Update: You might as well skip this post and go straight to the comments section where Remus made it clear why disk brakes are more efficient than V brakes.
There's a reason why disk brakes are better than V brakes on a mountain bike and it has nothing to do with the usual arguments: no mud and no water reaches the disks thus the braking is more efficient - unlike the rims which easily gather mud, sand and water while crossing streams, making it harder for the cyclist to stop the bike (not only that but the brake pads and the rims themselves are worn out by the abrasive materials caught between them, which not cool...).
But the real reason rim brakes are less efficient than disk brakes consists of a simple truth pertaining to physics: it's a lot easier to stop an object moving in slow speed than it is to bring to a halt an object moving in higher velocity.
The vectorized image above depicts a mountain bike wheel equipped with a disk brake system. Notice that the two points (A and B) travel different distances in the same time unit until they reach their destination positions (noted here as A' and B'). Based on this observation it can be stated that, given a certain spoke (radius) of a bicycle wheel (circle), each point of the spoke starting from the hub (center of circle) to the outer margin of the wheel (rim) has its own speed when the wheel is spinning. The further from the center, the higher its speed.
Think of it this way: no matter how fast the wheel is turning, the rim will always spin faster than the brake disk (which is smaller in diameter and has the same center) even though both the disk and the rim (and all points on any given radius for that matter) perform the same number of rotations per time unit. Example: One full spin of the wheel happens in one second - the speed of the valve (taken as a point on the rim) is about 200 cm/second while the speed of the corresponding point on the disk brake is about 60 cm/second.
Since the braking force is applied so close to the wheel's hub there is a need for sturdy spokes in the wheels that have disk brakes on them. Unlike the normal wheels on which the rim is designed to withstand erosion and friction (features needed for V brakes), disk brake wheels come with stronger spokes (double spokes) that distribute the braking force towards the tire without breaking the spokes themselves.
As a side note you may have the disappointment to find that medium bikes that have disk brakes (mechanical system, not hydraulic) do not have such strong rims. So while your bike brakes efficiently you might see that the wheels loose the round shape easily after bumpy downhill rides. This happened to a friend of mine who has a Kona Fire Mountain. A smart thing you can do is replace the rims (or the whole wheels) with ones that are designed for V brakes - they are stronger due to thicker walls; also the sharp U or V section profile makes them hard to bend/break.
Another reason the disk brakes are desirable is because they are thin and ventilated. Also the travel of the brake pads is shorter than in the case of V brakes - the pads are closely positioned sideways from the disk as to apply friction on it after a travel of not more than 2 mm. The holes in the disks allow a better ventilation letting the heat (caused by friction) escape more efficiently.
The rims get hot (after a long downhill ride with V brakes) and transfer the heat to the tires which is not necessarily a smart way of getting rid of heat. While I haven't heard of tires exploding or melting due to hot rims I think we all agree the disk brakes manage to take care both of stopping the bike faster and not damaging the tires through heat transfer.
The same physics law that makes the rim of the wheel move faster than any point situated closer to the center of the wheel is responsible for the noise of the helicopter rotor blade. Some of the noise that we hear in a flying helicopter (bang bang bang) is a series of sonic bangs caused by the tips of the blade which travel through air at supersonic speeds. The longer the blade, the faster the speed of its tip (rotations per minute being constant). Military forces are working to reduce this noise for stealth operations -long-range propagation of helicopter noise can alert an enemy to an incoming helicopter in time to re-orient defenses.Read more about helicopter noise reduction and rotor noise (see page 4). |
Want to learn more? Check out a cool article where you can learn about the benefits of having disk brakes on your mountain bike - weight and other technical aspects are being taken into account as well.
Keep riding your bike. And since you love biking so much, why not learn to design your own mountain bike T-shirt or jersey?
Cheers!
