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  1. #1
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    Questions about Specialized HotRocks.

    Currently my son is riding the 16" Spec Hot Rock. We've been very happy with the bike and he has enjoyed riding it.

    Like others who post here, i've been on the painful journey of trying to find the next bike for him. I've collected alot of information in this forum about different directions i could go for his next bike. I have not decided yet, but i do have a few questions about the HotRock line-up.

    1.) there is a 20" coaster HotRock for sale locally. I don't like the crank on the model. I feel it would be too short for my son. Is it possible to inexpensively (<$80) to swap out the crank.....?.....also is it possible to swap out the rear cog on that coaster hub....?

    2.) this question concerns the 6 speed HotRock. Is it possible to put a 7 speed cassette on that hub to give him a greater gear range.....?.....or would that require a new hub...the 7 speed would allow for a 32t the 6-speed offerings only allow for 28t.

    I'm also looking at some other bikes but i'd like to understand my options with HotRock line-up first.

  2. #2
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    How long is the too-short crank? Concensus is that most all kid bikes come with cranks that are way to long in proportion to their legs, something in the range of 110mm-140mm is probably good for a 20" kid bike.
    Never done it myself, but I dont think that swapping a cog on a coaster hub would be a problem to get lower gear. Also consider swapping to a smaller chainring.

    6-speed hotrock is a freewheel hub, not a casettr. You can easily put on another 6-or 7 speed freewheel with more suitable gearing. (Changing to 7-speed freewheel will requre changing shifter).

  3. #3
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    Quote Originally Posted by GrayJay View Post
    How long is the too-short crank? Concensus is that most all kid bikes come with cranks that are way to long in proportion to their legs, something in the range of 110mm-140mm is probably good for a 20" kid bike.
    Never done it myself, but I dont think that swapping a cog on a coaster hub would be a problem to get lower gear. Also consider swapping to a smaller chainring.

    6-speed hotrock is a freewheel hub, not a casettr. You can easily put on another 6-or 7 speed freewheel with more suitable gearing. (Changing to 7-speed freewheel will requre changing shifter).
    I think the Coaster 20" cranks length is 117mm, it just looks a little short. I think the crank on his 16" is too short, he ends up spinning like a hamster.

    I wasn't sure if the 6 speed hub would take a 7 speed cassette. Yes, would need new shifter.

  4. #4
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    Longer cranks would only slow the cadence if you also increased gearing, short cranks are actually easier to spin fast.

    You might want to review the distinction between freewheel and casette, same link has suggestions for converting from 6s to 7s. Worse case, you might beed to add a spacer-washer to the axel to move the locknut out slightly.
    Freewheels (Thread on Type) for Bicycles from Harris Cyclery

  5. #5
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    Quote Originally Posted by GrayJay View Post
    Longer cranks would only slow the cadence if you also increased gearing, short cranks are actually easier to spin fast.

    You might want to review the distinction between freewheel and casette, same link has suggestions for converting from 6s to 7s. Worse case, you might beed to add a spacer-washer to the axel to move the locknut out slightly.
    Freewheels (Thread on Type) for Bicycles from Harris Cyclery
    aren't shorter cranks harder to pedal......?...

    thanks for the link.

  6. #6
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    Shorter cranks are better for high-RPM spinning. Relativly long cranks are better for low-RPM, high-torque grinding/climbing. Going too long will exceed the natural range of motion for the legs, make for inefficient fit. If your kids coaster bike is geared low and he is spinning fast, it will be easier & more efficient to spin with short cranks, he does not need the extra leverage that long cranks provide.

