How to cut a seatpost?

Back by popular demand, the general all-things Road forum!

Moderator: robbosmans

Post Reply
User avatar
Lightweenie
Posts: 199
Joined: Sun Aug 03, 2014 1:00 pm

by Lightweenie

Hi all,

my ritchey carbon seatpost is about 8-9cm too long, so I was thinking to cut this part off (I wonder how much weight this would save). The seatpost ends at an angle (I guess 45 degrees?).

The question is what is the reason for this angle? Is it important to make the new cut at the same angle, or can I just use the tool and saw that I use for cutting steerer tubes and make a 90-degree-angle-cut?

In case it is important to cut at the same angle, do you have some tipps on how to get it right?

User avatar
pdlpsher1
Posts: 4013
Joined: Thu Jul 19, 2012 6:09 pm
Location: CO

by pdlpsher1

I would use a 90 degree cut to save more weight. Btw I use a power chop saw with a carbide blade designed for wood. It works great for carbon or aluminum.


Sent from my iPhone using Tapatalk

by Weenie


Visit starbike.com Online Retailer for HighEnd cycling components
Great Prices ✓    Broad Selection ✓    Worldwide Delivery ✓

www.starbike.com



sungod
Posts: 1702
Joined: Fri Dec 24, 2010 9:37 pm

by sungod

the angle doesn't matter

hacksaw with a 48tpi blade is also good for cf, once cut, use a piece of abrasive paper to smooth inner/outer edges

take care with the dust/other debris from the operation, it's not something you want to breath in, and tiny splinters of cf can easily penetrate the skin


if the post has a defined area for where it can be clamped, make sure this'll line up correctly after cutting!

User avatar
ms6073
Posts: 4288
Joined: Fri Jul 23, 2004 8:24 pm
Location: Houston, Texas

by ms6073

pdlpsher1 wrote:Btw I use a power chop saw with a carbide blade designed for wood.

Ditto - carbon fork steer tubes as well.
- Michael
"People should stop expecting normal from me... seriously, we all know it's never going to happen"

bremerradkurier
Posts: 419
Joined: Mon Jun 13, 2016 4:18 pm

by bremerradkurier

I used a Performance steertube saw cut guide on a 31.6mm seatpost; worked great, and should fit smaller diameter seatposts as well.

User avatar
kkibbler
Posts: 905
Joined: Tue Sep 09, 2014 9:30 am

by kkibbler

The angle cut is not necessary but it might serve 2 purposes.

1. Easier insertion into the seat tube

2. On my stock seatpost the angle cut is towards the back i.e. front of the seatpost is longer, back is shorter. My seat tube tapers as it approaches the BB shell. In case of catastrophic clamp failure resulting in seat post slip, I suppose the front of the seatpost would hit the top bottle cage bolt/boss and prevent the back of the seat post from blowing out the tapering seat tube. /shrug

Example (not my bike, note seat tube taper and top bottle cage boss)
Image

skiezo
Posts: 141
Joined: Fri Dec 26, 2008 2:32 am
Location: Mid. PA. USA

by skiezo

When I cut one of my old cf post I used a 4" angle grinder with a thin wheel (.040 thickness). I cut mine at an angle of some sort and I found the post started a bit easier that way as my ST seemed a bit snug. I than taped just a few mm above the cut and used a fine round file on both the inside and outside of the cut. The tape keeps the cf from splintering on the outside of the post.

Oswald
Posts: 794
Joined: Thu Jan 08, 2004 10:11 pm

by Oswald

Make sure to measure a couple of times before you cut! Just don't cut it too short. You may want to switch to another saddle in the future that is less high and may need more seatpost sticking out. Better to have it 2cm too long then 0.5cm too short...

I usually cut them at 90 degrees. Although I have cut some at 45 also. I use a wooden saw guide for that, something from my dad's workshop.

I put some tape around the place I want to cut, so the carbon doesn't fray. Then put a marking on the tape to clearly see where I want to cut. I stuff a wet cloth inside the seatpost, a bit deeper then where I need to cut, to catch the carbon dust from entering the seatpost. Then I make the cut using a saw guide (I have the Park Tool) and a sawblade with at least 32tpi. I cut 95% thru it without stopping. Then go from a slightly different angle for the last 5%. Then, after cutting, I gently use a very thin file to make the edge a bit smoother, making sure to go from the outside to the inside of the seatpost diameter.

