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Finding the right seatpost

Old 04-30-17, 05:09 AM
  #1  
rubiconazoid
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Finding the right seatpost

I have a Trek 720 that I'm trying to make some seat adjustments on. The current saddle is too high in the front for comfort with the stock seatpost. I believed the diameter of the seatpost to be 27.2mm and ordered one, but that size is too big. I really believe that I need an adjustable seatpost that I can change the angle of the seat with. Does anyone know of an adjustable seatpost that would fit this bike.

My guess is that looking at how large the 27.2mm post seems to be, that it would take at least 1mm less in OD than the 27.2mm post.

Much thanks
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Old 04-30-17, 05:22 AM
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.... all seatposts let you adjust the saddle angle
you loosen the bolt underneath that holds the clamps around the saddle rails
angle those clamp plates up or down
retighten the bolt

and given that it's a Trek
27.2 is almost Certainly the correct size
maybe your seattube is slightly warped making it hard to insert
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Old 04-30-17, 06:09 AM
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Originally Posted by xenologer
.... all seatposts let you adjust the saddle angle
you loosen the bolt underneath that holds the clamps around the saddle rails
angle those clamp plates up or down
retighten the bolt

and given that it's a Trek
27.2 is almost Certainly the correct size
maybe your seattube is slightly warped making it hard to insert
It is a 1990's era bike and I'm almost certain it's not warped. I gotta believe it's gonna take a smaller size. Maybe I could shim the clamps or something.
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Old 04-30-17, 06:15 AM
  #4  
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One accurate measurement is worth 1,000 assumptions.

The first thing that you should to is to look at the bottom of your stock seatpost to see if the diameter is stamped there. It usually is. If it isn't, get a caliper or micrometer and measure your seatpost accurately. Seatpost diameters come in 0.2mm increment and even a tiny bit off makes a huge difference.
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Old 04-30-17, 06:40 AM
  #5  
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I'm certain the stock seatpost on your Trek is adjustable for saddle angle and fore-and-aft location. Even the seatposts that come on Big-Box store bikes are adjustable and those on bike shop quality bikes always are. Ask a knowledgable friend or even go to a bike shop to be shown how.

I have had Treks from the early '80's through the late '90's and the all had adjustable seatposts. There were no other kind.
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Old 04-30-17, 06:45 AM
  #6  
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Originally Posted by Retro Grouch
One accurate measurement is worth 1,000 assumptions.

The first thing that you should to is to look at the bottom of your stock seatpost to see if the diameter is stamped there. It usually is. If it isn't, get a caliper or micrometer and measure your seatpost accurately. Seatpost diameters come in 0.2mm increment and even a tiny bit off makes a huge difference.
+1 Trying to clamp a too-small post can distort your seat tube. Many cycle shops have a seat post sizing tool, which has a stepped diameter and can be used to determine the correct size. Like this: https://www.amazon.co.uk/Cyclo-Tools.../dp/B00JQN4JCC
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Old 04-30-17, 07:22 AM
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Originally Posted by dsbrantjr
+1 Trying to clamp a too-small post can distort your seat tube.....
I think we've gotten too fixated on the OP's seatpost diameter question and ignored the high probability that there is nothing wrong with his current one. He just has to learn how to adjust the saddle clamp.
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Old 04-30-17, 08:35 AM
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Picture of the top of the seatpost where the seat is attached would be helpful.
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Old 04-30-17, 09:26 AM
  #9  
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All Trek 720s were reamed to accept a 27.2mm post. If yours does seem to allow that size, it's likely because the top of the seat tube is no longer round, perhaps from a previous owner using an undersize post. Measure the diameter carefully at two locations about 90 apart. You'll likely find that they are different and that is the source of your problem.

Edit: the above information applies to 1980s Waterloo-built lugged Reynolds 531 Trek 720s, not Asian built TIG welded 720s from the 1990s. Those may well use a different size, but that was long after I left Trek, so I can't tell you want it might be. Why did Trek recycle old model numbers into entirely different new models? It's not as if we're going to run out of numbers, after all.

