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I purchased a Hamax child seat (not unlike this one), but when attaching it to the frame the child seat is too low and is touching the rear wheel.

As you can see in the photo, I don't think I can secure it higher up in the frame.

Can't secure it higher up in the frame

Is it safe if I attach it to the seat post? Can I attach child seat to seat post?

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    Does not look like there is enough space on the seat post – Argenti Apparatus Jun 23 '18 at 18:56
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    Note the seat post is a narrower outside diameter than the seat tube. – Criggie Jun 24 '18 at 5:57
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    The way the cable has to slide over the top brace makes me wince! – Carel Jun 24 '18 at 8:04
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    @Criggie if the bracket is the same as either of the 2 different Hamax brackets I've had, the plate that goes behind the seat tube doesn't have much of a hole in the middle for it to straddle the cable stop. – Chris H Jun 25 '18 at 17:25
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    @Criggie True. In fact as you can see in the picture, I had to wrap an inner tube around that part in order for it to fit tight because even that one wasn't thick enough. – zundi Jun 26 '18 at 16:10
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I would strongly recommend against it. I have a similarly designed seat from Yepp, and the amount of force exerted on the frame is substantial. Seat posts can shear, and would result in a bad time for everyone.

You will also find that having your child that high up lightens the steering a lot. Mine is install in the correct position, and I have to consciously shift forward to maintain firm wheel placement; putting in on the seat post would make this even worse.

There are some that are designed to clip to the seat post, but their design is very different: This one for example and they rest on a pre-fitted rack.

Have you considered a crossbar seat instead?

  • I must disagree with you that the Thule Yepp child seat you linked is attached to the saddle stem. The seat rests on the luggage rack (where it is securely attached) and the strap holding it to the saddle stem is an additional safety measure should the child seat become loose. – Mike Aug 3 '18 at 13:41
  • Editted to clarify. – Inoutguttiwutts Aug 3 '18 at 14:00
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Answering your question - perhaps the seat stem will hold both you and the child and the child seat all together. And maybe not - depends on the total weight of yourself, the child and the seat. Should it fail, not only you're putting your child in danger but also your ability to have another children (black humor). Furthermore, by putting the centre of weight higher, cornering will become more difficult, again putting yourself and the passenger in danger. Bottom line - I wouldn't do it.

Taking this answer a step further, I see the following solutions to your problem, provided you want to keep the seat and the bicycle:

  • you mentioned you had to thicken the seat tube. If you use some tapered tube (or two parts), thicker in the top, where you put the clamp, you effectively place the seat higher like you were changing the angle of the seat tube to be steeper.
  • you can reroute the front derailleur cable so it goes through the bottom of the frame (most likely you need to replace your front derailleur to down-pull model, put the cable guide on the bottom bracket and another one on the down tube) and cut off the cable guide on the seat tube making enough space for the clamp to be mounted higher.
  • using a bench vise you can bend the rods of the child seat so it effectively sits higher. This option depends on the material of the rods - I suppose it is steel as it must be flexible and spring. And you also void the warranty of the seat this way.

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