[JeepRage]

I picked up a used, but great shape Diamondbank Line 24" for my son last night. Only part missing was the chain guide, which was the main criticism on this bike to begin with. Has 8 speed SRAM x3 in good shape, and most feedback was to upgrade the chain ring to a Race Face narrow wide to fix the chain drop problem that he will certainly have.

Like any non-enthusiast owned bike, the chain wasn't lubed and easier to replace than restore, so I'll pick up a new one. Cranks are 155mm, my son is just 5'3 but plan to have the bike for the next few inches anyways, if not more. Was looking at 135mm cranks, but unsure what the go-to is on here. The bike isn't overly light, but not looking to add more weight...

135 the right length to start with? Any known combo that comes with a 8 spd compatible N/W ring? (square taper bottom bracket).

 

[svinyard]

Do you mean 4'3" instead of 5'3" tall? While we swapped out our 155mm cranks for 140mm Prevelo cranks...other highlevel kids have left the 155mm on there and been fine. You might try and see how it goes first. It ends up being 120$+ for new BB and cranks and then you gotta remove the old BB (with special tool) and install the new BB (with different special tool). It can get a bit expensive.

 

[JeepRage]

Do you mean 4'3" instead of 5'3" tall? While we swapped out our 155mm cranks for 140mm Prevelo cranks...other highlevel kids have left the 155mm on there and been fine. You might try and see how it goes first. It ends up being 120$+ for new BB and cranks and then you gotta remove the old BB (with special tool) and install the new BB (with different special tool). It can get a bit expensive.

Yep... 4'3 (or 51")... Serves me right from trying to type that up on my phone. Ended up with a Trek Roscoe for my daughter as well (twin, and about .5" shorter). They had 152.5mm cranks, so I'm going to end up doing both bikes, or leave them as-is.

Starting with a new chain and the narrow wide ring for him, colored grips for both. You know, the important stuff. Neither bike is light, but the DB seems to have a heavy rear. Might look at Ali carbon seat posts.

 

[TimTucker]

For 24", we did Prevelo 120mm cranks, which seem to be one of the few short options still in stock:

HEIR Crank

The HEIR Direct Mount Crank offers direct mount crank versatility and performance, in crank lengths that are optimized for young riders. Features Stiff and light cold forged design. Compatible with 3-bolt direct mount chain rings. Specifications COMPONENT WEIGHT HEIR 120mm Direct Mount Crank 523...

prevelo.com

In weight, they could definitely do better: they're a bit heavier than the 102mm Spawn cranks + Ti square taper BB that we used for 20".

I run 152mm cranks on my own bike and generally feel like shorter is better. The lack of pedal strikes even with a bottom bracket closer to the ground on kids bikes helps a lot.

For chainring we with went with direct mount 0mm offset oval rings from USA Made components (I have one on my bike too). Their "shark tooth" design is a little odd, but we haven't had much issue with chain drops and they're only $17 on Amazon.

 

[rton20s]

The Prevelo Heir cranks suggested by svinyard and Tim are a great option. When they are available. Seems everything is harder to come by these days.

I believe both the Line and Roscoe come equipped with a square taper bottom bracket. There used to be a few budget options for 140mm crank arms (Source/Jet) that worked with square taper, but they're becoming harder to find. LDC still offers arms for $100 and they should be available as they are made in the US. And you can always swap the stock BB for a Ti model like Tim mentioned. Not cool, but they work just fine for kids. (I rode a square taper setup for years.)

https://ldcbmx.com/products/junior-4-bolt-cranks-130mm-thru-155mm?variant=40540572156073

 

[ladljon]

Been riding, racing since 83' 25" inseam. Went from 170mm to 135mm 8yrs ago.....never going longer.

 

[JeepRage]

Is the downside to square taper BMX cranks (135mm common) that they are 5 bolt vs 4? Certainly more common and inexpensive. Haven't hit trails yet, and could be a while before pedal strikes are a problem (may never with my daughter not hitting extreme anything), but it does seem like a large circle for them to pedal in.

I guess if I replace them, I could swap one set of 152/155s to my wife's bike (also an older square taper BB).

 

[rton20s]

Is the downside to square taper BMX cranks (135mm common) that they are 5 bolt vs 4? Certainly more common and inexpensive. Haven't hit trails yet, and could be a while before pedal strikes are a problem (may never with my daughter not hitting extreme anything), but it does seem like a large circle for them to pedal in.

I guess if I replace them, I could swap one set of 152/155s to my wife's bike (also an older square taper BB).

There are a couple things to be cautious of on BMX cranks. First is the 5 bolt pattern you mentioned. You can typically only go down to a 34T chainring, and narrow-wide designs are less common. On a typical 4 bolt 104 BCD chainring you can get all the way down to a 26T.

The other potential negative is that many BMX race parts are made to be LIGHT above all else. Some are so light that anything beyond super smooth riding on a BMX track can lead to failure. As long as you stay away from the superlight stuff with weight warnings plastered all over them, a square taper crank (BMX or otherwise) will work fine. We currently have 3 kids bikes (16", 20" & 24") set up on square taper cranks, and my old 29" rigid MTB uses square taper, as well.