Ultimate 20" bike build!- Mtbr.com
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  1. #1
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    Ultimate 20" bike build!

    Update: Weight on initial build is 16.18lbs. should be under 16lbs soon.

    I set out to build the best bike I could for my 4 (soon to be 5) yr old. We live right next to an awesome black-rated bike trail here in Colorado and he already shreds it on his modded 14" commencal. My little guy is a massive bike enthusiast and has already developed some real skills.

    I'm hoping to build something with geometry and to shred a rowdy, techy downhill and light enough to get back up.

    I had two ideas on the build that I think might be unique. Time and testing will tell whether they are good ideas or not!

    First is to make it a 24" front/20" rear wheel mixer setup, using a 20" frame and a ridged (non-corrected) 24" fork and wheel. The axle to crown on the 24" fork is tighter than a 20" suspension fork so that once it's sitting on a 24" wheel the geometry should be nearly the same. This should save about 2lbs and roll better.

    Second, I'm running a DH drivetrain because 1) it's meaningfully lighter than anything else and 2) the rear mech is really short, providing much-needed clearance when used with a 20" rear wheel.

    The building should commence this weekend, parts list below, let me know if I'm missing anything major or if there are lighter parts I should think about:


    • Flow frame with rear dropouts in the low-bb, 20" setting. I went with this frame because Flow was really helpful and great to work with. It's super light and has what looks to be great geo numbers. Might end up moving the dropouts to raise the BB, testing will determine.
    • Exotic carbon fork from the UK (heavier than it needs to be, but the only thing I could find with the right geo)
    • DT240S hubs 28h, XD driver (thought hard about Extralight hubs, but the DT's durability and cost savings swayed me over the weight savings)
    • Ti 14g BMX race spokes and DT alu nipples
    • Crest mk3 24"front/mischief 20" rear tubeless rims (to my knowledge this is the only light weight, tubeless 20" rim available and it was back ordered for wks, just came in!)
    • Ti/alu skewers
    • Answer slider 122mm bottom bracket Ti/alu
    • Narrow (600mm) Carbon MTB riser bars from china (they are super light, the right size, and relatively cheap, but I wouldn't trust them under adult use, for a 45lbs kid I think they'll be plenty strong
    • Answer atac 30mm stem (should have gone with a wren)
    • Thompson seatpost (not a dropper, but may try and fit one eventually). May swap to carbon, but most carbon posts don't seem to be lighter than the classic thompson. Also we'll be sliding it up and down a ton, so not sure how carbon would hold up.
    • Ti/Alu seat post QR
    • Lizard skins dsp grips (most grips are shockingly heavy!)
    • Aest/Rockbros ti/magnesium pedals (we run these on the BMX race bike--super light and grippy and the right size for little feet)
    • Turn3 billet 120mm bmx cranks with custom 64mm bcd spider
    • XO1 DH 7spd drivetrain w 24 tooth front chainring
    • Magura MT trail carbon brakes (planning on 160f/140r rotors)
    • SDG kids saddle (heavy but I couldn't find anything lighter in kid size)
    • Lil Joe rear and Rocket Ron front tires
    Last edited by BikePilot1; 01-11-2019 at 09:04 PM. Reason: Updated with weight

  2. #2
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    Flow makes a heck of a bike. Any reason you are not doing a 10sp drive train with some nice Trailcraft/spawn direct mount cranks and BB?

  3. #3
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    FYI, Tioga makes a 140g pivotal saddle that's almost the same size as the SDG (235x125 vs 235x122):
    D-Spyder S-Spec Pivotal – TIOGA BMX Race

  4. #4
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    Thanks! I did the 7spd to save weight and get a more compact rear mech. With the 7spd dh setup the cassette is 10-24 so I needed a small (22-24T) front chainring which I wasn't sure I could get on the spawn or trailcraft cranks. Also on those cranks there's no way to adjust to different q factors and I couldn't find the q factor listed for these cranks or the frame so wasn't sure it would clear. They look like great cranks though! We run the Turn3 cranks on a couple of bmx race bikes. They are light and nicely machined. I'm not sure this route is better but that's my reasoning .

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  5. #5
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    Here's another source for cheap narrow wide 64bcd chainrings if you're looking for 22t / 24t options (I'm using one on an old Trek I use to pull a bike trailer):
    https://usamade.myshopify.com/collec...-pro-64mm-74mm

  6. #6
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    Thanks! I thought hard about the railed version of that Tioga but wasn't sure how the little guy would do without padding. Less worried about sit bones than him jabbing himself on the tip or edges. I think I'll need railed rather than pivotal to get the seat far enough forward, but if the ergos work out with pivotal I'm inclined to try it for the weight savings (the small railed Tioga is heavier).

