# The Naked Scientists Forum

### Author Topic: Why does water fly off my bike wheels?  (Read 1646 times)

#### thedoc

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##### Why does water fly off my bike wheels?
« on: 31/12/2014 08:30:01 »
darren nazer asked the Naked Scientists:

I noticed that when cycling down a hill the surface water from the front wheel starts to come towards me after coming of off the wheel.

what is the speed needed for this to happen and if i go fast enough would i get a mouthful of road water?

By the way love the naked scientist to bits.

What do you think?
« Last Edit: 31/12/2014 08:30:01 by _system »

#### David Cooper

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##### Re: Why does water fly off my bike wheels?
« Reply #1 on: 31/12/2014 17:52:13 »
No matter how fast you go, the bottom of the wheel is always stationary, but it then begins to move upwards and gets whipped forwards until when it becomes the top of the wheel it is moving at twice the speed of the bicycle. The faster you go, the sooner the water will detach from the tyre as the acceleration becomes more extreme, so even without taking air resistance into account you should still see this effect of the stream of water coming off the tyre at different angles at different speeds. The actual angles and speeds will depend to some degree on the design of the tyre, but the variation may not be great. This sounds like the kind of thing that could qualify for an Ig Nobel Prize, so you ought to consider doing the experiments, then work out formulae to describe what you find, and then write it all up.

#### CliffordK

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##### Re: Why does water fly off my bike wheels?
« Reply #2 on: 01/01/2015 23:09:40 »
This sounds like the kind of thing that could qualify for an Ig Nobel Prize, so you ought to consider doing the experiments, then work out formulae to describe what you find, and then write it all up.
It would be an interesting question.  I've seen water appear to arc up from my tire (without fenders).  The faster I go the higher the arc seems to be.

There are several factors.  With centrifugal force, the water has to come off of the tire at a tangent to the tire.  The water also has to clear the fork.  This would put a limit to the upward angle of the water as that tangential angle that clears the fork.

The water then is acted on gravity (causing the drops to fall, 9.8 m/s2)

And wind resistance (slowing them from the forward acceleration and leading them to appear to arc back into your face.  And, wind resistance, of course, increases with speed (the square of the speed?)

You also have the rider's speed (let's assume the wind isn't blowing).  Catch up to the drops too quickly, and they don't get very high.

Can we also ignore air currents around the bicycle, tire, and rider?

#### The Naked Scientists Forum

##### Re: Why does water fly off my bike wheels?
« Reply #2 on: 01/01/2015 23:09:40 »