What Is A Seat Post Shim And Why Do You Need It?

When some people hear about seat post shims, they think they are unsafe and waste money. Others feel differently and believe they save money as you don’t need to buy a new seat post. But what is a seat post shim exactly?

A seat post shim is a plastic or metal adapter that allows you to fit a narrow seat post into a broad seat tube. 

You probably are not sure if you need a shim. Perhaps, it’s the working mechanism that is not clear. You may also be asking, ‘must I spend money on a shim?’

Well, I had the same concerns, and since they are no longer there, I’ll love to help clear yours.

Let’s dig deeper into our title.

what is a seat post shim

What Is A Seat Post Shim? Do You Need It?

As I mentioned, a seat post shim is a plastic or metal adapter that joins a narrow seat post to the seat tube with a bigger diameter.

In most cases, these adapters or tubing come in steel, aluminum, or PVC constructions. Shims feature a cut-off that makes them stretchable for easy fitting.

But why is a shim necessary? Do you need it?

Well, if your seat tube is wider than the seat post and you don’t have the money to get a new seat post, then yes, you need a shim.

If you have two bicycles at home and one has a slimmer seat tube and requires a matching seat post, you also need a shim.

So, what are the advantages of seat post shims?

A shim allows you to unite a narrow seat post with a broader seat tube, which means you don’t have to get a new seat post. From a financial perspective, that means you save money.

The other advantage is that a shim allows you to swap seat posts regardless of their thickness. Moreover, it increases the number of potential seat posts which could fit your seat tube.

How Does A Seatpost Shim Work?

Seat post shims have a simple working mechanism. They simply wrap around a narrow seat post and makes it as thick as the target seat tube.

For example, the Cane Creek Seatpost Shim converts a 28.6mm seat post to 31.6mm thick to fit into a 31.6mm seat tube.

On the other hand, the CYSKY Seatpost Shim converts a 27.2mmm seat post to 30.8mm thick to fit into a 30.8mm seat tube.

Note that most shims work both ways. For example, when the Fouriers Seatpost Shim 31.6 to 30.9mm comes with the measurements 31.6mm-30.9mm and 30.9mm-31.6mm, it means the following:

  • It can reduce the diameter of a 31.6mm seat tube to 30.9mm
  • It can also increase a seat post thickness from 30.9mm to 31.6mm
best seatpost shim

Can You Shim A Dropper Post?

A dropper post is just a seat post, which means you can shim it just like any seat post. Besides, dropper posts are a trend nowadays, given their adjustability, making them a target by shim manufacturers.

How Do You Install A Seatpost Shim?

Before you can buy a seat post shim, you must consider these two variables.

  • Your seat post thickness
  • Your seat tube diameter

If your seat post is 30.9mm thick, for example, and your seat tube is 31.6mm wide, then your shim should measure 30.9-31.6mm.

Now, once you have the right shim, here’s how to install seatpost shim:

  • Cover the shim’s inside with a carbon paste (if it’s a carbon seat post) or regular grease (if it’s a steel or aluminum seat post)
  • Slide your seat post into your shim and adjust the shim until it reaches the seat tube’s top part
  • Apply some grease on the shim’s outer before putting the seat post into the seat tube
  • Align the seat properly before tightening the collar

How To Make A Seatpost Shim – Homemade Seatpost Shim

You don’t necessarily have to buy a shim for your narrow seat post if you can make one. All that matters is getting something that can unify your broad seat tube with a slim seat post.

That means you can use an aluminum can, preferably a beer can, as I explain below.

How To Shim A Post With An Aluminum Can

Here are the steps to follow:

  • Find a pair of sharp scissors and use them to cut the top and bottom part of a beer can or any other aluminum can.
  • Remember to protect your hands with gloves when doing it.
  • Slice your aluminum can (on one side) to turn it into a thin aluminum roll.
  • Slide your narrow seat post into the wide seat tube without adding the shim.
  • Mark on the seat post the point where you’ll clamp the shim.
  • Apply a thin layer of bike grease inside the bike frame and on the seat post.
  • Now, wrap your shim (the thin aluminum roll) around the bike seat post and flatten all its edges to make it safe.
  • You can even align its top part with a screwdriver blade.
  • Once the shim flattens, tighten the seat post clamp to ensure that the shim doesn’t move.
how does a seatpost shim work

Congratulations! You just made and fitted a shim on your bike.

Note, however, that a homemade shim has its flaws, which means it’s not a perfect replacement for commercial shims.

What Flaws Exactly?

For starters, it lacks an effective lip stop. As a result, there’s always the risk of the seat post dropping into the seat tube.

It also has rough edges, which means they are not 100% safe.

Other concerns include imperfect aesthetics and lubrication difficulties. Homemade shims are not the easiest to lubricate, and they also don’t look aesthetically pleasing unless you paint them.

Also, some bike manufacturers may not accept warranty claims if you’ve fitted a shim. They see it as a considerable modification to the bike frame, which voids its warranty.

So, if you can, get the best seatpost shim online. After all, they are budget-friendly. You can get a decent option from $10.

Note that the shims you find online come in different sizes and so you only need to pick one that matches your seat tube diameter and seat post thickness.

Are Seatpost Shims Safe?

Commercial shims, more so metal shims, have one problem, and that is rust. Their fix, however, is to get PVC options as they don’t rust.

So, there isn’t much to worry about commercial shims. That means they are generally safe if you get a rust-proof option.

I cannot vouch for DIY shims, however. DIY shims, especially those made of beer cans, have rough edges that make them unsafe.

They also don’t have a stop lip, which you find on commercial shims. As a result, the seat post can drop into the seat tube, resulting in an out-of-balance seat.


Closing Thought:

In conclusion, what is a seat post shim, and why do you need it?

A seat post shim is a tubing adapter that combines your narrow seat post with a much bigger seat tube. So, you need it to fit your slimmer seat post into a seat tube with a large diameter.