Petit Cargo Bike Geometry Development

Education
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Hi All,

I’m looking to dip my toe into framebuilding, but am not quite sure where to start as far as making something that fits what I’m interested in, but also is rideable.

I’ve taken a lot of inspiration from the very talented Colin Woof, as well as Arian Bronner’s Apriko fork, and while I’m not worried about the fabrication details, I have no idea how to determine if I’m setting myself up for something terrible to ride.

What I’ve drawn out below is a hodgepodge of dims from my current commuter bike, which is the only bike I’m really familiar with, as well as some input based on the Velo Orange Neutrino (simply because it’s a minivelo with pretty well documented geometry) and from the Omnium Mini.

The goal is a mini cargo bike that doesn’t take up much space and can haul a window air conditioner unit (20L x 16H x 14W ~45lbs). If it can fit that, it will happily fit all sorts of other things I’d want to haul around.

The lines here approximate the center lines of the tubing. The plan is to braze or tig this up from straight gauge 4130 steel, something a little heavy just so it’s easier to work (.049in wall). I might need to buy a seat tube, doesn’t seem like any stock tubes work nicely with common seat post diameters. Probably a simple segmented or biplane fork at the front, trying to avoid any bending for now.

Aside from input on the geometry, I’m also wondering about whether it would make more sense to have the bend in the bottom tube before the steering/head tube (a la the Omnium Mini), and whether I’m setting myself up for clearance issues with my heels and the short chain stays.

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Beauty! Looks like you are starting out with Fusion? I think that’s a great first step for these non-traditional designs. It will make the construction much easier.

For seat tubes, there are two styles I would recommend:

29mm for 27.2 seat posts:

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“low geo” seat tube can be chopped really short.

35mm for 31.6 seat posts:

If you are using a 32mm top tube, I would go with the 35mm seat tube. It will be easier to build with (thicker topper and more room to land tubes). If weight and comfort is your objective, I would go with the 29mm seat tubes.

Whether the bend happens before or after the fork’s headtube, I think this comes down to an aesthetic choice and which one would be easier to bend and fixture. If you were using a butted tube, you would need to make sure the bend falls into the thick butt, but with straight gauge, its not a problem.

Say you are using a 38mm tube and bending it ~ 15deg with a 230mm CLR. The bend will take up quite a bit of space. ~10-20mm before and after the bend will have some deformation. To make life easier, I would try to place the bend between the handlebar headtube and the fork headtube.

You would also need to verify what your bender or bend contractor can do (tube diameter, CLR, and max bend angle).

That is a really good point. It will probably depend on what hub and crank standards you use. If you use quick-release, mountain bike cranks, and flat pedals, you will probably be fine. You can always draw this out to verify.

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Love this. The idea of starting with a cargo bike with heavier tubing is smart. I import Omniums to Australia and am a little familiar with the Mini. I’ve also ridden a Mini-Max a heap which is actually the closer bike to your design. A few thoughts:

  • I would go with the larger seat tube that Dan reccomends. Thicker and also dropper posts on cargo bikes are totally underrated. Easier to find good ones in 31.6.

  • I would go with slightly longer stays, maybe around 420-440. I know the Neutrino is 380 but that bike has a much shorter wheelbase than yours. In my experience chainstay length is all about front-rear balance. The front end might feel a bit vague with stays that short.

  • If you want to avoid a bend in the downtube you could just make the front section its own triangle. I like this design from Schon Studio. I think mitring the headtube though the downtube is a difficult operation otherwise. But also a good challenge.

  • We/Omnium sell replacement hardware so you could get a front fork with the steerer arm attachment, a steering arm with eye bolts (can be cut shorter) and a steerer tube with the same arm attachment point if you wanted. A few less things to worry about fabricating.

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If you model your design in BikeCAD as described at Bakfiets style cargo bikes | www.bikecad.ca others could more easily collaborate with you on the project. To save your design to share with others, see: Saving BikeCAD Designs Online | www.bikecad.ca .

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BikeCAD Air conditioner mover | www.bikecad.ca

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Thanks for the detailed responses! Some notes below but I’ll post the updated model when I have a minute to work on it.

Yes, doing all work in Fusion to nail everything down. It will help when I go to produce the fixtures and jigs as well, plus full size drawings for any bends, etc… I try not to go into the shop without a plan.

I am still trying to select tubing for the bike, but was probably going to go with 1in for the top tube, 5/8 for chain stays, 1/2 or 3/8 for seat stays, and 36mm handlebar/head.

Noted on the bend location, I was originally thinking of doing the down tube in 2in/50mm tube, but I’d have to buy dies, where if I go 1.5in, I have plenty of tooling. It won’t be as sturdy, but maybe I bump up to .065 wall there to compensate a bit. I’m anticipating gusseting between the bend and the steerer tube regardless.

