Saturday, June 25, 2016

24" MTB Build: "Kona Queso"

It all started with a free bike, a 24" Kona Hula (12" frame) that my friend's son had outgrown.  The frame is really on the large side for my son, who is only a year into his Isla Bein 20" Small.  While the bike as it was was a perfectly ridable bike, I decided that we would turn this into a bike-building project.  My son loved this idea.  And we set right to collecting parts for this too-large bicycle, because to do anything else would have required patience and deliberation.

The project goals were pretty simple:
  1. To have fun personalizing and customizing the bike.
  2. To work on it together (hopefully he'd learn a bit about the mechanics of it)
  3. And to drop 10lbs from the finished product (original weighed in at 30lbs; goal was 20lbs). 10lbs is pretty significant to a 50lb rider.
The constraint was to try to stay within budget of what his next (24" bike) would have been.  Luckily my wife and I have separate allowance funds; this way the discretionary expenses (the majority of the costs here) wouldn't cause marital discord :)

The plan for the bike was:

  • Convert to disc brakes.  This was bold, but research suggested there were some options that didn't involve welding mounting tabs to the [aluminum] frame.
  • Convert to rigid fork.  The affordable kids suspension forks don't actually compress for little 50lb children.  The high-end forks would exceed the budget (and how!).
  • Convert to 1x10 drivetrain for simplicity (and weight savings) but with enough range to ride any terrain around here.

The Frame

Here's the original frame.  It's a Kona Hula, a 12" frame size according to the sticker.  Made of 7005 aluminum alloy.  Thankfully, very standard specs: 27.2mm seatpost, 1 1/8" headtube, 68mm BSA (threaded) bottom bracket. 

"We" (I) decided early on that we'd convert this to disc brakes, so we ensured we couldn't change our mind by sawing off the brake bosses.

I removed the material with a dremel.  Luckily my friend Kurt has a pro-grade workshop and helped grind this down to finish this like a pro:

I drilled out the cable stops so we could run full-length housing for the brakes and derailleur.

I let my son choose the color.  We used Nova Powder Coat which uses (among others) Prismatic Powders; my son chose Hot Yellow.  Good choice, kid.  (I encouraged a high-viz color choice.)

Part of the fun was getting to bike down to Chantilly to pick it up.

The finished product really looked fantastic:

We got some Kona stickers off eBay.  They're not the same as the original ones, but they look good.

The Brakes

Before starting this project, I had no idea that people actually converted rim-brake frames to disc-brakes.  (Often the preference is to run a "mullet" with disc front and rim-brake rear.)  I decided to investigate, though, since I had a set of cable brakes from my commuter (before switching to hydro) that I wanted to use. I learned that there are several ways to convert a rim-brake bike to disc-brakes; the one I opted for was the A2Z DM-UNI adapter.  Found one for $20 from the UK, which was a lot cheaper than the domestic options (mostly ebay).
Before we sawed off the brake bosses, needed to make sure that this idea would actually work.  After spending some time with a file flattening out the dropout lip on the inside of the dropout, the adapter sits flat against the dropout and fits quite well. However, you can see that there's some space at the bottom (between the lower left bolt and the frame):

While this wouldn't be a problem for regular braking from forward motion, it would be a problem if applying the brake while rolling backwards.  That didn't seem like a great state of affairs.

Without any fancy tools, my solution was to hunt down some big aluminum washers in just the right diameter that would eat up the extra space.  This actually looked like it would work fine, if a little ugly:

Kurt rescued me again here with a much better idea, involving drilling through the frame and tapping out the thick aluminum backplate to the adapter for an M6 bolt.  This is a much swankier solution.  You can see the extra M6 bolt right above the QR skewer:

(A little preview of the finished product there.)  You can see in the photo above that I'm using TRP Spyre disc brake calipers.  This is worth mentioning, because these are road brakes, not mountain bike.  This actually is perfect for a build for a little person, though, since BMX brake levers are (typically) short-throw, just like road levers.

The Fork

I spent awhile investigating the options for forks.  There are a few suspension forks that are decent for this size wheel, but they're pricey.  The off-the-shelf suspension forks don't compress under the weight of tiny people, so they're really just good for weighing down the bike.  Many people also use 26" forks.  I measured the axle-to-crown distance on the suspension fork that was on the bike; it was around 425mm.  After scouring the various carbon fiber forks on ebay, I found a 26" fork with a 415mm axle-to-crown measurement, so while this might be a little taller than perfect for the 24" wheelsize, it was going to be 1cm lower than the RST fork it was replacing.