Originally Posted by rubiconazoid
It is a 1990's era bike and I'm almost certain it's not warped. I gotta believe it's gonna take a smaller size. Maybe I could shim the clamps or something.

Last edited by JohnDThompson; 04-30-17 at 09:31 AM.
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Old 04-30-17, 09:32 AM
  #10  
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Mid '90s Trek Multi-Track 720's have 26.6mm seat post diameters per the 1994 Trek Retail Technical Manual and my own Multi-Track 720.

Personally I like these two seat posts:
https://www.amazon.com/dp/B001CJV7AK
https://www.amazon.com/dp/B000AOZ1L4
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Old 04-30-17, 09:57 AM
  #11  
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Originally Posted by JohnDThompson
All Trek 720s were reamed to accept a 27.2mm post. If yours does seem to allow that size, it's likely because the top of the seat tube is no longer round, perhaps from a previous owner using an undersize post. Measure the diameter carefully at two locations about 90 apart. You'll likely find that they are different and that is the source of your problem.

Edit: the above information applies to 1980s Waterloo-built lugged Reynolds 531 Trek 720s, not Asian built TIG welded 720s from the 1990s. Those may well use a different size, but that was long after I left Trek, so I can't tell you want it might be. Why did Trek recycle old model numbers into entirely different new models? It's not as if we're going to run out of numbers, after all.
I bought it new and the seatpost it has is the original and has never been taken out of the frame until I tried to put the 27.2mm seatpost in for a replacement. It's just 2 big. I'll get a pic and try to post it up.
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Old 04-30-17, 10:21 AM
  #12  
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Originally Posted by rubiconazoid
I bought it new and the seatpost it has is the original and has never been taken out of the frame until I tried to put the 27.2mm seatpost in for a replacement. It's just 2 big. I'll get a pic and try to post it up.
When did you buy it? It looks like the answer to your question is in the post right above yours.

Originally Posted by nfmisso
Mid '90s Trek Multi-Track 720's have 26.6mm seat post diameters per the 1994 Trek Retail Technical Manual and my own Multi-Track 720.

Personally I like these two seat posts:
https://www.amazon.com/dp/B001CJV7AK
https://www.amazon.com/dp/B000AOZ1L4
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Old 04-30-17, 10:39 AM
  #13  
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Originally Posted by SquidPuppet
When did you buy it? It looks like the answer to your question is in the post right above yours.
Yes, the first link looks exactly like the seat post that came originally on the bike. With that particularly seatpost, how can I adjust the angle of the seat? From the looks of the clamp in the picture and the looks of the one on my bike, I'm just not figuring out how I can raise the rear of the seat.
Thanks!
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Old 04-30-17, 10:49 AM
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Originally Posted by rubiconazoid
Yes, the first link looks exactly like the seat post that came originally on the bike. With that particularly seatpost, how can I adjust the angle of the seat? From the looks of the clamp in the picture and the looks of the one on my bike, I'm just not figuring out how I can raise the rear of the seat.
Thanks!
Is this a trick question? Loosen the adjusting bolt. There is only ONE on the post. The clamp's hold on the saddle rails will become loose and sloppy. You can then tilt as you wish and also change the fore/aft position as well.
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Old 04-30-17, 11:08 AM
  #15  
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Originally Posted by rubiconazoid
Yes, the first link looks exactly like the seat post that came originally on the bike. With that particularly seatpost, how can I adjust the angle of the seat? From the looks of the clamp in the picture and the looks of the one on my bike, I'm just not figuring out how I can raise the rear of the seat.
Thanks!
Dude, you loosen the bolt/screw and then tilt the saddle up and down or slide it fore and aft or both. Not rocket science.
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Old 04-30-17, 12:59 PM
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Originally Posted by rubiconazoid
I bought it new and the seatpost it has is the original and has never been taken out of the frame until I tried to put the 27.2mm seatpost in for a replacement. It's just 2 big. I'll get a pic and try to post it up.
Is the size marked on the original post?
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Old 04-30-17, 03:37 PM
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Originally Posted by rubiconazoid
Yes, the first link looks exactly like the seat post that came originally on the bike. With that particularly seatpost, how can I adjust the angle of the seat? From the looks of the clamp in the picture and the looks of the one on my bike, I'm just not figuring out how I can raise the rear of the seat.
Thanks!
The bolt under the seat holds the two clamp parts in a curved cradle. If you loosen it they can slid forward or back in the cradle to adust tilt. This picture of a different seatpost shows it better. Sometimes corrosion fuses the parts, but if you loosen the bolt a bit and jerk the seat or whack it with your fist you can loosen it.

