Project Mjölnir - 20" Kids Bike

This build report actually predates my older son's Raleigh RX 24 build, but it was a more extensive build (and finished more recently), so it's getting published a bit out of order.

Since I'm both happy with my bikes and have no room for anything additional, I've decided to have some fun with the next bike for my [recently-turned] 4yo. He's been riding an Isla CNOC 14, which is great, but having gears is a complete game changer on our hilly roads.

This project wasn't really something I was looking for, but someone down the street decided to throw away a Scott Scale 20 kids bike ... and so I had no choice but to pull it out of the trash.

Anything steel has rusted out. The fork is terrible (and putting a fork on a kids bike is ridiculous). And it must weigh 30lbs. But the frame seems decent. The specs are standard (seat tube, head tube, BB). We can fix this.

The plans for this project:

1. Convert to disc brakes to maximize stopping power with least amount of hand force.
2. Wide range gears for MTB (and our neighborhood hills)
3. Lighten it up! -- Adding disc brakes will add weight, but we should be able to save some significant weight on the build.
4. Personalize it.  My second son is always getting hand-me-downs, so the goal is to make an exception.  We're going with a Thor theme here, since that's my son's favorite superhero.

Frame Modifications

The first step in the project was just stripping it down to the frame. Any parts that seem viable we'd donate to a local bike charity; the frame is all I really planned to use. As it turns out, we kept a few of the components, but most were unable to be salvaged.

The unmodified frame weighed in at 1588g (3.5lbs), which is not very light (it weighs almost as much as my 59cm titanium gravel frame), but it seems nicely built and it'll weigh a little less when we're done with it.

After stripping off the components, my friend Kurt helped me finish what I started with a Dremel; we ground off the brake bosses (we're going disc brakes here, so those are useless) and kickstand plate.

Sorry, kids; no, you can't have a kickstand on your bike.

Showed those brake bosses who's the boss.

Removed kickstand plate.

Kurt also helped me with installing the A2Z DM-UNI adapter -- the key to turning this into a disc-brake bike.  This time we modified the adapter a bit to make it a little easier to take wheel on/off.  We used the existing fender eyelets to fasten the adapter onto the frame.  This turned out really tidy.

Frame Finish

Painting

My original plan was to paint this myself using Montana cans. I figured the total cost of doing that would be around $70 (including frame stripper, necessary supplies, etc.). So not exactly cheap, but cheaper than powdercoat. I started by using a can of aircraft stripper I already had on hand to remove the existing paint. This is nasty stuff.

Covered in aircraft stripper.  Too bad it's not a scratch-n-sniff photo.

In the end, though, I have absolutely no place to do this work and do a decent job of it (a place to let it try w/o dust, etc.), so I decided to pony up and pay the extra (about double that price) for the powdercoating. (I didn't get a picture of the final result of stripping it, since it was never fully finished before giving up.)

Sadly, the guy that painted my previous bike-project frame is no longer in business, so I had to shop around a bit. Ended up taking this to ASCO; they were super nice, did the work right on schedule, and ended up giving a lower price than originally quoted once they saw in person both the (small) size of the frame and the amount of work I'd already (needlessly) done in stripping it down. They did a great job masking everything off, etc. It turned out fantastic!

Gold!

The painted frame weighed in at 1446g.  So we lost some weight and then likely gained a little back with the thicker powder coat.  Anyway, that'll have to do!

Decals

Since the goal is to make this bike more personalized, I opted not to look for Scott decals, and instead headed over to doityourselflettering.com to get an appropriate Thor-related sticker. I chose silver for the decals, and spec'd the reflective vinyl lettering for added low-light visibility.

Reflective decals.

Fork

For the fork, there are actually more options with 20" (thanks to folding bikes) than for 24". I found a few carbon options, but chose one that had the largest axle-to-crown measurements so it wouldn't mess up the geometry of the bike too much. (Though I'm not sure how much the original frame was designed around the suspension fork axle-to-crown.) This is definitely shorter than the original suspension fork, but I think it ends up looking a lot more normal geometry than the original fork. The fork has brake bosses for v-brake and disc-brake post mount. I just used brake boss plugs to fill in the holes in the front. (I actually have some cosplay EVA foam sample squares that I might use to make spikes to fill these holes, but I haven't done that yet.)

