[earlan357]

My 8-year old has been asking for a full suspension bike pretty much the minute after he learned to ride. Unfortunately nothing good really exists outside of spending $1,500 for a bike he'll outgrow in a year. I thought building a bike would be a great project that I could share with little man so after scouring classifieds for months I finally found a 2010 Giant Yukon FX 26" frameset in size X-Small from a seller near Whistler. Looking at the geo, with a reach figure of 380mm it fit in between his current 24" Hotrock hardtail(reach 350mm) and a modern x-small 27.5" (reach 400mm). It's short, high, and steep, but since it's for a kid on mostly single track, I wasn't worried about the outdated geo. I figured 24" wheels and a slammed dropper would reduce the standover enough for my son to comfortably mount with his 24.5" inseam. With the saddle forward on the rails and a short stem, we had room to extend the cockpit and go to bigger 26" wheels as he grew. It also came with a well-used Fox F100 fork, incomplete Shimano MT-315 brakes, and some other assorted parts. While I waited for it to clear customs, I started putting together a parts list. I had a set of Tektro Gemini disc brakes leftover from my Stumpjumper and the 140mm crankset and dropper post from his current bike. 24" disc wheels are nearly impossible to find so I jumped on a set from Ebay. 26mm id rims are rare in this size and matched up nicely with the Maxxis DHR2/Rekon combo. He wanted tires that matched dad's bike. The hubs are by ARC, a generic brand comparable to Novatec. I'll have the wheels re-laced onto some better hubs when they inevitably wear out.

When the frame arrived I tore everything down and gave it a good cleaning and inspection. As expected from a bike from near Whistler, the frame had a lot of nicks and scratches but it was straight and the welds looked good. The worst wear was from where the chain had probably come off the original 3x chainring and gouged up the chainstay's main pivot area. The suspension bearings were also mostly shot so I ordered a full set of Enduro Max bearings. I also made a set of bearing press tools at work while I waited for them. The new bearings were a breeze for him to swap out, demonstrating the value of having the right tools.

The external routing had exposed cables so I used a screwdriver guide to extend a drill bit to open them up to allow cables to fully pass through. The guide could sit fixed against the frame while the bit/adapter spun freely inside to prevent further scratching the frame.

Looking at the 70.5 degree head angle, I ordered a -2 degree EC44/EC44 angleset from Works Components. I also ordered a set of 40-series headset bearings from CaneCreek. The bearings from Works have crappy seals and mine have gotten notchy within a few rides. The Works bearings also use 45 degree inner bevels so I ordered a 36 degree CaneCreek tapered to straight steer crownrace and compression rings to match the new bearings. I had my son turn the press while I confirmed and held the alignment of the cups. An easy job with two people. I also found a Shimano Zee 10-spd rear mech, Deore shifter, and an SLX 11-36 cassette. The Freeride version of the Zee can accommodate up to 36T cassettes, plenty for a kid's bike, and the short cage tucks up nicely. Plus it's a DH derailleur so it should be pretty bullet proof. I reused his 140mm BMX cranks and 30T oval chainring. I'm a big fan of shorter cranks and oval rings on kids bikes since they tend to run their saddles low and this combo puts less stress on the knees. As an aside, I love how nice 10 speed parts can be had for below entry level 12 speed prices. We replaced the worn square taper BB with a new one. The chain was lining up with 6th gear. Since I was using 140mm cranks, the extra distance from the chainstay allowed me to use a narrower 115mm BB for slightly reduced Q factor and brought the chainline even with 5th gear. Perfect.

The fork had a lot of scratches on the stanchions. I picked up a seal kit and a generic 32mm seal driver tool. Since the new seals are a flangeless design, I would hate to think of how much a pita it would be to install them without this $20 tool. I sanded the burrs off the scratches, filled them with acrylic, and wet sanded and polished them until they were smooth. We changed out the lower seals and replaced the air and damper side oil( Fox10wt Green) which it needed badly. While I had it apart I dropped the forks travel spacer to the 80mm position. We topped it off with a blue decal set from Fox.

The shock is a Giant branded air shock, but it was in surprisingly good shape. I ordered a 3mm offset bushing and machined a 7mm nylon spacer to go above the stanchion top out bumper. This reduced the rear travel from 100mm to 80mm to match the front and lowered the BB offset from -10mm to -34mm. BB height is about 277mm. The shock is an older model with no negative air chamber, so reducing the stroke didn't change the characteristics enough that we could tell. My son gets 20% sag at 70 psi. At this pressure, rebound is very effective so no concerns with harsh top out. At his request, we put the Fox decals on the Giant air shock to make it match the frame. We'll replace it with a newer shock when he's ready for more travel.