And then, after weighing the cut off part, I usually try to crush the cut off part by smacking it with a hamer. Seriously, you should try that. You will be suprised how long it lasts before it breaks. Smacking it with a hamer isn't enough, it will only chip the carbon a little bit. You have to use a vise to crush it. :mrgreen:

User avatar
pdlpsher1
Posts: 4013
Joined: Thu Jul 19, 2012 6:09 pm
Location: CO

by pdlpsher1

I always keep the cut stub piece. One day I might decide to sell the post. Then I will glue the stub piece back on :-)


Sent from my iPhone using Tapatalk

User avatar
wheelsONfire
Posts: 6280
Joined: Mon Jul 07, 2014 8:15 am
Location: NorthEU

by wheelsONfire

Easy, just buy a carbon saw blade for the work.
http://www.parktool.com/product/carbon-saw-blade-csb-1

If you also have a guide, so you do not slip with blade.
You can use tape if you are stable at the hand. If not, use something that prevent the blade from sliding when you saw.

Remember, different saddles have different stack height, you also need to check what is minimal seatpost length insert on your frame!
It usually is 9-10 cm. I noted Cipollini had 14cm on NK1K!

When done, use a file or sand paper to smoothen edges of seat post.
Bikes:

Ax Lightness Vial EVO Race (2019.01.03)
Open *UP* (2016.04.14)
Paduano Racing Fidia (kind of shelved)


Ex bike; Vial EVO D, Vial EVO Ultra, Scott Foil, Paduano ti bike.

glepore
Posts: 1408
Joined: Thu Mar 28, 2013 4:42 pm
Location: Virginia USA

by glepore

Oswald wrote:Make sure to measure a couple of times before you cut! Just don't cut it too short. You may want to switch to another saddle in the future that is less high and may need more seatpost sticking out. Better to have it 2cm too long then 0.5cm too short...

I usually cut them at 90 degrees. Although I have cut some at 45 also. I use a wooden saw guide for that, something from my dad's workshop.

I put some tape around the place I want to cut, so the carbon doesn't fray. Then put a marking on the tape to clearly see where I want to cut. I stuff a wet cloth inside the seatpost, a bit deeper then where I need to cut, to catch the carbon dust from entering the seatpost. Then I make the cut using a saw guide (I have the Park Tool) and a sawblade with at least 32tpi. I cut 95% thru it without stopping. Then go from a slightly different angle for the last 5%. Then, after cutting, I gently use a very thin file to make the edge a bit smoother, making sure to go from the outside to the inside of the seatpost diameter.

And then, after weighing the cut off part, I usually try to crush the cut off part by smacking it with a hamer. Seriously, you should try that. You will be suprised how long it lasts before it breaks. Smacking it with a hamer isn't enough, it will only chip the carbon a little bit. You have to use a vise to crush it. :mrgreen:


OCD post of the week :D
Cysco Ti custom Campy SR mechanical (6.9);Berk custom (5.6); Serotta Ottrott(6.8) ; Anvil Custom steel Etap;1996 Colnago Technos Record

User avatar
cyclespeed
Posts: 1113
Joined: Mon Jun 06, 2016 8:45 am

by cyclespeed

A pipe cutter is excellent for tracing out the cut. People say don't use them as it can crush the carbon, but as long as you tighten it gently and in multiple small steps, it's fine.

You will then have a perfectly round and square groove to cut to. Use a fine hacksaw to finish off.

topflightpro
Posts: 829
Joined: Tue Jan 13, 2009 2:35 am

by topflightpro

I too wrap my posts and steertubes with masking tape before cutting to limit the chance of splintering.

User avatar
DMF
Posts: 1062
Joined: Sat Dec 06, 2008 10:14 am
Location: Sweden

by DMF

The 45 degree cut is because from the frames point of view the front side is the effective seat post length, and what matters from the aspect of stress induced on the seat cluster/junction of the frame. If you put your weight on the seat, there will be practically no pressure from the backside of the seat post towards the rear facing part of the frames seatstay/toptube cluster, so support from the seat post on that behalf doesn't really make a serious difference, if we are talking about the part of the frame cluster at where the seat post normally ends. Closer to the entry there will of course be plenty of pressure applied ;)

I.e a post that is 38cm on the rear side and 40cm on the front aide. Is always 40cm in every way the frame sees it. From a stress/pressure perspective at least.

It's a cheap/smart way to save weight with no downsides.

User avatar
Lightweenie
Posts: 199
Joined: Sun Aug 03, 2014 1:00 pm

by Lightweenie

Thans a lot for all the responses!

DMF's explanation seems the most convincing to me...
So I guess I can just measure from the longest part and then if easy add a small angle to save a couple of grams more. Of course I will keep 2-3cm more on the seatpost than I actually need, for the case I need to adjust or want to get a flatter saddle later.

by Weenie


Visit starbike.com Online Retailer for HighEnd cycling components
Great Prices ✓    Broad Selection ✓    Worldwide Delivery ✓

www.starbike.com



Post Reply