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Discussion Starter · #1 ·
One of my other hobbies used to be scale modeling airplanes. At a flea market I saw a little bicycle decoration made out of wire, that was just a decoration, but it was built to scale, and looked like a little city bike, and I thought it was cute, so the evil thought occurred to me, what about building a scale model that actually works, whether or not anyone could ride it.

I've built a bunch of bikes (8 total) for my kids, and scaled them down as much as possible. So for functional reasons I'm already doing the scale-down thing when it comes to handlebars (1/2 and 5/8 tubing), hubs (45mm OLN - 110OLN), cranks (40mm wide BB's), and sometimes custom pedals, but otherwise I never tried to make them LOOK to scale, and I used standard parts wherever humanly possible (1" headsets, off-the-shelf brakes, etc.) so they can look a bit whacky, but who cares about looks? But I just kept thinking about it....

A 60% scale model would be a 16er, that means either 305 or 349. I was able to find rims and tires on eBay for both sizes and decided to go with the 305. Unfortunately the rims are red, and the 32m tires are much fatter than I wanted, but if anyone else can find 25-305 tires anywhere in the world, I would be really happy to hear about it. There's nothing I can do about the fat tires so it's going to look a bit balloon-tired...

Next problem: figure out how to make scaled-down headset, cranks, chain? and brakes...

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Discussion Starter · #2 ·
Hubs....

For the front hub I found a 65mm wide folding-bike hub on eBay. It's not as narrow as I would like, but it's super light and should be OK with the fat tires.

For the rear hub I made one on a standard pattern I had from other bikes...75mm wide with track threads. In theory, I planned these so they can be run either fixed or with a 16t freewheel. But so far, I've never switched one to freewheel because the kids like their fixed bikes better. It can either take a QR skewer or 6mm bolts. The bolts weigh exactly the same as the skewer, and I kind of like the bolts.

I'm pretty happy that my red anodizing dye is a somewhat decent match for the rims.

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I don't understand why your showing parts being weighed on the scale. That tells me nothing. It would seem that there are about 1000 things more important than showing that.
 

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Discussion Starter · #5 ·
3D printer is a good way to visualize parts. I took some regular pedals, modeled in FreeCAD to life size, then scaled to match the wheel size. I have made aluminum pedals before, but 3D printed ones might be good enough for a model.
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Discussion Starter · #6 ·
According to my reckoning I need a 5/8 steer tube to look about right. For the headset, I was all ready to make a threaded steer tube, and bearing races and everything, I even bought a bar of O-1 steel for the races. But then I found out that you can get angular contact ball bearings in 15mm, which is pretty close to 5/8. It's even a fairly common bike bearing, apparently for suspension. 71802AC. They are a little bigger OD than I wanted, but this is a prototype anyway. So I'll make a threadless headset which will be a lot easier. I still think a tiny quill stem would be next-level adorable, so I want to make a threadful headset for a future bike. This is just a prototype.

It turns out typical mixte top tubes are 9/16. When scaled down it means 3/8 is too big and 1/4 is too small. So I had to order some 5/16 x 0.035 chromo for the top tubes. Which is thicker than I wanted, but it's more important for it to look right. It's a prototype anyway.

In the meantime I bent some bars. I slacked by doing an integrated stem... but it's a prototype anyway. I still don't know if this will be anything ridable. I'm just kinda in build-first-dont-even-ask-questions-later mode.
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Discussion Starter · #8 ·
Minus the drop bars, this is one of the bikes I'm using as inspiration. A random mixte I saw at the zoo and suspiciously took a hundred pictures of. Although it's looking like I need to shoot for a small/xs size frame actually, based on the wheel and tubing sizes I can get.

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While waiting for tubing and laser cut parts to come, I've been 3D printing other bits. I don't normally start with the seat but the seat is actually going to be a problem. I wanted to make a cool leather seat but can you tell how I'm getting lazy? It might have to just stay 3D printed for this one.

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Discussion Starter · #9 ·
I was able to come up with a head tube and steer tube. The bearing cups are too big, because of using cartridge bearings, but like I said I'll make a proper loose-ball headset next time.

Technically it could have a slop problem because I don't have a wedge cone thing. However I honed the top bearing seat to a very close fit. If you warm up the steer tube with your hand the bearing won't slide on, if you warm up the bearing in your hand it slides right on. That means it's within a fraction of a thousanth. With a coat of grease on the steer tube I don't think there will be any perceptible play.

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Discussion Starter · #10 · (Edited)
I found a Primo Comet 305-32 tire on the internet. It's a little bit smaller than my other Kenda tires, but it's quite a bit heavier, and I already designed the frame to fit the Kendas, so I think I will stick with the fatter ones.

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I also got a batch of laser-cut parts. Dropouts, fork crowns, and sprockets.

I have two ideas for the sprocket...either a 58mm spider and 22t-24t chainring, or a single 22t steel sprocket. Since a single sprocket will take up less space, it will probably be the single sprocket.

