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  1. #1
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    16" Hotrock Dirt Jump build.

    New to the forum, but got a lot of inspiration here, so figured I would post what I have gotten out of it. Credit where credit is due, Demo9 inspired a lot of this, but I think I contributed one really cool thing to his builds.

    With almost exactly a month to go before his birthday, I started the next kid build. My younger Grom is really in for a huge surprise. The kid has been rocking an $85 Walmart bike that weighs almost as much as him for way too long now. His skills have surpassed that little bike by a good bit, and he deserves to ride something that will no longer hold him back.

    Hopefully, this will be it.

    I had previously build up an Avent Mini BMX as a dirt jumper for my daughter. Those BMX mini frames have sizes and angles that are pretty spot on for dirt jumpers MTBs that are just mini versions of the ones that the big boys ride, so her build made a really nice little bike that she adores. After getting her new bike, I had a Specialized Hotrock 16 that she no longer needed. I looked around and saw some really amazing Hotrock 16 builds out there, with Demo9's being a really good example. Here is what the bike looked like when we bought it.

    16" Hotrock Dirt Jump build.-hotrock-16.jpg

    This frame will make a really cool little dirt jumper.

    Original Planned Build:
    Frame - Specialized Hotrock 16"
    Fork - Answer MAG Pro 20"
    Headset - VP sealed with Morphine Deadbolt stemlock
    Wheels - Sun CR18 16" hoops laced with titanium spokes and Sapim inverted brass nipples on Rockman 32h sealed ISO disc hubs with 100mm/110mm Al axles, front and back.
    Cranks - Sinz 125mm with 34t ring and Ti chainring bolts
    BB - Sinz titanium with American BB reducer
    Pedals - Wellgo Mini
    Chain - KMC Superlight
    Freewheel - 16t GT 120 POC ratcheting
    Brakes - Hygia Usagi hydraulic discs with 140mm Ashima Airotor discs, front and back. Planned to make custom bolt on disc tabs.
    Stem - Truvative
    Bars - Origin8 2 piece
    Grips - Odi BMX mini lock-ons
    Saddle - Sinz mini

    The important design elements were:
    1. Light weight frame and components. Weight means so much more when you are 40lb.
    2. Disc brakes. My kid is pretty fast and rides some pretty technical stuff. His hands are not super strong yet, so adjustable reach and hydro modulation was key. The Hygia Usagi lightweight cross country brakes with 140mm rotors will be like a crazy big downhill setup for him, but still not weigh a ton. Also makes his bike look legit for a freeride/dirt jumper.
    3. Disc hubs are hard to find for these little frames, and the commonly cited versions are really expensive. I found that mod trials bikes have 110mm rear axle spacing, are lightweight, and not stupid expensive. TartyBikes has a good selection, and their shipping/customer service is unmatched. This may be my contribution to the Hotrock 16 custom build culture. I have never heard of anyone using trials hubs. The other solutions I have seen are much heavier and are a clunky workaround bolt on to a BMX flip flop hub. This is much nicer.
    4. Sinz makes cranks with crankarm length from 125mm and up. Lighweight and short enough. Second build with these.
    5. Old school 20" BMX forks are lightweight, and with the burly crown, have the look of suspension forks, and slackens the headtube angle just a bit. Makes my Grom feel like his bike is just like Dad's. Relatively cheap if found used.

    I already had most parts, some were still on order. The only things to remain of the original bike will be the frame, the tires/tubes, and the seat clamp.

    I got yellow, black, and anodized purple paint, as well as white and black vinyl decals to round it out.

    Bike frame was broken down and made ready for stripping. I removed all unnecessary tabs. After many hours of grinding, sanding, painting, sanding, more painting, etc, I had something I liked.


    16" Hotrock Dirt Jump build.-photo-1.jpg16" Hotrock Dirt Jump build.-photo-2.jpg16" Hotrock Dirt Jump build.-photo-3.jpg

    By this time, most of the other parts had come in. There were a few snafus. The origina plan for a VP sealed headset fell through because the ebay seller didn't send the bearings, and in the end, proved that they knew nothing about bikes. I sent it back for refund. The replacement was a Crank Brothers Opium C headset. This thing is pretty trick. 66g! Weight is a super huge factor for building kids bikes, and this headset was perfect for this. If it holds up well, this may be my go-to headset in the future.