Wheels

20" Wheels: Paired Spoke Lacing

The stock wheels on this bike are the standard 20" (406 ETRTO) size, I ordered a set of Spank Spoon 28 rims in 32h drilling, since I found these on sale. I would have much preferred to find 24h drilling, as 32h is way overkill for a wheel this size and rider this tiny, but as this is a bmx wheel size, it is very difficult to find anything less than 32h -- and even 32h is far less common than 36h.

But, to solve this problem, I decided to go with 32h rims and hubs and just use half as many spokes, since 16 is also a nice divisible-by-4 number. The only challenge using half as many spokes presents is lacing and what to do with the extra holes. More on that in a bit.

For the hubs, BDOP has been having a sale on some old stock Novatec hubs. For this frame I need 130mm rear spacing and disc hubs; that's a bit of a unicorn now that the standard settled on 135 OLD for disc, so it is lucky that the D352 hub was available in that clearance bin. For front I got the excellent, but discontinued, D711 QR hub. I really like Novatec hubs, so picking these up for $80, per set (with Japanese bearings) was a great deal. I bought 2 sets ... more on that later.

In order to ensure that I wasn't lacing spokes from one side of hub to holes angled for the other side of the hub, I opted for a paired-spoke lacing. I was inspired by -- and followed the directions -- described here:

http://faqload.com/faqs/bicycle-components/wheels-and-tyres/paired-spoke-lacing

(There is actually a nds/ds-reversed mistake in the lacing instructions; I need to email the author to suggest correcting it.)

I used Sapim Laser spokes. Those spokes can be cut as short as 175mm, which worked for this build (I think my shortest length was 177mm).

To fill in the holes (to keep out dirt, water, etc.) I used 5mm plastic plugs for furniture, etc. This idea was also not original, I read about it first here:

http://forums.mtbr.com/fat-bikes/filling-those-unused-spoke-holes-rims-860362.html

A little out of focus, sorry.

Novatec D352 hubs are 130mm OLD, which is otherwise hard to find now.

(This wheel has not been tensioned yet.)

Gold nipples!

Using furniture plugs to fill the extra holes.

The hubs are light, but the rims, being BMX rims aren't the lightest.  1244g isn't terrible for a wheelset, but I suspect with a little patience, shopping and expanded budget this could drop a couple hundred grams.

16" Wheels: Some Ideas Prove to be Bad Ideas

To make this frame last longer, I had what felt like a clever idea to build it with 2 weelsets: The 20" (406) for which the frame is designed, and also a set of 16" -- though I went with the 349 ETRTO size, typically used on recumbents -- not the more-common 16" BMX (305 ETRTO). Sizing up a bit, since I don't need them that small -- and hopefully these will look less out-of-place in the frame. We shall see. (Spoiler alert: this was a dumb idea, but the build was still interesting.)

Because of how the holes are offset on one side of the hub vs. the other, using every other hole is actually less trivial than it sounds. It requires 3 spoke lengths. I chose the NDS as the side that would get the uncompensated length and then for the DS I adjusted the cross "factor" by 0.125 (i.e. I used a 1.0-0.125x for one set of spokes and 1.0+0.125x for the other). I thought I had a paper-napkin sketch that made this make sense, but I think maybe the correct adjustment would have been 0.25. If the wheels were for me, I probably would have replaced them as half of the DS spokes might not really be long enough, but since it's for a 40lb kid, I'm not going to worry about it.

I played around with the lacing a bit -- usually alternating sides, but since this is 1x lacing, you can also do either all heads-in or heads-out. I did the front DS heads-out, for no particular reason. (Mostly just with such tiny rims, I didn't see any reason to move the spokes further outboard, since the bracing angle is already pretty extreme.)

