seat posts and compact frames

[KirkKaas]

Does anyone have an idea of the minimum amount of seatpost that should be inserted in the seat tube of a compact frame? My thompson post doesn't a line or anything.

Thanks

Kirk

[Too Tall]

Regardless of the frame the seatpost should extend at least one inch if not more below the bottom of the top tube. That would be a min. recommend.

[Keith A]

According to Thomson's website their seatposts do have a Max Insertion line on them. I would suggest contacting them to find out what this should be for your particular seatpost. I have contacted them in the past, and they have been prompt and helpful with their replies.

What exact seatpost do you have? (length, diameter, setback?) Someone here might have the same post and could measure where the Max Insertion line is located.

[KirkKaas]

It is Thompson elite seatpost 27.2 no setback
it does seem to have about an 1inch and 1/2 below the bottom of the top tube. So if someone can find a minimun line on their post and measure it and send it to me that would be helpful.

thanks again

kirk

[Keith A]

What's the length of the post shaft?

[Jeremy]

It is Thompson elite seatpost 27.2 no setback
it does seem to have about an 1inch and 1/2 below the bottom of the top tube. So if someone can find a minimun line on their post and measure it and send it to me that would be helpful.

thanks again

kirk

What Too Tall said is correct. Also, minimum insertion lines are essentially meaningless because the length of the seat tube above the top tube varies so much on modern frames. Just make sure you have adequate insertion (3cm or more) beneath the bottom of the top tube/seat tube juncture and you should be fine.

Jeremy

[Keith A]

Also, minimum insertion lines are essentially meaningless because the length of the seat tube above the top tube varies so much on modern frames.I would expect that most of the manufactures realize this and might even design and test this aspect of their seaposts. I would also think (but don't know for sure) that they might even adjust their maximum insertion lines based on the length of the seatpost, type of material used, etc.

[Jeremy]

I would expect that most of the manufactures realize this and might even design and test this aspect of their seaposts. I would also think (but don't know for sure) that they might even adjust their maximum insertion lines based on the length of the seatpost, type of material used, etc.


The minimum insertion line on many seatposts will not be safe on many modern frames because they measure from the top of the seat tube. The length of seat tube above the top tube varies from less than 1cm to more than 5cm. Some seatposts use a thinner wall at the top of the post, or even a tapered section at the top of the post to reduce weight. So, the maximum insertion point is important. However, in practice it is rarely an issue. If you are using a long seatpost on a horizontal top tube frame, it might be a problem. But, if you are riding a compact frame that fits, there will always be enough post sticking out to make a maximim insertion line irrelevant.

Jeremy

[Keith A]

So you are saying that the line used that allows the seatpost to be at the maximum length outside of the frame is based on a horizontal top tube? I guess I have a hard time believing this is true since seatpost manufactures have been dealing with sloping top tubes for a long time with mountain bikes and would seem irresponsible (and not logical) to assume a horizontal top tube when placing "the line" on their seatposts.

BTW, in my previous comments the "max insertion line" is the one at the bottom of the seatpost that allows the maximum amount of seatpost to be exposed. At least Thomson calls this their "max line" and this is stamped on their post and stated as such in their documentation.

[Jeremy]

Hi Keith,

I often have a hard time believing some of the things that happen in this industry. But no, I do not mean that the insertion line is based on a horizontal top tube. It is the variability of the length of the seat tube above the top tube that makes an insertion line an unreliable guide. This doesn't have anything to do with whether the frame has a sloping or horizontal top tube. If your frame has 4cm of seat tube above the top tube and the insertion line on your seat post is 6cm from the bottom of the post then, at the max height, your seat post will not extend beneath the bottom of your top tube at all. I agree, you would think that all of the manufacturers would take these things into account. However, frame design varies so much now, that it is not uncommon for the above scenario to exist. The only point I was trying to make is that an insertion line is an unrelaible guide for determining whether you have enough post inside of the frame. If you want to be safe, measure the distance from the seat rail clamp (with your seat adjusted to the height that you need) down the seat tube to a point at least 3cm beneath the bottom of the top tube/seat tube juncture. Make sure that your seat post is at least that long.

Jeremy

[Keith A]

Gotcha. In this case, I would think that you could use the top edge of the clamping device (whether this is an actual clamp or part of the seat tube) to be your reference point to be compare against the max insertion line to make sure you have enough seatpost below this point.

Along the same lines, I would be careful of using any seatpost that has been cut for the reasons that Jeremy listed before. The seatpost designs often use different wall thicknesses to save weight in the center part of the seatpost. Therefore, the same relative location of the max insertion line on a trimmed seatpost might not apply.

[Dave]

6cm is the distance from end of the post to the minimum insertion line, but as others noted, it's far more important that the post extend below the bottom of the TT. 2cm should be plenty.

What has also been noted is that there are some odd frames out there with seat tubes that extend far more than the common 2cm, which is just enough for a standard seat post clamp. In this case, the minimum insertion line might be OK for the post, but not for the frame. Be sure it extends below the bottom of the TT.

[Peter P.]

The post should extend a minimum of 25mm BELOW the lower intersection of the seat and top tubes, REGARDLESS of the frame design. The result you're trying to achieve is to not stress the seat tube/top tube joint. By supporting the seat post below the joint as well as at the seat clamp, you minimize the potential to damage the frame at this junction.

The 25mm length is somewhat arbitrary on my part; I measured an American Classic road length post I had on hand and it's minimum insertion mark was at 8cm, which looked excessively long when lined up next to my 1" steel lugged, level top tubed frame. I'd say 25-50mm would be about right. One reason why you might want to stray near the minimum length is, the less post inside the frame, the easier it will be to remove should it seize in the frame.

[dave thompson]

Does anyone have an idea of the minimum amount of seatpost that should be inserted in the seat tube of a compact frame? My thompson post doesn't a line or anything.

Thanks

Kirk
My Thomson seat post has a minimum insertion line 65MM from the bottom of the post.

[bcm119]

Exposing more than 300mm of post greatly increases your chances of being struck by lightning. ;)