Are Seatpost Shims Safe?

Nothing is more effective than a seatpost shim when fitting a slimmer seatpost into a broader seat tube. But their effectiveness notwithstanding, are seatpost shims safe?

Provided you use a seatpost shim that’s longer than your seatpost’s nominal (minimal) insertion length, commercial seatpost shims work just fine, hence safe. In contrast, DIY options have rough edges that make them unsafe.   

We’ll compare DIY shims with commercial options to help you understand both sides.

But first, let’s understand what a seatpost shim is.

are seatpost shims safe

So, What is a Seatpost Shim?

A seatpost shim is an adapter or short tubing, usually aluminum, steel or PVC, that allows you to widen your seatpost diameter. As a result, it allows you to fit a narrow seatpost into a broader seat tube.

Are Seatpost Shims Safe?

Generally, bike seatposts feature an insertion length that protects them and the bike frame from cracking. So, as long as the seatpost shim length is longer than the insertion length, then it (the shim) will work fine.

It becomes a problem when you use a shim that’s shorter than the seatpost’s insertion length.

Overall, you only need to get the seatpost bottom part past the point of contact between the top tube and the seat tube. If you do that, then your seatpost shim will be safe to use.

But do seatpost shims work, and if yes, do you need one? Let’s answer that below

How Does A Seatpost Shim Work?

Seatpost shims work by wrapping around the bike seatpost and making it wider or thicker. That allows the seatpost to match the diameter of the seat tube.

An option like the Fouriers Seatpost Shim reduces the seat tube diameter from 31.60mm to 30.90mm and increases the seatpost thickness from 30.90mm to 31.60mm.

So, it works both ways, making it a fantastic adapter for your uneven seatpost and seat tube.

Do I Need a Seatpost Shim?

If you have a narrow seatpost that you want to fit into a broader seat tube, you need a seatpost shim. It allows you to convert the seatpost diameter to match that of the seat tube.

Are Seatpost Shims 100% Safe?

Seatpost shims are not 100% safe, especially when they are metal in build. Unless you grease the shim’s outer surface, there is always the risk of corrosion.

You can also treat the shim’s outer surface with an anti-rust spray like the Finish Line 1-Step Lubricant. It cleanses the shim’s surface and offers rust protection.

Do I Need a Seatpost Shim

Alternatively, consider switching to a PVC shim, given that plastic doesn’t rust. And if you are the DIY type, you can make your plastic shim using PVC pipe.

How About DIY Seatpost Shims? Are they Safe?

As I mentioned, you can make your seatpost shims from regular PVC pipes. Additionally, you could make one from beer cans, which saves you cash in the long run.

You are probably celebrating now. But before you can be ecstatic about it, you should be wary of the problems that come with DIY seatpost shims, especially beer can options.

The most common issues include:

  • Rough edges

Beer cans and aluminum cans, in general, remain with a rough edge after cutting, which could be a safety concern.

So, this makes DIY options unsafe. It’s also the same case with PVC shims after cutting.

  • Hard to Lubricate

DIY seatpost shims are generally hard to grease due to rough edges.

  • Missing stop lip

It’s easy for a DIY shim to slide into the seat tube as it lacks the stop lip. Commercial shims, in contrast, don’t miss the stop lip and a perfect example is the GANNOPER 100mm Seatpost Shim.

But with such issues, do seatpost shims work?

Yes, the concerns are there, but it doesn’t mean that DIY seatposts don’t work.

But to avoid all the above issues, buy a professionally designed seatpost shim. You can get one excellent seatpost shim from amazon.

Now, here is a table comparing commercial seatpost shims to DIY options:

 DetailsCommercial ShimDIY Shim
Aesthetic The shim barely shows, which means minor loss of the seatpost’s aestheticThe seatpost is likely to lose its aesthetic upon fitting the shim
Saving It saves you from having to buy a new seatpostIt not only saves you from getting a new post but also from buying a commercial seatpost shim
Lubrication Easy to lubricateDifficult to lubricate
Stop LipPresentAbsent
Shim EdgesSmooth edgesRough edges

Should I Use A Seatpost Shim? Pros & Cons

Pros – What We Like About Seatpost Shims

Why spend money on a seatpost shim? And why should you go through the trouble of fitting one?

If those two questions concern you, here are the reasons why you should get a seatpost shim.

  • Opportunity to Swap Seatposts

Just imagine this for a second;

You have two excellent bikes that you wouldn’t want to let go of. One sadly has a smaller seat tube, which means you need a narrower seatpost.

In such a case, a shim offers you the flexibility of using the same seatpost on the two bicycles.

An option like the CYSKY Seatpost Adapter Shim allows you to swap all bike seatposts. MTBs, Fixies, road bikes, BMX –name it.

Should I Use A Seatpost Shim
  • Saves You Money

With a seatpost shim, you don’t have to get a new seatpost if what you have is just slim. So, it saves you cash at the end of the day.

While a decent seatpost costs $50-$100, you only need to spend $10-$20 on a seatpost shim.

Surely! It makes financial sense to get a shim and fit it instead of investing in a new seatpost.

  • More Seat Options

A seatpost shim allows you to experiment with different seatposts to find your perfect match. You get to try out seatpost models that you wouldn’t have considered trying.

  • The Perfect Combo

Lastly, a seatpost shim is one thing that could unify any seatpost and seat tube with an odd diameter. So, it makes the perfect combo regardless of seatpost or seat tube diameter.

The Cons – What We Don’t Like About Seatpost Shims

While seatpost shims are handy in joining a slimmer seatpost with a broader seat tube, they aren’t flawless. I’m not talking about corrosion, which is avoidable.

No, I’m talking about the loss of frame warranty and more maintenance.

Sadly, some bike frame manufacturers render frame warranties invalid if a shim is involved. So, you may not hold them accountable for frame defects.

Furthermore, you have to do more lubrication on a shim seat tube. While a shim-less seatpost only requires one layer of lubrication, you have to lubricate both sides of a shim seatpost.

So, Should You Get A Seatpost Shim? Our Verdict

Other than the loss of the frame warranty and greasing the frame more, there isn’t much to concern you regarding seatpost shims.

That’s unless you go DIY, which, as we saw, is not 100% safe.

Overall, a shim saves you money and allows you to fit your narrow seatpost in a broader seat tube.

That alone should totally convince you to get this adapter. So, if you want to use a narrow seatpost, consider getting a thin shim. After all, it’s economical and convenient to do so.

How Do I Install A Seatpost Shim?

Installing a seatpost shim usually is straightforward, given that most shims come with installation instructions that you can follow easily.

So, check out the instruction guide and use it to fit the seatpost shim.

Overall, however, you can follow these steps to install a seatpost shim on a bike seatpost:

  • Cover the shim’s inside with carbon paste if you fit the shim on a carbon seatpost
  • But if it’s a steel or aluminum steel post, cover the contact points between the shim and seatpost with standard bike grease.
  • Now, slide the bike seatpost into the seatpost shim until the shim’s top part reaches the seatpost’s part that extends over the bike frame.
  • Cover the shim’s outer side with bike grease
  • Fit the seatpost into the bike frame and align it properly before tightening the collar


Closing Thought:

So, are seatpost shims safe? There’s no doubt that commercial shims are safe and thus a great purchase. However, you have to pick a rust-protected option to be guaranteed absolute safety.

But still, be wary of DIY options that often have rough edges, and as a result, are not safe.