seat post minimum insertion distance
 

All my seat posts are marked with minimum insert marks. Some time ago, it was pointed out here that this is to avoid trashing the frame - the seat post has to be below the top tube where it joins the seat tube. Now, on my one bike, the post is 27.2mm and the seat tube is 30.2 or something like that, so it has a shim.

Soooooo, why is that shim so short??? Far shorter than the minimum insert length? This completely trashes the idea because the stresses are taken entirely over the shim length.

Bad design?

(This would not be the only instance on this particular bike: The bottle cage mounts, one on down tube and one on seat tube are too close together so you can't actually get 2 bottles in. Dickheads.)

Probably the 2 mm difference does not matter. If any flex occurs, the seatpost below the shim will still contact the wall of the seat tube and add some support. I think as long as your shimmed seat post is inserted past the minimum insertion mark you are safe. Part of the idea may be to not have a lot of exposed seat post, making a long lever arm with lots more force on the seat tube/top tube, seat stay area. I am not a mechanical engineer and I have not stayed in a Holiday Inn Express, so these are just my guesses.

I think this is a good question, and I don't have a good answer. But I think the inconsistency is greaterthan jsharr thinks.

First, its 3mm difference not 2mm, 1.5mm on each side, and no way would you want the post to move that much while in the frame. If its an aluminum frame, the situation is even worse. Just get the right seatpost.

Have had bad luck with shimmed seatposts, real creaky, noise drives me nuts especially in a TI frame. Finally broke down and upgrades to the proper diameter. Mo betta.......

Was not saying to use the seat post unshimmed, just saying that though flex would occur, probably not catastrophic if post is shimmed and inserted past minimum distance.

[QUOTE=jur]- the seat post has to be below the top tube where it joins the seat tube."

Not true. Think back to the days when girls' bikes had top tubes that were 20"+ below the top of the seat tube.

The idea is to insert the seat post deep enough so that it has enough purchase to hold securely. If it is not in deep enough it can work its way out, leading to possible injury to the rider, and damage to the seat tube.

"Soooooo, why is that shim so short???"

Just how short is the shim? You haven't told us. Shims do not need to be the same length as the seat post they fit around. (Imagine a seatpost that sits 20" inside the seat tube, with a 3" shim. That should be plenty safe).

Bob

No way. An inch and a half would be more than enough for that.
As to the short shim dilemma, I don't know. How far does it go into the frame?

[QUOTE=Bobby Lex]Those girls bikes still have/had seatstays that help brace near the top of seat tube so that do't fly. Nor does anyone buy the notion of "deep enough...purchase to hold securely." Its about levering forces being excerted on a post thats being weighted at an angle. Shims are a better temp fix than a perm solution to a mismatched seatpost/tube problem. 20" inside a seat tube? Kindof an extreme example.

if it is a steel frame i would want the shime to be at least 2.5 inches long. if aluminum then 4.0 inches long. personally i would not use a shime setup at all but would opt for the correct diameter seatpost.

More details about the dimensions:

The seat post minimum mark is about 100mm, while the shim is about 50mm. The top tube is about 75mm below the top of the seat tube. So, a seat post by itself would be past the top tube joint, while the shim ends half-way along the joint.

Some time ago someone reported a broken seat tube - it was broken off right at the top of joint of the top tube. It was caused by an insufficiently inserted seat post - the guy had just an inch-and-a-half inserted. The ensuing discussion had it clear from the experts that it is not about the seat post, but about trashing the frame that the seat post has to be inserted deep enough. I can't find that thread, it had some photos also.

Ha! found it.

http://www.bikeforums.net/showthread.php?t=143548

My Thomsom post as a lot less tube below the min. mark than some of the less expensive types. Why?