Badzilla

Talking about the tube the seat post slips down into.
The bike looks like this.
How much of that short seat tube above the weld is necessary?
How much of that seat tube (above the weld) would you necessarily leave in order to feel the structure/strength is not degraded?
How much could you cut off without feeling unsafe?
Asking about feelings.
Thanks!
PS... A seat will in fact be used, but I'm not concerned about how the seat will be attached.

DiabloScott

Pic assist



There is probably a gap in the back that allows for a little flexibility to get the seatpost in there, and then the clamp keeps the post from slipping.

If you're going to run it without a seatpost, you could safely saw off almost all the way down to the weld. You'd want to be really careful not to get sawdust down in the BB though.

Not saying this is a good idea, I have no idea what your objective is... but it wouldn't affect the potentially deforming stresses on your frame. Oh, an observed trials bike maybe?

maddog34

you don't need a seat?
Cut away.

Have you bought that bike, or are you just dreaming about buying it?
I feel you should Dream about something worth buying instead of that tragically low end bike.

3alarmer

.
...you could probably lose all of it, for all the good it contributes to structural integrity. But you really do need some way to clamp the seat post in the frame, and for that bicycle, that section of the tube is how you do it. BTW, the seat post needs to extend below the spot where the welds join the seat tube to the rear triangle, just in case you didn't know that.

Andrew R Stewart 

If the seat and associated binder slot are to be retained the length of that slot is a dimensional factor to keep in mind. When the seat tube binder slot gets much less that 1" in length the clamping ability suffers. Slipping down over time or posts that rotate off center from the leg pressure are common results of poorly clamped posts.

The bottom of the slot wants to be above the seat stay attachment weld beads. The bottom is best with a rounded end (think upside down keyhole) to minimize future cracking.

I agree with the advise other gave, to seek a bike that either fits your stature or your intended use better than one that needs irreversible mods to work. Andy

SurferRosa

Stating that in the op would be far too helpful.

Badzilla

Tthat might be helpful to some, personally. I could've also posted my life story, about how I have been designing and building things for decades, about how I make my own tools to make things (sometimes I make tools to make tools to make things). The only help I need in this thread, and already got, is determining whether shortening the seat tube reduces the frame strength. Apparently nobody disagrees with the idea that it doesn't. That's good! I'm satisfied. How are you doing today?

Kai Winters

Your arrogance is noted...no suggestions forthcoming...do what you want...I don't care...

shelbyfv

And yet you find that "bike" worthy of effort....

Badzilla

That's the very kind of noise mutual blocking would prevent. Maybe someday.

Badzilla

Sounds like you are a spoiled brat.
The frame is aluminum, the welds are great, and the wheels are already trued. I really don't see what the problem is, besides the obvious "a more expensive bike would be a better bike". It won't be used as a mountain bike, typically there won't be a lot of strain on it. I'm satisfied. How it will work for the intended purpose is yet to be determined. But I am having fun.

Mr. 66

I feel great thank you for asking!

However people asking for purpose when you aware asking about safety is completely normal.

I guess you could give your life story, weird that with all your knowledge that you even feel the need to ask. So buy your POS modify and do what ever the puck you want to do with it.

Badzilla

A spoiled brat with an attitude...

Kai Winters

As long as I don't have to read your twaddle...it's all good

Mr. 66

From the guy that had a secret use for bike that is asking about a potential safety hazard after modifying.

rosefarts

Give the guy a break. You’d be irritable too if you were riding around with a seat mast up your butt.

sweeks

A few years ago, I actually saw a guy riding without a saddle (or seatpost!) on a 75-mile organized ride. He didn't seem irritable at all.

shelbyfv

Magic carpet ride?

Badzilla

Unlikely. Actually ignoring somebody defeats the purpose of whining about them.

Preventing yourself from seeing somebody else's posts is self-defeating. Mutual blocking would cure the problem. I could prevent posters who just like to make noise from commenting in the thread. Given mutual blocking... Not-quite-grown-up posters would find themselves in an ever smaller echo chamber. It would help civilize forums and practically eliminate the need for censorship. Users should decide for themselves who they associate with. Moderators do not want to give up their power of supervising this garbage, otherwise mutual blocking would happen.

Badzilla

That "secret use" is riding. Besides... I asked about "feelings feelings feelings", not technical expertise. Besides... If there were any safety concern, it could be noted in the reply. You know, like... "Not if you are planning to go to the moon on it".
Difficult to believe that needs to be spelled out. That is easily assumed by anyone who is capable of posting useful information in this forum.

Some of the useful replies assumed no seat would be used. That's why I immediately edited the post to make clear that a seat would in fact be used. The purpose is easily understood by grown-ups.

Badzilla

Building stuff is hard. Anyone who builds stuff has respect for even the simplest designs. Anyone who mods stuff doesn't go for expensive equipment (that would be *foolish*).

DiabloScott

You don't seem to have the appropriate attitude about this. People who design stuff have to submit their designs for technical review and quality control. You came in here and asked an unusual question. You didn't correctly explain what you wanted to do, and you didn't explain why you wanted to do it. You got some criticism; take it or don't, but your defensiveness comes across as a weakness, not a strength.

If you cut the seat tube and expect the existing seatpost and clamp to function properly, you might well fail.

Mr. 66

I should change my handle to 'brat with an attitude'. Of course this is sparked from what some may perceive as the attitude of a brat.

Lol

Badzilla

Says who? I'm satisfied with the sum of replies.

Even if that's true for somebody, that's certainly not true for everybody. I BUILD and USE my own designs. Anyone who BUILDS their own designs becomes very familiar with the need to design things right. The devil is in the details. Just amazing how many revisions one can go through before actually feeling good about building it. That's especially true if you want to reduce work (lazy), and if efficiency is one's primary concern. I recently came up with a flats and slots cutter. Very accurate for the low cost. I immediately used it and the results are fantastic. I needed it for way too many years before figuring out the easy way to make such a jig. Now I can make couplings between my motor and bike, and did. It's accurate to within a few hundredths of a millimeter, from one square drive edge-to-edge compared to the other pair of edges.

Kontact

You designed this $200 Mongoose? Congratulations!