I could easily do a unicrown fork, but some sort of crown would be more aesthetically correct for this type of bike, so I went for a plate crown. Of course I couldn't do tapered fork legs...I checked some tapered stays that I have, but they were still too big, so I just had to use straight tubing.

I edited out the pictures of scales, so as not to trigger the sensitive.

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I'm tentatively happy with the fork so far. Of course I included rack tabs...I think I could use model airplane pushrod hardware to make a tiny rack in the future.
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Discussion Starter · #11 ·
I normally do kid cranks with 15mm spindles with pinch bolts, but a proper vintage city bike should really have cottered cranks, so I had to design a cottered crank/bb. Normal cottered cranks use about a 17mm spindle. That scales nicely to 10mm. Seems small, but everything about this project seems small. 10mm spindle allows a sealed bearing bottom bracket exactly matching a standard 68mm BB. I tried two different bearings for giggles.

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Of course there's no way normal 7 or 8mm cotters will work, but it looks it will be easy enough to file 6mm cotters.

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It's really no wonder old bikes used cottered cranks. To make the crankarm you literally just need to drill 2 holes in about the right spots. No other precision machining, threads, splines, or anything. And the spindle can just be a round rod with some flats on it. Very stone-age technology, in a good, like why-isnt-everything-still-this-simple way.

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Very cool work. I'll admit that I was a bit 'eh' on it when I saw the wheels, but there is some nice work going on here.

You've already cut the sprockets, but for future reference, chain drives are available in a ton of different pitches.

It's really no wonder old bikes used cottered cranks. To make the crankarm you literally just need to drill 2 holes in about the right spots. No other precision machining, threads, splines, or anything. And the spindle can just be a round rod with some flats on it. Very stone-age technology, in a good, like why-isnt-everything-still-this-simple way.
Cotters are great right up until they're not. Under conditions expected on a working bicycle (that is, a vehicle, not sporting equipment), they work fine. The worst to contend with there is someone not installing them correctly. On sport bikes--especially under powerful riders--the cotters would deform and be quite difficult to remove. Also, having worked on bikes that are 60 years old with cotters to match...screw cotters.
 

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Discussion Starter · #13 ·
I bought a few lengths of smaller-pitch chain to test. I could have cut a front sprocket for it, or even made a rear cog, but that would rule out ever using a freewheel. Plus, what sealed it for me was the smaller-pitch chain was actually wider and heavier per length. I still think it would look better. I even thought about using 3D printer belts and making a belt drive. But I had extra 11spd chain from my cargo bike and that sealed the deal.
 

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Didn't think i'd care about this, but have been completely drawn in. Keep up the good work.

p.s. I miss the scales. :-(
 

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Discussion Starter · #18 ·
Well, after doing some pre-assembly, I realized the braking problem.

I have a lever design for 1/2" handlebars already, where I make a custom perch but use an off the shelf cyclocross lever. I think that will work OK here.

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Calipers though, regular road calipers fit the 30mm tires, but look dumb, just way wrong. And there's no hope of fitting a caliper on the rear either. It doesn't help I chose a frame design that has challenging rear brake placement anyway. I knew this, but yeah no denying it now.

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Scale modeling is weird because one item that looks out of scale will destroy the visual effectiveness of the model, literally one conspicuous bolt that is way out of scale can just ruin the effect. Another weird thing is that building exactly to scale doesn't always work either. Almost all scale airplane models have cockpit bubbles or wing chords that are significantly bigger than scale, in order to "look right". If you really built it to scale, it would paradoxically "look wrong". So it's just about what looks right. The full size brakes are wrong enough that it just ruin the whole thing. So it looks like I'm making custom brakes. Which is a bigger project in itself than making the whole rest of the bike, but luckily it's autumn and I'm booking this time as hobby time and not working time. Plus, I have other bikes with brake clearance issues, and there's a sliver of a chance that the R&D might actually pay off somehow for "real" bikes.
 

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Discussion Starter · #19 ·
Update on the "miniature bike project" which is now a "making custom tiny bike parts like brakes project".

I decided center-pull brakes would be best; they are common on Mixte frames, and the rear cable routing is simple and clean.

Before going further with the brakes themselves I worked out the cable routing. This required finishing the top-tubes and making teeny-tiny brake bridges / cable stops.

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I didn't have any 5mm barrel adjusters, so I had to stop and make some homemade barrel adjusters. With matching anodizing of course.

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After making the brake stops, I realized an off-the-shelf straddle cable looked way out of scale. It won't do to have miniature brakes and use a big ugly normal straddle cable, now will it? How to miniaturize a straddle cable? That took a day by itself.

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The brake arms themselves are fairly easy to make on my 3D printer. After a couple more iterations in plastic, I will get the outlines laser-cut and finish them with files. But it is a TIGHT squeeze in there.


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bloody good work, the lengths you're going to are amazing (to me anyway)
 
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