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My wife and I used to both commute by bike, and now that our child is old enough to go on a bike, we'd like to do so again. She has already installed a front-mounted baby seat on her dutch-style bike, the kid loves to ride in it!

However, it seems silly to adapt my road bike for a child seat when we already have a tandem bike made for two adults that would offer a much smoother ride for the child in the stoker's position. Is there such a thing as a seat-post adapter to carry a child seat instead of a saddle, or a baby seat that could replace the saddle?

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  • Can’t you put the baby on the rear rack? That would be the easiest and safest - and leaves open a triplet-bike. :)
    – RoboKaren
    Oct 16, 2017 at 0:36
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    Why not just use a stem mounted seat, mounted to the top tube?
    – whatsisname
    Oct 16, 2017 at 1:49
  • @whatsisname +1 the advantage of a stemmounted baby seat is that the center of balance won’t be too problematic even if there is no stocker.
    – RoboKaren
    Oct 16, 2017 at 2:42
  • @whatsisname This is actually the closest solution I've come to... I took a short seatpost and inserted it almost all the way, screwed it tight. and attached a Yepp Mini seat to sticking up seatpost using the SlimFit adapter. This seats the smaller one... and the bigger one goes on the back rack. But this still puts the weight very far back on the bike. Something more centered would be better
    – Tadhg
    Feb 12, 2019 at 20:09

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I don't know of a child seat which replaces the seat-post and I think it's unlikely that it exists as a product as this would be such an unusual case. The normal Hamax seats which attach to the seat tube will work fine and the design shown here provides some suspension.

Child sear (Hamax Siesta) on rear of tandem

I've ridden the bike like this, without a stoker and it does take some extra care due to the weight high up, especially at low speed and when pushing/parking the bike. In this photo I have the rear brake locked on with a strap, to stop the bike slipping. If you were certain you'd never have a stoker riding then, depending on the frame design, you could probably attach it to the captain's seat tube, but on a tandem it can be helpful to have a bit of weight over the back wheel for traction.

I know that doesn't exactly answer your question, but hopefully it's some help.

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  • You've got your siesta (if my eyes don't deceive me) mounted about as high as mine, but without a rack underneath - any particular reason? Now my daughter is a bit heavier I'm finding an effect on the handling
    – Chris H
    Oct 16, 2017 at 8:01
  • Can't remember if it's the shape of the frame/closeness of the mudguard which necessitated this. We may have had the rack fitted until she got heavy enough to bounce on it, but didn't move the bracket. But fair point, lower is typically better for balance for the child seat, tandem or not.
    – James Bradbury
    Oct 16, 2017 at 8:04
  • Did you try fitting sproglet's seat to the captain's seatpost, so it would be between the arms of the stoker? That would improve the handling and weight distribution.
    – Criggie ♦
    Oct 16, 2017 at 8:30
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    I think the question is explicitly about having child in child seat instead of stoker.
    – ojs
    Oct 16, 2017 at 8:59
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    @Criggie - on a good ride on a warm day, wouldn't that mounting position result in a soggy sprogey due to proximity to a sweaty stoker? ;)
    – FreeMan
    Oct 18, 2017 at 13:57
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So, in the end the answer is yes, it works. You can even add 2 children and it rides just fine... but I don't do it.

The reason is, it's really hard to handle the tandem with 20-50kg of deadweight on the back once you get off. You basically always need to stop beside a wall. Turning around when not on the bike is always a challenge on a tandem, it's nearly impossible with 2 kids on it - and you really don't want it to fall!

Trying it out with 2 child seats. Here's the setup...

The setup here is 2 seats - the Hamax rear seat mounted to the captain's seat tube and a Yepp Mini mounted to the stoker's seatpost. There's still space for another seat on the back rack :D

I guess it could work with an elastic to keep the back brake closed, and two-legged kickstand under the captain's seat tube, but I never found a compatible kickstand. (A kickstand under the stoker's seat won't work - you can't get back there and keep the bike steady.)

In the end it's safer and easier to tow a 2-child trailer behind my regular bike, even if it's less fun for the kids!

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  • Great work - thank you for coming back to provide closure and share your experiences. Feel free to mark this answer as "accepted" with the green tick/check mark to the left.
    – Criggie ♦
    Mar 7, 2020 at 21:12
  • Shame it didn't work out, but nice primary research and write up!
    – Swifty
    Mar 7, 2020 at 21:45

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