Everything was going pretty smoothly until I realized I'd made a major oversight. The suspension pivot bearing runs right through the middle of the seattube. This limits the max insertion depth of the dropper to 170mm. His current dropper (Brand-X Ascend Kid's 70mm) was over 200mm below the collar I found a Lev 80mm dropper in 30.9 with a 165mm max insertion depth. KS Levs have adjustable air pressure and are one of a handful of droppers a sub 60lbs rider can activate. Perfect, or so I though. As it turns out, the seat post was only honed 135mm deep, and the seatpost was hanging up on a step in the seattube. So I popped in a fixed seatpost as a place holder.

After assembling everything, we took the bike outside for a quick bounce test. We noticed the Fox fork was way to progressive. It seemed like the airspring and coil negative spring tune was way too heavy. Set at just 50 psi, my 190 lbs still had trouble getting it to bottom out and the too-strong negative coil was "sucking down" the fork. I pulled the airspring shaft, shortened it 20mm and drilled a new hole for the piston. This increased the size of the air chamber so I could run higher pressure to combat the negative coil and still get full travel, but the fork just wasn't great. Talking to a suspension tuner at work, we came to the conclusion that the fork would be better served if I shortened the airspring shaft another 20mm and reduced the stroke to 55mm. I machined a 25mm travel spacer and swapped the fork over to my son's old Hotrock and replaced the heavy Suntour spring fork. In that application it was perfect but I'd need to swap the rim brakes to disc. Fortunately I had the MT-200s that came with the frame. I installed an old set of 160mm rotors and put the old bike up on the classifieds. A very happy dad and son are the new owners.

Back to the Yukon, I snagged a mint condition Manitou Markor 26" 100mm fork. When it arrived I installed the 80mm travel spacer. The Manitou is very smooth and is much better suited to a lightweight rider and the steer tube was previously cut just long enough. We also ran into a problem with the Tektro brakes. They just couldn't generate much stopping power despite new SLX 160mm rotors, Shimano B03S pads, and a full bleed even after I bedded them in. I ripped off the Tektros and sourced a set of MT-500 brakes. We could use the same pads and rotors, and he could take advantage of the shorter reach and extra power of the servo wave levers. Being able to stop the bike on command with one finger is a huge confidence booster compared to death gripping rim brakes on.

With such a steep head angle, I was afraid the 35mm stem was going to make it too twitchy. The EC44 lower cup of the -2deg headset and slightly longer fork axle to crown also increased the stack height on paper, but swapping to a CaneCreek bearing and crown race (1-1/2"-1-1/8" reducer) meant the fork sat a lot deeper into the lower cup. All in all, it only added 6mm to the A2C which is mostly offset by the slacker angle.

Since he'd outgrown his SDG Fly Jr saddle, I picked up a NukeProof Urchin seat which he finds more comfortable despite the flat profile. Thin ODI Ruffian grips with engraved collars, RaceFace Ride shin-friendly plastic pedals, and a new handle bar trimmed to 620mm finished off the cockpit.

All that's left to do is solve the seatpost depth problem. As it sits now, the seat slammed is still higher on the 14" frame than his old 70mm dropper fully extended on the old 11" framed Hotrock. I found a used 30.9 seattube reamer for about what my local shop would charge to ream it, but it's in Germany and still a week or two out.It's currently too cold to ride the trails so he's been riding around the neighborhood with the seat in "dirt jumper mode" and hopping curbs with reckless abandon.

I've easily spent as much if not more as a new bike, but I got to spend a lot of time with my kid, taught him some stuff, and we have a kickass bike that he can ride at least until middle school.

Current Build List:
Frame: 2010 Giant Yukon FX x-small
Headset: Works -2 angle set EC44/EC424w/ CaneCreek 40 Headset Bearings and Crown Race
Fork: Manitou Markhor 26" 100mm set to 80mm
Brakes: Shimano MT-500 w/ Shimano B03S Resin Pads
Rotors: Shimano SLX RT66 160mm
Shifter: Shimano Deore 10-Spd
Grips: Odi Ruffian 100mm Lock-On
Dropper Remote: Giant Connect
Stem: Funn Equalizer 35mm Length 10mm Rise
Bars: Brand-X 15mm rise 620mm wide
BB: Shimano U300 68x115 Square Taper
Cranks: Jet 140mm 104 BCD
Pedal: RaceFace Ride Composite
Chainring: OneUp Oval 30T
Chain: KMC
Cassette: SLX 11-36 10-Spd
Derailleur: Shimano Zee Freeride 10-Spd Med. Cage
Saddle: Nukeproof Urchin
Seatpost: KS Lev 30.9 80mm (eventually)
Wheelset: 24” SunRingle MTX 33 26mm ID 32H
Hubs: ARC MT-001 100/135 QR
Tires: 24” Maxxis DHR2 front, Rekon rear
Shock: Giant Air 165x38 w/ 7.7mm length/stroke reduction
Offset Bushing: 3mm
Pivot Bearings: Enduro Max
Bottle Cage: Bontrager Sideload

Current Geo: BTW, this is a great site if you want to play around with geo numbers. bike-stats - everything about bike geometry

Update:
The 30.9 seatpost reamer arrived from Germany. The retaining bolt was too long and hitting the linkage bolt tube. I found an M16x1.5 bolt at the hardware store and faced down the bolt head on a lathe.