Still sorting out hub and cranks and pedals. Probably a cheap internal gear hub. I kind of want to buy the future components so I can get good measurements, but I have no idea. Something decent but not flashy - this bike is going to live outside in NYC. I was hoping to keep my life simple and do straight chain stays, but playing around with clearances with some of the info I can find online, I don’t think that will work out so well. Maybe if I lengthen them as Bushtrucker suggests.

Bushtrucker, I don’t understand droppers, but I’m willing to learn. I’ve used one on a mountain bike ride but mostly ended up confused. I’m open to it, I just don’t know what it’s supposed to do for me? I thought they were to make going downhill standing on the pedals more comfortable.

Noted on the stays, I’ll play with that. Looking to keep the overall length similar to my current bike for storage reasons.

I love bending, just intimidated in doing it in thinner walled tube with my setup, but as noted above, I might just thicken up that tube. However, that Schon bike is beautiful!

For the fork - noted, but I want to use a 16in/305 wheel on the front. Do Omnium’s go that small?

Lastly, I realize BikeCAD is a valuable tool for many people, but I’m far more comfortable just noodling around in Fusion/Solidworks, and already pay for it, plus I can build out jigs and fixturing, 3d printed parts, lasercut/waterjet files, etc… all in the same model.

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Droppers on cargo bikes give you a more secure stance at a stop light when the bike is heavily loaded. You can sit on the seat and have both feet flat on the ground. They can also make it easier to throw a leg over the bike. Totally worth it (and almost essential if you’ll ever have a kid up front on a Yepp Mini or Mac-Ride seat).

I built a similar cycle truck (but without remote steering) as my first cargo bike. I had a 26” rear wheel which let me use a free 90s MTB as a donor frame. I cut off the existing headtube and downtube but kept the rear triangle and top tube, then worked from there. That worked out so well that I helped a friend make a similar one, and both bikes still get regular use.

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Yes to what @Alex said regarding dropper posts. Plus they make it easier to share the bike with different sized riders.

Omniums use a 20”/406 wheel so probably best to make ya own for if goin 16”.

I would probably go larger than 1” for the top tube and stick to a max 0.9mm wall thickness. Should be easy enough to weld with a bit of practice. And the DT will be plenty stiff enough to resist any torsion generated buy the cargo.

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@jellywerker

Very honoured to be named in the first post. I’m blushing a little.

Re: bent tubes. Avoid where possible, I’m currently working at a bike company which uses one bent tube as the main element. Bending (especially thinwall large diameter) is the least controllable process in a bike frame so you need to be careful where you’re mounting the tube in your jigs and then how you’re cutting the mitres to ensure the end result of the bike frame is as designed. It’s quicker and has a higher process capability (ability to hold a tolerance) to cut separate mitres when you have that as an option.

Beware a chainstay as short as that:
Shimano will only guarantee their derailleur gearing down to 405mm chainstay-length the last I checked. So if you ever decided to change from a hubgear you would have issues.
Or, i mean, You could always pull some cogs off and reduce the gearing range as it is the sidebow of the chain that is the limiting factor. (Sidebow is how much the chain can be pulled sideways).

This bike is cool! I like it! I’m definitely following along.

@Alex I loved following along your cargoturds, it’s one of the things that made me want to make cargobikes

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Thanks!

After making the first two bikes I always planned to make one with remote steering like this. It never got farther along than the plans:

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Almost all of my cargo now is made up of kids, so I ride bucket bikes and long tails instead of cycle trucks. Both of mine are commercial designs: a R&M Packster 40 and a Benno Carry-On midtail. I did make a custom rack for that second one that is lighter, stiffer, and holds two kids (the stock rack was only designed to hold one).

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Also, I was inspired here by David Wilson who built a few longer Ominium-like bikes. Here is an ancient website with some info. He moved to Philadelphia over a decade ago and co-owns the best cargo bike shop in the city.

http://davidwilsonindustries.com/cargo/index.html
http://davidwilsonindustries.com/gallery/cargo/borracho/index.html

They offered a small version called the Shorty that is pretty inspiring:

I rode the prototype of that and it was a great little bike. Tons of capacity for its size.

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I am a huge fan of the Ursus Jumbo dual kickstands for cargo bikes. Make a dedicated mount for those! :grin: They come stock on two of the cargo bikes from Bombtrack.

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Definitely planning a kickstand plate!

This has been quiet as I was running into issues determining clearances. I’ve picked up most of the parts at this point and am finishing up building the wheels and should be able to translate that all back into CAD soon.

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I found out about these while adding a rear baby seat to my beach cruiser years ago.

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Has there been an update on your project since your last post? I’m keen to know more about your process. Currently I’m planing on constructing a biplane fork for a cargo bike I’m putting together.

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Heya!

Got the wheels built up so I could start taking some realistic measurements, and then got caught up with a bunch of life stuff. They’re still sitting beside my desk mocking my hubris.

Lots of playing around in CAD, but no real progress. Itching to get back to it after the turn of the year however.

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great. i look forward to your documentation here :slight_smile: