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How to offset sprocket from crank arm

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How to offset sprocket from crank arm

Old 03-09-23, 06:12 AM
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How to offset sprocket from crank arm

I am trying to build a bike frame where the sprocket could be hidden inside a multi-piece frame. However I am having an issue, where most, if not all sprockets are fastened directly to the crank arm and can not be separated. My idea is to fully hide the sprocket inside a metal space and have only crank arms visible to the outside.
This means that there needs to be a mechanism to allow offset on both sides of the bottom bracket to the crank arms.

Any suggestions on which parts to look out for that could allow this? Small customization of parts is not a problem.
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Old 03-09-23, 07:01 AM
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I'm having difficulty understanding this concept. It sounds like you are in need of stand-offs and extended custom bolts? Your bottom bracket shaft would need to be wider too? Maybe I'm way off track here. How about a sketch?
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Old 03-09-23, 05:17 PM
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Sounds like a fully enclosed chainguard to me, like on the old Raleigh DL1.
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Old 03-09-23, 11:49 PM
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There have been bikes (tandems) long ago that had a central to the frame chain path. Think birdcage like. Of course the cranks and rings were spec made.

I have to admit that as the last 50ish years have gone past i find it interesting in how aesthetics, design and function have played against each other so many times. Like I say our business is cyclic. Andy
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Old 03-10-23, 07:44 AM
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Thank you for the thoughts.

Since I am a new user in the forums, I cannot post pictures at the moment.

The old Raleigh DL1 seems to have the correct mechanisms I am looking for. However my idea is to completely change the shape of the cycle, where on initial view it does not even look bicycle-like, but resembles more a moped or a bike. It would still be driven by chain drive, but fully hidden from view.
I did some more research and it seems that I am looking for something BMX-like or MTB-like where the chainring is shaft-driven, for example the 48 tooth-21mm standard BB size. Then I could use the parts to space out the chainring from the crank, depending on how it would fit.

Currently I see that most of the BMX and MTB parts are high quality and high price, where a lot of focus has gone into low weight. This means that chainrings are made from aluminium, which I think does not have the best durability on long commutes.
In my case, it would not be a problem to fit a steel chainring and cheaper crankset, but I have not yet found one that fits the criteria.
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Old 03-10-23, 08:56 AM
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you can upload pictures to a gallery on your profile. Let us know and we can post them here
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Old 03-10-23, 09:52 AM
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I have uploaded a sketch on my profile.

It is a simplified sketch showing a top-down view of a bike frame, BB, BB bearings and cranks. Different parts shown in different colors.

There are 2 possible versions that I can think of for me - one version has a chainring between BB bearings and second version on the side.

I can solve the issue of BB bearing mounts, meaning I can manufacture my own frame, even my own crank middle shaft if needed, however I can not manufacture my own crank arms or heat-treated chainrings.
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Old 03-10-23, 12:01 PM
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I don't see it, do you? It says you haven't created any albums. Just post 3 more times and it will let you put it here.
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Old 03-11-23, 08:10 AM
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Here's the sketch of three versions - mainstream bike, concept 1 and concept 2.
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Old 03-11-23, 08:53 AM
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From a strictly problem solving point of view, concept 2 could be executed with bearings press-fit into the walls of a 4x8 piece of aluminum square channel.

The maximum chainring size would determine the extruded aluminum square channel minimum dimensions.

You might have luck cloning the direct mount spider or chainring scheme popular with mountain bikes. 5x110mm BCD spider arms welded to a spindle seems like it would be hard to manufacture by hand and wouldn't fit in the diameter of hole the bearing presses into...But a small direct mount interface welded onto a spindle, installed through a large bearing hole could be feasible.

I make no claims to expertise in any of the above. Merely a thought exercise. It'll be interesting to see this very unusual idea come to fruition.
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Old 03-15-23, 03:29 PM
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Do you want the chain to be hidden from view as well? Concept 1 would be much easier to execute, as you should be able to get away with just modifying off the shelf parts for the most part. Concept 2 would require a much more complicated frame. Concept 2 would also probably result in a much wider pedaling stance (q-factor), so it may pedal more like a fatbike if it matters.
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Old 03-16-23, 06:48 AM
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I would like to hide the chain and both front and rear chainrings.
It will be a single gear bicycle for city use.
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Old 03-16-23, 07:02 AM
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as a designer I'm curious about this, but still having difficulty completely understanding it. I'm looking forward to more info.
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Old 03-16-23, 10:18 AM
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I'm a bit unclear on where you would like the chain to go. Do you want the chainring, chain, and freewheel all inside of an oversized chain stay? And how does the drivetrain clear the rear tire?

Assuming you want the bike to still be fun to ride as a single speed, you are constrained by needing a large chainring, and the chainring+chain need to clear the rear tire.

