My grandson has the kind of bike you pedal backwards to brake, and he goes riding with me on my days off. Yesterday, he asked me if we can convert his bike because we have looked everywhere and no one seems to have a gear-shifting bike for someone who is 8. The Problem is that he is having trouble keeping up with me and wearing out a lot earlier because of the hills. Are there bikes with 18" or less wheels that shift or is he still too small for that kind of riding? I am pretty sure that I can figure out how to mount the gears and get a small derailleur just to learn how to shift and I am pretty sure I can change his brake system to the handlebar levers, but not sure it would be so wise, if I could just get his parents, who are willing and more financially able, to replace the bike with one that is built to be a dirt bike and not a trick bike frame with attachments to help him go faster.

I am pretty sure it is possible and I may even be able to make it just like mine but not sure how cost effective it is if he can just buy one.

I am still learning what different bikes are called. But he is about 3ft tall and his bike is getting too small for him. His current bike has 12" wheels.

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    I got my first bike IIRC at my 7th birthday and it was 24" wheel size with 3 gears internal hub (also coaster brake). My guess would be that the 18" bike may be too small for him anyways. Commented Jul 22, 2019 at 18:02
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    He is still only 3 ft 4-6 inches tall, he is glued to his 12" bike now but I had to pull the seat all the way up and it was just right for him. I do not know, but he also took the training wheels off himself when he was 6 so he could take that bike to the skate park, so he might be able to handle a 20 or more Commented Jul 22, 2019 at 18:06
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    Yeah, I did a google search and found some I can order online from BMX that has 7-speed drivetrain, I like your idea about 18" then 24" it will give him time to grow. And he takes pretty good care of his stuff, for an 8 year old so spending higher for something may work out good Commented Jul 22, 2019 at 18:33
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    Are you able to lower your own output to match his?
    – Criggie
    Commented Jul 22, 2019 at 20:32
  • 1
    Why not let him set the pace?
    – Reid
    Commented Jul 23, 2019 at 19:24

5 Answers 5


If your grandson is that young, he's likely to have trouble keeping up with you no matter what bike he's riding. It may be that it's your expectations, not his bike, that should be modified.

It seems really unlikely that you could retrofit a derailleur onto his current bike. A frame that small won't have room for a wider rear-wheel assembly. Mounting rim brakes would be tough if the frame isn't already set up for it. Even if you did succeed, he'd still be driving it with eight-year-old legs and an eight-year-old's heart and lungs.

Might you consider a tagalong bike, mounted to your bike? Your grandson might balk at this if he's used to his own separate bike, but it would ensure that he keeps up with you.

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    I do not know, @jeffB. He comes really close to keeping up now, just wears out from pedaling so much. You may be right though, that was part of why I am ok with him riding now, if he stays with it, by the time he is old enough to really ride with me, he will still want to. converting his bike may be too much. I actually think a tag-along is great idea and he might be up for it more since I already slow way down to ride with anyway, so me doing most of the work would be fine and reinforce his idea that PopPop is superman, lol Commented Jul 22, 2019 at 18:01
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    "Really unlikely" is probably understatement - I think 20' is the smallest you can do it safely (i.e. look at Navara Duster 20 - there is really not much space left between derailleur and ground). These bikes get very little use - so should be easy to find used one in like-new condition for price of new derailleur alone. Commented Jul 23, 2019 at 1:52
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    8 is too old for a tagalong bike. He should get a real one.
    – Nobody
    Commented Jul 23, 2019 at 6:10
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    @Nobody With respect, the child in question is off the height-distribution charts, and "one-size-fits-all" advice is unlikely to be helpful.
    – jeffB
    Commented Jul 23, 2019 at 16:14
  • +1 for "your expectations".
    – Reid
    Commented Jul 23, 2019 at 19:24

Is he extremely short? An 8-year-old is almost 9 years old, and: "26 inch mountain bikes are suited for kids that are 9-13 years old." Don't fix up the 12 incher. Head into a local bike shop and get a proper sizing.

  • Cheeky, but go in to the shop to get the right size, then get one 2nd hand for 1/10th the price, if you think he might put on a growth spurt. Any decent bike shop will be used to sizing for kids that will grow so will suggest the largest bike he can handle now so that he has growing room.
    – fdomn-m
    Commented Jul 23, 2019 at 7:47
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    He is super short for his age, between 3 and 3 and half feet tall. He has always been really small. Commented Jul 23, 2019 at 15:36
  • @LorenStevenson I hadn't realized just how unusually short your grandson is for his age. You can throw all the typical-bike-size-for-age advice out the window. Much will depend on his leg length, and whether he has other health issues.
    – jeffB
    Commented Jul 23, 2019 at 16:18
  • Yeah @jeffB he is super tiny Commented Jul 23, 2019 at 17:57
  • @jeffB, for context, my son just turned two years old, and he's nearly as tall as OPs grandson. Kids have a pretty wide range of dimensions. Commented Jul 24, 2019 at 4:09

If you are absolutely set on converting the bike, use an internal gear hub (IGH). There are IGHs with three gears (the usual kid's first IGH), and there are IGHs with seven gears (suitable for many adults), and both of them are cheap when bought second-hand. The last IGH I bought was a seven gear with coaster brake which I got second hand for 25 Euros from my local bike shop.