These are straight-gauge (2.0mm) titanium spokes in gold. Red Sapim Polyax aluminum nipples. I used motor oil as lubricant, as usual, but wondered if ti spokes might need anti-seize. Too late! They're properly tensioned, though, so I don't expect to ever need to adjust them.

The spoke sizes made this a fun project. Too bad I failed to get the math right; I clearly need to dust off my trigonometry.

Lesson Learned: See the Forest for the Trees

So, these wheels built up fine, but in the end they dropped the bike too much to be usable (pedals would hit the ground with even a minor-leaning turn).  I should have considered that more thoroughly.  The 140mm crank arms I'm using are about the shortest I can find, and there's even pedal strike risk with the normal 20" wheels (though right now the rear tire is a smaller 1.6" tire).

Brakes & Drivetrain

For the brakes, I ended up finding a set of Shimano BR-M395 (Acera) hydraulic brakes with new pads for $40.  While the levers aren't the same single/double-finger design as the fancier (and newer) Shimano levers, the levers do have reach adjust -- and I find my older son is still reluctant to use less than two fingers for braking even with the newer lever designs.  So I didn't really take that into consideration.

I'm sure these are also not the lightest; I didn't bother comparing the weights.  I also suspected that they would be easy to bleed and work great. They were and they do.

For the drivetrain, I decided to go a little experimental.  I figured grip shift still makes sense for him, if the actuation is light enough.  Finding a 10sp SRAM grip shift at an affordable price is pretty tricky (but 11-speed is no problem!).  While 11-speed is always tempting, I decided to go with a SRAM 9-speed setup.  I found a lightly-used 11-34t 9-speed XTR cassette, which seemed very sane, and a SRAM X0 grip shifter.  Problem is there aren't any 9-speed clutch rear derailleurs; however, Shimano 10-speed and SRAM 9-speed both use [almost the] same pull ratio (1.1:1 vs 1:1) and many people successfully use a Shimano 10-speed rear derailleur with SRAM 9speed levers.  I found a short-cage Shimano Zee (the Free-Ride edition supports 36t).  Done & done.

For the crankset, I went back to Trailcraft and got their direct-mount cranks in 140mm size with a tiny 28t cog.  With the 11-34t cassette that should be a big range with plenty of easy gears for those little legs.  I love that Trailcraft sell their components for folks like me that are building up or converting other bikes.

Cockpit

For the handlebars, I picked up some inexpensive carbon MTB bars from China. For my own mountain bike, I probably would be a little more conservative, but I don't weigh 40lbs like my son does.  I cut these (way) down for his narrow shoulders.

For the saddle, I got a Junior Well saddle; I found this on sale for around the same price as other kids saddle options; it weighs a reasonable (and lighter-than-alternatives) 225g and the SMP "eagle beak" nose helps keep shorts from getting snagged.  Plus the red fits our color scheme.

For the grips I used ESI Racer's Edge grips, since they're very thin/low-profile for little hands and very easy to cut down -- and I needed one shorty grip to complement the SRAM grip shifter.

Final Result

All built up, the bike is fantastic.  It works great, looks great, and my son absolutely loves it.  It's still too big, but he can make the tips of his feet touch the ground, so it's the only bike he'll ride now.  He can climb any hill sitting down now -- which is both good and bad since I need him to get more comfortable getting out of the saddle.

I opted for BMX tires on this bike, which should be superior on-road and just fine off-road (since we're not riding muddy trails anyway).

Found a dragon head badge on eBay.

Brakes dialed in all the way works great for 4yo fingers.  XO shifter is easy to twist.

Zee rear derailleur pairs nicely w/ SRAM 9sp.

Zipp road skewers (this is 130 OLD) and rainbow bolts! (My son always pointed out how much he liked these on my wheels.)

The new Trailcraft DM crankset is very nice.

Oh, and final weight was 15lbs 14oz (w/o pedals).  I had wanted to try to get it down to 14lbs.  I don't think that's possible with this frame (at least without going with boutique components), but this is very light.