    The next thing to be switched out was the Origin8 2 piece bars. They just looked bad. Good thing I had a set of 2" alloy risers that would work.

    The next interesting mixup is that I was sent 130mm Sinz crankarms instead of 125mm. Oh well, kid will have to deal. I'm sure he will have no problem.

    I also found a set of A2Z brake tabs for relatively cheap, so I ordered them, figuring that the custom brake tabs would be a backup. I felt a bit stressed about time, so these should just bolt up and go, no fabrication necessary.

    I am still waiting on the Sapim inverted nipples. This is stressing me out. I have tried to lace these rims previously, and the spoke holes are very tight on the spoke nipples. The tight holes means that the steeper angles on this small of a wheel make it hard to get everything to lace up nicely. The inverted nipples mean that I can pass the much thinner spoke through the hole, and the nipple remains inside the rim. This should make the steeper spoke angles much less of a problem. Hopefully they get here very soon. Time is running out.

    Here is my stash of loot as it stands.

    16" Hotrock Dirt Jump build.-package-copy.jpg16" Hotrock Dirt Jump build.-parts-copy.jpg

    I have already put everything together that I can at this point. No pics yet, but trust me, this little bike is super trick. It will be probably in the 14-15lb range when done. It has short reach hydro disc brakes. It is single speed and full rigid, but I am sorta skeptical about the need for either of these things for a 5 year old. The gearing would be just extra weight, and he motors up hills on his heavy Walmart bike already. Cutting the bike weight in half should make it even easier. The only suspension forks worth anything for a 16" wheeled bike are White Bros, and that is a pricy little toy. I'll wait until next build for that.

    Next post should probably be of the completed bike. Wheel build is the only major thing holding me up. I am taking it to the bike shop tomorrow to have the brake hoses shortened from their adult size. I have never done this, and I don't have time to experiment and get it wrong before his birthday. I'll let the pros do it right the first time.

    Can't wait to see this project finished. His little bike will be the coolest in the house when done. Then it will be time for a rebuild on mine.
    Last edited by IdahoLee; 04-09-2014 at 07:20 AM. Reason: typo

  2. #2
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    I can't wait to see this project finished either!

  3. #3
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    Incredible looking build. Love the amount of thought and attention to detail that has gone into this.

  4. #4
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    Holy Cow! that thing is gonna look sweet! Nice work.

  5. #5
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    dream build for my son. he's on a super heavy Specialized Fuse 16" bike I'd love to build him a light bike. maybe when he needs a bigger bike I'll hook him up with a custom build.



    -=Jason=-

  6. #6
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    Next snafu to report. Bike shop won't cut Ti spokes. Says that anything softer than stainless will wreck his spoke cutter. Sounds like BS to me. Good thing I didn't spend much on them. I'll ebay them and recoup what I got in them. Anyway, wheels being built with stainless, brake lines being shortened, and I can't wait to get the bike back.

  7. #7
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    Ti spokes are overrated IMO-you're better off with SS.

    Thing's going to be sweet.
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  8. #8
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    I have never been a weight weenie, but when I started building bikes for kids, I became one. If an average adult rider weighs in the 160-200lb range, and the average kid I am building bikes for is in the 40lb range, then for every ounce I save, it is like saving 4-5 ounces for an adult. That's plain huge. And the engineering for weight weenie parts is with adult riders in mind. You can consider super light structural components to be engineered as DH components for kids. So Ti spokes are a pretty tech piece of bling for a kids bike. A spoke set that weighs half as much as normal is a big drop in weight, especially rotating weight.

