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Here are 2 ideas for a makeshift design. The first one is no not that good. The second idea seems more doable using 2 heavy duty seatpost attached luggage racks.

The seatpost is 33mm diameter, dunno if that the official size but it's as close as I could measure it. The seat is great. The bike is aluminium alloy so I wish to keep it light.

Is there a pro solution to putting the seat back like this?

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  • 11
    Why do you need that amount of adjustment ? The seat rails should provide more than enough adjustment for a decent fitting bike. I wouldn't want to sit on anything that would potentially skewer my crown jewels in an accident or a bolt snapping.
    – Dan K
    Commented Jan 27, 2021 at 6:53
  • 6
    BTW this is a good question and I have upvoted it, despite being a bad idea in my opinion. Thank you for describing clearly your idea.
    – Criggie ♦
    Commented Jan 27, 2021 at 9:08
  • 5
    The seatpost clamped luggage racks have maximum loads much, much less than a person's weight. Using one will put a huge bending load on the seat post.
    – ojs
    Commented Jan 27, 2021 at 17:11
  • 10
    This whole thread looks like a massive case of the XY problem
    – user622505
    Commented Jan 27, 2021 at 21:44
  • 6
    @user622505 it's a common theme that the users who ask about homemade e-bikes really want a motorbike, and preferably one they're somehow allowed to ride drunk and without helmet, insurance or driver's license or at least significantly cheaper than real motorcycle. It never occurs to them that those don't exist for a reason.
    – ojs
    Commented Jan 28, 2021 at 7:34

3 Answers 3

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The only safe answer is to buy a bigger bike frame.

When your adaptor fails, you will descend rapidly onto the rear wheel. The remaining original seatpost has an excellent chance of tearing your chest open, and impaling your head up through the jawbone.

While this is definitely creative, its absolutely not the right answer.

Buy a bigger bike frame.

If you're under 20, then there's a good chance you're still growing. Even more reason to buy a larger frame bike.

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    You forgot to mention the part that's likely to matter the most to a lot of younger guys: first it destroys the family jewels. Then if you're particularly unfortunate, it might not kill you...
    – Jerry Coffin
    Commented Jan 27, 2021 at 6:57
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    @EricHuelin a layback seatpost is quite different from your mockup picture. For a start, a seatpost is one piece of metal, whereas the image suggests two stems mounted mirrored. All your weight is on 2 or 4 critical bolts, which are probably M5x0.8 and not really capable of supporting an adult body weight in tension. When used as a stem, these bolts and threads are under shear force.
    – Criggie ♦
    Commented Jan 27, 2021 at 9:06
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    Sure, go ahead. It's a bad idea and don't come to blame us when it fails, but at least the racks are less likely to snap in the middle and the remaining stub to stab you in the crotch.
    – ojs
    Commented Jan 27, 2021 at 17:28
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    @Eric Huelin You’re talking about 200ft•lb of torque or greater on the rack-to-seatpost joint, which will almost certainly break it. Running such a large amount of setback is also biomechanically sub-optimal. Just learn to move your butt forwards off the saddle and hovering over the top tube before stopping. Full squish MTBs usually lack the necessary rack mounting holes anyways on the dropouts, and the frame is not guaranteed to be strong enough to handle clamped on ones.
    – MaplePanda
    Commented Jan 27, 2021 at 18:26
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    @EricHuelin How does moving the seat back at the same height help your feet reach the ground?
    – Andrew Morton
    Commented Jan 28, 2021 at 19:44
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The classic solution is banana seat and sissy bars. You'll lose the rear suspension, but the chances are that it's not going to work anyway because you'd be sitting way more back than what the suspension was designed for.

Edit: Looks like suspension sissy bars do exist. It's going to be an interesting problem to balance the frame and seat suspension.

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  • 2
    Excellent point - it would be something like riding a bicycle while sitting on its parcel rack.
    – Criggie ♦
    Commented Jan 27, 2021 at 9:22
  • 2
    When I was 7, sitting on the rack and pulling off awesome wheelies was the second best thing after Nintendo.
    – ojs
    Commented Jan 27, 2021 at 14:30
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There is a simple, pro solution to this and it is have a framebuilder make you a seatpost that can do this.

A simple iteration would be a bunch of segments of chromoly tubes, which may need the part that goes in the frame to be turned, and then have the last segment be a 22.2mm one such that it can take a cheap saddle clamp. A more sophisticated version could have guts transplanted from another seatpost.

There is an open question with all of this whether your frame can handle this kind of leverage at the seatpost opening without flaring open. My guess is probably not, and there's no way to solve that problem. There are bikes that are designed to have much more setback than others, i.e. most Electras and their many copies, so you are not playing in a new sandbox here, but those frames have slacker seattubes to keep the aforementioned leverage down.

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  • I investigated these slacker seatubes and beach cruisers, all you get is hard tails and fixies. I have hills and bad roads that my legs and my pelvis can't handle. So I went for the closest I could find to my criteria list I found nothing, except this one bike but it was just a few inches off my ideal bike.
    – Eric Huelin
    Commented Jan 28, 2021 at 23:51

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