Raleigh RX24 - 24" Kids Road/CX Bike Project

Constraints breed creativity.  Or so they say.  Well, my bike storage is quite constrained.  And there are these growing kids with bigger and bigger bikes -- and expectations that they get to share that limited space.  So, while being constrained to 3 bikes sounds an extravagance of 2 (or 3) bikes to most of the world, it has made me hone those choices to maximize the range of cycling activities I'm equipped to enjoy.  I still change things around every now and then, but I've pretty much figured out (and designed) exactly what I want.  And so, if changing my own bikes doesn't sound at all appealing, how do I find outlet for my desire to fritter away time and money researching and sourcing bike components for exciting and different bike builds?  The kids' bikes, of course!

My older son has been saying (or said at least once, which was all it really took to seed the plan) that he wanted a road bike.  And he's agreed to give the juniors cyclocross team a try when that becomes an option next year (we suggested that he needs to pick something one sport-like thing to do for extra-curriculars next year, and this was the least offensive to him).   We might be a year and a size of clothes away, but it's obviously time to start on that road/cx bike project!

The Field

There are a few road bikes for kids.  Specialized Allex, Felt F25, Scott Speedster 24.  But if this really is to do dual duty, it needs to be a CX bike (have clearance for 'cross tires).  And, so this rules out the road bikes.

In the world of 24" kids 'cross bikes, there are also a few options.  Most have rim-brakes, but there are a few disc-brake options.  (Disc brakes are pretty much a hard requirement for me on this bike, for reasons that will become clear.)  But standing out head-and-shoulders above the other options, value-wise, is the Raleigh RX24.  So - spoiler alert - that is what I bought.

(Here's a good survey of kids CX bike options: https://rascalrides.com/the-best-kids-cyclocross-bikes/)

Project Overview

So for most people -- smarter, less frivolous people -- adding the $340 RX24 to cart and having it arrive 2 days later (Amazon Prime), would be the end of the story.  The bike, after all, has a solid set of components spec'd; it doesn't really need to be changed at all.

That's boring.

Surely we can find something that "needs" changing.

The highlights of what I set out to change / improve:

1. The wheels -- and the wheel size.  Not only build something that will be better and lighter, but rethink the wheel dimensions to work better for a road-bike setup.
2. The brakes.  The stock brakes would probably be fine, but for smaller/weaker hands, I'd like to improve the braking performance.  Especially if thinking about road speeds.
3. The drivetrain.  Switch to a drop-free 1x setup.  Expand the gear range and top-end speed.

And a sidecar goal to these overhaul changes was also a desire to reduce weight.  At claimed 21lbs (actual 22lbs, w/o pedals) this bike isn't excessively heavy, but I think is a few lbs heavier than it should be.  And when that's 1/3 of your body weight, saving some weight makes a noticeable difference.

And of course to do this all as economically as possible.  It's coming out of my fun-money account, which means these things have to compete with other bike-related purchases.  Informally my goal is to get this to something that is comparable (in terms of features, maybe weight) with the Isla Luath 24 Pro Series, but costing significantly less.

The Wheels

A tangent on wheel size

So, there are actually two different, incompatible wheel sizes being used to refer to "24-inch" bicycles.   The definitive online reference on this is Sheldon Brown's page, but the short-version is that we have bikes like the Isla Luath 24 that are using the kids mountain-bike 507 ETRTO size and bikes like the Raleigh RX24 that are using the British youth 540 ETRTO size. ... Also shared by wheelchairs.

That 540 size is pretty tricky if you actually want to find tires.  Well, other than wheelchair tires.  And this difficulty provided inspiration for one of the key goals of this bike project.

Can we change the wheel size?

Since this is a disc-brake bike, there's nothing stopping you from running a smaller wheel (well, almost -- see below).   The ETRTO number refers to the diameter between the bead shelves on the rim.  A 540 (mm) ETRTO is obviously quite a bit larger than a 507mm ETRTO that is the more common size, so putting smaller wheels in this frame is certainly an option.   But frames are designed with bottom bracket drops/heights for a specific size wheel, so while it may work fine in theory, in practice it might increase the risk of pedal strike (pedal hitting the ground when turning) if you drop the bike by 16mm ((540-507)/2).

