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I've noticed that, when I'm descending quickly on my road bike without pedaling, the pedals will sometimes spin too much freely, which seems like they have no impact whatsoever on the wheels.

Is it imperative to switch to a higher gear on the cassette in this case, or is this problem caused by failing mechanism in the bike (e.g. freehub)?

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    If you are not pedaling, how are the pedals spinning? Or do you mean when you begin to pedal it seems to have no effect as you are already going so fast?
    – Nate W
    Commented Jun 30, 2017 at 21:37
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    That is in fact do to with the gearing, and by not switching to a higher gear, you are beyond the top speed of that particular gear ratio. Generally if someone wanted to go faster they would switch to a higher gear. if you do not have any gears left you are nearing the maximum mechanically influenced speed of the bike.
    – Nate W
    Commented Jun 30, 2017 at 21:46
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    To be slightly clearer: if you're in the little ring and near the small end of the cassette, change up by changing into the big ring. That will avoid cross chaining.
    – Chris H
    Commented Jul 1, 2017 at 11:51
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    @h4k1m Cross-chaining briefly while not putting a lot of power through the pedals really isn't a problem. But it's better to plan ahead: if you're on the second-smallest gear on your cassette, you should usually have already changed up to the big chainring. As a general guide, if you're in the big chainring and getting towards the big end of your cassette, consider straightening things out by going to the small chainring and whatever gear on the back gives you about the same ratio (you'll get used to what that gear will be, by feel, as you ride the bike more). Commented Jul 1, 2017 at 15:47
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    @h4k1m Put simply, use the big chainring whenever you want gears bigger than the small one can provide. Don't think in terms of "When should I be using each chainring?"; rather, use whatever chainring gives you the gears you need for the situation you're in. Typically, there's some overlap between the big and small chainring: small chainring plus smallest cog is going to be equivalent to the big chainring and, say the third-biggest cog. Commented Jul 1, 2017 at 20:50

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No, the behaviour you're observing doesn't mean there's anything wrong with your bike. You're just travelling faster than you can pedal in a particular gear.

For a moment, let's forget about descending and suppose that you're travelling on a level road, so the only thing that's pushing you forward is your pedalling. In principle, you can travel at any speed in any gear, as long as you can pedal fast enough. In reality, of course, there's a limit to how fast you can spin the pedals: your legs can only move so fast.

Now, think about what happens if you stop pedalling while you're moving forwards: the bike starts to freewheel. In fact, this will happen whenever you're pedalling slower than the bike is moving. You can try this out. As you're cycling along, pedal slower for a moment, and you'll notice two things: first, you'll hear the clicks of the freewheel as the bike coasts; second, pedalling is very easy because all you're doing is turning the cranks, chainrings, chain and rear cassette. None of your effort is going into moving the bike forwards.

This is exactly what's happening while you're descending: you're not pedalling fast enough to match the actual speed of your bike, so you're still coasting and the pedals are very easy to turn.

This is a perfectly normal situation: there's nothing wrong with the bike and you don't have to do anything about it. However, even if you're freewheeling down a descent, I would recommend that you change into a gear that's appropriate for the speed you're travelling at, or your highest gear if you don't have a gear high enough. At some point, you will want to pedal again, either because you slowed down (corner, traffic, lesser gradient, bottom of the hill, etc.) or just because you want to go faster. If you're in an appropriate gear, you can just pedal and drive the bike forward. If you're in too low a gear, the pedals will spin with no resistance, which can be a huge surprise that throws you off-balance and causes a crash. Correspondingly, when you do start to pedal again, always start somewhat tentatively to make sure your gear is appropriate. It's when you push hard against no resistance that you're thrown off balance.

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    Agreed - Being in the highest gear possible means that you're ready to push and carry momentum through any inclines in an undulating descent, or through any short level parts of the descent.
    – Criggie
    Commented Jul 1, 2017 at 22:52

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