How short is long enough?
 

I recently bought a new seatpost (Nitto S-83); a great deal at 35% off. Unfortunately when it arrived it was a 300mm and I ordered a 250mm. I emailed the vender and they said they would pay postage for the return but they don't have any 250mm to replace it with. Due to the fact that I can just cut it down I declined. The issue now is how short. The post the new post is replacing was a 250mm, but even it was long longer than it needed. The bike has standard geometry and can use a very short seatpost, i.e. less than 180-200mm would work with plenty to spare. Is there any reason I should not cut it down to 200mm?

You need roughly 60mm inside the frame, or enough to reach 25mm below the bottom of the top tube (whichever is deeper). Knowing that you can feel free to cut any extra length off, but leave yourself a margin for error, since the added weight of 10mm of post in negligible.

So set your final seat height and mark the post where it emerges from the frame. Put the post next to the seat tube and determine where the bottom needs to reach and draw your cut line anywhere below that. The only thing you're giving up is some ability to resell the bike to a taller person.

Why cut it at all? Leaving it full length helps resale possibilities.

+1 for leaving it full length if you can fit it far enough into the frame. I don't see any downside to leaving it longer except for the minor weight savings.

i'm kind of sensitive about excess weight on my bikes, but even I might hesitate to cut just 50mm.

For error, resale, and also changes in saddles, shoes, pedals, crank, etc., which can affect how much post you need.

Same here. I'm a bit of a weenie in that I like to know what everything weighs, but I won't sacrifice other things like useability or function to save weight. The only reason I'd cut down a post is if it didn't fit all the way in the frame.

How much would you save on ~100mm of post? 50g maybe?

Are you sensitive about excess weight on your bike's motor? :roflmao:

The seatpost is a close fit so I figure the shorter the better; weight hasn't even a consideration. Besides I have several longer seatposts that will fit if I ever want to sell the bike; but this one is special to me. http://bikeforums.net/attachment.php...hmentid=243377


BTW, I am retired with a nice pension and have 19 bikes in my garage. I have only sold one bike ever, which was an old Nishiki my 10 YO son excitedly found rusting in front of a house saying "Take me and give me some love" and he charged me with fixing it up. I turned the bike into fixie including lots of old school parts I had laying around, powder coating, stainless steel spokes and a fixie hub. One of my riding worked with a guy who wanted a fixie in a bad way. I showed him the bike just to show how you can build one up and it was love at first sight; who was I to stand between them?

In that case, when you cut the tube, cut it on an angle to maximise the removed material.

I wouldn't go much past what would be level when it's inserted, though.

I had short stem on my bike once, and rather than getting a new one, I took it past the max lines.

One day while riding to work, riding with no hands on the handle bars, I couldn't help but laugh as my butt hit the ground and the bike kept rolling on as if nothing happened. Man, I had that bike dialed in, it went at least thirty yards before it fell over.

Lucky for me and my bike it wasn't a busy street, and no cars were driving by at the time.

So I'd just recommend making sure that you don't take too much off if you cut it down.

This confused the hell out of me until I realised you were referring to the seatpost...

The stem is the bit connecting the bars to the forks.

Cutting off the excess length is no problem and easy with an decent fine tooth hacksaw and you don't even have to worry about the cut being exactly 90° they way you would with a steerer.

Insert the seatpost into your frame and mark it at the desired saddle height. Then remove it and measure the distance from the end of the post to the minimum insertion line (typically 60 to 75 mm). Add that distance below the mark you made on the installed post and cut a few cm below that for some future adjustability.

I have cut down a couple of 350 mm Easton aluminum MTB posts to 250mm, which is still about 50 mm longer than I need, using a hacksaw and a steerer cutting guide (because I happen to have one) and saved 60 grams on a 31.6 mm seatpost.

Now, before anyone asks why I didn't just buy a road seatpost to begin with, I couldn't find decent quality two-bolt road posts in the diameters I needed at reasonable cost and Jenson had the Easton MTB posts on sale at a not-to-be passed up sale price.

Only if the braze-ons in the seat tube prevent getting the saddle low enough,
should sawing off the seat post be considered,

there is a dont go above this mark on many seat posts,

rule of thumb
it needs to insert below the bottom of the seat tube/top tube join .

Bah

Believe me this was a serious concern, but my situation is quite different. The amount of seatpost I need is only 80mm above the crown. I cut the 300mm post down to 200mm; there is 120mm of post in the seat tube. Speaking of max lines, Nitto seems to have a lot of faith in seat tubes as the max line on the 300mm post is only 65mm; looking at it it just didn't seem to be enough (if you look at the photo I attached closely you can see the max line; it's the second line from the bottom.

65cm is plenty of insertion depth for a post into a tube. It exceeds 2 diameters which is the rule of thumb for the first condition of safe fit. When you see deeper insertion depths such as on certain Thomson posts, it isn't because the post would tear out of the tube. It's because the OEM customer has a seat tube that extends above the seat tube, so the minumum insertion is increased to get the bottom of the post to below the bottom of the top tube (second condition of correct fit).

Yeah, sorry....was a little too amused thinking back to that incident.

So is minimum insertion meant to be measured at the top of the seat tube or in the seat tube even with the top tube.