There's another option, which would be fitting larger wheels in the frame.  As it turns out, the very common 26" MTB wheel size is not that much larger -- its ETRTO is 559mm.  So 9.5mm increase in radius.  But for a road tire, that might actually work out perfectly.

The nominal size of the stock tires is 37-540  (37mm tires), which theoretically indicates an overall diameter of:

37 * 2 + 540 = 614

If I were to fit a set of 28mm road tires on 559 rims, the theoretical diameter would be:

28 * 2 + 559 = 615

That's pretty close.

In practice, though, nominal tire sizes and measured tire sizes are only loosely related, so this isn't quite as cut and dry as it looked on paper.  (Spoiler alert) In practice everything measured smaller than claimed, so the new wheels are maybe a few mm bigger -- but it all works out just fine.

The Wheel Build

I'm always looking for an excuse to build wheels, so this is certainly a fun part.  Sourcing parts for a 26" road wheel (esp. w/ quick-release dropouts) is actually really easy.  For one thing evolution of trends plays into this nicely: older MTB rims were quite narrow by modern (MTB) standards -- and looked much more like what are now being sold as road-width rims.  And the using QR for disc-brake wheels is definitely a thing of the past, so it's quite easy to find hubs on clearance, etc.

Most 26" (MTB) rims, though, do tend to be drilled for 32 spokes.  I really wanted a 24-spoke wheel.  This is a road bike, after all.    Luckily there were some lightweight rims drilled 24h and 28h for XC racing.  I picked up a set of 24-hole Sun UFO rims for $50 shipped; these are eyeleted rims which weigh around 370g each.  For hubs, I turned to BDOP cycling clearance bin and picked up a Novatec D612SB and D711SB hub for around $70 total.  I laced these up with Sapim Laser spokes.  At 1377g, they are certainly some of the lightest "normal-sized" wheels I have built.

So, that wheel build was very economical [by wheel-build standards, anyway!], I ended up going completely off the deep end when it came to tire choices.  To be fair, the options for road tires in this size are fairly limited (though vast in comparison to the stock wheel 540 size).  After looking at ~$50 prices for a Schwalbe Pro or Conti Grand Prix tire, I ended up buying $78 Compass Elk Pass tires, which are absolutely the nicest tires any 26" road wheel could have.   And completely excessive.  But I kinda love my Compass tires and I'm happy to support this smaller outfit.

On these rims, which measure 17mm internally, the 32-559 Elk Pass tires measure around 27-28mm high.

I also picked up a set of the Isla Greim 26 tires for when this swaps to 'cross season.  On these rims these 31-559 tires measured ~30mm wide and~29mm high.

Tire Clearance

As hinted at earlier, the nominal size of the stock tires (37-540) was larger than they measured (closer to 32mm), but in the end even the Isla CX tires fit fine  Here are a few clearance photos with the Compass tires:

10mm of side clearance in chainstays.

I used some 10mm-thick rubber tracks to provide a visual on clearance.

Demonstrating that ~10mm of extra clearance exist at seatstay.

Plenty of room in fork.

Demonstrating clearance at chainstays (bike is upside down).

And here are a few with the Isla/Greim tire:

~8mm of clearance to the seat tube.

~8mm clearance from chain stays.

Photo of chainstay clearance.

So, these both fit quite comfortably, which is great since they should address the complete set of purposes for this bike.

The Brakes

If money were no object, I would have replaced the SRAM brake/shifter levers with a full-hydraulic 1x setup.  But that would be a $400+ investment, and that seemed a bit much even for me.  Plus, even though they offer reach-adjust, the SRAM road hydraulic brake hoods are really big for someone with such small hands.

At first I considered TRP Spyre calipers, but in the end decided that these probably weren't going to offer much of an improvement in braking over the stock Promax 300R calipers. 

I then turned my attention the cable-actuated hydraulic calipers.  These should provide better bite than stock calipers but would work with the existing SRAM S500/Apex lever set.  The TRP Hy/Rd was an obvious choice, but upon further research discovered the Juin Tech R1, which was not only cheaper but also was quite a bit lighter (as light as TRP Spyres).

