Engineers - Are seatpost shims frame-friendly??

[OldFatTi]

Been thinking about this...

Shimming a frame with a proper shim, 100mm long, and a smaller diameter post, will induce more flex of the post itself, but will this be beneficial or detremental to the life of the frame's seat tube/seat clamp area?

I'm thinking of arguments for both as the flex will absorb impact throughout it's flex, rather than transmit it directly and rigidly to the frame, but then again is the flex acting like a lever on the frame??

An example being Lynskey used to have problems with their seat tubes cracking on 31.6mm post fitments, then they shimmed their seat tubes down to 27.2mm (still 35mm OD frame) with less failures. So smaller seatpost ID, less stress at the seat tube??

What do you guys think? Any absolute engineering facts? Any experiences?

 

[M-Power]

Interesting. I can only think that shimming a 27.2 in a Lynskey acted to dissipate the stress riser creating load. Shimmed seat pins creak badly from my experience, resulting in a tendency to over-tighten things, which is not good. Better to find the right size post IMO. Ti posts are better at flexing, so maybe they protect the frame to some degree.

 

[cherrybomb]

A properly fitted shimmed seat post won't spread the force on the frame any more or less than one without a shim. You'll gain a little flex in a shimmed post because of the reduced diameter but this will all occur in the portion of the post outside the frame.
I guess it might reduce shock loads on the seat clamp area a little but I wouldn't have thought it would be significant.
Any issues with shimmed posts relate to poorly fitting components rather than the presence of the shim per-say. You also need to watch if there is a decent length of seat tube above the top tube junction because the shim needs to go well past this point.

 

[OldFatTi]

Yes, ignoring what some "less educated" folk may do such as torquing the seat clamp bolt with a 2ft breaker bar and/or fitting too short a shim to not go deep enough into the frame!

M-Power I've never had a problem using shims - correct shim for frame (with longest length possible and plenty of depth past the seat/stays/top tube junction), correct post for shim, assembly paste between everything and the correct seatbolt torque has always been fine and dandy. Yep the rigidity of the 31.6 diameter posts must have put more load on the frame on the Lynskeys, hence the better reliability record of the 27.2 spec'd frames.

Cherrybomb - wholeheartedley agree!

Many (most?) titanium frames are already shimmed from the factory - aforementioned Lynskey, Sandvik Ti Kona, Ti Fat, so a "correctly" sized stock size post will only interface with ~100mm (or whatever the length of the frames insert) of the seat tube anyway.

I suppose it's just whether seatpost flex helps to preserve a frame or contributes to a frame's fatigue. I agree and would think the former, as the load is being spread out over time rather than sharper, higher peaked forces. But it's an interesting one.

Forces on a flexy post are fore-aft too of course - that could be where a potential problem may lie -whereas a suspension post would be much more vertical. I'd say a sus-post would definitely preserve our beloved frames.

 

[trail_head]

Been thinking about this...

Shimming a frame with a proper shim, 100mm long, and a smaller diameter post, will induce more flex of the post itself, but will this be beneficial or detremental to the life of the frame's seat tube/seat clamp area?

I'm thinking of arguments for both as the flex will absorb impact throughout it's flex, rather than transmit it directly and rigidly to the frame, but then again is the flex acting like a lever on the frame??

An example being Lynskey used to have problems with their seat tubes cracking on 31.6mm post fitments, then they shimmed their seat tubes down to 27.2mm (still 35mm OD frame) with less failures. So smaller seatpost ID, less stress at the seat tube??

What do you guys think? Any absolute engineering facts? Any experiences?

When I use a shim, I also pin a nylon sleeve to the end of the post to minimize stress at the clamp are. this also means I can save weight on the shim;

 

[OldFatTi]

Great idea Mr trail_head

 

[feetabix]

Been thinking about this...

Shimming a frame with a proper shim, 100mm long, and a smaller diameter post, will induce more flex of the post itself, but will this be beneficial or detremental to the life of the frame's seat tube/seat clamp area?

I'm thinking of arguments for both as the flex will absorb impact throughout it's flex, rather than transmit it directly and rigidly to the frame, but then again is the flex acting like a lever on the frame??

An example being Lynskey used to have problems with their seat tubes cracking on 31.6mm post fitments, then they shimmed their seat tubes down to 27.2mm (still 35mm OD frame) with less failures. So smaller seatpost ID, less stress at the bseat tube??




What do you guys think? Any absolute engineering facts? Any experiences?

When I use a shim, I also pin a nylon sleeve to the end of the post to minimize stress at the clamp are. this also means I can save weight on the shim;

That's a great idea!.......the same principle used in various sus forks.....think of using a broom one then two handed.....

The physics of a post shim can all be assessed using newtons laws (force x perpendicular distance from fulcrum etc).....assuming the post is strong enough to do its job it's width is not that relevant, it's just the interface between shim and frame that counts......the same forces will be sent to the frame regardless of post width.......smaller shims will place greater forces per unit of frame area, and thus longer is better......that nylon shim effectively lengthens the shim, reduces stress on the top tube/seat stay area, and is good thinking.......I've several shimmed posts covering ti, steel and alloy frames....all of which support my fat ass without any problem......

 

[FMJ]

Overkill to the Nth degree. World's most expensive seat post shim.

 

[Woz]

Interesting idea that trial_head, but I'm of the school of cutting down the seat-post to the size of the shim. I once had an old AL Kona with a much too long post. The clamp area acted like a pivot and eventually the seat-tube visibly bowed where the end of the post was.

I agree that a seat-post with a bit of flex is a good thing; I prefer ones with an off-set as they are a little more forgiving.

Never had any problems with shims, including the beer can ones. Carlsberg or Guinness, but not Heineken as they have some embossed logos. Using soft drink cans is just wrong.

I drill a very small hole in the shim before inserting it - about 1cm below and opposite to the bottom of the seat-clamp slot. You can then easily remove the shim by inserting a small screwdriver at the bottom of the seat-clamp slot, through the shim slot, to the other side and into the shim hole - then push down on the screwdriver handle and voila the shim starts to pop out just enough to grab with fingers. I am an engineer

 

[FMJ]

Did you ever find the Titanio you were looking for Woz?