Hey, I’ve recently gotten into MTB and started learning the anatomy of a bike.
The rear hub makes a clicking noise when coasting but is silent when pedaling – this is caused by springs or ratchets in the hub either engaging with the rest of the system or going the opposite direction, causing the springs (ratchets) to click.
And here is what I cannot fully grasp: What makes the inside of the hub suddenly start going in the other direction when coasting?
Sorry if my description was somewhat vague but I did my best 😀
Thanks!
In: 23
It isn’t that the inside of the hub has started going backwards. It’s that the rest of the drive train (pedals, gears, chain) has STOPPED moving with respect to the wheel. So, from the perspective of the hub, the drive train is effectively moving in the opposite direction. This is why you get the same clicking noise if the bicycle is stopped, but you run the pedals backwards.
Imagine you are standing by a twirling kiddie merry go round, holding out your hand and letting the poles slap it as they go by. You’ll hear a slap-slap-slap-slap sound. Then you decide you want to speed it up, so you put your hand behind of the poles and start running around the merry go round, pushing it. No more slapping.
A bike hub is a bit like that. When you are coasting, parts that normally that work together inside the hub are slapping each other. Once you start to pedal, the parts engage and the chain starts to drive the wheel forward, just like you running and pushing the merry-go-round, and the slapping sound stops.
This is a tricky one… but I’ll take a crack at it.
There’s most definitely a few different designs. I can try my best to explain one popular design with enough detail.
There’s a center ring that the sprocket is fixed to, it has to rotate with the sprocket and chain. That center ring has sawtooth cuts on the sides both facing the same direction.
That center ring spins freely on the rear axel.
Then there’s a ring on both sides of that middle one.
One is just a ring with sawtooth cuts in the opposite direction of the center ring. But it has inner posts that ride inside a channel on the axel. So it has to rotate in unison with the axel, but can slide along the axel.
The other is similar, also sawtooth cuts, but larger as it holds the wheel assembly, and doesn’t slide down a channel along the axel. It only rotates.
The two outer rings are linked with a stretched spring that pulls them together and sandwiches the center ring in the middle.
Because of the sawtooth teeth on the sides of the rings, when a force is applied in one direction the flat faces of the teeth press together rings can push on other rings.
But force in the opposite direction, and the sloped faces of the teeth meet. As those faces are like a ramp, the ring teeth ride up those ramps.
The rings expand apart until the end of all the teeth run out. Then the teeth fall into the groove of the next slot and the spring slaps all the rings back closer together. Click.
The force can be applied to either the center ring, using the pedals, chain, and sprocket. Or similarly force can be applied to the outer rings through the wheel.
The wheel will behave just like the pedals will. In one direction, the ring teeth meet face to face and transfer force. In the other direction, they meet slope to slope and slip past another.
You can see that by flipping the bike upside-down. Spinning the wheel forwards and it’s a lot of slip-click-slip-click. But in reverse it stays engaged and transfers the rotation through the rings.
Former 6-year bicycle mechanic here.
A bike hub contains bearings that hug the axle of the bike and allow the wheel to spin independently of it, meaning that your wheel can spin while the axle stays in place relative to the frame of the bike.
There are two basic kinds of rear hubs in a bike.
One is just a normal hub without any ratcheting mechanism but has external threads on one side (almost always the right side) that someone can attach a freewheel to. A freewheel is a cassette (gear cluster on a bike) that has a ratcheting mechanism inside. When you replace the gears, you are also replacing the ratcheting mechanism. This is usually what’s used on cheap bikes.
The other is a longer hub that has a “freehub” attached. The ratcheting mechanism and an additional bearing or two are built into the freehub body. The cassette (just the gears this time, not the gears and the ratchet) is placed onto this freehub body. The freehub body can be replaced, but its lifespan is much longer than the lifespan of the gears that you would place on it which would be replaced once a year or so, so it goes longer without being serviced.
You’re correct that it is a ratcheting mechanism that keeps it engaged in only one direction. The clicking you are hearing are the pawls being sprung back into place – the actual “click” happens when they suddenly stop moving once they reach their reset position. The pawls are little teeth-looking things that are spring-loaded to only collapse when force is applied from a certain direction but to stay up when force comes from the other direction.
As for the inside of the hub moving the other direction – the reason you can’t grasp it is probably because this isn’t what’s happening – you’ve been right about the rest of it. The inside of the hub is going to move as long as the bike is rolling, it’s the *outside* of the hub (the freehub body) that stops when you stop pedaling. When you are pedaling the ratcheting action means the cassette and hub are moving together, when you are not pedaling the ratchet allows the freehub and hub to stop moving together which is when you’ll hear the pawls clicking back up after they’re allowed to be pushed down. It’s not moving the *other* direction, it’s just not moving in the *same* direction. It’s not moving at all, but the wheel is still moving relative to it.
A mechanic tip – as a general rule you can tell the quality of the freehub (and thus the bike, unless they’ve only upgraded the freehub for some reason) by how loud the freehub clicks. The louder the better. There are exceptions to this like with other stuff, but if you’re out on the ~~train~~ trail and hear a super loud ratchet in the distance, it’s probably a good bike.
A GIF is worth a thousand words here: https://www.notubes.com/media/wysiwyg/neo-product-1-2.gif
Take a look at the white and red pawls. See how they are sprung and hit the sawtooth-like static ring? That static ring is attached to the rest of the wheel, and what you see here is what’s happening when you backpedal. You get noise because the spring pushes the pawl back. When you pedal the pawls don’t have anywhere to go, they are pushing against the vertical part of the ramp, thus making the outer ring(remember it’s attached to the rest of the wheel) to move. When you coast, the same thing the GIF shows happen, but the inner part is static (you are holding it with your feet) and the outer is moving.
>And here is what I cannot fully grasp: What makes the inside of the hub suddenly start going in the other direction when coasting?
There are actually two spinny bits on the hub: The wheel, and the sprocket. Inside the wheel is a ratchet and pawl mechanism. Whenever the sprocket tries to spin faster than the wheel, the pawls lock into the ratchet, transferring the torque from the sprocket into the wheel.
Whenever the wheel is rotating faster than the sprocket, the pawls slip along the ratchet, which makes the clicking sound. Nothing starts going in the opposite direction, it’s just one part travelling faster than the other.
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