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Old 04-24-07, 08:30 PM   #1
parkeredwards
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Cycling Physics: Help with school project!

Hey, I need a little bit of help with my school project its about the physics of biking, i have done some research but i have found very little. So I'm asking the Bike Forums community. So i think for this project i will need to new more about how a bike actually stays upright when u ride?, why bigger disc brakes have more stopping power, how wider tires increase traction? etc...

I will continue researching and i would really appreciate if anyone could help me.

Parker Edwards
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Old 04-24-07, 08:51 PM   #2
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how a bike actually stays upright when u ride?

well alot of it is the balance of the rider...but if you wanted to put a science angle to it its kinda like havign 2 big ol gyroscopes aka the wheels. the faster you go...the faster they spin...the easier it is to balance...assuming you dont suck


why bigger disc brakes have more stopping power

what determins your stopping power is the surface areal between the pad and the disk. so a bigger rotor...the bigger caliper/pads you have....and the more contant you can make so you get more stopping power. also depends on how much pressure the the pistons can put to the rotor, so having more pistons doesnt nessearily mean more stopping power. look up how brakes in cars work


how wider tires increase traction?.

just like in a car...a wider tyre makes the bike be on contatact with more of the road. the more contact...the more traction. generally....diff compounds and diff pressures within the tyre ill also affect it. wider tyires ill give better cornering traction but ill create more drag on a straight. and vice versa

you should probably look more into the answeres i give, its hard to really put out alot of information into this.

use wikipedia.com and howtstuffworks.com

if you got more questions put them up!
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Old 04-24-07, 09:07 PM   #3
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Some things to look at

Wikipedia has a fairly accurate explanation of bike stability (gyroscopic effect is actually negligible) as well as a very large amount of information and references.

Here are some other aspects to consider

Gearing - mechanical advantage of gearing.

Wheel strength - how the opposing tension of spokes creates a light but strong structure.

Bearings - how they ease the effort needed to turn the wheels.

Aerodynamics - how it has allowed riders of specialized vehicles to reach amaing speeds.

Frame design - the multitude of approaches to comfort and speed.

I've often thought one could do an entire class on The Science of the Bicycle - physics, physiology, mathematics (you should see the formula for determining the correct length of spokes!)
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Old 04-24-07, 09:12 PM   #4
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yea i was kinda pulling the gyroscope thing out of my a$$...lol

intresting project though...id be cool to see all that info put in one place
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Old 04-24-07, 09:13 PM   #5
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Quote:
Originally Posted by DeadSailor
how a bike actually stays upright when u ride?

well alot of it is the balance of the rider...but if you wanted to put a science angle to it its kinda like havign 2 big ol gyroscopes aka the wheels. the faster you go...the faster they spin...the easier it is to balance...assuming you dont suck
Very interesting explanation but, unfortunately, it's wrong. It isn't "balance" as in a tightwire or circus act and it certainly isn't gyroscopic stability. If it's gyroscopic, how can you ride at two or three miles/hour where the wheels are barely turning but nearly anyone can do it.

The real physics are quite complex but the essence is that you are always "steering" the bike back under you.
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Old 04-24-07, 09:44 PM   #6
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Quote:
Originally Posted by parkeredwards
Hey, I need a little bit of help with my school project its about the physics of biking, i have done some research but i have found very little. So I'm asking the Bike Forums community. So i think for this project i will need to new more about how a bike actually stays upright when u ride?, why bigger disc brakes have more stopping power, how wider tires increase traction? etc...

I will continue researching and i would really appreciate if anyone could help me.

Parker Edwards
You might want to get a copy of "Bicycling Science", by Wilson, MIT press:

(http://www.amazon.com/Bicycling-Scie...7472812&sr=8-1)

The meat of the book is all about bicycle physics, with topics such as

Power and speed
Aerodynamics
Rolling
Braking
Steering and balancing
Mechanics and mechanisms (power transmission)
Material and stresses.

I think you picked a great subject. People are familiar with bicycles, but not many understand the science behind it.

Good luck!
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Old 04-24-07, 09:56 PM   #7
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I can answer two of these for you but do you mind if I ask what grade you're in?

Bigger disk breaks have more stopping power because of what's called the 'Law of the Lever'. The disk turns about its centre and the stopping force from the break pad is at the rim. The bigger breaks have a bigger radius and the 'stopping power' called torque is equal to the force from the breaking pads multiplied by their distance from the centre, i.e. the radius of the disk. The bigger the disk's radius, the bigger the torque.

