darksiderising

So I was sitting around and came up with this idea for a traditionally-geared fixed drivetrain.

-Take a rear bmx hub with the english threading on one side and metric (smaller) on the other and space it to whatever your frame takes.
-Space the hub for a multi-geared freewheel on the english threaded side.
-Build up a wheel around this hub. Thread the multi-geared freewheel on.
-Thread a single-speed freewheel onto the metric side.
-Set up the geared side just like a normal geared bike.
-Now, stick another drive-side crankarm and single chainring onto the left side of the bike. Ensure that the gear ratio on this left-side-reverse-drive is higher than or equal to any of the ratios on the right-side-drive. Hook up the left-side-reverse-drive. (For example, if the biggest ring on the right-side-drive matches the single ring on the left-side-reverse-drive, just make sure that the left-side-reverse-drive single speed freewheel has an equal to or smaller amount of teeth as the right-side-drive's smallest cog.

If my mind is working correctly, wouldn't the right-side-drive simply act normally? While forward pedaling is applied, the left-side-reverse-drive will just spin the freewheel. When reverse pressure is applied, the left-side-reverse-drive will engage the single speed freewheel and keep the wheel spinning. If I am thinking about this the right way, when attempting to 'coast,' the pedals will want to spin at a rate according to the ratio of the left-side-reverse-drive gear ratio.

I'm not saying this is useful for anything. I just need someone to check my thinking.

Dannihilator

Why bother, Sturmey Archer's 3 speed fixed is due out soon.

silsteve

Sounds right to me.

darksiderising

Please read the last line of my post. Thanks!

cc700

both cranks move at all times, so the wheel will effectively lock the cranks into position or drop the chain on the side that is moving faster. if the gearing is really higher on the left crank than ever possible on the right it may "work" but it will just be redundant... unless you mean metric threadling that is reverse in which case ...

basically no, your thinking is not correct.

depending on which way the freewheel on the left side engages, the bike will either have a redundant drivetrain or a drivetrain that locks itself into position and either brakes, drops, or binds the chain to the point where one fails or neither function.

darksiderising

^^^ Please explain how this will happen.

cc700

it's because you can't drive two different gear ratios with the same pulley. think of a pulley system where you pull a rope, but that rope splits into two different pulleys. one's larger than the other, but they're separate. now either the weights on the other sides of the pulley are going to rise at different rates, or there's going to be slack in the rope.


I don't understand which way you are putting the freewheel on the left side. if the pawls face opposite directions because one's reverse threaded it will turn the whole wheel into one big fixed cog. neither will give an inch to the other. if the pawls face the same direction, then they can each move just fine but they'll be redundant because the wheel is just going to spin free in the direction of the pawls. so it's either fixed and will drop or break the chain that has more load, or it's free and redundant.

either it's working to be free with the rotation of the wheel moving forward, or it's working to be free with the rotation of the wheel moving backward.


metric and english threading thread the same way. do you mean one side is reverse threaded? or do you just mean you have to use english on one side and metric on the other? because the threading size is irrelevant. what's relevant is which way the freewheel pawls engage.

darksiderising

^ The threading both uses a right-handed thread. My idea does not drive both sides at the same time, only one of the freewheels is ever engaged. The reason a bmx hub is very handy is that one side is a traditional english threading, for the geared freewheel, and the other side is the smaller metric threading, for the small single speed freewheel.

Let's say you have a 12-28 freewheel on one side. You'd want a 12-tooth or smaller single speed freewheel (which you can only find to fit the metric size) on the other side in order for the drive trains to not conflict and lock up.

EDIT: After some research, it appears that the smallest metric size is a 14-tooth freewheel. Luckily, most multi-geared freewheels appear to have a 14 as the smallest gear.

Here's an example pairing:
ss: https://norcalcycling.com/page.cfm?Pa...ils&sku=FW1200
geared: https://norcalcycling.com/page.cfm?Pa...ils&sku=FW8130

Dannihilator

Also you would have to put frame spacing into play.

darksiderising

^ Any idea how wide typical bmx hub bodies are? From what I have seen, they appear to be pretty narrow.

kemmer

I actually think this will work, but it's kind of dumb since you can only get higher gears and not lower ones. It might be kinda fun though...

EDIT: hold on... it won't work. I'm having a hard time explaining exactly why it won't work, but lets say you shift into your highest gear, the fixed drive side will not allow you to take advantage of that higher gear.

darksiderising

^ I think you understand why I might want to do this. Just for the sake of it.

Drwecki

Sheldon's site has a picture of something similar..too lazy to google right now, but there are directions and everything. I think the guy can even switch from geared to fixed".. Dunno.

darksiderising

The "fixed drive side" and the highest gear will be equal, so this will not be a problem. The highest gear will never be above that of the "fixed drive side" (I refer to this side as the left-side-reverse-drive in my earlier posts).

ADSR

I'd thought of something similar using single freewheels on either side set up so your skidding ratio would be lower than your pedalling ratio. It was just a meandering thought though. No idea if it works.

darksiderising

I think you might be talking about the converted internally geared hubs that Sheldon had a tutorial for.

darksiderising

This doesn't work because your drive side must be of equal or lower ("easier") gearing compared to your 'skid side'.

EDIT: This also makes me realize that you will not be able to ride backwards with my setup unless you are in a gear where both sides are equal (the high gear).