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  1. #1
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    lightweight 150-155mm cranks for kids?

    ok guys - i'm in desperate need of a suitable crankset for my 8 year old boys new bike.

    I still use the "cheapo-minibike" crankset off of his old 20" bike. I see that Scott 24"-bikes come with 152mm triple cranksets from Sugino but i think 3 chainrings are way too much. And on top of that they would be all steel, the square bottombracket would need to be 120mm wide...not really what i want on my kids bike. He is short legged...i want cranks with a low q-factor and max. 2 chainrings.

    The cranks we are using now have a 26t chainring and in the rear i mounted one of my 11-34 titanium cassettes. This setup suits him fine right now but if i could get something like 24/32 or 22/32 i think it would be perfect. Right now he still couldn't shift using the STI thumbshifters.He hasn't the power to push the gear in completelyso we are using just this one ring right now. I will try a grip-shifter and hopefully this allows him to change gears up front soon.

    So before i start to actually shorten a nice 175mm crankset to make it fit i thought i ask for your advice. Maybe someone already has gone throught the pain of finding suitable parts for his kid...ah - no BMX cranks! I also looked into them but they usually have 110mm chainrings and the lowest you can get is a 30t and this sure isn't low enough.And you can't fit 2 chainrings as well.

    How it all started:
    He took part in a local MTB-trainingsride with other children but found himself on the smallest bike of the group. All others had already 24" or even 26" wheels while he was still on a 20"...he is pretty short still so that bike was perfect BUT as you can imagine time has come to move over for a big wheeled bike with front suspension. Since i already got that sweet little 24" Scott frameset from a friend of mine in the basement for almost a year i finally took it out and by carefully selecting seatpost and saddle i found a setup that allows my son to barely (!) fit. His feets just make it to the ground....

    I had to make the bore of the seatpost wider. It had a 26,8 but in this weird size i couldn't find anything light and low. The clamping of the saddle had to be as low as possible to have the saddle as low as possible. Now it accepts 27,2mm and an old FRM carbon seatpost alongside a flat SLR was fitted.

    The stem is a 60mm with 38 degrees so pretty short and low as well. The handlebar is a lowriser which has the ends bent back slightly so he doesn't have to lean forward too much anymore. As is the position on the bike is pretty much perfect BUT i need some cranks to fit it too!


    As pictured the bike now weighs 8,6 kilos

    frameset: Scott Team Issue Superlite XS ca. 1330g
    fork: 24" fork weighs a hefty 1650g and i might replace it with an old Manitou Mach 5 which performs much better.There i will need to make an adapter to mount the V's. this fork should save about 300g total and add a huge amount of performance.The actual one is barely moving under my sons light weight...
    Headset: Amclassic 80g
    Stem: generic 60mm 115g
    handlebar: gereric lowrizer,shortened to 52cm
    grips: foam: 25g
    brakelevers: FRM DP-3 108g (these are outfitted with longer reach adjustement bolts to have the levers very close to the bars for his small hands
    brakes: Vuelta Magnesiums incl tuningkit 232g
    shifter: tuned STI (only right side)
    cables: Alligator I-Links
    seatpost: FRM carbon ca. 140g
    saddle: SLR XP ca. 180g
    Wheels: Scott 24"
    tubes: Eclipse 24" 48g !!
    tires: Schwalbe Mow Joe 24" ca. 400g
    rear derailleur: XTR shortcage 207g
    cassette: Nino's Titanium 11-34: 187g
    chain: KMC X10L ca. 225g
    pedals: generic flat ca. 250g (will be replaced with Ritchey SPD Titaniums as soon as he feels comfortable with the big bike. He already used them on his 20" bike)
    covers on top-tube and stem


    Did i already mention it is a bigger challendge to build a childrens bike than it is to do a big one?! You really have to be creative and have lots of spares and ideas to make a bike fit your needs. But seeing him go crazy on it afterwards is worth all the hassle
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  2. #2
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    Middleburn RS7 go as small as 160mm, and they can use all the Middleburn spiders including the DUO chainrings, you can get them in ISIS and square taper.
    I'm a member of NSMBA and IMBA Canada

  3. #3
    Northeastern Rider
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    Nino,
    When your boy outgrows that frame LMK. My daughter should be growing into a 24" by the time yours outgrows it. I might be willing to pick the frame up from you if you decide to let it go.
    "The original concept of freeriding was that there was no set course, goals or rules to abide by. "
    Mountain Riders

  4. #4
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    Nino I don't know if you have access to the proper equipment so here's an idea. Ask around at www.cnczone.com to see if someone can shorten an existing crank. I would start in the RFQ (request for quote) forum. When I had my custom AMP to standard IS disc mounts made I found someone there that did an excellent job.

