Frame failure: should I get a longer seatpost?

[roubaixtuesday]

Following on from discussion of tandem frame failure here:

https://www.cyclechat.net/threads/frame-failure-idle-speculation-on-root-cause.300835/

Thanks to all there for an interesting discussion. The new tandem has less upstand, a larger frame, so less exposed seatpost, and a slightly longer seatpost, so should be much better.

But is it good enough?

Following pics of the seatpost out of the frame, and in the frame, plus a pic of the whole machine for context.

In the shot "in the frame" my thumb shows the length of the post.

 

[Drago]

The only failures I have personally been have all been at the seat tube/top tube junction where the leverage from the seat post does it work. Being a heavy lad the first thing I do is fit a 400mm seatmpost (or longer if I cn source one) seat post to speed the load.

In your position I would be most inclined to replade that for a longer one.

 

[Yellow Saddle]

A longer seatpost will only prevent the type of frame failure in question if these two conditions are met:
1) The seat tube sections above and below the top tube should be of the same diameter as the seat post. If the lower section has a larger internal diameter, the top section will still flex.
2) The seat tube is sufficiently stiff to reduce flex.

In your case, you have a very long and highly-loaded section of post above the seat tube and therefore require a stronger (thicker walled) seatpost. If seems from your photos that your seat tube is at least of equal ID in both sections. Thereis nothing wrong witht he length of your seatpost and a longer one of the same material will not improve the outcome.

 

[twentysix by twentyfive]

I wouldn't feel safe with a thumb nail length bit of seat post. You most definitely need a much longer part of seat post in the frame. Currently you risk damaging the frame.

 

[roubaixtuesday]

I wouldn't feel safe with a thumb nail length bit of seat post. You most definitely need a much longer part of seat post in the frame. Currently you risk damaging the frame.

I think you've misinterpreted. It's where my thumbnail is on the pic, about 125mm into the frame.

 

[twentysix by twentyfive]

I think you've misinterpreted. It's where my thumbnail is on the pic, about 125mm into the frame.

Ooops. 125mm should be OK if all the correct diameters

Pleased I gave @Ming the Merciless a larf

 

[T4tomo]

You're about 2/3 up and 1/3 in which should be ample. Also the design of this frame is much better, the failed one had a vertical exposed seat tube extension which took the majority the flex forces, this one appears to finish flush to the top tube, so any "flex loadings" will dissipate into the top tube supporting it, I think*

*I expect @Yellow Saddle to have a bot of a laugh at my pseudo physics / engineering but it makes sense to me

 

[MichaelW2]

Does the cantilever luggage have any effect?

 

[roubaixtuesday]

the cantilever luggage

Do you mean the bar bag on the stoker's bars?