I'm looking at a bike where I'll almost definitely change to a Thompson Elite post. The only length available in that diameter will be 330mm and long enough with about 35 mm to spare. Currently im on a seatpost with about 100mm extra. I'm a heavy guy and feel safer with more seatpost than the required minimum. Reasoning that there will be less stress on the frame.

Am I right in wanting more than the minimum insertion or am I concerned about nothing?

Edit: 35mm to spare. As in BEYOND the 63.5mm minimum.

What is your frame? It is the seat tube of the frame that suffers from too short a post. (Though many posts have material removed from the inside at the end so if you raise one of those posts too high, the most critical point on the seatpost, where it leaves the frame, will now have less material.)

With lighter steel frames and insufficient seatpost insertion, hard bumps can lead to bulging the seat tube at the end of the post. I would not expect a CF frame to fare any better. If I were you, I'd wait until I found a longer post. (Or if you really like the frame and want the great Thompson clamp assembly, have a custom post made. Thompson sells the clamp parts. I have two custom TiCycles seatposts with Thompson clamps (to get 60mm setback, far more than any stock post).

Ben

minimum is about 2 1/2 to 3 inches on any frame type... and a bit longer for carbon frames.

and i'd say a bit extra is needed when rider weight or aggressive riding is considered.

MOST seat posts have a minimum insertion marker on the posts.

35mm simply will not be enough.... in any scenario.... and frankly, it sounds like your bike frame may be too small for you.

Eleven inches of exposed post?

Why do you want to use a Thomson post?

The proposed frame will be a KHS 747 2xl. It won't be too small.

The frame is steel. Minimum insertion for the Thompson 29.8 330mm is stated as 63.5mm. So there will be 100mm inserted.

I want to use a Thompson because it is a quality product and I've had great service from one currently.
It never slips.
Nicely made.
Great adjustability.

There will be 225mm exposed. About 8.85".

This is what I would be concerned with and the OP's intuition that a longer post is better is correct. The fulcrum is going to be near the binder bolt, so a longer post in the seat tube will spread that force out over a greater internal area.



So basically 4 inches. If by "heavy" you mean 250 - 300 lbs, that is probably sufficient.
If you mean 400 lbs, that could possibly damage the frame if you take bump head on without standing and putting some weight on the pedals. A lot depends on how precisely the seat tube is bored.

Your edit of your first post clears things up considerably. Carry on.

I'm about 250#. I'm sure I could find a longer post in a different brand but I really like the Thompson.

Ride it all day long. Don't be Evel Knievel. Should be fine.

Thanks for your advice. I just placed an order for the KHS 747. A Big bike for a big guy.

So, about 10cm of seatpost in the frame. That should be enough, and is probably about what the average insertion length was 20 years ago before going to the silly sloping top tubes. But, you do have a longer post to give it more leverage too.

What crank length are you using? 180mm cranks are generally available, and would drop the seat slightly over, say a 170mm crankset.

Oh, here are some comments about your KHS 747.
https://www.bikeforums.net/clydesdale...-seatpost.html

29.8 seatpost?

Anyway, that Skyscraper seatpost listed above is still available. Are you going with zero setback?

The 747 in a 2XL comes with 200mm cranks and that is taken into account in all the numbers I posted.

The crank length and a frame designed for them are the main drivers for the purchase. Plus I found an excellent price on one.

I've got a tall friend with one of those - he says it rides better than any other big bike he's had. He splurged on some extra nice wheels.

Thanks, that's encouraging.

What's wrong with your current post that you feel a need to replace it?

thank you for the edit to the top post.

sounded like you'd only have 35mm inserted... yikes! that ain't enough, eh? ;-)

As long as you have the minimum insertion and the seat post is past the top tube you should be good.
With some MTB frames, min insertion is still above the top tube and puts too much stress on the seat tube extension. I had to go with an extra long post for my Surly Krampus due to the sloping top tube and long seat tube.

With the extensions you are citing, I'm getting something like a 39-40" inseam. Just how big (tall) are you?

(PS I agree with the other posters: 4 inch insertion should be adequate for guys our weight - I'm about 240).

My thompson seatpost failed at the clamp after the warranty period. I weigh 180lb and and had much more then minimum insertion, Being by far the most expensive seatpost I have ever bought I consider them very poor value. I think there extra cost is due to their weight savings. I would never buy another under any circumstances. Personally If I were going to the maximum of a thompson I would consider something with more mass. I would buy a knock off from china before I would buy another thompson. At least if it fails I haven't lost so much money.

Thomson seatposts have a pretty solid reputation for quality. How is it that you think the insertion depth has anything to do with the clamp failure?

Not impossible that you got a defective unit, but more likely you damaged it yourself somehow.

Been using the same one on the same bike for 14 years.

The forums are great place to bring to light products strengths and weaknesses.

You are a fan of this mfg and I understand as I also have my tried and true. Though there is no reason to negatively attribute a messenger with things they did not say OR do in order to defend the reputation of a company.

I adjusted with 4 inch allen wrench in bare hand while on a ride. I was certainly surprised when my little allen wrench broke my $75 seatpost. Prior to that, I was a fan and would recommend the product.

For a 250 pound rider, I would recommend something more robust.

...here's the deal on bike frames where the seat tube extends above the top tube and clamps above like your bike. The insertion needs to extend a safe distance beyond the top tube junction, not just a certain designated depth.

Otherwise, it will cam out, especially with a substantially sized rider.

So if 100 mm is 'bout four inches, looking at the photo I would not feel like that was adequate depth on that particular bike if I were riding it. If you can get 100 mm of seat post below the top tube, you're good to go.

You might get away with four inches total. I personally would not do it, but I have been known to wear a belt and suspenders on a couple of occasions.