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Freewheels and Coasting Mechanism
#1
Hi Members
I'm a new member to this forum and this is my first post.
I always wondered how does one do Coasting ( continue to rotate the rear wheels ie continue riding without Pedaling).
How is this made possible. I mean normally what one would think is as the chains drive the sprockets which are attached to the Hub as ONE unit then HOW DOES THE HUB/WHEEL ROTATE EVEN AFTER ONE STOPS PEDALING for a while.
I came to know about the Freewheels mounting but didn't understand how it works. Specially the role of bearings to maintain Coasting. And secondly there seems to be a ratchet like mechanism which keeps the wheel motion unidirectional ( even if we pedal backwards the bike rear wheel maintains its forward rotation). Where is the Ratchet placed. And how it does the locking thing in one direction ie what part/component does it actually lock.
I didn't get a clear picture of this whole mechanism and how it works. I mean how does the hub rotate continuously where as the chain( no pedaling) keeps stationary.
Guys this may seem very basic for you all but I've always wondered how such a motion is possible and what mechanism in the bike makes it work. I would like to have some pictures or drawings a an easier way to understand. So can you guys be kind enough to help me understand this.
I searched a lot on the web but couldn't find anything beyond some obscure info and marketing products.
So Expecting your support and help to clear my doubt.
Thanks in advance and take care.
-Arrow.

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#2
One of those things that seems obvious...once you've seen one apart.
The freewheel uses a "ratchet" to allow it to turn "freely" in one direction.
Here's a general description of a ratchet:
http://en.wikipedia.org/wiki/Ratchet_(device)
The ratchets shown in that article have a toothed gear and "pawls" that engage the teeth in one direction, but will allow it to slip past it in the other. A freewheel's ratchet works in a similar fashion. But instead of a gear with teeth facing out, it has a ring with inward facing teeth.
Here's a few pics of a freewheel taken apart:
http://www.sheldonbrown.com/mega7/
On the 3rd photo down you can see the toothed ring on the right. The photo below that shows a close up of the pawls. When you pedal backwards or coast, the smooth side of the pawl is pushed down by the ramped side of the teeth on the ring. It the opposite direction, the pointed end of the pawl catches in the the sharp side of the teeth on the ring and locks them together.
When you're coasting, the click-click-click sound is the pawls popping back up after each tooth compresses them.
Simple and efficient.
Next challenge - simple description of how these things work:
http://www.sheldonbrown.com/harris/rohloff.html

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#3
Hey Arrow,
I've looked around the net and haven't found anything yet to help explain this. Basically it is a set of spring loaded pawls that are directional, so they catch when you are pedaling forward, but don't when you are coasting.
I know that is not a good explanation so I'm going to work on this today and see I can't find or make a better diagram for you Smile

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#4
There you go.... a much better explanation for you while I was posting mine... thanks Dave!

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#5
Thanks A Lot Dave and Alex for responding quickly to my question and helping me with the links. I feel welcomed and nice to be here.
Guys now I know that its a Ratchet and Pawl system to provide the unidirectional movement but can you please go a bit further and explain where the ratchet pawl system is placed in/on the Hub or Freewheel.
Where is its exact location and placement and how does this arrangement engage the wheel and turn it when the pedal is pushed forward.
Is the pawl itself fixed to the rotating part of the Hub/Freewheel ( or anywhere else on the rear wheel) and the pawl rotates along with the rotating rear wheel ( that's what comes to my mind as I've never opened unscrewed any bike components). Or else how.
Getting to know this will be a great help in understanding the mechanism's use specially in a bike.
Looking forward to your help and answers.
Thanks again and take care.
- Arrow.

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#6
A freewheel is basically two parts.The inner section screws onto the hub and holds the pawls. So the pawls are 'fixed' to the hub and rotate with it. The outer part of the freewheel has the toothed ring. So the outer part can rotate independently of the wheel (as in when you pedal backwards). But when you pedal forward, the pawls lock into the teeth of the toothed ring and the two parts lock together and rotate as a single unit connected to the hub. There are two rings of bearings between the two parts which allow them to rotate separately.
A "freehub" works in the same basic way, but I am not sure of the pawl/toothed ring arrangement. The main difference on a freehub system is that the ratcheting mechanism is bolted to the hub itself and then the cogs are a separate piece that can be removed without removing the ratchet (cassette).
Not sure how much more specific I can get than that without diagrams. A little tough to describe these things just with words.

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