A few quick turns and voilà, full dropper insertion.

Also found some nifty blue sag rings.

 

[brentos]

Wow, this is great. I am going through a similar project with my son and this is a great reminder to keep him involved with each step of the process.

One question (I'll have more eventually). Did you get any pictures of the air spring shaft and your modification? I'm fighting the same thing and looking for a way to A) Shorten the Coil Negative Spring, and B) Increase air volume.

I had considered increasing volume with Schrader valve extenders on the top, but it seems hokey.

 

[earlan357]

Wow, this is great. I am going through a similar project with my son and this is a great reminder to keep him involved with each step of the process.

One question (I'll have more eventually). Did you get any pictures of the air spring shaft and your modification? I'm fighting the same thing and looking for a way to A) Shorten the Coil Negative Spring, and B) Increase air volume.

I had considered increasing volume with Schrader valve extenders on the top, but it seems hokey.

I don’t have pics but here’s what it kind of looks like mocked up using a Rhythm 36 air shaft, a 20mm spacer I made, and a random spring to stand in as the negative coil. See how the white piston is secured with a roll pin? I punched out the pins, pulled everything off, slipped the spacer between the negative seat and a travel limiter also secured with a similar pin. I chopped off the end of the shaft and drilled a new hole for the piston pin. I shortened it 20mm, which worked out to about 12 CCs, similar to removing 2 tokens.

 

[earlan357]

I work in Nascar and I asked one of our shock dyno operators for some advise on the Fox shock. He basically worked out that the negative coil was too heavy. A lighter spring would require a custom spring and small bump would suffer due to the reduced force fighting stiction. Instead he recommended keeping the heavy spring, allowing it to act as a negative throughout the majority of the stroke by reducing travel below 60 mm. It would be supple but ramp up appropriately for a sub 100 lbs rider. But that would reduce the axle to crown too much. That’s why I moved it to the old hardtail and bought a Markhor. Really nice for the money and just works better at lower psi.
Rear shock was easy since there’s no negative air chamber. 10 minutes on a lathe with some nylon 6/6. I matched the shaft diameter, but made it wider than the rubber bumper so the bumper wouldn’t inadvertently stretch over the spacer when squished.
If you have a Rockshox shock or similar that uses an external topout bumper, I can make you a stroke limiter to try out.

 

[Kookmyers]

I’m impressed and jealous. Great work! I’m waiting on a 24 frame for my 7yr old‘s next bike to replace his 20. We went hardtail. Figured we’d get a year out and then switch to full. This one‘s costing a fortune, and you may have read my woes about what the 100mm fork is going to do to the geometry but lets not rehash that here!

 

[brentos]

I don’t have pics but here’s what it kind of looks like mocked up using a Rhythm 36 air shaft, a 20mm spacer I made, and a random spring to stand in as the negative coil. See how the white piston is secured with a roll pin? I punched out the pins, pulled everything off, slipped the spacer between the negative seat and a travel limiter also secured with a similar pin. I chopped off the end of the shaft and drilled a new hole for the piston pin. I shortened it 20mm, which worked out to about 12 CCs, similar to removing 2 tokens.

Nice. So to add volume, without changing travel, I simply have to remove roll pin, cut shaft, drill new hole for role pin, reassemble.

It seems in some configurations this will also shorten the shock at top out, but others it will not. We'll have to open his up to see what it has!

Thank you!

 

[brentos]

Following up on the Fox fork setup for a light rider.

My son’s bike has a 2012 Fox F100 FIT RLC. The issue was that it a) ramped up to quickly and b) the negative spring would eat up 20 mm of travel when set to the 45 psi appropriate for his weight.

I tore it apart to tale a look at the air spring and see what I could do. A simple change to the negative spring perch to the 120mm position, with a 10 mm spacer to drop travel back down to 110 seemed to mostly resolve the issues. It required removal/reinstallation of the roll pins holding the piston and negative spring perch. It now settles around 90 mm and progresses less, much because there was at least 10cc float fluid on top of the piston. I put 5cc back in, giving more volume.

He hasn’t ridden it yet, but it seems to have gone from terrible to pretty awesome.

I did check damper travel and Fox warns to not extend their older 100mm forks, but damper travel topped out right on the hairy edge of 120mm, so I set it to 110mm. At his pressures, it would never top out, but I didn’t want to worry about it and come back to reduce travel when he when he needs more fork pressure.

The end result for the air spring including 10mm spacer is pictured above (110mm travel configuration). Note the added 10mm spacer between the spring perch and the negative coil, and the spring perch moved left 20mm by relocation of the spring seat/roll pin.