LarrySellerz

Hey guys, I’m doing some work on a bike that I’m going to give to my friend, she’s 5’5. The bike is a 47 and I’m wondering what seat/seatpost combo to go with. I broke the one it came with in another bike, and was going to cut the nasty part off and mark another indicator line. Is this a wild idea?

KCT1986

It may be possible to cut and re-use. See if extends far enough to be below the junction of the seatstay and the seattube when properly fit for rider height

If the seattube has an inner sleeve at the junction of the top tube and widens below the sleeve, then the seatpost should extend at least to the bottom of the sleeve or reasonably close. Any further than the sleeve does nothing.

HillRider

"Minimum Insertion" marks on seatposts I've measured are 60 to 100mm above the bottom of the post. This allows enough to extend below the top tube/seat tube lower edges in most frames when the height is set for the rider. Long seatposts usually have higher MI marks due to their potential increased leverage

WizardOfBoz

If you use the same minimum insertion distance from where you cut off to your new "min insertion depth", and the rest of the post is ok, your modded post should be ok. I'm basing this on 1) The same insertion distance gives the same joint strength, 2) an assumption that your girlfriend is a) smaller (lighter) than you and rides less agressively (i.e. she doesn't break seatposts off routinely while riding, and 3) the shorter post with the same insertion means that the extension is less so you have a shorter moment arm.

Again, I'm assuming that all damage to the post was on the part that you are cutting off.

2old

I purposely cut down a "too long" seatpost once and used the logic above. Many years later it's still functional.

Mr. 66

I would trash it, I would not want to tell anyone 'hey it was broke but it will be ok for you'.

I would just find a post that you don't need to question.

FBinNY 

There is never a problem using a shorter post as long as you have about 2.5" inserted into the frame.

This is based on a long standing guideline of 2 diameters minimum overlap in tube junctions.

The markings are the result of 50 year old CPSC rules intended to prevent consumers without engineering degrees from exceeding that guideline when raising the saddle. This was a serious issue causing injuries when parents tried to avoid buying new bikes for their growing children.

In your case, if all the damage is in the section you cut off, then it's perfectly OK to use the post. No need to mark it unless your friend is still growing.

squirtdad

question: after allowing for for a minimum insertion (and for that I would go to spec for the seatpost and use same is original plus a CM or so.) is there enough seat post left to position height correctly? Seems pretty short

Hondo6

Same here, but for a different reason.

CF laminates can sometimes have internal damage that isn't obvious externally. This one has been stressed enough to break.

I'd worry about it having unseen (and possibly un-seeable without x-rays or similar remote sensing) internal cracks or de-lamination above where I'd assumed there was no further damage. For that reason I'd never trust it.

And I damn sure wouldn't provide it to a friend and assure them it was OK if I couldn't be positive it was.

mpetry912 

Exactly right, if it was an aluminum post, sure, but not a CF one.

also beware, there are counterfeit name brand seatposts and other components out there. I bought one that was badged "Ritchey" even the packaging, was cheap like 30 buck, not even good enough to be a cheap knockoff. Nt Wrth Sht as they say

I would not give that post to a friend, you would never forgive yourself if it failed.

/markp

maddog34

+ 1 on this... and being who you are, you will not cut the Carbon Fiber correctly anyway, Larry.

PS.. a 47 frame is too small for your friend. Find a 50 or 52 for her, depending on her torso/leg ratio...

shelbyfv

Yep. Best look for a way to gracefully extricate yourself from attempting to provide bike assistance. The seatpost would be just the tip of the iceberg and there is no outcome where you'll end up being seen in a positive light. Maybe there is some other way you can ease yourself into her life.

veganbikes

This is from an old thread where most people said DON'T USE THAT SEATPOST. Now he is claiming to want to hurt..errr ""help"" a friend.

If your seat post especially a carbon one is damaged don't ever give it to your friend and cut it yourself unless you know what you are doing but probably don't use it. In this case the seatpost was clearly damaged by the OP putting it way too high above the minimum insertion line and cracking it and they attempted to blame a bike shop for doing that. Really bad form.

smd4

Seat posts start at about $15 bucks. Pry open that wallet if you want to be her friend...

Shadco 

Perfect example of when to look a gift horse in the mouth.

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