I believe that the gear ratios can be different. I thought about it this way:
If the two concentric circles are the two chainrings, and the two cogs on the outside are the rear cogs. Because they freewheel in opposite directions, they cannot produce torque against each other, so they have no physical mechanism that connects their motion.... so I drew it this way to make it more clear. Obviously the two gear ratios above are different.So clearly the angular velocity has to be the same for both chainrings, being constrained by the bottom bracket. But if you then express the cogs' angular velocities through the gear ratios, you see that the only way that omega2 = omega1 is if r2/r3 = r1/r4, or if the gear ratios are the same.
Physically, if you had a big gear for going forward, and a small gear for stopping, or going backward, then the whole time you are moving forward, the other freewheel (on the non-drive side, or I should say the left) will be freewheeling like crazy, but because its going backwards it shouldn't matter. If you get a cheap freewheel like I have on my single speeds, then it might sap a few watts out though!
Anyway, if you disagree with my conclusions, let me know... I'd be curious if anyone sees this differently...
6 comments:
Think about this...
With regular fixed gears:
Big gears mean a lower cadence
Small gears mean a higher cadence
And that is univeral for slowing down/speeding up
So...
If you were pedaling the bigger gear forward at a cadence of 90 rpm. and the only mechanism you had to stop was to slow your cadence, how would that ever activate the smaller gears higher cadence for slowing down.
I still dont have it figured out!
Because once you stop pedaling forward, and start pedaling backward, you'll have a second where nothing happens, like the article said, because the other freewheel hasn't yet engaged its pawl (unless you happen to start pedaling backward right when it had last clicked over). Then the forward gear is freewheeling, so its ratio does not matter, and you're driving against the other freewheel, which because it has a smaller gear will be easier to push back, but won't stop you as powerfully as if you just had a fixed gear with the big gear. But that is what you want right? A small gear to just slow down that won't blow your knees?
yeah, I am talking about what happens when you are slowing down. Not actually going backwards. I can see that the opposite side would freewheel when going in the wrong direction.
you are talking about pedaling backwards. I am talking about just slowing down your cadence... think about that
what happens when you start slowing your cadence. I am starting to think that something would break or unscrew. Or maybe it wouldnt turn... hmmmmmm
No, there is no difference between going backward, and just slowing your cadence. As soon as your cadence is not enough for the forward freewheel to keep putting energy into the wheel, it will freewheel. If you just "coast" the forward freewheel will start to click over (or might just rotate through if you are really coasting because some of the cheap freewheels do that) while the backward freewheel will then drive the pedals. At that point if you were to start applying some force backward, then you'd be pushing against the smaller gear via the backwards freewheel, as the forward freewheel will not certainly be freewheeling. Don't make me take the Bianchi and space out the BB so I can do this for real....
The most difficult thing if you have different gear ratios is to get it set up so the chains will have similar tension. You might run into the case where one drivetrain would want to be one link of the chain further back or something.... that'd be the biggest headache I think.
You might need to. There is something not right.
Think about when you just roll a regular freewheel bike backwards. It locks to the cranks and turns them backwards. So its gear ratio defines the omega, in the opposite direction.
So if you had only a backwards freewheel on a bike it would define the omega of the bike when rolling forwards.
Think about what that means to this experiment. I dont have my head completely around it, but I am pretty sure that we are complete dorks.
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