Are you familiar with the concept of gear-inches? (I can't post links yet, but a quick google will pull up wikipedia or Sheldon Brown's site for explanation) Decide what gear-inch pedaling ratio is best for your single speed, and then based on your intended wheel size, determine which chainring and freewheel combination will give you your gear-inch ratio. Once you have your ideal wheel size, tire width and chainring size determined, you might have a better idea of what the frame and custom crankset will look like around these components.
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Old 03-16-23, 11:05 AM
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This is a good way to put it - yes, chainring, chain and freewheel should all be inside of an oversized chain stay, which should be oversized enough to clear the drivetrain and covered from the outside.

I have looked at the gearing - it looks like from available parts, I will be using 13T rear sprocket with around 32T front sprocket, 47-507 tire, which will work best at 13.75mp/h or 22km/h. The idea is, that I need to use as small front sprocket as possible to fit inside the frame, which means I need to go to smallest possible with the rear one. 11-28 would give the same result, but I can't find any 11T freewheels currently.

I am currently trying to make it work with rear hub spacing less than 100mm with the single gear. For example I could try making 90mm hub work - which do exist as I have read online. This is another part I am having difficulty finding components for.
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Old 03-16-23, 08:53 PM
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As a life long wrench I hope that whatever design this OP comes up with is easy to work on. Common hardware as much as possible. Little need to remove serious parts (bearings, frame members) to fix a flat tire. Enclosed drivetrains are all fine till something wears down or goes wrong. Then the unique parts and assembly's cost will be truly known. Andy
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Old 03-16-23, 09:32 PM
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The front is actually the easy part of this to accomplish. You can use a 2 piece crank designed for a direct mount chainring, you'd just need to create a spacer that would interface with the crank arm's spline on one side and the chainring's spline on the other. Using a 73mm crank in a 68mm bike would let you space the chainring 2.5mm away from the crankarm, with the normal gap between the ring and crank there should be more than enough room for a cover to hide it all. The bigger issue is that the rear cog sits inboard of the frame and to cover it inside a frame design would require having the axle nut and cog in each other's location and that's the hurdle I see difficult to overcome.
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Old 03-20-23, 10:17 AM
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Dummy post to get over 10 post threshold, apologies as I'm sure this breaks the rules

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Old 03-20-23, 10:20 AM
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Would you consider a design like this (Ignore the suspension bits)? The chainring is on the non-drive side, driving an intermediate shaft, which turns a sprocket on the drive side which connects to the freewheel. This would allow you to have two small sprockets on the drive side.

https://www.pinkbike.com/news/starli...d-dh-bike.html

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Old 03-20-23, 02:57 PM
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I suppose a shaft drive is out of the question. Andy
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Old 09-13-23, 06:41 AM
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Thanks for all the thoughts guys! Sorry for not giving out responses.

6 months has passed and the prototype bike is now ready. I will share a few pictures and give a quick overview, I am open to suggestions/ideas/thoughts if you guys have any.
This is now a fully functional and truly unique city e-bike, having 250W mid-motor and 630Wh battery back. Total bike weight came to about 25.5kg. Can ride up to 100km/62mi on a single battery charge, however with an empty battery it's still an ease to ride, just unnecessarily heavy for a regular bike.
Single speed 30-12 with a chain drive, comfort zone being in around 20km/h or 12.5mp/h for city rides.

Currently we have done around 200km/124mi testing on it and have ironed out all clunks and rattles on it, however there is a lot to discover surely.

The bottom bracket issue was solved with using a mid-drive e-bike motor. With this I could attach a 30T chainring and cover it all with our frame. This also meant that I didn't need to include bottom bracket frame part, just the mid-drive motor attachment points were necessary.
Fork and wheels are from a regular city bike, wheel covers are plastic and just for aesthetic reasons.
1 inch threaded headset, covered with a tube for aesthetic reasons again, however the tube detaches along with the quill stem for easy access to headset if necessary.
Standard bars, grips, brake lever, standard quick-adjust seat tube and standard seat.
Custom frame (Front part is pie-cut style welded because it was impossible to find a tube bender for this application)
Mid motor mounted upside down - bulk of it is towards the rear wheel
Battery mounted in front of the motor and behind the front wheel, giving very low centre of gravity for an e-bike
12T single speed rear cassette
Currently using cable operated drum rear brake, however would like to convert to 140mm discs, which can be fully hidden inside the frame.
Custom rear wheel bolt and hub inner shaft, however it works just like a thru-axle, enabling for a fully seamless connection without any parts sticking out of the frame.
The large seat tube is empty on the inside.
Custom front basket
Custom rear mudguard
Custom rear light






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Old 09-13-23, 06:46 AM
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Originally Posted by Andrew R Stewart
I suppose a shaft drive is out of the question. Andy
Drive shaft is not out of the question and would surely like to learn more about the technology, however there is very little information out there and it's difficult to utilize on a prototype. It would actually be ideal solution if it could solve our maintenance issues with chains or belts.
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Old 09-13-23, 09:17 AM
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Looks like a fun ride. Don't know if I'd want to work on it...
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Old 09-13-23, 11:46 AM
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Drive shafts haven't been a big success in cycling. People keep trying to bring them back though. This frame might be stiff enough to make it work, but integrating it with one of the available mid drives is likely going to be a problem.
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Old 09-13-23, 12:37 PM
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Whoa! That's cool lookin'

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