What do you do when your bike’s seatpost doesn’t fit the frame, and you don’t want to buy a new one? A seatpost shim is an effective solution to solve this problem, but what is a seatpost shim, you may ask?
A seatpost shim is a short tubing made of steel, thin aluminum, or PVC used to raise a seat post’s diameter. It fills the space between the frame’s inside diameter and the outside diameter of the seat post. Shims are made with a cutoff that renders them expandable and enables the installation process.
There is more to know about Seatpost shims besides what they are, and I will get into details in this article.
Should I Use A Seatpost Shim?
There are multiple reasons to use a Seatpost shim.
1. It Saves You Money
As I mentioned earlier, a Seatpost shim comes in handy when you have a bike with a seat post that’s too small for your frame and don’t want to buy a new seat post. Without a Seatpost shim, you would have to purchase a seat post which costs money.
2. It Allows Seat post Swapping
In addition, it allows you to swap your seat post between bikes with different seat tube diameters. In that case, it isn’t necessary to have multiple seat posts for your bikes, whether you own two or more bikes.
3. It Facilitates The Use Of Different Seat posts Types
The number of seat posts that can fit into your bike is considerably lower if you don’t have a seatpost shim. This is because seat posts come in different diameters.
Since you can fit different seat posts to your bike when you have a shim, it lets you experiment on different models that wouldn’t be possible without it and also take advantage of discounts.
4. Unusual Mixture Size
A seatpost shim is helpful if the seat post or seat tube of your bike has an unusual diameter because it makes the combination work. For instance, you can connect a 27.2mm seat post with a 30mm seat tube with the help of a shim.
What Are The Downsides Of Seatpost Shims?
While Seatpost shims offer the benefits mentioned above, they also come with some downsides. Knowing them will help you know what to expect if you choose to use a Seatpost shim.
1. Warranty Loss
Some bike frame manufactures, especially those who make carbon frames, can render your warranty invalid if you use a Seatpost shim. This means you’ll have to cater to the costs of damages incurred, and that could set you back financially.
Before fitting a Seatpost shim into your bike, make sure you find out the impact it will have on the warranty.
2. Extra Maintenance
A Seatpost shim is another piece of equipment you have to deal with, and it can be quite annoying, especially if you frequently adjust your seat post’s height.
This equipment also needs extra maintenance. While a seat post without a shim needs a single lubrication layer, the presence of a shim means you have to lubricate both sides.
3. Aesthetics
If the aesthetics of your bike are of utmost importance, you might not like a seatpost shim. However, the adapter is barely visible, so it shouldn’t be a huge problem.
4. Corrosion
To prevent corrosions, you must grease the shim’s outside if you use a metal shim on a metal frame. Consequently, an assembly paste is crucial when using a metal shim on a carbon frame.
To avoid this problem altogether, go for an aluminum alloy Seatpost shim like CYSKY Seatpost Adapter Shim, Bicycle Seat Post Tube Adapter (Link to Amazon). This product is size 25.4mm-27.2mm, durable, anti-rust, and suitable for various bikes mountain bikes and road bikes.
When you weigh between the pros and cons of a Seatpost shim, the benefits outweigh the downsides. Therefore, it is worth getting a shim.
How Do You Shim A Seatpost?
Seatpost shims are relatively simple equipment to install. However, to make sure it will solve your problem, consider the seat post’s thickness and seat tube diameter when buying a shim.
For example, if your bike’s seat tube is made for 30.8mm seat posts, but you have a 27.2mm seat post, you will need a size 27.2mm 30.8m shim like Bnineteenteam Aluminum Alloy Bicycle Seatpost Tube Adapter. This shim is lightweight, durable, aluminum alloy-made, and quite easy to install.
The installation procedure is as follows:
a) Apply the contact region between the seatpost shim and the seat post using a standard bicycle grease if the seat post is steel or aluminum-made. In contrast, cover the shim’s inside with carbon paste if you have a carbon seat post.
b) Take the shim and slide your seat post into it until the shim’s upper portion gets to the seat post’s part that will be visible outside the bike’s frame.
c) If the frame is metal, cover the shim’s exterior with grease and a lubricant recommended by the manufacture if it is a carbon frame.
d) Put the seat post into your bike’s frame, making sure you align the seat well. Additionally, ensure the collar is tight to the manufacturer’s required setting, especially if it’s a carbon seat post since excessive clamping force can crush it.
Are Seatpost Shims Safe?
The seatpost shims safety depends on their length. Seat posts typically have the least insertion length to protect them and the frame from cracking.
The Seatpost shim will work well if it’s longer or the same length as the minimum insertion length. Ultimately, your bike’s frame won’t tell the difference between a seat post with a shim and one without, so as long as you use it properly, you’ll be safe.
In addition, you should note that frames also have an insertion point limit. Due to that, you are required to make sure the seat post’s bottom part is beyond the contact point between the seat tube and top tube.
To make sure you get the best service and stay on your bike, you should get a Seatpost shim with a stop lip to fixate the shim and keep it from falling into the bike’s seat tube.
For this reason, I suggest you get GANOPPER 100mm Long Seatpost Shim (Link to Amazon). This durable, anti-rust adapter with a CD texture surface is manufactured using 6061 aluminum alloy and CNC machining.
FAQs
1. Can A Shim A Seatpost?
It’s perfectly okay to shim a seat post. However, make sure you measure the seat tube and seat post precisely because even 0.11mm can make a difference.
2. Can You Use A Seatpost Shim In A Carbon Frame?
While it’s possible to use a shim on a carbon frame, some manufacturers are against it, and it may cost you your warranty.
This is because manufacturers don’t usually shim the frames when testing them for failures; hence there are insufficient data to fully conclude that Seatpost shims don’t compromise the integrity of frames.
3. What Can You Use As A Shim?
If you don’t have a proper Seatpost shim, you can use a make-shift shim made of a beer can to prevent your seat post from slipping. The problem with this method is it doesn’t have a stop lip, has poor aesthetics and rough edges, and doesn’t facilitate lubrication.
However, this method works better than using a foil.
4. Can You Shim A Dropper Post?
A shim can help you custom-tune your dropper post by minimizing your post’s extended and travel length. You just have to find a shim that will work well with your bike and dropper post.
Shims’ popularity went up when dropper posts were not trendy, and the available models weren’t enough to accommodate every seat post size. This is because riders would use them to fit a dropper post into bikes with diverse frame sizes.
To Conclude, What Is A Seatpost Shim?
I hope you now have a better understanding of this adapter and in a better place to buy one that will work for your bike and help save you the money you’d have otherwise spent on a new seat post.
A seatpost shim is convenient, saves you money, and functional, making it a worthy piece of equipment.