Critically, this fork was available in tapered or straight steerer.  We chose the green logo, which locked in our color scheme.


I am really impressed with this fork, actually.  The finish is top-notch and the weight is pretty incredible at 421g.

I would consider using this on a CX build (only slightly taller than the standard 400mm axle-to-crown measurement of a CX fork).  If only it came with thru-axle.

The Drivetrain

After doing a bit of research, I settled on a Shimano Deore SL-M591 (10-speed).  My son has SRAM (X4) grip shift on his current bike, which works fine, but folks on the interweb suggested that Shimano trigger shifters were easy for little hands -- and I found someone selling a take-off shifter and derailleur combo for $60.  Done!


I perplexed for a long time over the crankset.  I wanted one with short arms.  Ideally something like 145mm, which is what he has on his Isla 20" bike now.  Most cranksets with arms that short are BMX cranksets.  While a BMX crankset would work, it is hard to find BMX cranksets with 4-bolt 104mm BCD, since typically BMX chainrings are much bigger than the 30-32t I was hoping to fit.  Also BMX chainline is different from MTB; BMX bikes have narrower rear spacing.

In the end, I decided to splurge and buy the Trailcraft 1x crankset which includes a 30t ring.  The arms are a little longer, at 152mm, but this seemed fine.  Of course, we went with a green chainring to keep our color theme going!

Now this is a crankset that needs a square-taper bottom bracket.  The standard Shimano square taper bottom brackets are really heavy (300+grams), so I was really fortunate to find someone selling a used Race Face Taperlock BB on eBay for $25.  This has a titanium spindle and weighs in at a paltry 165g.

For the cassette, I really went overboard and picked up a (used) XTR 11-36t cassette.  While I would never buy myself a titanium wear-and-tear part like that, I figured that as lightly as he rides it'll probably last through both kids and while [the used one] cost me twice the price as a [new] standard cassette, the weight savings are huge (230g for the XTR vs ~400g for a standard SRAM PG1030).

The Wheels

I should really have led out with a discussion of the wheels, because this was really the keystone of the build: or at least the reason we were converting to disc-brakes.  I really wanted to build a wheelset using Stans Crest 24" rims and the leftover 28-hole Novatec D712SB / D771 hubset I had (the rear couldn't be converted to 11sp). 

We used Laser spokes and alloy nipples.  This is a very light wheelset.

This was one of the more fun parts of the build for me too -- and I think for my son; he did the bulk of the wheel lacing (with help & instruction).

We ordered Schwalbe Rocket Ron tires from Germany (it was cheaper than domestic prices, even paying 20EUR for shipping).  There aren't many tires options for 24" wheels; the Rocket Rons are generally agreed to be the best.  Mounting them tubeless was super easy, thanks to the excellent Stans rims.

Cockpit & Other Bits

Not too much to say here.  I traded a Ritchey WCS alloy road bar from my parts bin for a Easton EA90 SL carbon flat bar.  Another example of something I'd never buy myself.  My son helped cut them down a little narrower.

Found some lower profile ODI grips with green endcaps.  Probably could have gotten something even narrower for his hands, but these seem to fit fine.  Picked up some green BMX levers. 

For the seatpost, I was gonna use my old carbon Bontrager XXX seatpost, but it doesn't slam down far enough for him right now (yeah, the bike is a bit large), so we're using an Easton EA70 seatpost I had in the bin.

For saddle, I floundered around a bit.  Originally I found a great deal on a Bontrager Kovee Elite (ti rails), but this proved to be way too long a saddle to work.  Eventually I found the Selle SMP Junior Lite saddle for 50% off in the UK and ordered it. A bit pricey, but I think it'll be moving between bikes (and between kids) for many years.  It's probably more tailored to road riding, but my son says he likes it (we haven't been on any long rides yet, to be fair).

Finished Product

My son named his bike "Kona Queso", since the yellow/green/red color scheme was like District Taco and apparently my son can't get enough of DT queso :)

The weight came out better than expected at 17lbs 12oz (8.05kg).

Osiris (co-owner of District Taco) hooked us up with a DT sticker, which we cut out and placed on the toptube:

We haven't done any long rides yet, but it's been ridden a whole bunch since we finished.  Including some light mountain biking (with little brother who has just started riding the little Isla CNOC 14).