    Have you run one of these? Any issues with the no padding situation? All our bmx bikes have hard plastic seats, but it's basically impossible to sit down so we don't have much relevant experience.
    Quote Originally Posted by TimTucker View Post
    FYI, Tioga makes a 140g pivotal saddle that's almost the same size as the SDG (235x125 vs 235x122):
    D-Spyder S-Spec Pivotal – TIOGA BMX Race
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  7. #7
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    Killer, thanks!
    Quote Originally Posted by TimTucker View Post
    Here's another source for cheap narrow wide 64bcd chainrings if you're looking for 22t / 24t options (I'm using one on an old Trek I use to pull a bike trailer):
    https://usamade.myshopify.com/collec...-pro-64mm-74mm
    Sent from my Pixel 2 using Tapatalk

  8. #8
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    Quote Originally Posted by BikePilot1 View Post
    Have you run one of these? Any issues with the no padding situation? All our bmx bikes have hard plastic seats, but it's basically impossible to sit down so we don't have much relevant experience.
    I just ordered one to try, but most of the reviews I've seen say there's more flex to it than you'd think.

  9. #9
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    I just built up a 16'' flow around a 10 speed 11-32 xx cassette I had from years ago for my 3 year old (soon to be 4.)

    I did an x0 10 speed shifter with a red rear derailleur, and xx world cup brakes. Built some stolen rampage 28h bmx rims into shimano hubs, put schwalbe black jack 16 tires on it, and right now some basic alloy parts to finish it off.

    I'm looking forward to when she's big enough for 20''. I've got a pair of 24h hubs off a roval control sl wheel from a few years back - dt 240 internals, straight pull. Alienation is coming out with a carbon bmx rim, I've got my eye on those. If not those, Box sells a 290g 20'' rim. Finding 16'' stuff that is halfway decent is hard! Her 20 inch wheels will probably be a pound lighter than her (1540g) 16 inch wheels.

    Here she is in the park today - I'm letting her get used to hand brakes and a trigger shifter before we go to singletrack.
    Last edited by masont; 11-30-2018 at 02:25 AM.

  10. #10
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    Hello like-minded friend. A few things I've found that might interest you -

    Check these grips out - they're around the same weight as the lizard skinz without a lockring, only they have a lockring, and at 100mm are sized more appropriately for small kids CLICK GRIPS AND CLAMPS - Promax Components

    This stem knocks 30g off the atac, but it isn't available in 30mm, only 40 - https://wrensports.com/product/crazy...aluminum-stem/

    These pedals knock 80g off, and are size appropriate (and the price is right) - Dazz Lite – Tioga - The bearings were pretty tight when I got them, but some tri-flow got them rolling just fine, and they're plastic pins, but if they wear out I'll consider it a huge win she's ridden that much and happily buy another pair.

    Have you found any shorter cranks that are light in your research? I don't really want to go above 105.
    Last edited by masont; 11-30-2018 at 02:26 AM.

  11. #11
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    Howdy and thanks! The DSP grips are actually 40g lighter (https://lizardskins.com/mtb/grips/dsp/dsp-grips-303-mm) and I plan to cut to fit, we'll see how that goes. I think we have those promax grips on his Commencal now, definitely seem like good ones and have held up well to a lot of use;.

    I like that Wren stem and will move to that when a 40mm stem is suitable.

    Current pedals are 161g/pair (Aest magnesium body, ti axle--pretty fragile but have held up for us on the bmx race bike, we'll see how they do with the mtb useage).

    For light cranks in that size range look at LSR, Helium, Turn3, and Bombshell. I think they all go down to 100mm and Turn3 goes I think to 85mm. Turn3 tends to be the least expensive and I've been really happy with them (there's three sets in lengths varying from 95-120mm in my house right now). I weighed the 120mm Turn3s that I'm using on this Flow build at 320g (that includes the spider, but not bottom bracket or chainring). They are strong too. I've jumped on his other Turn3 equipped bikes in the driveway with the Turn3s and nothing bad happened (I'm 195lbs).

  12. #12
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    That video is awesome, she looks ready to rock!

