The Perfect Kids Mountain Bike and How to Build it…
This article presents the results of numerous hours of forum and internet search to design an asymptotically perfect kids mountain bike (MTB). It summarises the design concept, decision making, and some building peculiarities. It addresses parents and experts that would like to get familiar with the unique requirements kids bikes have, such as weight and ergonomics aspects. The result is a 14" frame, 26" wheel-size, 8543g, 100mm suspension fork, ultralight custom kids MTB at a material cost of approximately $1800.
There is a growing market for kids mountain bikes (MTB) that can compete with their parents’ high-end bikes. Good examples are from https://www.vpace.de or https://www.federleicht-bike.com. Companies like these are supplying kid-friendly geometries combined with light-weight designs. Nevertheless, I chose to build up a custom bike just because my son and myself decided to build one ourselves to understand the design and learn how light proper kids MTB could become.
Our goal was to develop and build a kids MTB that covers: age range: 7–12, size range: 130 cm — 145 cm, kids friendly geometry, front suspension, 1xX drive chain, mass < 9kg, material cost <$2000.
Weight considerations and kids-friendly ergonomics drove most of the design. I also balanced component cost and (second-hand) availability in comparison to weight loss.
The rule of thumb for bikes (except very downhill oriented ones) is, the lighter, the better they feel. Notably, we would like to have a light bike compared to our body mass. We can define a body to bike weight ratio (BBR) = rider weight/bike weight. I own a 170mm travel Enduro bike weighing just below 13kg. Most stock kids’ bikes are the same weight, by the way. My BBR is 5.85. My son’s BBR for an off-the-shelf bike would be 2.31, which means the bike is almost half his weight! In summary, because kids are much lighter, we would like to provide them with the lightest bike possible to feel comfortable and enjoy the ride with at least a BBR >3.
I start with the wheels because this is the first thing to think about when designing a bicycle. The bike industry is moving towards larger wheel sizes. Larger wheels are a great move, and they should also grow on kids’ bikes! The 29er concept scaled down to kids’ bikes. Despite that bikes for 130 to 145cm sized kids are usual 24", 26" wheels were a must for us. To reduce unsuspended and rotating masses, you really should get out the letter scale for the wheels. Design criteria were: lowest weight and largest size. To reduce weight, we are using the lightest CycloCross Hubs combined with 24hole (fewer spokes) carbon rims. The 26" wheelset comes to 1350g (without tires). Material costs are about $300 with cool looking straight-pull hubs. Also, the wheelset comes at the expense of two nights and two six-packs of beer for building and trueing them.
The problem with fork and suspension, in general, is that the future rider is light — very light, e.g., only 30kg. There are very few forks on the market that can be set to a reasonably low loosening torque. One of them is the RockShox Sid in the DualAir version. DualAir is important! If you can still get one (second-hand market: eBay, et cetera), I highly recommend it. The only problem with this fork (besides setup) is that the tapering was too long for our frame. I had to use a lathe to machine the top spacer of the headset to make it fit. If you cannot get a Sid DualAir, you are probably best off with (i) a fixed fork, or (ii) a fork specially tuned for the weight of the rider ($$$).
The decision was to have none — because 1) it adds a lot of weight, 2) you end up with (even more significant) problems with loosening torque, 3) makes the use more complicated (lockout settings), and 4) do you need it — says the owner of a high-end Enduro bike — when sticking to mostly flow trails.
I had almost settled on a VPACE MAX26. The frame is ideal in geometry, weight (1.420 g), and price tag (349€). However, it was hard to get at the time, so I started looking around for alternatives. I settled in onto the TideAce FM-M001 14" Kids Carbon Frame. Its geometry is not so different from the VPACE frame and can also take large 26" wheels, disc brakes, tapered forks, and has a BB92 bottom bracket. It is a bit lighter (1350g painted version) than MAX26 and with $300 also a bit cheaper.
Brakes are another difficult one. A no brainer is the disc size: simply fit the largest you can! We use 180mm in the front and 160mm in the rear. Finger strength and ergonomics of brakes are all but ideal when it comes to kids, so make their lives easier applying physics and use large discs! The weight addition is minimal. You may also want to consider V-Brakes. For those, more kids friendly versions are available. There are (very) few disc brakes on the market suited for kids. My recommendation is the Formula R1 (carbon). If you can get them second-hand — get them. Another option is Magura MT4. But check the R1 levers below, and you understand why this is the best fit for kids.
Cranks should be rider size divided by 10. If you have a BB92 bottom bracket, you are left with basically two options: Federleicht or VPACE (or a shortened arm) in today’s market. VPACE standard cranks are 573g @ 135mm length and €129. For almost 3x the cost, you can save 200g with Federleicht. I went for the VPACE 135mm crank arm and also ordered their kids-friendly pedals.
To me, it was clear that 1xX makes life easier for my son — the 1x design was fixed. From a price tag point of view, I would recommend looking for a 1x11 SRAM GX on the second-hand market. In my very subjective opinion, this is the sweet spot between cost, weight, and performance. 1x12 would be beautiful, but at the time, these were too expensive.
The saddle was my son’s choice — it had to be this light white carbon saddle. It is very light-weight indeed, and he hates to ride it without bike pants. If your child is not masochistic — the offerings by VPACE seem to be the much better choice.
Because of the frame geometry, I decided for the shortest stem possible and quite a few (carbon) spacers to further lift the handlebar in the future. The handlebar should be shoulder-width + 20cm, so we dialed in on a 52cm carbon car.
The lightest MTB tires on the market for 26" are Schwalbe’s Racing Ralph in a cumbersome tubeless setup, initially. Then decided to add tubes. I am not happy with this setup yet — too many thorn punctures.
This is unnecessary information — skip over it. If you continue reading: we use red anodized disc screws, red anodized spoke nipples, and mostly titanium screws on the bike. Of course, we added MORITZ decals to it.
Moritz’s bike is one of the lightest (8560g) kids MTB with front suspension out there, and he sincerely enjoys riding it. He started riding it at size 127cm, and now with 134cm, it is an excellent fit for him.
I hope this article is helpful to other parents to identify what to look after when buying or building a bike for their children.
Building a custom bike for (together with) my son was a fun project. Feel free to copy and modify our design if you are a talented bike builder and have access to and knowledge of a well-equipped workshop for some tasks. The full build specifications and individual component weights you can get from the spreadsheet at the end of this article.
Things that I do not consider ideal are (i) puncture sensitive tires, (ii) fixed Seatpost, and (iii) a high bottom bracket. Especially, bikes for kids should have a low bottom bracket to allow for a lower saddle to mount them more easily. The only modification after a year of riding the bike was a new magnetic bottle and holder (www.monkey-link.com).