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  1. #1
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    Cam's 24 inch bike build

    Part 1, Background

    Hey Dads and Moms, here's another kid's bike build thread. Cameron's eighth birthday is coming up in June, and he's ready to move up to 24 inch wheels, so I wanted to get a project rolling to have his bike ready by summer. I'll document all my work here so you can play along from home.

    Cameron and I have been mtb-ing together since he was five. His first bike was a Giant MTX125 20-inch full rigid, which was a pretty good first mountain bike. We ride a fair amount of roots and small rocks in the Mid-Atlantic area; I wanted to move him up to a bike with some form of suspension, so he got a Marin Hidden Canyon (which seems to be a favorite on this forum) for his birthday a year go. That was a big improvement - lighter, better components, a little bit of squish up front (not much, even after I dug into the fork and trimmed back the spacers to reduce the spring preload as much as possible). We did some upgrading on it, and got the weight down under 23 pounds, for very little investment.

    Here's a pic, mostly of the boy, but you can see some of the bike too:


    But he still struggles with some roots and rocks, even to the point of going over the bars now and then. He's usually a good sport about it, picking himelf up and dusting himself off without too much drama. But I've been dying to move him up to bigger wheels, and he's finally grown enough to fit a 24. Off to Craigslist I go.

    My requirements were simple: 24" bike-store-quality bike with a 1-1/8" headset, so we have upgrade options for the fork. Oh, and it can't be too expensive, either. We'll carry over some of the good parts from his Marin (seatpost and saddle, carbon handlebar & 60mm stem, light pedals), and hopefully make some judicious upgrades on other components, most of all the fork.

    In our next installment, a bike is found!

  2. #2
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    Here's the bike

    OK, so in our last installment, I had started looking for a 24 inch bike-store-quality bike with a 1-1/8" head tube. Tons of toy-store bikes out there, not many nice ones. Then I came across a Trek MT220 in fair shape -- it had been left outside some, had some rust, but the fundamentals were good. 3x7 drivetrain, 1-1/8" head tube (although it's threaded, that's no big deal). Here's a pic:



    That fork is pretty much useless. No adjustability, heavy, not much compliance, and the boots are cracked. I knew I wanted to upgrade him to a hydraulic/air fork, and this sealed it. I found a 10-year-old Marzocchi X Fly 80mm fork on eBay for $80 shipped. The A-C measurement is off by a bit, but not enough to worry about it:



    This fork is much lighter, actually works, and it would get us started on a weight loss program for the bike. But I didn't know exactly how much weight we could lose. So I decided to go a little nuts. I stripped the bike down to bare frame, and weighed every single component. I plugged it all into a spreadsheet, and planned out phases for upgrades, so I'd know exactly what each change would yield. Without further ado, my spreadsheet:

    Component

    Current

    Weight

    Phase 1

    Weight

    Phase 2

    Weight

    Frame

    Trek MT220 Alpha

    1420

    same

    1420

    same

    1420

    Fork

    SR Suntour

    2050

    Marz X Fly

    1630

    same

    1630

    Front wheel

    36 straight spoke bolt-on

    843

    Sun CR-18 36 spoke disc

    820

    same

    820

    Rear wheel

    36 straight spoke freewheel bolt-on

    955

    same

    955

    Sun CR-18 disc cassette

    950

    Front Tire

    Bontrager SX 24x2.1

    850

    Kenda SB-8 24x1.95

    500

    same

    500

    Rear Tire

    Bontrager SX 24x2.1

    850

    Kenda SB-8 24x1.95

    500

    same

    500

    Tubes

    Generic 24

    340

    same

    340

    tubeless? (valve)

