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What causes "balance"?

Riding in Oregon requires a lot of time on the rollers in the off-season. I ride before i start teaching (HS Biology teacher) in the gym of our high school. The physics teacher asked me to come into his class and ride the rollers so he could help explain rotational inertia to his class. Pretty Cool I thought.

I know that the rotational inertia of the wheels is what keeps the bike upright while riding on the rollers, but I wanted to know if there are any other factors keeping you upright while riding on a road. Hypothetically speaking, if we get our wheel weight down to 150g, would it be harder to ride in a straight line?

Very curious,
Doug
 

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hi, I'm Larry
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Yes, gyroscopic forces would be lower

I forget all the formulas (I'm an old forgetful engineer). But gyroscopic forces are one of the factors that keep a bicycle upright. The lower the rotational mass (weight) of the wheel the lower the gyroscopic forces. I believe the mass factor is linear. If you cut the mass in half the gyroscopic effect would be a half. It is the major reason a bicycle is unstable at low speeds and does not get stable until you are spinning a bit.

The other factors are turning the front wheel and where you have your bodys center of gravity in relationship to the bike (balance).

If you can rig up a bicycle wheel with handles on either side this makes for a great demonstration. Have the student sit in a swivel chair and the spin the wheel up while they are holding it perpendicular to the ground. You can pivot the chair right and left by tilting the wheel side to side. You can steer the chair using the wheel. More of a demo for gyroscopic effects than rotational mass but pretty cool if you are a tecky sort. You get to feel the force relationships versus looking at arrow and formulas on the board.
 

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The rotational inertia of the wheels does create a gyroscopic effect, and the effect is proportional to mass and proportional to speed squared. Therefore going twice as fast gives you 4 times the effect. There is a good discussion of this at:
http://www.analyticcycling.com/WheelsInertia_Page.html
You can even test your wheels with a simple pendulum test!!!

However, in practice, what keeps you up is balance (center of gravity) and not the gyroscopic effect. If you have ever played with a small gyroscope, you will notice how you can spin it at an angle and it stays at that angle and rotates slowly around it's base (a process called 'precession'). However, two spinning bicyle wheels are not enough to keep 150lbs of cyclist upright. Consider what happens on ice. If you hit a patch of ice, the rotational effects are still present, but you lose the ability to balance, and you will fall over.:(

So, the short answer is you can buy those nice new carbon wheels you're lusting after and not have to worry about falling off your bike because of them...;)
 

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Non non normal
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ninelittlepiggies said:
What causes "balance"?

Riding in Oregon requires a lot of time on the rollers in the off-season. I ride before i start teaching (HS Biology teacher) in the gym of our high school. The physics teacher asked me to come into his class and ride the rollers so he could help explain rotational inertia to his class. Pretty Cool I thought.

I know that the rotational inertia of the wheels is what keeps the bike upright while riding on the rollers, but I wanted to know if there are any other factors keeping you upright while riding on a road. Hypothetically speaking, if we get our wheel weight down to 150g, would it be harder to ride in a straight line?

Very curious,
Doug
The more rotational inertia provided reduces the precision required in balancing the bicycle.

A good experiment is to take the class and their bikes outside and see how many can ride in the parking lot, then put them all in close ranks and tell them to trackstand. Their balancing abilities haven't changed but their ability to stay on the bike sure has.

If the above poster is correct( I believe they are) then since rotational inertia increases to the square of the velocity then lighter wheels would have less stabililty at very low speeds but this difference would rapidly diminish the faster you go.

The moral of this whole story is teach your kid to ride a bike going downhill. They will ride longer but of course the crash will also be more spectacular.
 

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Miggity Mac Daddy
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I was working the Interbike show when Lew had introduced their KOM wheels (this was in 2001) weighing a total of 800-900 grams or something ridiculous. I was talking to a guy who rode them, he said they rode beautifully up the mountian, but going down was scary as hell. He said it was like riding on ice - because - you guessed it, the wheels weren't heavy enough to generate enough gyroscopic weight (or whatever the proper physical term is).

I'm just repeating what everyone else on the board said, just in a "real world" setting. Hope that helps.
 

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Many facts are stated but most miss the point

ninelittlepiggies said:
What causes "balance"?

Riding in Oregon requires a lot of time on the rollers in the off-season. I ride before i start teaching (HS Biology teacher) in the gym of our high school. The physics teacher asked me to come into his class and ride the rollers so he could help explain rotational inertia to his class. Pretty Cool I thought.

I know that the rotational inertia of the wheels is what keeps the bike upright while riding on the rollers, but I wanted to know if there are any other factors keeping you upright while riding on a road. Hypothetically speaking, if we get our wheel weight down to 150g, would it be harder to ride in a straight line?

Very curious,
Doug

Rotational inertia is not a significant force in balancing a bicycle. Most balancing comes from steering geometry. When weight shifts laterally and the bike is in motion (on road or rollers), the front wheel automatically steers in the correct direction to compensate and bring the bike directly beneath the center of gravity (as adjusted for lateral forces.) A similar principle applies to an ice skater on one skate where no rotating mass is present to confuse the mind.

The reference to the experiment of holding a spinning bicycle wheel is misleading as these forces are small as related to bicycle stability (think about a 200 lb rider with 3 lbs of wheels and tires.)

From what I've read hear and at other websites, if lightweight, trick wheels are unstable at high speed, it's because they are inherently overstressed and/or inadequately stiff or strong.
 

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Just a thought. What about frame vibration.

The heavier an object is the more momentum or inertia that object can create. And since all rotating objects create vibration wouldn't a set of light weight wheels feel more stable at high speeds than heavier wheels. Light weight wheels wouldn't be able to transfer as much vibration to the frame. I have a steel frame with fairly heavy wheels and at high speeds my frame shudders quite violently. It's not that I can see it flex, but it feels as though the frame is alive in a bad way. If your frame isn't very energy transferring than this may not be an issue.

I always thought that frame shudder plays a big role in stability. Wouldn't heavier wheels cause more frame shudder (vibration)?
 
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