The Juin Tech R1 calipers are also rebranded as the Yokozuna Motoko -- and I found a set of these  domestically for $160, which was quite close enough to the $150 that the Juin Tech R1 cost -- plus they would include the notoriously good Yokozuna Reaction housing (which sells for $75 by itself).

I did order separate Avid HS1 rotors, though.  The Avid rotors offer excellent stopping power and they're inexpensive (I got them lightly used on ebay for $20 for the set, though Amazon also has them new for not much more).

I haven't actually ridden this bike, but from what I can tell these do a great job stopping the bicycle.  I gave my son instructions for bedding in the pads/rotors; I think he followed them.

(I should probably mention that the brakes added a little net weight to the bike.  The stock Promax 300R calipers were around 130g, compared to the 142g for the Motoko calipers.  But the point here wasn't to save weight, but to improve braking.)

The Drivetrain

The goal was to expand the gear range, move to a modern 1x system (drop the bash guards sandwiching the front ring), and reduce system weight.

The stock bike came with a 32t chainring and an 11-32t cassette.  I found a lightly used XG-1080 11-36t cassette on ebay for under $80, which is a lot for a cassette, but they aren't quite wear-and-tear parts on kids bikes.  I paired that with a Deckas 38t, narrow-wide chainring.

And swapped out the Apex WiFli rear derailleur (max 32t cog) for a used SRAM GX short-cage, clutch rear derailleur (max 36t cog).

So we moved from a 291% gear range to a 327% gear range, while lightening up the overall system and quieting it down (from chain slap).

Other Odds and Ends

I found some used Bontrager 38cm carbon XXX bars on ebay for $40, which weighed about half of the stock 36cm bars with compact drop/reach.

The 60mm Uno stem didn't save much weight, but I had it on hand, and I like them.  (I've actually ordered a 40mm Wren stem to pull the bar in a bit, so I'll swap that in when it arrives.)

I kept the stock cranks, but replaced the square-taper bottom bracket with a "Week Eight" titanium + alloy model available on ebay for $39.  Generic square-taper bottom brackets are ridiculously heavy.

For the saddle, I did some research and settled on the WTB SL8 as a good option to anatomically fit young riders (narrow and not too long).  At $90 this was one of the more expensive components, but it seemed like a decent investment.  The stock saddle on the RX24 is probably one of the worst parts of the stock spec.

The seatpost is a Hylix 27.2mm carbon post.  Quite inexpensive on ebay/aliexpress.

The Final Product

I'm really happy with how it turned out.  The wheel size upgrade is a huge win for tire options/versatility -- and the wheels are much lighter.  The brakes are a nice improvement (or so I'll just have to assume based on all reviews) while using the existing levers and the drivetrain is expanded and lightened up.

Ready in all his mountain-biking gear to test out this road bike!

In terms of the weight savings, we got the bike down from 22.0 lbs to 17.3 lbs (both weights w/o pedals).  My son helped me do some part weighing so we could see what were the biggest wins weight-wise.  This is obviously not comprehensive, but there were a few surprising items.  There are certainly many places to save further weight, if someone were so inclined (and less budget constrained) -- e.g. the crank arms.  Unfortunately, there's no option that I'm aware of for a 3rd party carbon fork at this size; that would likely save significant weight.

Part	Stock Part Weight (g)	New Part Weight (g)
Stem	115	91
Saddle	321	189
Seatpost	294	156
Bottom Bracket	305	135
Cassette	329	213
Rear Derailleur	208	266
Handlebars	356	181

This was a fun project.  I don't think there is anything at this point that I would do differently, though certainly if I wasn't trying to stay in any sort of budget, I would have found a way to do full hydraulic brakes.  As it is, my son still isn't comfortable using the brakes from the hoods; it's just not enough leverage to quickly stop the bike from there.  Hydraulics, if he could fit his hands around the larger hoods easily, would not have that problem.  But he is happy to use the drops and I've positioned the bars a bit higher / less-aggressive to make that more comfortable.