Homemade Adjustable Kids Bikes and a High-Pivot - Crankworx Whistler 2023

Jul 27, 2023 at 0:02
by Dario DiGiulio  

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.

High single pivot with an idler.

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.

Chris King BB serving as the main pivot.

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
Reach: 425mm
Stack: 599.5mm
Chainstay: 436mm
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.

Square section tubing gets the job done.
Undo the preload, flip, and presto - new reach.

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.

24" Mode
Travel: 120mm (F) and 120mm (R)
Seat Tube: 300mm
ST Angle: 76deg
HT Angle: 65deg
Effective TT Length: 501mm
HT Length: 90mm
Wheelbase: 1073mm
Chainstay Length: 410mm
Stack: 546.5mm
Reach: 365mm
BB Height: 305mm
Standover: 557.3mm
Crank Length: 143mm
27.5" Mode
Travel: 140mm (F) and 125mm (R)
Seat Tube: 300mm
ST Angle: 75.5deg
HT Angle: 64.5deg
Effective TT Length: 525mm
HT Length: 90mm
Wheelbase: 1117mm
Chainstay Length: 426mm
Stack: 559mm
Reach: 380.5mm
BB Height: 350.4mm
Standover: 602.1mm
Crank Length: 155mm

Frame Weight: 8lbs
Total Weight: 29lbs

Very impressive work for a production run of three.

More images can be found in the high-res gallery here.

Author Info:
dariodigiulio avatar

Member since Dec 25, 2016
123 articles

  • 95 1
 Rad steel homebrew bikes. You love to see it.
  • 43 1
 34lb build is actually pretty amazing for a homemade steel full suspension. That's around the ballpark for what most peoples long travel carbon rigs come out to.
  • 29 10
 I don't believe that figure
  • 37 2
 It's the components. People often look at total bike weight and mostly attribute a heavy or light total weight to the frame, despite the frame being a third to a quarter of the total weight.

This bike is built mostly from a mix of XC, trail, and all-mountain category components. If I'm not mistaken, that's the XTR 11-speed compact drivetrain, XC carbon flat handlebar, foam grips ... weight was clearly a priority.

A high-end, enduro category frame (with similar shock) is maybe 6.5 - 9.5 lbs. This frame is 11.5 lbs, which is nearly double the lightest frames in the category. Put that lightweight parts kit on a chunky frame and 34.5 lbs isn't impossible. Put the same kit on a Last Tarvo (just over 6 lbs. with that shock) and would it be under 30 lbs? Seems plausible.

On the other hand, take an 8.5 lb carbon frame + shock and build it up with more typical parts - in particular, Double Down tires, or similar - and would it be 34 lbs? Again, seems plausible.
  • 3 0
 @R-M-R: Great breakdown. Little things add up!
  • 2 1
 @ironchefjon you're bang on. His home made bike is about 2 lbs lighter than my size large Druid... and that bike has carbon everything (handlebars, cranks, wheels, frame).
  • 2 0
 @R-M-R: Absolutely. I had aluminum Intense Tracer that was below 30lbs. The frame was pretty heavy. The components were light. Sum was under 30.
  • 1 0
 @mattg95: I do only because it’s basically a size small frame with 425mm reach. A large would probably be gettin near 37-38lbs or something, which sounds about right
  • 2 0
 Yeah I don't understand the focus on the full build weight?
If you terrain, style and weight requires DD tyres instead of EXO and a 38 instead of 36, having a magical bump devouring frame still won't make enough difference to let you get away with, let's say, the EXO tyres.

Frame weight is 5,2kg. Not saying it's too much or not worth the benefits of steel and high pivot, but let's just keep in mind that a carbon Gambler is about HALF of that.

One exception, where complete bike weight makes for relevant comparisons: when you're shopping "for a given price point", and thus see 2 equally priced and capable bikes, and you'd favour the lighter one.
  • 1 0
 @BamaBiscuits: no, between a small and large there won't be 3-4lbs of difference. Only 1, or max 2.

@R-M-R: exactly. Though frame weight is often even less, in my case 3,5/17,5kg, that's one fifth only (despite being Al frame)
  • 26 0
 Killing the dad game there, total respect
  • 9 0
 Way to go on making the rest of us feel like inadequate parents!
  • 3 0
 Too right. If I tried that it would resemble the bike equivalent of the car Homer Simpson designed.
  • 1 0
 @Super7: your bike would be monkey navigated?
  • 1 0
 @whydomylegshurt: I'd probably crash less if it was.
  • 20 1
 That headset is calling out for internal cable routing along with about half a house worth of bits
  • 15 0
 Great idea on the kids bikes. Bikes that grow with them and ride well no doubt.
  • 11 0
 Honestly it’s how premium bike brands should be developing kids bikes, but they’re in the business of selling bikes, not providing products good for a decade.

It’s good that this was developed for 24-275 as that’s the time when they grow the fastest - approximately 6yo-12yo.

