Longneck frames BEND easier?? :(

I dont knowhow long its been like this but my KH longneck has been bent at the neck. its not anything majorly huge but the degree of bending is noticable.

I use a stubby seatpost like 1.5 inch.

From a science/material standpoint does a super stubby seatpost put a lot of stressful leverage force in a long seattube like on the longneck frames?

alsoI heard having a shim inside your frame makes the neck bend easier too?

Please help me out :(:o

when you say 1.5 inches do you mean thats how much is in the frame?
If so i can imagine that putting a lot of force in that 1.5 inch area

If you can, a picture showing us exactly how it’s bent will help us help you figure out why it bent.

Yes, from an engineering standpoint, having a 1.5inch stubby stem means that you probably have a really large force on your frame. More detailed speaking, your seatpost acts as a lever when you put a force on it. So if the part of your seatpost sticking out of your frame is 3 times longer, than the part inside of your frame, the force on the end of your seatpost (inside the frame) will be three times higher than on the other side. So it would be better to use a seatpost that fits farther into your frame.

Using a shim can make the seatpost lean in an angle compared to having a proper fit stem. This doesnt necessarily enhance the force on your frame (in a way it does, a little bit, but my english is not proficient enough to explain this correctly). Anyways, as long as you just use a normal shin (some people use multiple shins) you should be ok.


few questions:

  1. Multiple shims? how is this possible?
  2. Say i use a 100mm shim and have a seatpost that is longer than 100mm, but have about 30mm sticking up from the top of the neck, does this put a lot of stress on the frameneck?

1.) Hmm, maybe we are talking about different things. A shim, I thought a shim is the little piece of sheet metal that you wrap around your seatpost in order to increase its diameter. Like this http://www.johnhenrybikes.com/images/products/SeatPost%20Shim-Large.jpg

If you were talking about the same thing, then it is very well possible to use multiple of those by simply sticking one inside the other.

2.) Im sorry but I dont really understand your question. As mentioned before English is not my mother’s tongue. Do you mean that the shim that you wrapped around your seatpost is sticking 30mm out of your frame? That would not be a big problem, because shims do not really increase stress on the frame in most cases. And in your case 70mm should be enough i suppose, but if you can it is of course better to put the hole shim inside the frame.

Where did your seat tube bend bend?

The maximum bending moment for the seat tube will be at the the bottom where it joins to the rest of the frame. The thing that determines this is the length of the ‘lever’, the combined post and seat tube length. It shouldn’t make a difference what material the ‘lever’ is made of.

Basically a higher seat will make the frame bend easier.

A short shim will act like a short seat post, with only that much length in full contact with the frame tube.

The more post you have inside your frame, the stronger that part of the frame will be. Generally there will always be a higher area of stress where the bottom of the seat post (or shim) is, inside the frame. Bends are likely to happen either there, or at the join between the seat tube and crown.

But a short shim does not increase the lever effect. It just reduces the contact area. But you have to keep in mind that a seatpost that “fits” does not have contact on its whole surface, neither. Because a stem that fits into the frame is always smaller than the frame, it just has contact on a fraction of its surface to the frame.