    50

    Crankset

    Generic triple, steel riveted rings

    1000

    Remove guards

    965

    Sugino 600 24-36

    720

    Bottom bracket

    generic square taper

    291

    same

    291

    Sinz Expert

    220

    Kickstand

    generic kickstand

    300

    none

    0

    none

    0

    Front Der

    Shimano triple

    151

    same

    151

    same

    151

    Rear Der

    SRAM X3 long cage

    280

    same

    280

    Sora 8 speed

    220

    Freewheel

    Shimano 14-28

    438

    same

    438

    SRAM PG850 cass

    300

    Chain

    Generic 7 speed

    291

    KMC 8 speed

    290

    KMC 8 speed

    290

    Pedals

    Plastic mountain

    320

    Plastic junior

    200

    same

    200

    Front brake

    V-brake

    188

    Hayes MX-4 disc + 160mm

    321

    same

    321

    Rear brake

    V-brake

    188

    same

    188

    same

    188

    grips

    generic grips

    50

    same

    50

    same

    50

    shifters w/cable

    SRAM 3.0 comp

    163

    same

    163

    SRAM MRX 8 speed

    163

    housing

    housing

    101

    same

    101

    same

    101

    brake levers w/cable

    Generic brake levers

    215

    same

    215

    same

    215

    handlebar

    Bontrager aluminum

    275

    Forte carbon

    170

    same

    170

    Stem

    Quill

    390

    Dimension 70mm

    140

    same

    140

    Headset

    “1-1/8 inch threaded”

    156

    WTB Momentum steel

    140

    same

    140

    Seatpost collar

    aluminum seatpost collar

    33

    same

    33

    same

    33

    seatpost

    Steel

    414

    Kalloy Uno

    220

    same

    220

    saddle

    Junior saddle

    340

    Origin8 Torque Lite

    200

    same

    200

    Grams

    *

    12892

    *

    10721

    *

    9912

    Pounds

    *

    28.40

    *

    23.61

    *

    21.83



    It's interesting to know exactly what goes into the weight of a bike.

  3. #3
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    Notes on individual components

    Those Bontrager tires are 850 grams each! That's the same weight as the Maxxis Ardent Tubeless tires on my 29ER! The crankset and seatpost are really heavy, too. And the freewheel is a boat anchor. Phase II will involve lacing up a custom rear wheel with a modern disc hub and 8-speed cassette; that should lose some good weight, even taking into account adding a disc caliper. I'm going to go install the threadless headset now, and see how much steer tube I need to cut for the fork. Will report back soon.

  4. #4
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    Looking forward to see him out on it. Based on the last time I saw him (October?) he'll probably good to go on a 24" this season. Shaving off so much weight is quite the game, most of which is easy pickings on one of these kids frames. Are you doing the wheel build yourself?

    I'm building up a new frame with Owen too. He has been helping to select parts and will be turning the wrench on this one with my guidance. Last month at the swap meet he got a new carbon bar for real cheap when he pulled out his own cash and the seller felt bad about the original price offer.

    He's several inches shorter than Cameron so I'm not sure exactly when he'll be able to handle the bike proficiently off-road. He'll probably ride the 20" until the end of the summer and then maybe try the new one. Also gives us more time to build it.

  5. #5
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    Ta Daaaaah!

    Here's the big reveal. I learned how to lace up a wheel, and built a custom 24" front disc wheel to go on the 26er fork. Sun Ringle CR-18 rim, generic front disc hub (kinda heavy), 36 straight 15-gauge spokes. Didn't get the weight savings I was hoping for, but it's still a really cool wheel.

    With the 26er fork, the front end is significantly longer than before, and it looks pretty raked out - kind of like a downhill bike. But that's the nice thing about an air fork - it will actually sag under his 60-lbs weight, unlike any coil spring kids' fork. It should be super plush for him, and provide actual hydraulically-damped travel, which just might blow his mind.

    One thing I did that wasn't in the initial plan - I drilled out the rivets on the outer chainring, and pulled it off, leaving him with a 2x7 setup. That saved something like 150 grams, for free! He doesn't need the third chainring, 32-14 should be plenty fast.

    As the bike sits now, a little more than a week from his birthday, we're at 23.9 lbs. I haven't swapped over his blingy carbon handlebar or superlight stem yet, because that might tip him off that an upgrade is coming.

