Condensed answer: Seatpost shims are adapters allowing the installation of seatposts that are too slim for a specific frame.
Shims do their job fairly well and therefore aren’t a bad idea if your seatpost is too small for your seat tube.
What Is a Seatpost Shim?
Seatpost shims are short pieces of tubing, usually made of PVC, thin aluminum or steel, used to increase the diameter of a seatpost.
Shims have a cutoff which makes them expandable and facilitates the installation process.
How Do Seatpost Shims Work?
Seatpost shims are a fairly simple piece of equipment. They simply wrap around the seatpost to make it thicker.
How To Install a Seatpost Shim
Note: Before buying a seatpost shim make sure that it can work in your particular situation. Two variables that have to be taken into consideration:
- The thickness of your seat post;
- The diameter of the seat tube;
Example: If you have a 27.2mm seatpost, but your seat tube is designed for 31.8mm seatposts, you’ll need a “27.2mm ->31.8mm” shim.
Once you have the right shim for your bike and seat post, proceed to installation.
The steps are as follows:
1. If you’re installing a shim on an aluminum or steel seat post, cover the contact area between the seat post and the shim with regular grease.
If you’re installing a shim on a carbon seat post, cover the inside of the shim with carbon paste.
2. Slide the seat post into the shim until the upper portion of the shim reaches the part of the seat post that will be showing outside of the frame.
3. If you’re installing the seat post on a metal frame, cover the outside of the shim with grease.
If you’re installing the seat post in a carbon frame, cover the outside of the shim with the lubrication recommended by the frame manufacturer.
Warning: Some carbon frame manufacturers won’t respect the warranty of the bike if you use a shim. Contact your producer for more information.
4. Put the seat post into the frame. Мake sure that the seat is aligned properly. Tighten the collar to the setting required by the manufacturer.
This step is particularly important for carbon seat posts because they can easily be crushed by too much clamping force.
Are Seatpost Shims Safe?
Seatposts have a minimum insertion length which protects the seatpost and the frame from cracks.
If the shim is as long or longer than the minimum insertion length required by the seatpost, it will work just fine.
At the end of the day, the frame can’t tell the difference between a shim-less seatpost and one with a shim.
Note: Frames have a minimum insertion point too, but it’s a lot shorter than that of a seatpost. Usually, the requirement is to have the bottom of the seatpost past the point where the seat tube and the top tube connect.
Can A Seatpost Shim Be Used With a Carbon Frame?
Some carbon frame manufacturers warn against shims and may ignore your warranty if you rely on a shim to install your seatpost.
To be on the safe side, contact the maker of your frame or the bike shop from which you’ve bought it.
The main reason for this phenomenon is that manufacturers rarely use a shim when testing the frame for failure.
In consequence, there isn’t enough data to conclude with 100% certainty that a shim doesn’t hurt the integrity of the frame.
Having said that, a great number of people use shims with carbon frames without problems.
What’s The Most Common Problem With Shims?
If you use a metal shim inserting in a metal frame, the outside of the shim has to be greased.
If you have a metal shim going into a carbon frame, the shim has to be treated with assembly paste to prevent corrosion.
Another solution is to install a PVC shim since plastic doesn’t corrode.
If you have the necessary tools, you can make your own shim from a PVC pipe.
Can You Use a Shim With a Dropper Post?
Installing a dropper post via a shim is common practice.
In the past, when dropper posts weren’t as trendy, there weren’t enough models to cover all seat post sizes. As a result, the popularity of shims exploded.
Are DIY Shims Made From Aluminum Beer Cans Any Good?
If the objective is to stop a seatpost from slipping, and there’s nothing else available, a shim made from an aluminum can could save the day.
However, this method has the following downsides:
- Lack of a “stop lip”
Standard seatpost shims have a “stop lip” which fixates the shim and prevents it from falling into the seat tube.
If you use a DIY shim made from a can, it may slide too deep into the seat tube.
Note: This downside can be negated to a certain extent by taping the shim with duct tape to the seat post. (Don’t wrap the entire body. Two points, top and bottom, are enough.)
- Difficult to apply lubrication
A homemade shim isn’t super secure. As a result, it’s a bit more difficult to apply the necessary lubrication.
This is a problem because without lubrication the shim will get stuck into the seat tube if used for a long time.
- Rough edges
Cut aluminum cans can easily scratch the frame and/or the seatpost.
- Poor aesthetics
A shim produced in a professional environment is more aesthetically pleasing than a DIY solution.
Ultimately, shims made from cans are good only as a temporary solution. If you intend to use a shim for a long time, consider buying a dedicated one.
What Are The Downsides of Using a Shim?
A shim is another bike part that you have to deal with it. This could be particularly annoying if you often change the height of your seat post.
Obviously, this point doesn’t apply to people who rarely if ever move their seat posts upon the initial adjustment.
2. Warranty Loss
As already mentioned, some frame manufacturers would declare a warranty invalid if a shim is used.
The looks of a shim bother some people even though the adapter is barely showing.
3. Extra maintenance
If you have a shim, you have to lubricate it on both sides whereas a shim-less seat post requires only one layer of lubrication.
What Are The Advantages of a Seatpost Shim?
1. Money savings
A cheap seatpost shim saves you the trouble of buying a new seatpost when the current one is too slim for your frame.
2. Seatpost Swapping
Imagine the following situation: You have 2 bikes. One of them has a smaller diameter seat tube and thus necessitates a slimmer seatpost.
A shim would allow you to use the same seatpost on both bikes if you so desire.
3. Unusual Size Mixture
If you have a seat tube or a seat post with an unusual diameter, a shim could make the combo work.
For example, if you’re trying to put a 27.2mm seat post in a 30mm seat tube, a shim will get the job done.
4. Wider variety of seat posts
A shim increases the number of seatposts that would fit on your bike. This allows you to benefit from discounts and to experiment with models that you wouldn’t be able to use otherwise.