The question about why a moving bike stays up is much harder (and it's why I asked your grade. I didn't learn it until I got to college). Anything which travels in a circle experiences a force away from the centre of the circle, this is called a centrifugal force. For example, the centrifugal force is what stops the planets from falling into the Sun, the gravity of the Sun pulls them in but since they're travelling in circles their centrifugal force pushes them back out and cancels out the Sun's gravity.

This is also why you have to lean your bicycle to the side when you take a corner quickly. When you take a corner you are traveling in a part of a circle and the centrifugal force pushes you away from the direction you are turning. By leaning into the corner you cancel out the force pushing you away from it.

When you ride along on the straight your bike naturally tends to fall one way or the other, the same as it would if it was still but since you're on it you can turn the handlebars a little to either side to turn in a very small part of a circle and use this centrifugal force to push you back up again. You can see this if you cycle in a straight line along wet grass. If you look back at your track you'll see that you were actually turning a little bit from side to side all the time.

The reason you can't balance on your bike when you're stopped is because when you turn the handlebars you're still not moving so you don't get any centrifugal force.


Let me know if you understand that and if you do I'll tell you some more.
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Old 04-25-07, 05:34 AM   #8
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Originally Posted by Bald Student
I can answer two of these for you but do you mind if I ask what grade you're in?

Bigger disk breaks
Especially if we're speaking to a student we should model correct spelling. The word is "brake," not "break!" If you use a dictionary, you'll see the two words do not mean the same thing.

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Old 04-25-07, 07:41 AM   #9
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The wheels have a tendancy to "self-balance" due to the magnitude, and more importantly the direction, of the angular momentum resulting from the spin of the tires as one pedals away.
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Old 04-25-07, 07:51 AM   #10
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Originally Posted by Bald Student
I can answer two of these for you but do you mind if I ask what grade you're in?

Bigger disk breaks have more stopping power because of what's called the 'Law of the Lever'. The disk turns about its centre and the stopping force from the break pad is at the rim. The bigger breaks have a bigger radius and the 'stopping power' called torque is equal to the force from the breaking pads multiplied by their distance from the centre, i.e. the radius of the disk. The bigger the disk's radius, the bigger the torque.

The question about why a moving bike stays up is much harder (and it's why I asked your grade. I didn't learn it until I got to college). Anything which travels in a circle experiences a force away from the centre of the circle, this is called a centrifugal force. For example, the centrifugal force is what stops the planets from falling into the Sun, the gravity of the Sun pulls them in but since they're travelling in circles their centrifugal force pushes them back out and cancels out the Sun's gravity.

This is also why you have to lean your bicycle to the side when you take a corner quickly. When you take a corner you are traveling in a part of a circle and the centrifugal force pushes you away from the direction you are turning. By leaning into the corner you cancel out the force pushing you away from it.

When you ride along on the straight your bike naturally tends to fall one way or the other, the same as it would if it was still but since you're on it you can turn the handlebars a little to either side to turn in a very small part of a circle and use this centrifugal force to push you back up again. You can see this if you cycle in a straight line along wet grass. If you look back at your track you'll see that you were actually turning a little bit from side to side all the time.

The reason you can't balance on your bike when you're stopped is because when you turn the handlebars you're still not moving so you don't get any centrifugal force.


Let me know if you understand that and if you do I'll tell you some more.
Actually it is called centripetal force not centrifugal. A very common mistake.

Mud
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Old 04-25-07, 08:28 AM   #11
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Originally Posted by Bald Student
When you ride along on the straight your bike naturally tends to fall one way or the other, the same as it would if it was still but since you're on it you can turn the handlebars a little to either side to turn in a very small part of a circle and use this centrifugal force to push you back up again. You can see this if you cycle in a straight line along wet grass. If you look back at your track you'll see that you were actually turning a little bit from side to side all the time.

The reason you can't balance on your bike when you're stopped is because when you turn the handlebars you're still not moving so you don't get any centrifugal force.
I don't know why a bike wobbles when one is riding, but it's probably due to slight deviations in the bike's wheels. They cannot be perfectly symmetrical, and the rider's grip on the handle bars might make it worse. Even slight movements of the hand will cause the steering to be less than 100% straight and true.

BUT this has absolutely nothing to do with balancing on a bike. Like I said, it all has to do with angular momentum, or somewhat of a gyroscopic effect.

The centripital effect is what causes the reflector on a bike's spokes to be pushed out radially away from the wheel hub. It is also, as Bald Student mentioned, responsible for why one has to turn into a curve. Otherwise if you take the curve too quickly and too vertically, the centripital force will overcome the friction of the tires and you're going to slip.

