It feels like every interesting home-brewed bike is coming out of the woodwork for this year's Crankworx. There's been no shortage of interesting creations floating around the show, and a pair of unusual-looking kids bikes caught our eye in the lineup. Alongside them was a garage-built high-pivot, all made by the same man. David has a background in mechanical engineering, most recently working at Space X designing things that are slightly more complicated than bicycles.
David spent time building bikes at his alma mater, Cal Poly San Luis Obispo (same as me), and found himself returning to the craft after wanting to experiment with the burgeoning trend of high-pivot trail bikes. That was a couple years ago, and now that the trend is fully en vogue, his bike is looking more current than ever.
He designed his all-mountain bike to be low-slung, slack, and fun on the descents. Taking geometry cues from his prior bike - a Transition Sentinel - he added some personal touch and some math to get the resulting frame. Kinematic info can be found in this Instagram post
, and the pertinent numbers are as follows.
Head Tube Angle: 63.5deg
Seat tube angle: 76deg (straight)
BB Height: 346mm
Crank Length: 165mm
Wheel Size: mullet 29/27.5
Travel: 145mm rear / 160mm front
Weight: 34.5lbs, 11.5lbs frame & shock
David did the machining and welding in his garage, and managed to use off the shelf parts for most of the linkage hardware, as well as the idler pulley. He's hoping to increase the torsional stiffness on the next iteration, as well as increase the BB height slightly. Another long term goal is getting a better sense of the design loads that come into play on the bike, primarily around the rear shock and linkage architecture. Those data points can be hard to figure out without proper testing equipment, so he feels that his current form is a bit overbuilt.
The kids' bikes are a little simpler, but took just as much thought and effort to bring to reality. Both are the same size, but due to their adaptable geometry the fit numbers can change quite a bit. Most of this adjustment is achieved by using a square headtube, not dissimilar to the racetrack shaped hardware that Guerilla Gravity uses on their Revved frames. Add to that sliding chainstays, some custom cut and tapped cranks, and a healthy dose of used parts off the BuySell, and you've got two very sick bikes for the kids.
The little frames can fit a water bottle; take notes, adult bike brands. David implemented a simple single pivot suspension design that was made to be as progressive as possible within the space allowed in the frame, resulting in a slightly over linear leverage rate.
Weight wasn't a primary concern in the initial build, instead aiming for bikes that rode well, grew with the kids, and didn't break the bank when all was said and done. The bikes can be run in 24" mode, or swapped to 27.5" by running a 140mm fork, fully extending the dropouts, and flipping the headset to the longer setting.
As we were shooting photos, one of David's kids mentioned the comments they've been getting in the lift line at the resort: "Everybody always asks about the square thing."
Well, here's hoping this clears some of those questions up.
Travel: 120mm (F) and 120mm (R)
Seat Tube: 300mm
ST Angle: 76deg
HT Angle: 65deg
Effective TT Length: 501mm
HT Length: 90mm
Chainstay Length: 410mm
BB Height: 305mm
Crank Length: 143mm
Travel: 140mm (F) and 125mm (R)
Seat Tube: 300mm
ST Angle: 75.5deg
HT Angle: 64.5deg
Effective TT Length: 525mm
HT Length: 90mm
Chainstay Length: 426mm
BB Height: 350.4mm
Crank Length: 155mm
: 8lbsTotal Weight
More images can be found in the high-res gallery here