Need advice - seatpost past minimum insert

[joseelias]

Hi!

I've searched the web for this question (read some discussions on here too) but I'm needing some particular advice in my case.

I've got a 1996 steel Kona Lava Dome and my seatpost broke at the clamp area (BBB Hightower 400mm), so I've decided to use one I had from the 90's.

I've used this seatpost in the past in this same bike without any problems. But upon reading so many diverse opinions I'd like for you to consider my case in particular.

The seatpost is a relatively cheap Kalloy. For me to have my ideal leg extension (very, very picky about this) I need to exceed the minimum insert line by 5/6mm above the seatpost clamp. The seatpost also has about 10cm (4 inches) under the minimum insert line and the bottom is located just under the bottom weld of the toptube. You may look at the pictures.

On an additional note, the Kalloy seatpost, despite being 5cm shorter than the BBB has roughly the same weight, which may mean that the tube is thicker/stronger or the clamp is heavier. Did not weight without the clamps.

So, considering the following, do you think this may pose a risk to use on MTB?:

- 5/6mm exceeding,
- the location of the bottom of the seatube on the frame,
- that this is a Kona with that seatube extension,
- steel frame,
- I weight about 82kg (182 pounds)

Thanks!

 

[al-onestare]

Re:

Does it pose a risk? Yes.

The question to ask is, if you are picky about leg extension then have you tried a larger frame? You may find it's the better solution all round.

 

[half cog]

Re:

So far as I remember Kona do not count the extension on the frame as structural. It is merely there to stiffen a long post. You need a post that will allow the minimum extension mark to be level with the top of the top tube when it is the right height for you. That means a second mark on the post above the original at the same measurement as the frame extension .I have seen frames run where the entire post has only been in the extension and they got away with it but its not right according to Kona .The post is acting as a long lever and pulling the frame tubes and joints around to an unacceptable level.So I would say either a bigger frame or a longer post to be sure

 

[mk one]

Seatposts are cheap nowadays, you could get a 450mm post and have plenty inside the frame, longer crank arms or like al said above, a larger frame.

As you have rode with it for years without problems you have already answered your question, manufactures will usually mark things up on the safe side to stop any failures.

Personally i would just pick another post up for a few quid and not have to worry about what ifs

Mark

 

[joseelias]

@al-onestare
No, I haven't tried a bigger frame. I own this frame since 1998 and it's the only bike I've owned since then. Looking back maybe it would have been better to by a size 20 instead of the 19 although with the current setup it feels great.

Plus, in Portugal it's almost impossible to buy frames from this era. They are extremely rare since MTB was almost at the beginning, bikes were incredible expensive so not a lot of people could afford them and I believe that about 90% or more of the frames sold here are size 18. Plus, a frame like this would probably be sold for over 100€, and honestly I cannot afford it.

The only alternative would be to buy a more generic and recent frame made of aluminum, but then I would need disc brakes wheels...

@half cog
If things are as you say than I need to find another solution. Need to see if I can find spares for the BBB seatpost or eventually buy a new one which isn't the best thing to happen now.

Anyway, I've always ridden in complete disrespect with those norms it seems. Maybe I got lucky. Up until a few years ago I weighted 10kg less and I've never been a very aggressive rider. More the long distance type.

@mkone
In fact I've ridden this seatpost for several years. I replaced it with a suspension seatpost from RockShox which had the same length as this one and used it for several years too. Still have this one too and it's still in great condition.

But sometime ago I started to think about this same issue, took off the Rockshox and bought the 400mm BBB that broke now. Since I never had any problems with the Kalloy or the RockShox I started to think if it wouldn't be okay to use them.

 

[cce]

I'd always recommend running the minimum insert mark close to the top tube on these konas with the extended seat tube, not for the sake of the seatpost, but for the frame

 

[half cog]

Re:

The extension acts to prevent the post snapping off at the frame. You are expected to run a long seat post with these and that causes what engineers call a stress riser. That is to say that the post will be flexing a great deal till it joins the frame at which point it will be held firm. It puts all the stress into that one point on the post.The frame extension stiffens the post over the last few inches thus allowing the flex to be gradually taken out.Run it as you will but that is the reason that extension is there and as said before it is not designed to be a structural part of the main frame merely a stiffening device

 

[half cog]

Re:

Sorry double post

 

[mynchiboy]

Re:

Pretty much with the others here. To be safe you need to have enough post in the frame to cover the areas where seat stays and top tube join with the seat tube - as has been said, don't read too much into the seat tube extension...

Out of interest how tall are you and what is your inside leg measurement? I'm thinking that your frame may also be a bit small, if you're running a lot of seatpost to get a straight leg extension?

 

[FluffyChicken]

Re:

Run it, many people have and do. Many run with shims also and little touches the frame past the point of the clamp and is loose to the point of the butting starts/shim where it looses complete contact.
The post will bend first, bit by bit if it cannot cope.

I have never seen any literature from Kona describing to ignore Post Manufacturers minimum inserts. A free mm is here nor there.