Anyways, when you're pedalling furiously, there is an angular momentum component from both wheels. The wheels are rotating from top to bottom, from a rider's perspective. Look at the way your right hand grips the handlebars. Now image sticking your thumb out to the left. Your fingers are curved in the direction of the wheel rotation and your thumb is then pointing in the direction of angular momentum. L = r x p, where r is the radius of the wheel and p is the momentum of it. The momentum is = mass * velocity. The velocity is in the forward direction.

r x p is read as "r cross p", meaning you take the cross-product (don't worry about this term) of the two vectors. If the bike is moving forward, the direction of the angular momentum is then going to be towards the rider's left (in agreement with the thumb that I had you imagine was sticking out parallel to the handlebar.

If you are not perfectly balanced on the bike, there will be a torque applied to it. If you're moving slow, the velocity and hence momentum of the bike will be very low, resulting in a small angular momentum of the wheels. Thus, the torque will overcome the angular momentum and you'll have balance issues. If you're moving quite quickly, the momentum will be high and this the angular momentum will also be large. Any force or weight imbalance will still induce a torque, but the angular momentum will overcome it easily and keep you balanced.

Let's say the rider is putting more weight over the left side of the bike. T = r x F. Thus, the torque component will be towards the back of the bike. This is why steering into a turn is a good practice for those on bikes and motorcycles. The angular momentum will want to follow and meet the torque and thus the wheels will turn slightly. You have to turn the handlebars b/c on a bike you're not going fast enough or creating much of a torque for the bike to well, steer itself.

To help you visualize this stuff, think about a rolling frisbee or quarter. A frisbee is better because its edge is usually rounded which makes it unbalanceable. The harder you roll the frisbee, the faster and more vertically true its spin. When it starts to slow down, it will wobble a little bit and then take on a slight weight imbalance. Then the frisbee will start to curve a bit and roll in a large arc until it loses its speed and falls down.

Are there any handlebars maintaining the balance of the spinning frisbee? No.

Angular momentum, or if you want, gyroscopic motion, is the reason bikes stay upright when being riden.

Wider tires result in higher traction since there is more contact area between the tire and the ground. Thus, there is a greater magnitude of static friction exerted on the wheel. The tire will not slip and lose traction unless the static frictional force is overcome. I'm not too familiar with tires and traction so I won't say anything else about it to avoid directing you with inaccurate answers.
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Old 04-25-07, 09:17 AM   #12
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Quote:
Originally Posted by HillRider
If it's gyroscopic, how can you ride at two or three miles/hour where the wheels are barely turning but nearly anyone can do it.
I usually like throw the "motionless" trackstands that anybody can do out there for them to chew on whenever someone mentions gyroscopic effect.

Quote:
Originally Posted by Bald Student
The reason you can't balance on your bike when you're stopped is because when you turn the handlebars you're still not moving so you don't get any centrifugal force.
100% false statement, see above.

http://www.youtube.com/watch?v=XLGnnIK8M8w

Quote:
I don't know why a bike wobbles when one is riding, but it's probably due to slight deviations in the bike's wheels. They cannot be perfectly symmetrical, and the rider's grip on the handle bars might make it worse. Even slight movements of the hand will cause the steering to be less than 100% straight and true.
I highly doubt the deviation in a bikes wheels causes it to wobble. Even if you locked the steering at 100%, it will still fall over to one side when you push the bike down the driveway (or ideally, on a perfectly smooth flat surface).
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Old 04-25-07, 09:47 AM   #13
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I usually like throw the "motionless" trackstands that anybody can do out there for them to chew on whenever someone mentions gyroscopic effect.
Apples and oranges.

Trackstands = balancing act in absence of angular momentum.

Riding a bike without falling = "gyroscopic" effect and presence of angular momentum components.

Motionless = human balance
Slow speed = mix of human balance and angular momentum
Higher speeds = angular momentum

When most people pedal furiously, they're not going to be capable of any subtle balancing - nobody's going to be able to react quick enough to have any effect on the bike's balance.
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Old 04-25-07, 10:54 AM   #14
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Originally Posted by parkeredwards
Hey, I need a little bit of help with my school project its about the physics of biking, i have done some research but i have found very little.
Google is your friend. I googled BICYCLE PHYSICS and here are the first two hits:

http://socrates.berkeley.edu/~fajans.../bicycles.html

http://www.angelfire.com/pq/bicycles/
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Old 04-25-07, 11:15 AM   #15
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I don't like that first link of yours.
Quote:
Most physicists think that angular momentum in the wheel is responsible for balancing.
DEMO
angular momentum with lead wheel and rotating chair.
NOT TRUE! Very little to do with actual behavior. Proved by construction of bike with counterrotating wheels that behaved virtually the same.