The main cost factor (either time or money for labor) will be rebuilding the wheel with the new hub.

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    I am reconsidering rebuilding given everyone's excellent advise But like the idea of getting a bike with an IGH Commented Jul 23, 2019 at 15:37
  • This is the best answer to the question as asked, and probably a good idea if OP feels up to doing it. Adding real brakes will also be needed and may be a bit of work. Commented Jul 24, 2019 at 5:15
  • @R.. Well, the coaster brake that comes with most IGHs is a real brake. Since it's actuated by the chain itself and not by a cable that can get stuck or break, coaster brakes are actually quite robust and fail-safe as long as they are not overheated. That is the only real problem with coaster brakes: They lack sufficient area for heat dissipation, and are thus not suitable to handle long descents. Of course, you always need two working brakes on a bike. Of these two, the coaster brake provides one, so there's only one brake left to change for a good one. Commented Jul 24, 2019 at 8:15
  • @cmaster: Ah, I wasn't aware IGH's had coaster brakes (and am sorta disgusted that they do, but depending on what OP's grandchild likes it may be a good or a bad thing). Commented Jul 24, 2019 at 16:05
  • @R.. There are IGHs with and without coaster brakes. The most expensive ones like the Rohloff don't have coaster brakes because you cannot combine a coaster brake with oil immersion lubrication. But the cheap vanilla IGHs that you see either on children's bikes or on city bikes usually have a coaster brake. I don't know what you find disgusting about coaster brakes, though. I've been riding bikes with such IGHs for thirty years now, and I've always seen them as a near-perfect, reliable, no-fail backup to stop my bike in case of an emergency. Commented Jul 24, 2019 at 16:20

Like cmaster mentioned, fitting internal gear hub (even 7-speed) wouldn't be too difficult. It would consist of:

  1. Rebuild the wheel around the internal geared hub. This is possible to do at home with basic equipment, takes a few hours to learn but in my experience is quite easy and there are many tutorials online. For a light-use bike, spoke tensiometers and such advanced tools are not mandatory.

  2. Route the gear shifting cable from back wheel to handlebar. I guess you could zip-tie it, if you don't find any better method of mounting it.

  3. Install gear shifting lever on handlebar.

But if you are aiming for the granddad-of-the-year award, install a front wheel e-bike assist motor also. That will make uphills a breeze. The common cheap kits would probably work, though you should pick one with lowest powered motor and smallest battery to avoid adding too much weight.

  • I would love to do that, the motor idea would work If I can afford it Commented Jul 23, 2019 at 15:48
  • If you go the electric motor route, you'll have to make sure that what you do is actually legally allowed. Commented Jul 23, 2019 at 18:19
  • Rebuilding a wheel is not trivial. You don't have to make the wheel strong for a kids bike, but you'll probably need different length spokes and the spoke length calculation is a pain. Motor kits are designed for full-sized bikes, there won't be enough room on a 12" wheel bike. Commented Jul 25, 2019 at 15:06
  • @ArgentiApparatus There are online calculators that make the spoke length calculation quite easy, it just requires care in taking measurements. Admittedly the short spokes (~150mm) for 12" wheels are less common, but available at least online. As for the ebike conversion, yeah, it will need some searching to find a suitable motor. However smallest geared motors (such as GBK-100F) in kits are ~120mm flange diameter, which still leaves plenty of space to spoke a 12 inch (300 mm) wheel. The fork width could be more of a problem, though there are special narrow motors also.
    – jpa
    Commented Jul 26, 2019 at 8:39
  • @I'm thinking of battery space as well as the hub motor Commented Jul 26, 2019 at 11:49

I think your best bet is buying a new child's bike off Craigslist, one with gears. Adding gears to an existing bike will likely cost more than a decent used bike and will not likely end up problem free.

I have an 8 year old and we just got her a new bike with gears, we live in an area that is quite hilly. She has a 24" hybrid bike (road tires with upright seating) with 14 speeds and grip shifters. We've gone as many as 12 miles with hills and she has no problems keeping up. 24" may seem big, but the seat on her bike is almost the same height as the tire.

  • 'Adding gears to an existing bike will likely cost a more than a decent used bike' absolutely, especially a 12" wheel bike. Commented Jul 25, 2019 at 15:09
  • Yeah that is what I spent this weekend explaining to my daughter and son-in-law. We are now looking at splitting the cost Commented Jul 27, 2019 at 15:14
  • To answer your question, yes there are some small bikes 18" or less with gears. I see a few 18" or less bikes with gears for $60 or less on my local Craigs List. Your luck may vary based on your location.
    – Gary Bak
    Commented Jul 27, 2019 at 18:11

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