    Here's how it looks:








  6. #6
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    Awesome! I'm jealous! I picked up a Scale RC for my 6yo (only $280!), to give him this Christmas. The only problem is that his 5-yo brother keeps right up with him, so I need to find another 24-inch bike (preferably another Scale RC, but I won't hold my breath!). I called on a Trek MT220 the other day but it was already gone. I even stripped down my old Marin Pine Mountain for components for the next 24 build. You've got me looking for that front shock already though!

  7. #7
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    Bike looks great. I did the same thing with a Gary Fisher Tyro I picked up on Craigslist. I ended up with a Manitou Axel coil fork with a light spring in it... Works ok.
    My son's bike is a little heavier than I'd like but he's going to be stepping up to a 26er soon enough so I'm not going to spend any more money on this one.
    I like turtles

  8. #8
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    Looks great Steven. I'm sure he'll be excited to show it off. Due to baseball and other scheduling conflicts we might actually make our first group ride on the 25th. Hope to see it there.

    Owen has a box of parts right now. Only thing we need are spokes. Last week we took it to get the frame faced/chased/reamed. I made him do the talking and we watched it on the stand while the mechanic worked. Hopefully he's learned something.

  9. #9
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    That sounds sweet. I'd really like to get Cameron more involved in maintenance, but so far he just seems to want to ride!

  10. #10
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    Nice job on the bike and the documentation. I am impressed that the frame is just 1420gr (3.1lbd) , definitly a good frame to start a lightweight project bike.

    My purely constructive criticism is that I think the fork may be a bit too much. You currently have around a 64° head tube angle though with some sag it might be around 65° while being ridden. You should try riding the bike around a bit yourself and see if the steering feel predicatable and safe, I would image that it has way too much fork trail and probably exhibits serious degree of wheel-flop. (BTDT). Wonky steering could make it difficult for a inexperienced rider to handle. A-C of the fork has the front end jacked up so high that bars are way above level of seat. Besides helping with the handlebar height, a shorter fork would help to lower the bottom bracket (by about 1/2 as much), make it easier for him to touch feet on the ground while on the saddle. On my daughters bike with a tall bottom bracket, I find that I need to keep the seat height lower than her leg length calls for, just in order for her to be able to comforably reach the ground. The fork also still outweights the frame considerably. You could probably source a rigid fork that is easily couple of pounds lighter, or modify a lighter 26" vintage fork to work with 24" wheels and get a lower A-C.

    You could save couple grams of rotational weight by lacing the wheels with only 18 spokes instead of 36 which is overkill for a kid rider.

    cheers-

  11. #11
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    Thanks for the feedback. You raise some valid points.

    I tried to get an older fork with a lower A-C measurement, but I wasn't going to spring $200+ for an old 63mm SID. With the fork sagged down 30%, the front end doesn't feel too floppy. We'll have to see how he likes it. He's not an inexperienced rider, and he's OK with only touching his toes to the ground. He sometimes rides an XS 26er around the neighborhood, so I'm not too worried about his being able to handle the height of the saddle.

    If it doesn't work, we'll revisit the fork. Rigid is not an option. We ride some roots and rocks that would turn him off pretty quickly without suspension. That's one of the major goals of this excercise--to have a working, hydraulically damped front suspension that has an appropriate spring for a 60-pound rider.

    By the way, in the photos, the saddle is not set to his height yet. He hasn't seen the bike. It's a surprise. I expect it will come up an inch or two.

  12. #12
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    Great Build! Do you happen to know what year that MT 220 is? I am buying a trashed 2008 MT 220, and I am just trying to figure out if your frame is a similar year. Your thread inspired me to build a similar bike to the one you built, so I may have some questions in the future.

  13. #13
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    Thanks. I think it's a 2008 - that's the earliest one I could find on Bikepedia that has an aluminum frame - earlier ones had steel frames. Here's an action shot from a kids' MTB race a few weeks ago:


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