Seat post nominal and actual size

Asked by: Steven Buck

There is typically a ~0.2 mm difference between the post size and the inner diameter of the tubing. For example, a common setup in good quality steel alloy frames is a seat tube with 28.6 outer diameter and 0.6 mm wall thickness at the top, giving an inner diameter of 27.4. These frames take 27.2 mm seatposts.

How do I know the size of my seatpost?

To find out the diameter of your seatpost, you must remove the seatpost from the bicycle frame and look for the size, which is stamped just below the “Minimum Insert” line toward the bottom. The number will be a three-digit number (such as 26.8 or 27.2 or 31.6).

What is standard seat post size?

27.2mm

The most common diameter is probably 27.2mm, but other sizes between 21.15 and 35mm are often found as well. Seatposts also vary in length. A shorter seatpost will typically be lighter in weight but less adjustable.

Why are there so many seatpost sizes?

Higher quality frames are usually made with thinned down tube walls to reduce the weight of the frame. This meant that a wider seatpost diameter usually meant a higher quality (and lighter) frame. So high-quality frames with 1 1/8″ seat tubes usually had 27.2 mm wide seatposts.

What size clamp do I need for a 27.2 seatpost?

Note: The seat clamp diameter needs to match the diameter of your seat tube rather than your seatpost, which is a little smaller. For example, a 27.2mm seatpost (a size commonly found on road bikes) fits inside a 28.6mm diameter seat tube, so it’s a 28.6mm clamp you need.

Will a 31.6 seatpost fit a 30.9 frame?

You can fit a narrower post into a wider frame using a shim (a thin sleeve of aluminium or plastic), for example a 30.9 mm dropper post into a 31.6 mm frame. However, there is no way to fit a wider post in a narrower frame, for example a 31.6 mm post into a 30.9 mm frame.

How do you measure a seatpost with a ruler?

Quote from video: Themselves. So what was available in this range you know we'll average that out at you know 31.7 millimeters the closest size we could find was 31.8.

What does 27.2 mean?

Yes 27.2 is about ‘going the extra mile‘ – somewhat cliched you might say, but as a metaphor it’s just trying to make a small statement. It’s not that difficult to go the extra one mile if you’ve already run 26.2 of them, and also – it takes a marathon effort to make a change.

Are bike seat posts universal?

Yes, most modern bike seats are universal and can be interchanged with different seat posts. The exceptions are if you have much older equipment or a specialized bike. It’s best to check the number of rails and the rail width on your saddle to ensure a correct fit to the seat adapter.

Are bike seats universal size?

While “most” bicycle seats are universal and interchangeable, not all bicycle seats are interchangeable or universal. However, the good news is that most modern bikes and brands have accepted that one fitting is best.

How do I choose a seatpost?

Quote from video: There's a very simple method which is to go ahead and loosen up your seat clamp bolt here slide your saddle. And your seat post out. Look at the seat post and see if there's a number printed on there.

Is 150mm dropper post enough?

The somewhere-in-the-middle 80-150mm travel posts are probably your best bet. Or are looking to spend a lot of time with your bike airing out over jumps, or dropping into super technical terrain where you just want the saddle completely out of the way? Then longer travel >150mm is probably your bag.

Can you shim a dropper post?

Quote from video: But be able to drop that seat as low as possible when you're actually going down the mountain. Now the way one of components. Does this is they have a plastic shim.

How do you shim a seatpost?

Quote from video: While holding the can tight against the post. Slide it in until there's about an inch exposed. Make your find adjustments while sinking the shim down to just above the clamp.

Are bike seatposts universal?

How do you measure a tube seat?

Seat tube length is the straight line distance between the centre of the bottom bracket and the top of the seat tube.

How do you measure bike seat height?

The saddle height is always measured with the crank arm pointed down and in line with the seat tube. The distance from the center of the pedal axle to the top of your saddle is your seat height.

Heel method

To start with, raise your saddle roughly to your hip. …
Sit down on the saddle and touch the pedal with your heel.

How do you measure a dropper seatpost?

To choose the correct travel dropper post, measure from the seatpost collar to the seat rail (in your climbing, or highest setting) on your current bike. Subtract 50mm from this measurement and the resulting number is the maximum travel length dropper post you can comfortably run.

Will any seatpost fit my bike?

The diameter of your current seat post will be the only size your frame can fit. There are 3 common sizes, 27.2mm, 30.9mm, and 31.6mm, with the latter two being the most common by far. 34.9mm is also becoming increasingly popular.

Are all droppers the same size?

The short answer to the question of the size of a drop is that the volume of a drop usually does vary, in fact depending on 10 different factors. This is of course relevant to alt. proc. because recipes often specify “20 drops”, “20 gtt” or “20 gt” which also means drops.