  7. #7
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    Quote Originally Posted by Cant Climb View Post
    I think the Coaster 20" cranks length is 117mm, it just looks a little short. I think the crank on his 16" is too short, he ends up spinning like a hamster.
    Spinning like a hamster is more a function of the tiny wheels and short gearing. It doesn't require much speed to get a 16" bike spun out. He'd also be spinning like a hampter if he had longer cranks.
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  8. #8
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    The current 20's come with disc brake tabs and a freewheel cassette, and what looks to be a 3 piece crank.
    If its an older model it probabley wont, but you could ditch the coaster and use either rim breaks or convert to disc. Either way it would need a new wheel and you will need breaks
    If the crank is the standard 1 piece you can use a USA/European BB adapter and a 63mm BB this then open up the range of cranks. Sinz BMX cranks start at 115mm
    This will cost more than what you've budgeted but you can save cash through eBay parts and all these mods go together ie difficult to do in stages.
    MTB:
    09 Stumpy Elite
    10 Enduro Pro Custom
    16" HotRock Custom

    Road:
    08 Jamis Xenith Comp
    13 Pinarello Dogma 65.1

  9. #9
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    I went through the same thing when going from a 16 to 20" bike for my son. The stock crank arm length on the 16" hotrock was 92mm, and 140mm on the 20" Giant XTC. I put 125mm Sinz crank arms on the Giant before he even through a leg over it, which was I think made a big difference. I just upgraded to 135mm Sinz crank arms last night and won't ever use the 125 arms again (square taper). Let me know if you're interested in them. For what it's worth, I needed to buy a new bottom bracket, chain ring, chain ring bolts, and chain guide. But through ebay and amazon, it isn't too expensive. The arms were $55, chain ring was $15, and bolts were $9 through ebay. I bought the BB from a local shop for probably ~$15-$20. And I found the 135mm Sinz crank arms on Amazon for $45.

  10. #10
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    maleonardphi - I am curious, at what age and/or inseam length did you determine that the 125mm cranks were too short for your son? Were there any odvious symptoms that necessitated changing from 125mm to 135mm?

  11. #11
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    He is a little over five and a half, and I think a little over 45" tall. I started on the 20" bike at 5 (too early in retrospect, but I thought he would benefit from the gears and v-brakes). I didn't do any measurements, just watched him riding around and felt like it was time. Plus, I figured the longer crank arms would give him a little more leverage for the climbs.

  12. #12
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    So we rode last night and today, and yesterday I was regretting swapping crank arms. But today, I moved his seat up a little, and back a little and he looked much better. Still though, I think he could have stayed on the 125mm crank arms a bit longer. For what it's worth, I just measured him and he is 45" tall, with an 18" inseam.

  13. #13
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    I had a professional bike fit last year for my road bike due to a bit of pain in my knees and lower back.
    Numerous things where brought up and addressed:
    1. Cleats in wrong place causing stress on knee and ankle
    2. Angle of foot very common and addressed with wedges on sole
    3. Seat & Bar position
    The above addressed the knee & back pain but then he moved onto the drive train which was quiet enlightening.

    He explained that humans are not design to cycle and the leverage needed to pedal places a lot of strain on the knee and ankle. I was running 172.5mm cranks at the time and he suggested a switch to 165mm.
    Being 6ft tall this didn't make sense but as i was paying for the specialists help and he had various cranks available to try i thought I'd give it ago.

    The change was dramatic, yes I felt like I couldn't apply as much pressure through the end of the crank, but I could spin the crank far easier and faster.
    He pointed out there is no actual proven rules around crank length but that in his experience shorter cranks helped to promote spinning rather than stomping on the pedals, and didn't stretch the ligaments as much which meant less strain on the joints, he also explained that shorter cranks mean improved ground clearance.
    Finally he pointed out the spinning cranks is less demanding/tiresome and can actually increase average power output as it forces you to use the correct gears.

    He loaned me the 165mm crank for 2 weeks to try and suggested that on my last ride I switched back to the longer crank for the day.
    When I did this I could actually feel the difference he talked about and I found myself staying in gears to climb but stomping on the pedals to turn them over. Dropping to a lower gear to try and spin didn't feel as comfortable.

    As a consequence of this I've now switched all my cranks to shorter lengths and look at my gearing to ensure that I can climb through the use of the correct gearing rather than bruit force.

    I don't know what others feel on this but thought the info may be useful to you.
    MTB:
    09 Stumpy Elite
    10 Enduro Pro Custom
    16" HotRock Custom

    Road:
    08 Jamis Xenith Comp
    13 Pinarello Dogma 65.1

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