    The bike will be super nice with SS spokes, but I had my hands on a set if Ti spokes, and they didn't run me the usual arm and a leg. I was just a little disappointed that the bike mechanic wouldn't cut and lace them. His reasoning sounded BS, he was just using his bias to bully me, and I am sorta in a bind. The wheels had to get built, and it was a very difficult build. I'm off on a rant, but as had been stated, this whole thing was a lot of planning, and the guy just didn't like my plan.

  9. #9
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    You can relax - he's right about it beating on the cutter. We do a ton of metal fabrication where I work, mostly stainless, but anytime we do something that involves tougher materials, the stuff wears out blades/wheels/etc like crazy. Only makes sense.

    My kid broke all sorts of spokes on his 16" (he really likes to jump). I had a hassle finding replacements, and the local shops couldn't make me fresh ones as their machines couldn't do them short enough. If the bike is really intended to be used mainly as a dirt jump machine, weight isn't anywhere near as important as it would be if you were building a trail/all-around bike. Of course, in that case, you also most likely not want to go with the plus-sized fork slacking it out either, and there's really no need for a front brake. (I'm thinking you're going for an do-everything ride more than an actual dedicated jump bike though, so feel free to ignore me).
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  10. #10
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    Yeah, this will be his everything bike.

    For the continuing saga of the pain in the rear wheel builder, I called the shop. The wheel builder isn't there but will be later this afternoon. He left a message with the shop tech to tell me he doesn't feel comfortable building the wheels. Something about too much liability. WTF?!?!? This guy is supposed to be a master wheel builder. He certainly has no customer service, so that can't be why he works at the high end bike shop here in town. I can't wait to hear his answer for why he can't build my wheels when he gets in. If he won't do it, I'll build them myself. I am now completely sure I can do it, even though the first attempt was not so good. Second try will make it work. Save me a bunch of cash as well.

    Deadlines looming for the build. I feel like I'm in an episode of American Chopper. Parts take too long to get here, contractors can't do their job, there is a deadline to meet, and I have a bad temper about it all. I am usually very level headed, but this is just spinning me up.

  11. #11
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    Well, hopefully I got everything worked out. The original wheel builder is off the job. Apparently it is his day off, and the shop is embarrassed about the whole thing. They are actually a good shop, and this whole thing is an outlier. With the shop and me both making calls for a backup, we now have a guy who builds wheels for a friend of mine lacing them. He builds wheels for pros, and I have seen his work. I am happy to have upgraded. He will lace them up tonight. The brake lines are being shortened as we speak. If I get it back tomorrow, it will give me at least a day before the birthday to tune, tighten, and grease everything to perfection. Then the surprise reveal at my boys' party at the local bike park. He's been hinting that he needs a new bike, but I really don't think that he has a clue that he will be getting one.

  12. #12
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    Nice paont job! Can't wait to see the finished product. Btw, where did you get the fork?

  13. #13
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    Ebay. Used Answer Mag Pro. Think I paid $50 for it last summer.

    You can find a carbon one, and two chromo ones on ebay right now. Mine are nicer than any on there in my opinion, but looked just as bad before I stripped, painted, and added stickers to it. Those ones could be made to look really nice with some TLC. The carbons would be pretty trick. You could also just keep your eye out for some. They show up some, especially at the start of race season.

  14. #14
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    Do you have the weight of the Sinz arms handy?
    I just got a set and they feel like boat anchors.
    *** --- *** --- ***

  15. #15
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    The sinz weigh close to a pound. Not the lightest, but pretty inexpensive, and come in the right lengths. A drill press could lighten them. I may have to do that later.

    Check this one out.

    16" Hotrock Dirt Jump build.-relieved-sinz.jpg

  16. #16
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    The wheels are not yet sorted out, but in an effort to get something rideable for my boy's birthday, I built up an alternate set of wheels. I had a set of freewheel hubs, the stock spokes, nipples, and rims, and an alloy v-brake plate I pilfered from my 24" Redline cruiser. I had to teach myself a 2 cross pattern to get the spokes to work with the new hubs, but it came out OK, and he is now riding it. I'll get the hydro discs sorted out later.
    16" Hotrock Dirt Jump build.-img_0544.jpg16" Hotrock Dirt Jump build.-sam-side.jpg16" Hotrock Dirt Jump build.-sam-diag.jpg

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    NICE! He looks psyched, good job Dad.
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    Birthday party went off perfectly. He hit the pump track at speeds much higher than before. He was even fast enough to catch some air on the table tops. This was never possible on a coaster brake bike. His big sister better watch out. She has been complaining that when she follows him through the pump track, she has to go super slow to not run him over. We didn't race today but I am pretty sure that, if he is not already faster than her, he probably will be soon. He's a little more motivated by his bike than she is, and that will end up making a huge difference in their speeds.