  5. #5
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    When I was a kid (13 years old) my parents bought me a adult 26" mountain bike from the discount box stores. It was so big that I had to walk the bike over to the curb so that I could have the bike on the road so I could get on from the grass side.

  6. #6
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    Nino, didn't you ask this question before? If it wasn't you, then someone else did, and I provided a link to a guy who shortens them. I don't have the link still, but I know I mentioned it on here.

    Edit: Nino, it was you who asked: 140mm Kids Crank??

    Here's the link: http://bikesmithdesign.com/Short_Cranks/shorten.html

  7. #7
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    Quote Originally Posted by nino
    ah - no BMX cranks!
    profile mini magnutanium 150mm arms: 368g
    Get the longer 6" ti spindle and alloy cups and the spider of your choice (they sell 94/58, & 104/64 4-arm)...

    Costs a lot, but kidproof...

  8. #8
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    Nino, how did you make the seat post up to 27.2 size? I want to do the same but it has me stumped and I can't find anything on web or forums so far. Cheers

  9. #9
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    Quote Originally Posted by paddler28
    Nino, how did you make the seat post up to 27.2 size? I want to do the same but it has me stumped and I can't find anything on web or forums so far. Cheers
    There is that tool, i think it is a "reamer" or something like that. I'm not english spoken so i don't know if i remember the name of that specific tool correct. Anyway - i was able to enlarge the bore to 27,2 where i now have my old Token Carbon installed. It weighed just 115g in full 320mm lenght but now is shortened to about 20cm so i guess it's more like 75g now.

    i just found a link-i was right: it's called a reamer:
    http://www.ceeway.com/economy%20work...mebuilding.htm

    The bike itself looks a bit different now. Pictured still with that FRM seatpost...and just last week the front rim "exploded". It's brake tracks were completely worn so when i inflated the tires the rim collapsed.Now i was faced with a huge problem as it turned out that 20-hole rims like the ones it uses are VERY hard to get. My wife couldn't believe it took me almost 1 week to finally lokate such a (lightweight) rim. I have a hard time to believe Scott (and also Merida) sell those kids Race-Bikes that aren't cheap at all...yet you can't get replacement wheels. It really took me 1 full week , several phonecalls and a lot of "vitamins" to finally get the parts needed.

    Full specs here:
    24" Kids Bike: 8,4 Kilos/ 18,5 lbs
    Attached Thumbnails Attached Thumbnails lightweight 150-155mm cranks for kids?-seat_tube_reamer.gif  

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    Last edited by nino; 04-17-2010 at 03:41 PM.

  10. #10
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    Nino, your kids bike looks amazing and the advice on the reamer was perfect. Much easier to find a light weight 27.2 rathe than a 26.8. The cranks you have adapted also are great and I have been looking at doing the same thing myself. Do you know what model of Truvativ cranks you used? as some of them are hollow arms and I don't want to buy some and get it wrong.
    Cheers

  11. #11
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    Quote Originally Posted by paddler28
    Nino, your kids bike looks amazing and the advice on the reamer was perfect. Much easier to find a light weight 27.2 rathe than a 26.8. The cranks you have adapted also are great and I have been looking at doing the same thing myself. Do you know what model of Truvativ cranks you used? as some of them are hollow arms and I don't want to buy some and get it wrong.
    Cheers
    Thanks for the compliments!

    I wouldn't do the cranks on my own.Just give that guy a call and order them there.They are well worth the money and not that expensive!
    I think they are listed on that site.I don't remember which ones that was but you'll see them on the site.

    I just looked at the site again - It's the Isoflow:
    http://bikesmithdesign.com/Short_Cranks/IsoFlow.html

    good luck!

    bye
    nino

  12. #12
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    Quote Originally Posted by paddler28
    Nino, your kids bike looks amazing and the advice on the reamer was perfect. Much easier to find a light weight 27.2 rathe than a 26.8. The cranks you have adapted also are great and I have been looking at doing the same thing myself. Do you know what model of Truvativ cranks you used? as some of them are hollow arms and I don't want to buy some and get it wrong.
    Cheers
    You can get a Thompson Elite seatpost in 26.8.Replace the top cradle with KCNC yokes & Ti bolts,My sons then weighed 150 grams for his Spark RC 24.Went with Middleburn 160mm cranks & 22/32 rings with BBG bash guard,Race Face Ti BB 107mm,this gives a nice light set up with a Q-factor of 162mm.