Makes me wonder if a GG Shreddog couldn’t do something similar.
  • 8 0
 "Travel: Head Tube Angle: 63.5deg"
  • 4 0
 it's great they added that because the head angle on my trek is always changing when I go out of town
  • 1 0
  • 3 0
 In the working sector, I mean work tools, a situation could not exist in which so many moving parts are not protected by a plastic or sheet metal cover. These high pivot bikes seem made to grab your clothes, your skin, your fingers….
  • 2 0
 Haven't drivetrains already been doing that for decades?
  • 1 0
 @DCF: this is getting complicated too much Big Grin
  • 6 0
 Chris King bb as the main pivot is genius level
  • 1 2
 Other than the durability issues I have seen with those bottom brackets.
  • 2 0
 Right?!? Seems overly simple but at the same time very serviceable.
  • 2 1
 Pivot bearings are usually not installed with threads, and while I'm intrigued, I'm also concerned because mechanical precession means that the BB will want to tighten itself in one direction of rotation (compression or extension, depending on which way you install it), and loosen itself in the other direction.
  • 4 3
 @barp: you do know how bb’s are threaded?
  • 3 3
 @somebody-else: do you pedal a tiny bit in one direction and then a tiny bit backwards? I don’t. I pedal 99.8% forward.
  • 1 1
 @somebody-else: I absolutely do, and that's why I understand how mechanical precession allows them to stay tight rather than coming loose (unless you have an Italian threaded BB where both sides are right-hand threaded). @husstler also seems to know this.
  • 2 1
 @barp: did you ever work on old cars and try to take the wheels off?
  • 3 0
 @somebody-else: Yep. Have you ever seen a man eat his own head?
  • 1 1
 @barp: and new cars?
  • 3 0
 @barp: my BB threads to the rear, I pedal forward, never come loose even when I used to just snug it with a BB wrench and not torque it down. Don't see how what you are saying is an issue with BBs on a bike....
  • 3 2
 @IanGilbertoud: As @husstler said, BBs are threaded the way they are (opposite threads on opposite sides) because 99.8% (or more) of pedaling is done in one direction. So, mechanical precession (look it up if you haven't read about it already) means that they are inclined to tighten themselves, not loosen themselves. Same reason pedals also have opposite threads (but the left and right threads are swapped on the sides of the bike, because the crank that the pedal threads into is a rotating frame of reference). But a frame pivot by definition is going to have half of its rotation in one direction and half in the other. I feel like I'm repeating myself but I can't see how to make this any clearer.
  • 1 3
 @barp: if only there were ball bearings inside to spin instead of your BB shell...wonder why press fit BBs don't spin
  • 1 0
 @IanGilbertoud: Sounds like you're determined to just keep on wondering, not learning.
  • 2 0
 Folks don't like the head tube, but I see some potential. There was a guy back in the '90s that would do custom sculpture on the front of the head tube, usually a very goofy-looking face. Can't recall the brand but it was boutique steel. I could see a kid's bike with a raging Godzilla 'stretching through' the front face of that box.
  • 1 0
 That little vertical tube below the idler that's part of the swingarm structure - seems like a good point to anchor the idler to, so as to lower the chainline (as much as possible without the chain touching the chainstays) and get a little better pedalling performance.
  • 3 0
 "Alongside them was a garage-built high-pivot, all made by the same man." uhhhhhhhhhhhhhhhhhhhhhhh
  • 3 0
 I love that the kids bikes actully grow in every dimension, both fit and handling wise.
  • 4 0
 That head tube far from attractive but it is pretty trick.
  • 2 0
 The amount of photos in the hi-res gallery with these bikes laying on their drive side is making my eye twitch - sick builds though
  • 1 0
 Curious if there's some bicycle frame engineer out there who already knows that the headtube will fail if you tried to put a full-sized person on there. ... or if they're rapidly working to patent that headset right now.
  • 3 0
 That’s awesome! Nice work
  • 6 2
 26 inch?
  • 18 3
  • 1 0
 Super Awesome! Can't help but wonder if he took some design cues for the HP from Evan Turpen's (Contra) earlier protos.
  • 2 0
 Room for a -10 degree angleset!
  • 1 0
 Must have saw a can of SPAM and said “hold my beer.” This, a headtube was born. Smile
  • 1 0
 That's where you swipe your credit card every time you want to go for a ride in the new pay per ride MTB standard.
  • 2 0
 In frame storage... in the head tube! Worlds first?
  • 1 0
 Nice work! Those adjustable kids bikes are super smart…
  • 2 0
 Very cool.
  • 8 7
 GG no longer has the ugliest headtube in existence
  • 1 0
 high pivot evils after they copy his homework.
  • 1 0
 The high pivot using the old gamblers linkage design is awesome
  • 1 0
 You lost me at that head tub ...
  • 1 0
 Another quality bike from a SLO grad!
  • 1 0
 What a wonderful job! I would have liked to have a dad like this ^^
  • 2 1
 sory the head tube is just fucking hideous
  • 2 1
 Nice work Dave!
  • 1 0
 I second that
  • 1 0
 Talented Dad
  • 2 1
 Absolutely amazing
  • 1 0
 The hp looks incredible

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