The angular momentum demo with the wheel and rotating chair is used to demonstrate the motion of a spinning bike wheel. It's been used to show that angular momentum is responsible for bike stability as well.

Also, bikes with counterrotating wheels have also been used to prove FOR the argument that angular momentum is responsible for bike stability.

I fall into the category of stubborn physicist though. Mathematics and physics supports the angular momentum rationale even though Fajans argues against it.
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Old 04-25-07, 06:53 PM   #16
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Quote:
Originally Posted by Mudu93
Actually it is called centripetal force not centrifugal. A very common mistake.

Mud
Bald Student was correct in using the word 'centrifugal'. Wikipedia says,

The centripetal force is the external force required to make a body follow a circular path at constant speed. The force is directed inward, toward the center of the circle. Hence it is a force requirement, not a particular kind of force. Any force (gravitational, electromagnetic, etc.) can act as a centripetal force. The term centripetal force comes from the Latin words centrum ("center") and petere ("tend towards").
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Old 04-25-07, 07:55 PM   #17
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Bald Student was correct in using the word 'centrifugal'.
To be fair, both forces are confusing and I have simplified a little bit in my explanation above.

Aside from that (and a spelling correction which I'm happy to accept) I am pretty certain that what I wrote about the physics of cycling is correct.
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Old 04-25-07, 08:28 PM   #18
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"why bigger disc brakes have more stopping power, how wider tires increase traction? etc..."

Why are you assuming those are always true?
All you need are brakes that are "big enough. After that, it's the tire/road interface.
A bigger disk gives you more "leverage" than a smaller disk, but std. rim brakes provide even more. Must be something else in the equation.....How about hydraulic rim brakes (if they existed) vs hydraulic disks?
A wider tire might increase traction on loose surfaces, but how about ice?
Does a 700x23C tire lack traction in deep sand? Can you dig yourself out of the sand to find out?
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Old 04-25-07, 08:35 PM   #19
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May I ask what college you went to? The reason Im asking is because anyone with any knowledge of physics will tell you there is no such thing as the centrifugal force. In fact, the only force acting on earth as it circles the sun is gravity. Gravity, like most forces acts to produce an acceleration, which if you remember is a change in velocity. The acceleration the earth is experiencing due to gravity is a change in direction.

Furthermore, when youre taking a turn on a bike, there is no centrifugal force. What you feel pushing you out of a turn is (gasp) the desire of a body in motion to remain in motion. In otherwords, before the turn youre moving straight ahead, and a force is requiered to change your velocity. You experience this force from contact with the bike.

So, theres no need to make up this magical centrifugal force to explain circular motion.
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Old 04-25-07, 08:49 PM   #20
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Originally Posted by tirreno
May I ask what college you went to? The reason Im asking is because anyone with any knowledge of physics will tell you there is no such thing as the centrifugal force.

So, theres no need to make up this magical centrifugal force to explain circular motion.
Well put. Every intro to physics text that I've seen has a paragraph about this at the beginning of the rotational motion chapter.

I'm still not convinced that angular momentum is not the reason bikes are easily balanced when being riden. The rolled quarter/frisbee analogy seems to support it quite well.
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Old 04-26-07, 01:31 PM   #21
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May I ask what college you went to? The reason Im asking is because anyone with any knowledge of physics will tell you there is no such thing as the centrifugal force.
You may, University College Dublin, Ireland and you are correct in what you say but it is outside the scope of a High School physics project.
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Old 04-26-07, 01:40 PM   #22
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Try this weblink


http://library.thinkquest.org/J002670/parts.htm
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Old 04-26-07, 03:02 PM   #23
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hydraulic rim brakes do exist

Quote:
Originally Posted by Bill Kapaun
How about hydraulic rim brakes (if they existed) vs hydraulic disks?
Magura has been making hydraulic rim brakes for many years.

http://www.universalcycles.com/shopp...22&category=39

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Old 04-26-07, 03:09 PM   #24
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Originally Posted by spide
Magura has been making hydraulic rim brakes for many years.
I like the way the brake booster is part of the brake. Man, you're gonna need the reinforcement.......I believe I've heard stories of rims collapsing from the pressure-
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Old 04-26-07, 04:00 PM   #25
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Another thing to add about brake rotor size is this. What a brake actually does is turn the energy from the forward momentum into heat energy. Remember, that the energy has to go somewhere. Brakes, however, can only work effectively up to a certain temperature, after that they fade. A larger rotor can dissipate more heat. It has more surface area and one point on the rotor is in the cooling phase longer. You can have all of the clamping power in the world, but if the pads overheat, the brakes will do very little.

As far a balance on a bike goes, scientists still argue that subject.
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