  13. #13
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    Quote Originally Posted by xc71
    You can get a Thompson Elite seatpost in 26.8.Replace the top cradle with KCNC yokes & Ti bolts,My sons then weighed 150 grams for his Spark RC 24.Went with Middleburn 160mm cranks & 22/32 rings with BBG bash guard,Race Face Ti BB 107mm,this gives a nice light set up with a Q-factor of 162mm.
    I think a Token 27,2 carbon costs much less than your Thomson+tuningparts AND is lighter...(137g in full 350mm lenght). But then you save the hassle of getting that diameter up....so it's definitely a easy fix.

    160mm would have been too long for my son.

  14. #14
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    Quote Originally Posted by nino
    I think a Token 27,2 carbon costs much less than your Thomson+tuningparts AND is lighter...(137g in full 350mm lenght). But then you save the hassle of getting that diameter up....so it's definitely a easy fix.

    160mm would have been too long for my son.
    I would re check your math, the cost of a Token carbon post & the Reamer is way more than the Thompson plus the tuning parts.

  15. #15
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    Quote Originally Posted by xc71
    I would re check your math, the cost of a Token carbon post & the Reamer is way more than the Thompson plus the tuning parts.
    correct - i forgot the reamer.

  16. #16
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    Nino,on the Spark RC 24 it comes with Deore shifters & XT Shadow.As you mentioned this is not ideal for fast down shifts - for kids, to much thumb pressure required.Do you know the weight of Deore shifters as I'm either going to go with the 96 gr Sec gripshift or low normal rear del.

  17. #17
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    Quote Originally Posted by xc71
    Nino,on the Spark RC 24 it comes with Deore shifters & XT Shadow.As you mentioned this is not ideal for fast down shifts - for kids, to much thumb pressure required.Do you know the weight of Deore shifters as I'm either going to go with the 96 gr Sec gripshift or low normal rear del.
    Deores for sure will be a bit heavier than let's say XT which are about 220g/pair (without gear display)
    With kids it is really a matter of trying things out.You would think that pushing some TIs shifter pods would be easy but they can't push them into the click...But at the same time using Grip-Shifters might also be a bad choice if they can't grip the throttle hard enough to twist....you really have to try!

    What is really, really important on kids bikes is to have the slickest cables possible. The little handforce they have can't be wasted in friction, nor on shifters and definitely not on the brakes. I-links proved to be the perfect cables once again with slick performance all year long.

  18. #18
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    Thanks for the advice guys.

    At the moment my boy is riding on a scott contessa jr 20" which is running 34-42 teeth rings. He's on that as in 2008 it was lighter than the scale, although all the stickers to cover the girl decals have probably added some weight! Anyway I am going to change the cranks on this for my girl, and my guy is going onto a new scott scale 24 inch, which will also get it's cranks changed.

    OK final decision time. Which crankset do you think would be lighter. The Truvativ isoflow (shortened to 162mm) with salsa alloy and no bash guard, or the Middleburn 160mm cranks. Either way I am going to have to import them so am trying to get this right.

    Mark who shortens the Truvativ cranks also gave the option of 24 34 rings. Would this be a better combo for either size bike?

    Any help appreciated as over here the local bike shop thinks I'm a bit nuts doing this to a kids bike, and it is great to have some like minded people.

  19. #19
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    Quote Originally Posted by paddler28
    Thanks for the advice guys.

    At the moment my boy is riding on a scott contessa jr 20" which is running 34-42 teeth rings. He's on that as in 2008 it was lighter than the scale, although all the stickers to cover the girl decals have probably added some weight! Anyway I am going to change the cranks on this for my girl, and my guy is going onto a new scott scale 24 inch, which will also get it's cranks changed.

    OK final decision time. Which crankset do you think would be lighter. The Truvativ isoflow (shortened to 162mm) with salsa alloy and no bash guard, or the Middleburn 160mm cranks. Either way I am going to have to import them so am trying to get this right.

    Mark who shortens the Truvativ cranks also gave the option of 24 34 rings. Would this be a better combo for either size bike?

