrepower (Sep 7, 2009)

I just bought a 30.6mm Salsa to install since the inner diameter of my Cannondale seat clamp measures 30.6ish mm. I also got some friction paste for aluminum/carbon which I will apply as well. I am also going to get a new seat post. So I am taking all remedies and applying as I want a first try success. 

Thanks again for the the help guys.


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## Berkley (May 21, 2007)

hyrepower said:


> The cannondale seatpost measures 27.3mm-27.33mm, the micrometer is an off brand.
> 
> The Thudbuster measures 27.3..-27.36mm using the same tool.
> 
> ...


Are these measurements correct? If I'm reading this right, then your frame I.D. is smaller than the post and that can't be the case.

31mm is definitely an odd size. 32 will be way too big and 30.6 will be really tight. I'd try to find one that's 30.9 or 31.0mm. They seem t be uncommon though.


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## theMeat (Jan 5, 2010)

hyrepower said:


> The cannondale seatpost measures 27.3mm-27.33mm, the micrometer is an off brand.
> 
> The Thudbuster measures 27.3..-27.36mm using the same tool.
> 
> ...


I'd guess measuring and/or your tool is off and your post is 27.2, and the frame is 31.8, just a guess from over the internet. Why not call Cannondale, head to lbs, maybe even the one where you bought it and find out the clamp size.
BTW Zebra's right about having something on the post to stop ceasing.


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## dwt (Jul 19, 2009)

Back in the day, I used to run a suspension post which required a shim to fit my seat tube. No matter how hard I torqued the clamp, the post would slip.

The solution I found was to fasten a hose clamp on the post at the junction with the clamp. It was ugly but it didn't slip and it kept the post from slipping









Good luck.

Sent from my iPhone using Tapatalk


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## SpyderPride (Jul 22, 2008)

Yes, all metals oxidize. Even stainless steel which is simply rust-resistant. All aluminum you see which hasn't just been cut has an oxide layer on it, which is actually harder and protects it from further oxidation. Or, it is anodized/powdercoated/painted/etc.

So you grease it to keep the electrolyte off or you manually remove the electrolyte. If I were worried about corrosion and wanted to skip the cleanings, I would use something non-lubricating since I too have slippage issues. I may actually look into it.

--
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## hyrepower (Sep 7, 2009)

Berkley said:


> Are these measurements correct? If I'm reading this right, then your frame I.D. is smaller than the post and that can't be the case.
> 
> 31mm is definitely an odd size. 32 will be way too big and 30.6 will be really tight. I'd try to find one that's 30.9 or 31.0mm. They seem t be uncommon though.


I corrected the post. It should say 27.23


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## Glide the Clyde (Nov 12, 2009)

dwt said:


>


That's just silly.


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## mbmb65 (Jan 13, 2004)

car bone said:


> MEASURE IT! seriously you have to get a "vernier caliper", otherwise you are only guessing, and we are too. You need to know the dimensions. Look up on youtube how to use a vernier caliper. the cheapest **** on earth except plastic will do for you. for good stuff it needs to say mitutoyo on it. I only have mitutoyo. and they only cost like 30-40 bux.
> 
> You need to measure the outside diameter of the frame where the post goes in. that affects your purchase of a seatpost clamp. 0.1mm to large means it won't work. ok.


A vernier caliper? Why? Does anyone even use vernier calipers anymore? A good dial caliper is the answer. Or digital even.


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## natzx7 (May 30, 2007)

This is what I have:
6 Inch LCD Digital Caliper with Extra Battery and Case: Amazon.com: Industrial & Scientific
It's cheap, works fine for the home mechanic. I would be totally guessing without it. It even came with a spare battery!


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## dwt (Jul 19, 2009)

Malibu412 said:


> That's just silly.


But it worked. What do I care if it's silly?

Sent from my iPhone using Tapatalk


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## theMeat (Jan 5, 2010)

dwt said:


> But it worked. What do I care if it's silly?


Caring or not doesn't change it being silly. The right sizes working together with a decent clamp and some lube works. But I guess so would welding, drilling a bolt throu, glue, or a hose clamp, etc., hey maybe some vise grips for adjustment on the fly.


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## junior1210 (Sep 9, 2011)

Once again you could buy on of these;


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## dropspace (Jan 1, 2007)

1 red bull can and a pair of scissors. By the time the red bull kicks in you can be on the trail.

happens to me all the time with Thomson posts. I think their posts are made to tighter tolerances than the average mass produced mtb frames.

Honestly though, if you cut it the right size and set it properly you can shim it and you cant even tell its shimmed. I can even adjust the post and the shim stays put if you (lightly) grease both sides of it.


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## dwt (Jul 19, 2009)

junior1210 said:


> Once again you could buy on of these;


You could, but it is just a pretty and pricey hose clamp. So therefore "silly" according to certain snobby wrenches in this thread. I'm all about low tech, cheap and simple if doing it by the book fails. Hose clamp, Red Bull or Guinness shim, whatever works.

Sent from my iPhone using Tapatalk


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## hyrepower (Sep 7, 2009)

dwt said:


> You could, but it is just a pretty and pricey hose clamp. So therefore "silly" according to certain snobby wrenches in this thread. I'm all about low tech, cheap and simple if doing it by the book fails. Hose clamp, Red Bull or Guinness shim, whatever works.
> 
> Sent from my iPhone using Tapatalk


LOL. Well I just bought some friction paste and a Salsa Clamp. If that does not work, I will try the $.01 fix as opposed to the $25 I just spent on the paste and clamp, haha.


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## theMeat (Jan 5, 2010)

dwt said:


> You could, but it is just a pretty and pricey hose clamp. So therefore "silly" according to certain snobby wrenches in this thread.


"Snobby", hmm, that's a new one for me. Is it snobby or silly to fix it the right way? I don't really get what you do with the hose clamp either, do you put it in place of the seat clamp where it can't be nearly as strong as a decent clamp, or on the post itself above the frame to stop it from sliding down but it can still rotate and scratch off the paint?


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## natzx7 (May 30, 2007)

Not to jump on the bandwagon here, but as someone who has spent 25 years in boat engine rooms.....I have more hose clamp scars that I can count. Those things hurt.


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## dwt (Jul 19, 2009)

theMeat said:


> Or on the post itself above the frame to stop it from sliding down but it can still rotate and scratch off the paint?


This.

It did not rotate or slide. As I posted, the clamp was used on a suspension post (U.S.E.) which came with a shim, and no matter how tight I torqued the seatpost clamp, the post never held in place without the hose clamp.

This was years ago, I no longer have the post. Now run a dropper where this problem does not exist by design.

Sent from my iPhone using Tapatalk


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## patineto (Oct 28, 2005)

A warning...:madman:

You need to be careful if running a seat post is not the right size, you can deform the seat tube to the point it can generate stress cracks, plus is not like Cannondale is fun of replacing frames do to "Miss use", just take it to the shop you got it from and start from there, specially if you have no clue how to use a measuring device.


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## rz79 (Jan 19, 2011)

Sooo... Did the clamp work???!!!


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## swanhild (Oct 1, 2013)

I guess it has something to do with the diameter post. Well hopefully I hope you got that one fixed by a techncian nearby.


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## FastBanana (Aug 29, 2013)

A little chalk where it clamps will work good if it is the right size.


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