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  1. #1
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    More kids bike crankset questions!

    My son is currently on a 20" bike with SINZ 5 arm cranks with the old style square bottom bracket axle - these have been awesome and work great for a 1x9 setup (we are using a 36t front chainring with 11-36 rear cassette).

    I am currently planning his 24" bike and would like to go to a 4-Arm crankset so I can mount a smaller single front chainring (32t). I have noticed that there is a much broader range of BMX cranks available now which have an integrated axle. Take for example the Bombshell Mini Cranks which are sized from 135 to 150:
    Bombshell Spinnergy Mini Cranks at Danscomp

    My questions are:
    1. Does anyone know what the chainline of such bmx racing bikes/cranks is? Will this work with 1x10 geared setup? Will probably need some creative spacing!
    2. Will these BMX Cranks fit the typical 68/73mm MTB bottom bracket shell width? I have no idea what shell width the BMX racing frames use!

    Thanks in advance!
    Cheers Anthony

  2. #2
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    Are you going 1x on his 24 also?
    13 Lenz Lunchbox punkass

  3. #3
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    Quote Originally Posted by TwoTone View Post
    Are you going 1x on his 24 also?
    Yep - I am interested in the lil shredder phenom frame, and this won't take a front derailleur. This isn't a problem though, as having used SRAM XX1 this year on my own bike, I am sold on the function of the 1x11spd setup. It's pretty over the top, but I will probably build the bike with an 11 speed SRAM X01 10-42 cassette and rear mech, and use a 30 or 32t front chainring. The SRAM cranks only go down to 165mm, thus the need for something shorter (probably 140-150mm...he has just turned 8)

  4. #4
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    How tall is he?

  5. #5
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    Quote Originally Posted by onetothepowerofn View Post
    Yep - I am interested in the lil shredder phenom frame, and this won't take a front derailleur. This isn't a problem though, as having used SRAM XX1 this year on my own bike, I am sold on the function of the 1x11spd setup. It's pretty over the top, but I will probably build the bike with an 11 speed SRAM X01 10-42 cassette and rear mech, and use a 30 or 32t front chainring. The SRAM cranks only go down to 165mm, thus the need for something shorter (probably 140-150mm...he has just turned 8)
    I assume your area isn't hilly? While the 1x11 may work for you, based on experience with my two kids, no way in hell they are climbing some of the hills we have with a 1x11 and they aren't that big.
    13 Lenz Lunchbox punkass

  6. #6
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    Quote Originally Posted by onetothepowerofn View Post

    I am currently planning his 24" bike and would like to go to a 4-Arm crankset so I can mount a smaller single front chainring (32t). I have noticed that there is a much broader range of BMX cranks available now which have an integrated axle. Take for example the Bombshell Mini Cranks which are sized from 135 to 150:
    Bombshell Spinnergy Mini Cranks at Danscomp

    Sorry, I cannot answer your original questions, but I have some other remarks. I am considering a similar project for my kids 24 inch bike, so have considered some options.

    At 218 USD these Bombshell Mini Cranks are sure very expensive. And judging from the published information I don't see significant weight savings compared to your 110 BCD Sinz expert cranks (but there may be some weight difference when you account for with BB etc.). If you want to go 104 BCD, a cheaper option might be getting MTB cranks shortened. Bikesmithdesign indicates that currently SRAM S600 crankset is a a good option for shortening. Google shows Bikesmithdesign as unsafe site currently, so I am linking to an archive snapshot of the relevant page. It contains info in S600 BB sizing. S600 come in Square taper and Powerspline BB.
    SRAM S600 MTB crankarms Shortened

    You may also have further advantage with shortened MTB 104/64 BCD cranks versus BMX cranks (which I believe are 104 BCD only). The most common smallest size for 104 BCD is 32T, but you can get as low as 30T on 104 BCD and as low as 28T to fit on the 64 BCD granny ring position (but these 30T and 28T may or may not be 11 speed specific wide-narrow options that you are looking for).

    I have not done any conversions like above myself yet, all of the above is from my internet browsing.

    The other issue is what is the required minimum front chainring for your application.

    My point of reference for kids 24 inch MTB is Islabikes. Islabikes Craig 24 (which is their "real" kids MTB) runs 28T chainring with 140mm crank arm and rear 10 speed 11-36 mech, their Beinn 24, which is a kind of their everyday MTB-ish kids bike, runs 32T from with 140mm crank arms and rear 8 speed 11-32 mech.

    On this subject also see this thread
    Need some advice on a crankset for kids bike....

  7. #7
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    Quote Originally Posted by onetothepowerofn View Post
    My questions are:

    2. Will these BMX Cranks fit the typical 68/73mm MTB bottom bracket shell width? I have no idea what shell width the BMX racing frames use!

    Thanks in advance!
    Cheers Anthony
    In the somewhat bizzaro world of BMX, a road/MTB threaded english/ISO bottom bracket is refered to as "euro", as opposed to the various press-fit BMX bottom bracket standards usually refered to as American, MID, Spanish....

    In the case of this crankset, it has a 24mm integral spindel which should be compatable with a regular road/MTB threaded bottom bracket shell by using a standard external cup BB (such as a shimano 24mm diameter). THere are likely bearing solutions to allow use of the same crank in the BMX press-fit BBs as well.

  8. #8
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    You really want to go with a 104 bcd 4 bolt crank. The BMX are 110mm BCD 5 bolt. Hard to get anything smaller than 34t. OK for 20" bikes, not so good for 24" depending where you ride.

    Best bet is to shorten (or have shortened) some cranks. I've personally done a few sets now in sizes from 125mm to 150mm. You can't go much longer than 150mm if shortening 175mm cranks. Truvative 5D cranks work well for shortening and you should be able to pick up a second hand set for next to nothing. 104mm BCD will allow you to run a narrow-wide style chainring which is just the thing for 1x setups. Smallest is a 30t raceface narrow-wide ring. Otherwise, you can use the granny ring mounts and go down below that, but your chainline may suffer (can be cured with a wider BB but the q-factor will blow out).

  9. #9
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    I realize I am a little late to the party, but my 2 cents is to go to Profile Racing Cranks which come in 145mm and can use either spline drive or bolt on chainrings. Profile and others make chainrings for 3/32" chains from 19 to 40 teeth. You can use the profile BB or put some spanish bearings in a raceface BB to deal with the 19mm crank spindle.

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