In which location of a bicycle is friction useful ? Explain its function
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There are many sources of friction (or drag) on a bicycle.
If you're going fast, the biggest is aerodynamic - the friction of the air against you and your bike as you attempt to push through it. If you look at a time trial bike you can see the sources of aerodynamic drag they've tried to minimize, by adopting a very uncomfortable but low body position and handlebars that let you stretch your hands out in front of you, by using solid disk wheels where possible because of the drag of the spokes, by using narrow high pressure tires, by shrouding the brake calipers and cables as much as possible, etc. Road bikes are getting more aerodynamic features because top riders are hitting such high speeds, especially in the finish sprint, that aerodynamic drag can mean the difference between winning and second place. But all of those aerodynamic things are trade-off. I mentioned the uncomfortable body position, but also time trial bikes don't handle very well. Disk wheels are a nightmare in a cross wind (you'll notice that mostly they only have disks on the back in order to make the front more controllable) and the high pressure tires don't corner very well, especially in the rain.
Another big one is the drag from the tires. Much of that friction comes from the deformation of the side walls and that's why fast bikes use very high tire pressures. There appears to be a bit of a change towards lower pressures to give the rider more comfort and control, but that's at the expense of lower friction because everything is a trade-off.
The last one worth worrying about is mechanical friction in the drive train. Every mechanical system has some, and bike drive trains are no exception. Bearings and chains and derailleurs and freehubs all make friction. Companies like Ceramic Speed promise you a watt or two of reduced friction loss by switching to ceramic bearings, or by using their rear derailleur wheel or their special chain or lubricant. There's always a trade off and in this case (as in many) it's money versus watts. If you're a top level racer and you've already got a great bike and you're as lean as you can possibly be, maybe those bearings will make sense to you
If you're going fast, the biggest is aerodynamic - the friction of the air against you and your bike as you attempt to push through it. If you look at a time trial bike you can see the sources of aerodynamic drag they've tried to minimize, by adopting a very uncomfortable but low body position and handlebars that let you stretch your hands out in front of you, by using solid disk wheels where possible because of the drag of the spokes, by using narrow high pressure tires, by shrouding the brake calipers and cables as much as possible, etc. Road bikes are getting more aerodynamic features because top riders are hitting such high speeds, especially in the finish sprint, that aerodynamic drag can mean the difference between winning and second place. But all of those aerodynamic things are trade-off. I mentioned the uncomfortable body position, but also time trial bikes don't handle very well. Disk wheels are a nightmare in a cross wind (you'll notice that mostly they only have disks on the back in order to make the front more controllable) and the high pressure tires don't corner very well, especially in the rain.
Another big one is the drag from the tires. Much of that friction comes from the deformation of the side walls and that's why fast bikes use very high tire pressures. There appears to be a bit of a change towards lower pressures to give the rider more comfort and control, but that's at the expense of lower friction because everything is a trade-off.
The last one worth worrying about is mechanical friction in the drive train. Every mechanical system has some, and bike drive trains are no exception. Bearings and chains and derailleurs and freehubs all make friction. Companies like Ceramic Speed promise you a watt or two of reduced friction loss by switching to ceramic bearings, or by using their rear derailleur wheel or their special chain or lubricant. There's always a trade off and in this case (as in many) it's money versus watts. If you're a top level racer and you've already got a great bike and you're as lean as you can possibly be, maybe those bearings will make sense to you
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