Condensed answer: A bicycle seat post can be cut as long as the new size respects the minimum insertion length of the seat post.
Ignoring the minimum insertion length and cutting too much of the seat post will result in extra stress on the frame (seat tube) and may cause a crack.
How To Cut a Seat Post
Step 1: Measuring and Cutting
Before cutting, it’s necessary to first take three measurements:
- The total length of the seat post’s tube
- The minimum insertion length
- The length of the seat post that shows above the seat tube
To find out how much you can cut, subtract the minimum insert length and the seat post that shows from the total length of the seat post.
Maximum Cut = Total Seat Post Length – (Minimum Insertion Length + Seat Post Showing Above Seat Tube)
For example, if the total length of the seat post is 36cm, the minimum insertion length is 10cm, and the seat post above the seat tube is 15cm, the formula gives us:
Maximum cut = 36 – (10+15) = 11cm.
If the seat post is made of metal (aluminum or steel), you can use the following tools to cut it:
- A Hacksaw + a guide (The guide is optional but recommended to ensure a straight cut).
- A Pipe cutter
- A Dremel
The pipe cutter makes a cleaner cut with less residue but could be expensive, especially if you only need it once. (Note: A pipe cutter can also be used to cut fork steerers.)
If the seat post is made of carbon, it’s recommended to use a hacksaw with a blade designed specifically for carbon.
The usual choice is a 32 TPI blade. In this case, TPI stands for Teeth Per Inch. Since carbon is fine material, one needs lots of teeth per inch to make a smooth cut. Regular hacksaw blades with fewer teeth won’t do it.
Do not use a pipe cutter for a carbon post. Carbon is not resistant to squeezing force/compression and the cutter may crush it.
Step 2: Mark The New Minimum Insertion Length
After you’re done with the cutting, mark the new minimum insertion length on the seat post. You can use a permanent marker and/or make a small scratch on the seat post.
(Don’t scratch the seat post if it isn’t made of metal.)
Step 3: Filing
Once the seat post is cut, it’s highly recommended to soften the edges of the cut end. Otherwise, the seat post can damage the inner side of the seat tube.
The Benefits of Cutting a Bicycle Seat Post
The main incentive to cut a seat post is to lower it more than the frame allows and thus make the bike fit shorter riders.
If the seat post is long, and the rider has to drop the seat post a lot, the water bottle braze-ons on the seat tube may prevent the seat post from going deeper.
One can solve this problem by shortening the seat post.
2. Weight savings
Cutting the seat post results in weight savings. However, the gains are too minuscule to matter. Therefore, if the seat post fits fine, it’s highly questionable whether the effort and time needed to cut it are worth it.
The Downsides of Cutting a Bicycle Seat Post
By cutting the seat post, you will prevent taller people from riding the bike. If the bike is used by a single person, this isn’t a problem. But if you plan on passing it to someone else, it may be wiser to keep the seat post as long as possible.
Lower Resale Value
By modifying the seat post, you will lower its resale value.
Can I Damage The Frame By Cutting My Seat Post
If the minimum insertion length is respected, then cutting the seat post will not result in damage to the seat post or the frame.
There’s a point beyond which the extra length of а seat post has no tangible effect on the frame’s structure. Or in other words, more does not equal more strength.
Therefore, it’s fairly safe to cut your seat post as long as you take into consideration the minimum insertion length and file the ends.
- Regular seat posts have a beveled end. It’s recommended to recreate it with a file or sandpaper after cutting the seat post.
- To minimize the chances of damaging the seat post and spreading metal bits, one can use masking tape to cover the working area.
- Some people like to spray water on the post while making the cut to minimize carbon dust or the spread of metal pieces into the air.
- You can make a “practice cut” by first cutting a small piece. The goal is to get a feel for the material and operation before making the final cut.
- Wear a dust mark or at least a scarf to avoid breathing the dust.
Summary: What You Need To Know
- A seat post can be shortened as long as the new size respects the minimum insertion length of the seat post. If you cut the seat post too much, you risk damaging the seat tube.
- If you have a metal seat post, you can use a hacksaw, a pipe cutter or a Dremel to make the cut.
- If you have a carbon seat post, a hacksaw blade with lots of teeth will be needed as pipe cutters can damage the seat post due to the high compression force.
- After cutting the seat post, it’s recommended to file the end and mark the new minimum insertion length.