# maximum seat pot height vs frame size



## zenithkhu (Aug 11, 2010)

is there any formula between maximum seat post height and frame size?
i just bought Giant MTB extra small size. For paddeling, I've to adjust seat to 60cm.
Today I saw a label at the frame said less than 80cm inside the frame could cause frame damage. Is that important? What should i do?
thanks.


----------



## jeffj (Jan 13, 2004)

Even though frame sizes are listed by seat tube length, all other things being equal, you should fit yourself to a frame based primarily on the top tube length.

Then adjust the saddle height to fit, even if you need a longer than normal seatpost (like a 400mm or 410mm which are fairly common). Many seatposts have a minimum insertion line marked on them. They should definitely extend far enough into the frame to go below the junction of the top tube and the seat tube on a traditionally designed frame. Some frames have a top tube that meets the seat tube further down, but have a gusset or brace like the pic below:










In that case, the 80-100mm suggestion should be suitable.


----------



## Squash (Jul 20, 2003)

zenithkhu said:


> is there any formula between maximum seat post height and frame size?
> i just bought Giant MTB extra small size. For paddeling, I've to adjust seat to 60cm.
> Today I saw a label at the frame said less than 80cm inside the frame could cause frame damage. Is that important? What should i do?
> thanks.


Yes it is VERY important! Minimum seatpost insertion requirements from a frame manufacturer are to prevent the frame from cracking at the seat tube/toptube junction. However I think that you may be confusing centimeters (CM) with milimeters (MM). 60cm would be 600mm, there are 10mm per cm. That would mean your seatpost is extended to 600mm. Nobody I know of makes a seatpost that long except for custom jobs for folding bikes and the like.

Heres what you can do to check and see if you actually have enough seatpost in the frame. Remove the seatpost and measure from the bottom of the seat clamp to the end of the post. Reinsert the seatpost into the frame to your required height, then measure the exposed post from the bottom of the seat clamp to where it disapears into the seat tube. Subtract the length of the exposed post from the over all length, and that's what you have inside the frame. If it's not at least as much as the manufacturers recommended minimum it's be easy to figure out how much longer of a seatpost you'll need. Example: Full seat post length, 100mm, exposed post, 60mm, 100 - 60 = 40mm. From there, minimum insert requirement, 80mm, actuall insert length, 40mm, 80 - 40 = 40mm. So you'd need a seat post at least 40mm longer to meat the minimum insert requirements. This is only an example, your figures will obviously be quite different. If you don't have enough seat post in the frame the simple solution is to buy a longer post of the correct diameter. Problem solved. Unless the bike is WAY small for you, you should be able to find a post that will work. As jeffj noted, MTB seat posts are readily available up to 400 or 410mm long.

Anyway, get busy and figure it out. You deffinately want to know if you've got enough seat post in your frame. Not having enough can certainly cause frame damage. It likely won't happen immediately, but over time you'll likely have a problem.

Good Dirt


----------



## Iboated (May 12, 2010)

I got improperly sized for a frame, I got a 16" but I had to buy a 400mm post to get enough leg into it.. Even now with the 400, I still don't have much in the tube... I hope it cracks so I can hope to get the 18"..


----------



## zenithkhu (Aug 11, 2010)

thanks Jeffj and Squash. 
i took out the seat post and i found there is also another line saying Minimum insertion line. 
i guess 80mm sign is just warning and the minimum insertion line is more important right?
now my seat post is set at 60mm is that reallly bad for frame?


----------



## mitzikatzi (Sep 9, 2008)

Short answer. Yes it matters.
Buy a longer post  As others have said the general consensus seems to be it is best to have the seat post to extend below the top tube seat tube junction. Only having 60mm in the frame means you have a lot of post showing which will result in a lot or torque being applied to the seat tube. A longer post with more in the frame will be stronger. Otherwise you might crack the frame.
Often the frame and post state different abouts of minimum insertion. Go with the bigger number usually 80 to 100mm. 

Using 60mm of insertion In a couple of months you may have a forum post "my frame cracked" Why?


----------