  19. #19
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    Did you stack spacers for the stem or did you cut down the fork.

  20. #20
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    Spacers. Gave him a little more upright ride.

  21. #21
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    Quote Originally Posted by IdahoLee View Post
    The wheels are not yet sorted out, but in an effort to get something rideable for my boy's birthday, I built up an alternate set of wheels. I had a set of freewheel hubs, the stock spokes, nipples, and rims, and an alloy v-brake plate I pilfered from my 24" Redline cruiser. I had to teach myself a 2 cross pattern to get the spokes to work with the new hubs, but it came out OK, and he is now riding it. I'll get the hydro discs sorted out later.
    What brake lever are you using and how do you like it? Also, how do you feel like he's doing with just the rear V brake?

    I've been back and forth about how to modify our Hotrock 16 before it gets passed on to our soon to be five year old. Now that I know Dan's Comp has these V brake plates for $25 I'm more willing to go that route than the $60 clamp on adapters I was leaning towards.

    But since I'll likely be buying a new rear hub anyways I can't get it out of my head to go ahead and go the disc route. I have access to a full machine shop so a mount is something I think I can figure out.

    Was your original plan to run front and rear discs? I see two rotors and levers in the original post. How were you going to add the front brake?

    Also, did you ever check and see if the A2Z adapter would work in the rear? I thought someone had tried it on a Hotrock but had issues and ended up fabbing their own.

  22. #22
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    Sweet little ride. Nice job!! Did you ever get those wheels built? I am on the same quest for a disc freewheel set for my Hotrock mod. I actually don't see a Rockman rear with the 110mm spacing. Am I just missing it? I see a beautiful Hope pro 2 front though. If you have solved this hub puzzle I will be stoked!

    I have another question. How is the clearance on that crank? Looks like those pedals would hit for sure?

  23. #23
    CJH
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    Quote Originally Posted by bikin222 View Post
    I am on the same quest for a disc freewheel set for my Hotrock mod. I actually don't see a Rockman rear with the 110mm spacing.
    I have that same question. I suspect he plans to use the 116 mm rear and make it fit. I'm pretty sure there's another thread on here that mentions this and I'm looking for it right now.

  24. #24
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    Wheels are built, just have not gotten around to putting them on and working out the last little bugs. The rims are not the same size as the stock rims, so I need a different set of 16" tires to make it work. The v-brake plate works well enough for now. The v-brakes are not as solid as a disc, so eventually I want to swap it out. I am just at the end of the year as a resident physician, I am applying for a fellowship, and I am an administrative chief, so my workload is actually pretty maxed out. I work 80 hours a week in the hospital, plus my administrative duties, as well as extra time with my fellowship application. The little time I have left goes into riding with the kids, not fixing kids bikes. The wheel swap will have to wait for now.

    The plan was discs front and back, and the disc tabs were planned as custom machined, but I found a cheap set of A2Z that turned out to work well, so I went with them instead. The discs worked fine in the rear, but the fork was a little narrow for the front. I have a little modification to do to the discs to make it work in the front. The hubs are the rockman 116mm, and worked just fine with the 110mm spacing. It will be a few months before I have time to put the disc set on the bike, and in the mean time, my boy is having a blast riding with a single rear v-brake.

  25. #25
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    Thanks for the info. Life sounds busy. Spend it riding. Bike looks great. If you ever get it sorted out please post a pic. I think I am going to try for your wheel build. Did you end going with the inverted nipple?

    Thanks again!

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