Last edited by cooker; 04-30-17 at 03:41 PM.
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Old 04-30-17, 04:33 PM
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Originally Posted by JohnDThompson
Is the size marked on the original post?
The size doesn't matter. If he can't figure out how to adjust the saddle clamp on his current post, he won't know how to do it on a different one either.
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Old 05-01-17, 03:27 PM
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Originally Posted by HillRider
size doesn't matter.
Whew!
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Old 05-02-17, 03:40 PM
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Thanks for all the replies. I finally had a little time to break it apart and look closely at it and figured it out. Exactly as someone said, the bottow piece that is loose is somewhat curved and the base of the seatpost has a series of grooves in it which allow the adjustment of the seat angle. I was able to make a change by loosening the one allen bolt and moving it slightly back and forth.

I'm a little embarrassed I didn't look closer to begin with, but hey, it's not the first time I've proved how dumb I can be and I'm positive it won't be the last. Please forgive and thanks for the help!
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Old 05-02-17, 04:10 PM
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Originally Posted by rubiconazoid
Thanks for all the replies. I finally had a little time to break it apart and look closely at it and figured it out. Exactly as someone said, the bottow piece that is loose is somewhat curved and the base of the seatpost has a series of grooves in it which allow the adjustment of the seat angle. I was able to make a change by loosening the one allen bolt and moving it slightly back and forth.

I'm a little embarrassed I didn't look closer to begin with, but hey, it's not the first time I've proved how dumb I can be and I'm positive it won't be the last. Please forgive and thanks for the help!
No need for apologies - it's not obvious and it has come up from time to time here before.
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Old 05-03-17, 02:08 AM
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This thread is a perfect example of people telling others a spec that is an inaccurate assumption. I didn't know Trek used 26.6 posts on those bikes.
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Old 05-03-17, 06:34 AM
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Originally Posted by mtnbke
This thread is a perfect example of people telling others a spec that is an inaccurate assumption. I didn't know Trek used 26.6 posts on those bikes.
That's correct but immaterial to the OP's original problem. The seatpost's diameter was assumed incorrectly since almost all Treks of that period, including both of mine, used a 27.2 mm seatpost so that was a reasonable answer, even if wrong in this specific case.

But that wasn't the problem, adjusting the saddle clamp was.
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Old 05-03-17, 07:22 AM
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Originally Posted by mtnbke
This thread is a perfect example of people telling others a spec that is an inaccurate assumption. I didn't know Trek used 26.6 posts on those bikes.
He had already bought the wrong sized post, and discovered it was wrong, and multiple people advised him or her to check the size of the original post, or measure with calipers or at a bike shop, and not to try to fit a wrong post. Nobody gave bad advice.
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Old 05-03-17, 10:39 AM
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Originally Posted by xenologer
.... all seatposts let you adjust the saddle angle
you loosen the bolt underneath that holds the clamps around the saddle rails
angle those clamp plates up or down
retighten the bolt

and given that it's a Trek
27.2 is almost Certainly the correct size
maybe your seattube is slightly warped making it hard to insert
This was incorrect advice.

People shouldn't just assume they know. Reminds me of Old Potatoe offering his expert opinion on what taper Mavic bottom brackets were. Then when challenged becoming adamant that he knew. He just thought he knew and he was wrong.
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