How much seat post do I need?
 

I have a Thompson seat post, pretty sure it's Al. There is a line on it that says "Max." I assume that I should not cut above this line? It still seems like a lot of dead weight to me.

Max line should be 100mm from the end. Therefore, cut it so that 100mm is inserted in your frame's seat tube. It has to clear the top tube.

Unless your seat post is cast iron, I wouldn't cut any of it.

Measure from the max line to the end of the seatpost that goes into the frame. You can cut the post to any length as long as you maintain this minimum insertion. For example, if you want to cut two inches off the post, mark a new max line two inches above where the old one was. But I agree with the other poster who said leave it alone. Even three or four inches of an aluminum seatpost doesn't weigh very much. Plus, there is always the possibility that you might cut off so much that you can't raise the saddle later if you want to.

The Max line denotes the least post that should inside the frame. Stupid nomenclature, right? It should be Min written upside down. Go figure. Unless you have a crazy long MTB post(400 mm) on a road bike, leave it alone.

Instead of cutting pieces off... or drilling holes to save a few grams of weight... maybe you could look at Carbon Fiber replacement parts.

+1

Or ride the bike more!

.
...Max called and he wants his post back. He says please don't cut it.

You can cut it but probably shouldn't pass the post on in the future. Whatever the distance from end to the "Max" line offers a conservative guide for the minimum insertion depth. But the principle of the max line is based on a very rough calculation of possible rider weight, saddle position, terrain, and other considerations. It is very conservative.

I'm all for making parts your own and personally would have no issue with cutting the post. A 400mm post can be safely shortened to 300mm if the minimum insertion depth is followed. The post becomes less likely to bend when shortened so, theoretically, you could even raise it up a bit. Haha

^this. You just don't want to be on your bike when the seatpost/seat collar fail. That's what happens if you do this wrong.


J.

Take a swig of water and you'll save as much weight as cutting a nice Thompson post... not worth the effort and lack of resale value

Max makes sense to me. That's the maximum amount of sestpost that should show

I must spend too much time standing on my head.

I think the stress put on the seat tube by a short post is going to cause the tube to fail , not the seat post. The further the post is inserted the more the stress is distributed.

http://fcdn.mtbr.com/attachments/ari...301_065037.jpg

...^^^note the minimum insertion markings, clearly visible.

Lot's of folks can't follow directions.

I rode a vintage Sakae/Ringyo CT P5E Alloy for five years measured at 180mm long (measurement not from max height line). I bought a 300mm post a couple weeks ago and it takes like 3 DAYS for me to get the seatpost all the way in. Don't cut under 100mm. I'm going to cut my post once I get the tool.

in that age of bike i'd be more afraid it was an odd size like 26.8mm that you forced a 27.2 into.

You won't get a 27.2 post into a 26.8 seattube, but spending three days to get it into a 27.0 frame like my Raleigh Carlton sounds very believable.

26.8 an odd size? It's by far the most common for the steel Japanese of the 70s and 80s.

Ben

well... odd for those of used to modern bikes with either 27.2 or 31.8 ;)

but ya 27.0 sounds more likely

Wouldn't happen with carbon

I can't believe how many people are saying not to cut the post. As mentioned, just make sure you will have enough left to safely maintain the minimum insertion. There's nothing magical about a seatpost that makes it a bad idea to cut down as long as you do it properly.

It is mostly a waste of time and effort. Plus if he ever wants to get rid of it or changes bikes it will be worthless. Thompson is already one of the lightest alloy posts out there so cutting off a few inches isn't going to do a damn thing.

plus his problem seems to be a diameter issue rather than length.

Time and effort are relative. I can cut through a aluminum seatpost in about 45 seconds with my hand hacksaw. Now my time IS valuable but I still come out ahead by doing the work myself.:lol:

The reason to avoid recommending cutting is, IMHO, an overabundance of caution. You can "save" someone every time you warn them off. It's nice to "save" people.

There's an old saying, "You can never be too rich or too thin." Thin is like light weight. Of course it does take considerable restraint to avoid taking this ridiculous saying seriously.

I surfed Sheldon Brown's website for awhile and my Bridgestone 400 has a 27mm diameter seatpost. I was joking about it taking 3 days haha. It takes about 1 minute for me to get the 300mm seatpost all the way down into the downtube. I would have kept using the Sakae/Ringyo CT P5E which I can fit in about 20 seconds but it was all perma-dirty (no signs of rust) from 30 years of use.