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I think 42 teeth is the maximum cassette size for a 20” bike. I built my son a 20” bike with 2.125” tyres and an 11-42T cassette driven by a Box 4 derailleur and 32 tooth chainring, and it works perfectly and is great for climbing and difficult terrain. Tech support at Box told me with any of their wider cassettes the derailleur would strike the ground/tyre on a 20” wheel bike.

Now I am looking to upgrade my son to a 24” bike. I have seen 24” bikes for sale running 9 and 10 speed 11-48, but does anyone know if an 11 speed 11-51 or 12 speed 10-50 will work please?

20” Bike with 11-42T cassette and 32T chainring

Speed at Cadence

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  • Hi, welcome to bicycles! You'll need the specs of the whole drivetrain, most specifically the derailleur capacity. (Unless you're planning on replacing that too.)
    – DavidW
    Commented May 19, 2023 at 20:01
  • Yes I am planning on changing the derailleur - what I need to know is simply what i asked, the maximum size cassette that fits on 24”. Commented May 20, 2023 at 7:17
  • Sorry for a slightly off topic question, but why are you looking for such a low gear? A 51t on an 24" wheel gives you a MUCH smaller effective gear than it would on a normal 27.5 or 29" wheel. In my experience young kids are pretty light and generally climb relatively well - they just lack raw power. Happy for this to get moved to a chat room if others are interested
    – Andy P
    Commented Jun 29, 2023 at 11:06
  • Andy P: I reckon I have more experience of kids cycling that you. My son did a 1,300m vertical ascent over 20km of Mount Olympus in Cyprus at age 6, and I can tell you that to make that climb he needed much lower gearing than me. Likewise when we are difficult terrain, he need to keep his cadence much higher than me to avoid stalling. It's not just my son, who is admittedly rather exceptional, I have taken many kids cycling. I also think it is soemthing the market is waking up to and i see kids bike cassette sizes increasing in the markeplace. Commented Jun 29, 2023 at 11:14
  • Difficult terrain is definitely exponentially more difficult for young riders. Less momentum, less raw power and small wheels don't roll over stuff nearly as well.
    – Andy P
    Commented Jun 29, 2023 at 11:44

2 Answers 2

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11-50 Cassette on 24" Wheel

In answer to the question: The largest cassette that fits on a 24" wheel bike with 12 speeds is at least 50T.

Take a look at this Fezzari Lone Peak 24 Kids Mountain Bike with 24" wheels, 30T chainring, 1x12 SRAM SX Eage deraileur and 11-50T cassette.

Recently released, this is the widest range and most impressively geared 24" production bike I have seen so far, though there are many other that come close in terms of range at least, e.g. many kids mountain bikes now run 9 speed 11-48 Box Prime 3, or 10 speed 11-48 MicroShift Advent X.

I calculate the Fezzari Lone Peak 24 will have a speed of 30kmph at cadence of 90 with the smallest 11 tooth cog and a speed of 6.5kmph at a cadence of 90 with the biggest 50 tooth cog. That is the same as my son’s 20” bike pictured above in the lowest gear, but 5kmph faster in the highest gear. I think those numbers are perfect for a 8-10 year old kids mountain bike. I want to build something similar, but with a much lighter rigid carbon front fork.

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    Small side note. 34/11 on small wheels is going to spin out very easily on down slopes
    – Andy P
    Commented Jun 29, 2023 at 11:08
  • That derailleur looks almost low enough to strike on a tight corner. It's certainly going to be mowing grass if ridden in a field like the one pictured.
    – DavidW
    Commented Jun 29, 2023 at 17:24
  • @DaveW The derailleur on my kids 20” bike hangs about as low as the one in the picture in its lowest position- just above the rim of the wheel- and it does pick up grass and mud- but honestly my kid rides 50km+ a week on roads and trails, and its never really been an issue, just a bit of extra cleaning and lube at end of day. Commented Jun 29, 2023 at 19:01
  • derailleur picks up grass on normal size wheels too. its kinda just part of MTB'ing in grassy areas
    – Andy P
    Commented Jun 29, 2023 at 22:16
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The limiting factor is more how close to the road your lower jockey wheel gets. If you just ride plain flat roads, then less clearance is workable. If you ride through anything less-flat than a road, you'll want some more clearance to avoid derailleur-strike.

Another consequence is that the lower the derailleur, the dirtier the bike gets. My 20" bike's transmission gets horribly dirty even on dry rides compared to physically larger bikes. I presume this is because of proximity to the ground.

I have a 34 tooth cassette on a bike with 20" wheels.

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  • @EdwardHooper It's not likely to be done very often as that makes for very low gearing (but it sounds like he's used to that from the 20"). The cassette will go on the wheel assuming you have the right sort of hub, so derailleur strike really is the issue.
    – Chris H
    Commented May 20, 2023 at 7:26
  • @EdwardHooper I can't give you a straight number answer. This was pointing out the risk of ground strike from any longer derailleur, depending more on the irregularity of the terrain. Using a gear calculator suggests that the 20" wheel will do 5 and 6 km/h at 90 RPM with a 51 and 42 tooth rear cog, and the 24" will do 6 and 7 km/h respectively.
    – Criggie
    Commented May 20, 2023 at 8:06
  • @EdwardHooper Sadly I think the only option is to get a suitable derailleur, hold it up to the bike/s and see where the bottom ends up relative to the ground when extended down (ie, a middle of cassette gear selection) A high gear will fold the derailleur up, and a low gear will stretch it forward, both being away from the ground. A middle-ish gear will be your lowest case.
    – Criggie
    Commented May 20, 2023 at 8:07

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