  13. #13
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    Quote Originally Posted by BikePilot1 View Post
    For light cranks in that size range look at LSR, Helium, Turn3, and Bombshell. I think they all go down to 100mm and Turn3 goes I think to 85mm. Turn3 tends to be the least expensive and I've been really happy with them (there's three sets in lengths varying from 95-120mm in my house right now). I weighed the 120mm Turn3s that I'm using on this Flow build at 320g (that includes the spider, but not bottom bracket or chainring). They are strong too. I've jumped on his other Turn3 equipped bikes in the driveway with the Turn3s and nothing bad happened (I'm 195lbs).
    My boys are on Turn3 cranks right now for BMX racing, but I have been looking at the VSR ones (Van sothen racing) if I can find some used or something. I think this is what you meant by LSR and maybe Helium. Helium makes the super "light and tight" bottom brackets, and they have some really nice pedals, too. The pedals weight about the same as the Aest, but are more durable (and more expensive, like $70-80).

  14. #14
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    Thanks, VSR is what I meant!

  15. #15
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    Quote Originally Posted by BikePilot1 View Post
    Howdy and thanks! The DSP grips are actually 40g lighter (https://lizardskins.com/mtb/grips/dsp/dsp-grips-303-mm) and I plan to cut to fit, we'll see how that goes. I think we have those promax grips on his Commencal now, definitely seem like good ones and have held up well to a lot of use;.

    I like that Wren stem and will move to that when a 40mm stem is suitable.

    Current pedals are 161g/pair (Aest magnesium body, ti axle--pretty fragile but have held up for us on the bmx race bike, we'll see how they do with the mtb useage).

    For light cranks in that size range look at LSR, Helium, Turn3, and Bombshell. I think they all go down to 100mm and Turn3 goes I think to 85mm. Turn3 tends to be the least expensive and I've been really happy with them (there's three sets in lengths varying from 95-120mm in my house right now). I weighed the 120mm Turn3s that I'm using on this Flow build at 320g (that includes the spider, but not bottom bracket or chainring). They are strong too. I've jumped on his other Turn3 equipped bikes in the driveway with the Turn3s and nothing bad happened (I'm 195lbs).
    My dude, I wish I'd have talked to you before I started this project. I googled the wrong pedals and looked at the wrong lizardskinz grips. I've been doing nicest possible bike with a seatbelt, but I think I'm going to make some upgrades.

    I just talked to the owner of Turn3, and I'm going to get a 100mm crankset and a ti bottom bracket, should knock a bunch of weight off that way.

    Was there enough frame clearance to run a 68x108mm BB and a 30ish tooth chainring on the Flow? I'd like to get the chainring as inboard as possible because, well, chainline on a 16'' bike.

    Have you run into a light 20'' 406 rim in a 24 hole drilling? (tubeless if possible, but I'd take tubed if I could find it.

    Oh, and also - "You must spread some Reputation around before giving it to BikePilot1 again."

  16. #16
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    Thanks! On the flow it looks like I'll have to use a 119mm bottom bracket to keep the crank arms from hitting the chain stays (and even that will require a little bit of grinding on the back of the cranks to clear). I'm not sure on chainring clearance but hope to get all that installed this weekend. Generally I use the narrowest spindle possible to keep the q factor as tight as possible, then space the chainring in or out as needed.

    100mm cranks should clear better than my 120s since the chainstays get wider as you go back.

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  17. #17
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    Re a light 406mm rim, I don't recall anything in 24h though there is one Chinese carbon company that I think will do any drilling you want. I can't remember the name, I'll try and find it.

    The lightest I could in 406 tubeless is the Alienation mischief. It's listed as a front only because it's not rim brake compatible.

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  18. #18
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    I think this is the place that does carbon 20" rims to order. http://www.smcbike.com/.

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  19. #19
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    Quote Originally Posted by TimTucker View Post
    I just ordered one to try, but most of the reviews I've seen say there's more flex to it than you'd think.
    Update in the Tioga saddle: it's pretty hard and real world weight measured out to around 147g (I suspect the pivotal bolt might not be included in the 140g listing). Not something I'd use if I were going to be hitting bumps or riding much distance sitting on it.

  20. #20
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    Quote Originally Posted by TimTucker View Post
    Update in the Tioga saddle: it's pretty hard and real world weight measured out to around 147g (I suspect the pivotal bolt might not be included in the 140g listing). Not something I'd use if I were going to be hitting bumps or riding much distance sitting on it.
    Trailcraft sells a kids saddle. Not sure how it compares

  21. #21
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    Quote Originally Posted by BikePilot1 View Post
    I think this is the place that does carbon 20" rims to order. MIni Size Carbon Rims and Wheels for Brompton, Birdy, Moulton, Tern, Tyrell, BMX Race, Recumbent, Handbike - SMC Bike.

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    This is a solid profile and tubeless ready. I haven't seen that combo yet. SMC 20" 406 BMX Racing Clincher Tubeless Carbon Rim 32mm Wide - SMC Bike

  22. #22
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    Thanks for the update! We have a ton of bumps so I'll probably stick with the SDG for now. I have a padded supercross pivotal seat that I should weigh. It's probably lighter than the railed seats like the SDG I'm using.

