6

I want to place a crank on a bike at a different location than the one the bike is originally located.

This is similar to what i want to do, this is an addon crank for a smaller person to pedal with a tandem bike sold as a kit. But the link is 404.
enter image description here

I want to buy this piece (black piece) it's a crank that can be bolted on but "bolt on crank" in google result in anything but. The image was found on Pinterest so that doesn't help me in finding this part. enter image description here

17
  • Get a custom build – Paul H Dec 4 '20 at 20:11
  • 1
    Moving the crank forward would be impractical, in most cases. But note that the bike in the top picture appears to be cobbled from two BMX bikes that were joined by using the seat post of the front bike to fit into the head tube of the rear one. – Daniel R Hicks Dec 4 '20 at 21:03
  • 1
    @Criggie Interesting google image search for "swing bike" Yes, it has the same layout as one but does not swing. – Eric Huelin Dec 4 '20 at 21:36
  • 3
    That's an enormous edit to the question and has now invalidated a bunch of comments. Consider appending rather than replacing significant parts of the question, to avoid confusion. – Criggie Dec 5 '20 at 1:14
  • 1
    I recommend to edit many parts of the original question back in, as it makes the whole project much clearer. At least the photo of the combined frames and the search for how to build this into a feet forward bike, rather than a tandem with kiddie seat in the back. – Willeke Dec 5 '20 at 11:50
9

TL;DR: The search terms to use may be "kidback adapter," "child stoker adapter," or "child stoker kit."


Googling for "tandem child bicycle crank" produced a link to Sheldon Brown's site, which had a page on tandems and children, and which had this to say:

For smaller children, (or larger tandems) a "kidback" attachment is needed. This consists of a "bottom bracket" that can clamp onto the seat tube of a conventional tandem, so that even a toddler can reach the pedals. Most tandem manufacturers offer kidback kits as an option, and there are also aftermarket units available from various sources.

The "bottom bracket" of a kidback is typically 1 1/2-2" forward of the seat tube, so it is best to move the stoker saddle as far forward as possible, either by reversing the saddle clamp if a "pipe" type seatpost is used, or by using one of the forward seatpost adaptors made for use with aerobars. In addition to improving the child stoker's leg angle, the forward saddle position reduces the reach to the rear handlebars.

An extra-long stoker stem is also called for with a kidback, preferably one that is adjustable for length. Many kidback users use drop type handlebars flipped over so that the child can hold onto the outside of the "drops". The U-shaped handlebars supplied with many small-wheel and children's bikes also can extend back to where the child can reach them.

A child as young as 2 1/2 can stoke a tandem with a kidback attachment, although only for relatively short rides. With time, growth and experience, your stokid will gradually be able to happily complete longer and longer rides.

A subsequent search for "kidback" showed, among others, a link to Rodriguez Bicycles, which sells tandems and these adaptors. There may be other companies selling them. This is just the first site I found, and I didn't search further. Note that per the FAQ, we strongly prefer not to make specific product recommendations. Also, kidback may have been Rodriguez's brand name, and a more general term may be "child stoker kit" or "child stoker adaptor". I would assume that many tandem manufacturers will offer these kits; Burley, when it offered tandems, seems to have done so (as described on Rodriguez Bicycles' site), Precision Tandems does (and the link goes to an installation guide), and Santana Tandems does also.

I see the question got at least one close vote, but sometimes, knowing what to Google for is not trivial, especially if you lack other specialized bicycle terminology.


Technical notes: The stoker adapters clamp around your stoker seat tube, and they contain a bottom bracket inside. I don't know tandem terminology, but you do run a short length of chain from the child's crank to the actual stoker crank. Below is a photo of the actual stoker crank, taken from Ray Dobbins:

enter image description here

In purchasing one of these, you'd need to match the clamp's diameter to your seat tube diameter. It's probably worth reminding users to adhere to torque specifications here. The Precision Tandems link above shows the possible consequences. For steel tandems, particularly with thicker tubing, I'd assume it's harder to over-torque the clamp. I'd be more careful with lightweight racing tandems. Unfortunately, I have not come across suggested torque specifications. You could start with the torque specs for seatpost clamps, probably 4-8 Nm (but check if your tandem has a different spec). Given the expected power output of a small child, something in this range should be enough to secure the clamp.

I'm not sure what lengths kids' cranks come in, but I'm sure it's safe to assume a child stoker adapter kit would come with a crankset of appropriate length. Santana Tandems (link above) gives you a 125mm crankset. An alternative is that you can purchase crank shorteners and use an adult-sized crank. I'd assume a kit would come with a length of chain, but if you're buying a second hand kit, you might need to get a chain of more appropriate length to your setup.

4
  • 2
    The Googling is for my own ends, the education on the terminology is the query. The answer is helpful and appreciated. – Eric Huelin Dec 5 '20 at 1:02
  • How is the small person getting on and off the saddle? How are they going to stop at a red traffic light? – Michael Dec 5 '20 at 9:12
  • 1
    @Michael, the same as a child gets on the back of a bike where they ride a child seat or just the rack, the adult holds the bike and the child is either lifted on or climbs up. And when stopped for a short while the child stays on the seat with their feet on the pedals. The adult has to spend a bit more effort keeping the bike upright but all parents used to having a kid in a seat on the bike are used to that. – Willeke Dec 5 '20 at 10:48
  • 2
    As an alternative search string, try self building recumbent bikes. I have not done it but I do know that many self build recumbents use this kind of parts, often with less traditional shapes. – Willeke Dec 5 '20 at 10:50

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.