Who likes Crank-drives?

[Deleted member 4366]

There's nothing to stop you bolting a belt drive sprocket to a normal single-speed free-wheel sprocket. There would be plenty of room on a normal bike with 135mm drop-outs. You could also bolt a belt-drive sprocket to a chain-wheel. It should be an easy conversion, but you need a roller tensioner somewhere.

 

[Arbol]

It should be an easy conversion, but you need a roller tensioner somewhere.

Would a roller tensioner be needed if a gates-belt is coupled with a gates-belt sprocket?

I do not see a tensioner in:

http://dirtragmag.com/review-gates-centertrack-carbon-belt-drive-system/

which is I believe a typical configuration of a belt and a belt-adapted sprocket.

 

[Arbol]

You could also bolt a belt-drive sprocket to a chain-wheel

A chain-wheel is the sprocket of a crankset, right?

If yes, then it would be necessary (not only optional) to use a belt-adapted chain-wheel, if using a belt and a belt-adapted sprocket, right?

 

[Deleted member 4366]

The one in the link has sliding horizontal drop-outs to tension the belt. How would you tension yours with vertical drop-outs.?

 

[Arbol]

OK, so a bike with horizontal drop-outs works without tensioner, for example:

http://dirtragmag.com/review-gates-centertrack-carbon-belt-drive-system/
http://www.singletracks.com/blog/mtb-gear/the-gates-carbon-drive-chain-replacement/

But there are images of apparently vertical drop-outs, without anything which resembles a tensioner (granted, I may have missed it):

http://paketabike.com/?page=belt_drive

But it seems I am wrong, because that webpage says:

Lastly, there must be a way to tension the belt. [as d8veh said] We build the single speed frames with sliding rear dropouts to tension the belt.

But the more I look at it, the less I see a sliding rear dropout there.

 

[Arbol]

Also, look at this:

http://www.cyclingnews.com/features/photos/chain-drive-or-belt-drive-whos-faster/247313

There is no tensioner, and this is not a horizontal dropout.

 

[Blew it]

OK, so a bike with horizontal drop-outs works without tensioner, for example:

http://dirtragmag.com/review-gates-centertrack-carbon-belt-drive-system/
http://www.singletracks.com/blog/mtb-gear/the-gates-carbon-drive-chain-replacement/

But there are images of apparently vertical drop-outs, without anything which resembles a tensioner (granted, I may have missed it):

http://paketabike.com/?page=belt_drive

But it seems I am wrong, because that webpage says:

Lastly, there must be a way to tension the belt. [as d8veh said] We build the single speed frames with sliding rear dropouts to tension the belt.

But the more I look at it, the less I see a sliding rear dropout there.

Belt tensioning is facilitated by the eccentric bottom bracket cartridge.

 

[flecc]

And the frame and sprocket combination is designed for a specific available belt length. That's why these belt drives are normally only seen on bikes designed for them.

 

[Arbol]

And the frame and sprocket combination is designed for a specific available belt length. That's why these belt drives are normally only seen on bikes designed for them.

And also the frame has to be "breakable", so yes, definitely the frame has to be designed ab initio. One cannot use a standard frame. But that does not need to be bad.

 

[flecc]

This type of single rear arm frame for belt drive can be designed to be not "breakable" and use standard cycle components. Can also have rear suspension:

 

[Arbol]

This idea reminds me of a swing arm.

 

[flecc]

Yes, that's why I remarked it could also have rear suspension. There have been a number of folding bikes like this. If the swing arm pivot is centred on the bottom bracket, the belt tension would be constant.

 

[Arbol]

Let me tell you what I have in mind:

a 24'' foldable with rear hinge (like the Rhino Joey; I also could consider 20'' wheels), a triangular box instead of a triangle (to put 18650s) and a front wheel with the folding mechanism as the Ori (or directly removable with a QR). Foldable handlebar and pedals. Fixed saddle with security pinhead (or similar). A singlespeed motor (eg Xiongda) and a belt to avoid grease. LCD on top of the box. Only two visible cables from the handlebar: brakes.

It would be a dream to build such a bike, for urban usage.

 

[Arbol]

Yes, that's why I remarked it could also have rear suspension. There have been a number of folding bikes like this. If the swing arm pivot is centred on the bottom bracket, the belt tension would be constant.

If the structure had a rear suspension, linked to the frame in a "detachable" way, could there be a rotation around the BB to turn the whole rear of the bike towards below the triangle frame, as Joey / Brompton / Birdy style?

Or is it necessary (in order to turn the rear wheel, such that it ends up below the frame) the rotation point is different from the BB?

 

[flecc]

Brompton/Birdy style rear frame fold rotation can easily be arranged with the pivot around the bottom bracket. The pivot would be heavier since it would surround both sides of the frame bottom bracket, but completely practical. This would maintain belt tension both for suspension action and when folded.

A bike like the one you imagine can definitely be achieved, but it would be expensive to produce such a one-off design.

 

[Arbol]

What would be the most expensive / complex part to produce such a design?

The engineering / CAD process, or the real metal building and soldering?

Locally, I know two guys building bike frames, one mostly alloy, the other mostly cromoly.

But I do not know an engineer who could build the CAD to show to the building guys.

May I ask your opinion on that?

 

[trex]

you may hit an existing patent on the fold.

 

[flecc]

Yes, the patent problem could well crop up. It wouldn't really matter if truly a home build, but asking professional engineers to produce something breaching someone's rights is a problem.

I don't know of anyone who could carry out the CAD side for you.

Costs can run away on a project like this, into several thousands of euros. It's the unusual features that can be the most expensive, for example the design and manufacture of the swing arm bearing surrounding the bottom bracket. Either each side could have a detachable half cap for the bearing, ic engine conrod fashion, or the swing arm could have separatable left and right sides to mount on the bottom bracket housing. There are also other possibilities for this, all special and taking much design and manufacturing time.

Sometimes on a prototype project like this a feature can prove unsatisfactory once built, necessitating starting again with the work and expenditure.

 

[Arbol]

Very useful and insightful comments, flecc and trex. Thanks.

Taking into account your comments, let us KISS:

What about getting rid of the rear suspension / swing arm (which would be nice, but really not an integral part of my original idea) and building a hardtail (but still with a fold for the rear triangle)?

Then, the pivot of the rear triangle would not need to coincide with the BB, since the rear part would always be rigid (except when folding, of course). This would be similar to the rear folding of Brompton / Ori / Birdy / Airnimal Joey.

Would this simplification still face potential patent problems? I mean, are Brompton / Ori / ... paying for a patent by using that folding method?

Then, the remaining question is if it would be tolerable from a engineering point of view that when the fold of the rear triangle takes place, since the pivot of the rear triangle does not coincide with the BB, tension would be (slightly?) distorted. I remember watching a youtube video of the Ori suggesting to place the chain at the smallest sprocket before folding. Would this problem of the tension be a deal breaker, for a belt (I assume chains are more "flexible" regarding tensions)? Or it could be feasible?

To sum up, two questions:

1. If I get rid of the idea of BB = swing arm pivot, and I use a typical, non-concentric solution (such as Brompton / Ori ...), is there still a potential patent problem?
2. Is the use of a non-concentric solution a deal breaker for the tension of a belt (unlike a chain), when the rear wheel folds, around a pivot point which is not exactly the BB?

 

[Arbol]

Edit: stupid question. I have watched an Ori fold, and clearly the chain suffers a massive transformation in geometry. It could not work with a belt.