    On initial mockup it looks like best geo will be had with the seat all the way forward on the rails. Little kid bikes haven't joined the steep sta trend yet it seems.
    Quote Originally Posted by TimTucker View Post
    Update in the Tioga saddle: it's pretty hard and real world weight measured out to around 147g (I suspect the pivotal bolt might not be included in the 140g listing). Not something I'd use if I were going to be hitting bumps or riding much distance sitting on it.
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  23. #23
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    Just finished the build and little dude took it for a quick spin in the driveway. We will hit the trails on it soon! Confirmed that the 24" fork and wheel fit and work. We will soon find out how well!

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  24. #24
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    16.18LBS! Even lighter than I expected!!! And that's with the heavy sdg saddle.

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  25. #25
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    Maybe its just the camera angle, but those little joe tires looks huge despite only being 20x2.0. What do you think of those tires? My kids bikes have all come with small block eights, which are a bit narrow despite being 20x2.125.

  26. #26
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    It's a good size tire and seems to have a nice grippy rubber compound and reasonable tread pattern. They have a smaller little joe (20x1.4) and a larger one (20x2.0). I bought one of each. The smaller one is much lower volume and ~60g lighter. The wider one is also much taller/higher volume. That's the one that's in the pics and I think will be the one we'll run most often. I'll swap the small one on if we have some huge climbing or smoother terrain to cover. I don't think I've seen a small block eight in person so not sure how it would compare. From pictures it looks like the little joe has side knobs likely to grip better in the dirt while the SB8 looks to have smaller knobs, particularly when it comes to the side, cornering knobs. The Spawn/Brood tires are bigger and more aggressive still, but for our uses I think the little joe strike a nice balance between weight, rolling, volume and grip. I'll report more once we get more off road miles on it.

    They also setup tubeless without any fuss.

    The front tire in the pics is a 24" rocket ron.

  27. #27
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    Maiden voyage was a success! We rode lookout mountain trail to chimney gulch then through downtown Golden for lunch. Mom picked us up an spared us the 2k' climb back home! My little guy is a big fan of bridges.

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  28. #28
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    Bump for fitek

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  29. #29
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    Thanks!

  30. #30
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    Quote Originally Posted by BikePilot1 View Post
    Confirmed that the 24" fork and wheel fit and work. We will soon find out how well!
    This is pretty cool. Any issues with toe overlap with the much larger 24" wheel? Looks like it might cause issues based on the pic, or maybe the pedals just look really large. Is he spinning out the gearing on the flats or is gearing OK?
    Unique looking build for sure, great job!

  31. #31
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    Quote Originally Posted by GSJ1973 View Post
    This is pretty cool. Any issues with toe overlap with the much larger 24" wheel? Looks like it might cause issues based on the pic, or maybe the pedals just look really large. Is he spinning out the gearing on the flats or is gearing OK? Unique looking build for sure, great job!
    Thanks! No toe issues so far, but he has little feet. The reach can be adjusted via headset ecentrics so if we end up needing more space it's possible to do. I don't think it'll be an issue though as he's not clipped the tire once yet and he has done a fair bit of tight, slow playing around.

    The gearing is a bit light I think. He spins out easily on the road and hasn't used the largest rear cog yet. I have another chainring to try that's a couple of teeth bigger. We have 2' of snow on the ground now and will be spending a lot more time on skis than wheels for a while though!

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  32. #32
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    Quote Originally Posted by BikePilot1 View Post
    Thanks! No toe issues so far, but he has little feet. The reach can be adjusted via headset ecentrics so if we end up needing more space it's possible to do. I don't think it'll be an issue though as he's not clipped the tire once yet and he has done a fair bit of tight, slow playing around.

    The gearing is a bit light I think. He spins out easily on the road and hasn't used the largest rear cog yet. I have another chainring to try that's a couple of teeth bigger. We have 2' of snow on the ground now and will be spending a lot more time on skis than wheels for a while though!

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    Cool. Is the rear derailleur of the SRAM DH stuff smaller than say a short cage rear X5/X7/X9/Zee like the other 20" bikes seem to be using? Or was this spec more of a weight savings measure? I'm sure a road 10/11 speed cassette and short cage derailleur could be about as light, right?

  33. #33
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    Yes it's quite a bit shorter. Similar to the old short cage 8spd xtr mech from the 1990s. It can only be used with their matched 10-24 cassette or so they say.

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  34. #34
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    I suspect the road setup could be a good one. I never really looked into it. I did run an 8spd road setup on a DH bike in the 1990s with good results!

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