    Any help appreciated as over here the local bike shop thinks I'm a bit nuts doing this to a kids bike, and it is great to have some like minded people.
    I would go Middleburn.The stock crank on the Spark RC 24 is a FSA Alpha 165mm with 22/32/42=775 grams plus the FSA ISIS BB = 353 gr for a total of 1028 gr.
    The Middleburn 160mm with spider & 22/32/BBG guard, Alum bolts= 580 gr.The 22/32/rings are the Hardcoat models.
    Just the Middleburn cranks & 4 bolt spider are 440 gr.
    Race Face Ti square taper 68/73 x 107 = 161 gr. plus 580 gr for a total of 741 gr
    Really happy with the Middleburn set up.With a 11/32 cassette this is perfect gearing for my 9 year old on technical mountain trails with some steep climbs.To bad your LBS doesn't realize if you put a kid on a decent light bike with proper length cranks & gearing that they will actually enjoy challenging rides.The only place you will find 160mm Middleburns is mtbtandems.com they have them custom made.

  20. #20
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    Thanks heaps for the advice.
    Actually the savings are even more. 775 g for the FSA cranks, and FSA BB 353g is 1128gr. Nearly 400g heavier than the Middleburn. That makes it worth while for me.

  21. #21
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    Quote Originally Posted by paddler28
    Thanks heaps for the advice.
    Actually the savings are even more. 775 g for the FSA cranks, and FSA BB 353g is 1128gr. Nearly 400g heavier than the Middleburn. That makes it worth while for me.
    One thing I should mention [I assume the Scale has the same width BB as the Spark ]the Race Face BB has the adjustable cups & you can adjust the chain line with this style BB,so I was able to get away with a 107mm BB.If you go with the fixed style like Shimano & others that tighten against the frame you will probably need a 112 or 113mm BB.

  22. #22
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    Quote Originally Posted by nino
    correct - i forgot the reamer.

    OK guys. I can end the debate. My local bike store actually had two reamers and let me borrow one for a couple of days. I shop there a lot so no cost for that. The trick ws I had to ask one ofthe guys that had been there for over 10 years as it is a tool that is now little used, but they used to use them a lot.

    I ended up using a KCNC seat post, with the cut weight of 120g, that save 100g compared to the carbon 26.8 post I was going to run. I am a happy chicky and feel a slightly manic laugh coming on as it all worked perfectly. Now I just need to find a light weight quick release seatpost. Any suggestions?

    Thanks heaps for all the advice. I think you are my new hero.

  23. #23
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    Quote Originally Posted by nino
    Deores for sure will be a bit heavier than let's say XT which are about 220g/pair (without gear display)
    With kids it is really a matter of trying things out.You would think that pushing some TIs shifter pods would be easy but they can't push them into the click...But at the same time using Grip-Shifters might also be a bad choice if they can't grip the throttle hard enough to twist....you really have to try!

    What is really, really important on kids bikes is to have the slickest cables possible. The little handforce they have can't be wasted in friction, nor on shifters and definitely not on the brakes. I-links proved to be the perfect cables once again with slick performance all year long.

    Can I get some more advice. I want to follow what you did with the cranks as the only other option seems to be Middleburn, which are very light (possibly lighter than the isoflows?) but also 160mm long which I think is too long. The problem is that in New Zealand there seems to be a lack of any titanium BB, except for some BMX ones which are 68mm. Do you know the except specs of what you used or any good sources, as will have to order first and hope it is right. I have even been looking at other options of cranks as the ISIS BB's are easy to get here, but most seems to have a groove out from the back of the crank and I'm not sure how it would go. Sorry to be a pain, just I suspect I am the only one doing this out here. Cheers.

  24. #24
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    Quote Originally Posted by xc71
    One thing I should mention [I assume the Scale has the same width BB as the Spark ]the Race Face BB has the adjustable cups & you can adjust the chain line with this style BB,so I was able to get away with a 107mm BB.If you go with the fixed style like Shimano & others that tighten against the frame you will probably need a 112 or 113mm BB.
    Have just run into that interesting question this week re 107 or 112 BB size, and you had already answered it for me. Thanks heaps for that.

  25. #25
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    So, for the latest batch of parents working on this topic: Canfield Brothers is making relatively light cranks in the 155mm length. I've got a pair and putting them on my sons XS Demo 8. Just working out chain retention, and he'll be using them this year

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